Maker Pro
Maker Pro

1980 Soviet Radio Cassette amplifier problem.

duke37

Jan 9, 2011
5,364
Joined
Jan 9, 2011
Messages
5,364
A few mm of unshieded wire will not pick up significant signal if properly terminated.
The FM signal can be seen at VT11 collector and the AM signal at the collector of VT12. If these signals are clean , then the AM and FM radios are OK.

Oscillation at any frequency is probably due to bad smoothing capacitors. Are you running on battery or mains? Is the low frequency oscillation mains frequency?
 

Richard9025

May 24, 2016
205
Joined
May 24, 2016
Messages
205
A few mm of unshieded wire will not pick up significant signal if properly terminated.
Yes, a few millimetres, but I use ~30-50 centimetres (long crocodile leads).
Oscillation at any frequency is probably due to bad smoothing capacitors. Are you running on battery or mains? Is the low frequency oscillation mains frequency?
Anything and everything runs on batteries.

The FM signal can be seen at VT11 collector and the AM signal at the collector of VT12. If these signals are clean , then the AM and FM radios are OK.
Oh, you've reminded me of FM.
See the following image:
8RR0x2 - Copy.jpg
One paradox that I can't understand:
Why if I hook an amplifier to the signal present at the - of C57, point KT, (On FM) it sounds very good, without any form of distortion, and if I insert music there, I get very good audio BUT if connecting them, I obtain a very distorted sound thru the original good amplifier... :confused:

I'll redo all the tests regarding inserting audio, hooking the present audio, tomorrow. I'll do the following: C59 (on AM), C57 (on FM) and point 29.
Are there any other points where I can insert audio/ hook to another amplifier? Thanks!
 

duke37

Jan 9, 2011
5,364
Joined
Jan 9, 2011
Messages
5,364
Why complicate things? You have a good FM radio and a good amplifier which work well together. What does it sound like?
You have an AM radio which is distorted. The distortion could be due to variation of the power supply or a faulty VT5 (unlikely). I have not worked out the AM AGC.
Look at the AM power supply and AGC line with the scope. Do not stuff funny signals in from outside.
You could use a medium wave oscillator modulated with a pure audio tone to look at the AM output.
 

Richard9025

May 24, 2016
205
Joined
May 24, 2016
Messages
205
You could use a medium wave oscillator modulated with a pure audio tone to look at the AM output.
Oh, the function generator app on my phone can also generate, apart from sine, square, triangular waves, with different amplitudes, frequencies, phases, it can also modulate (am and fm) a signal at different frequencies.
At frequency do I modulate the signal? (the IF of this radio is 465khz)
How do I insert it? like an audio signal? where?
Look at the AM power supply and AGC line with the scope. Do not stuff funny signals in from outside
Will do
If you supply me with the exact point of where I need to use the scope, it would be really helpful.
You have a good FM radio and a good amplifier which work well together
They don't work together, that's the "paradox".
What does it sound like?
Before writing more, I want to clarify how everything sounds:
I picked some numbers:
8RR0x2 - Copy - Copy.jpg
All these numbers mean points in which the audio present in these points is run through another amplifier.

On FM, the station that it's receiving is a very weak one, so don't expect high-quality audio.

number 1: https://vocaroo.com/i/s1eGHzLbJM6w
number 2: https://vocaroo.com/i/s0cn0NtLOtTU
number 3: https://vocaroo.com/i/s0sbqfX7O00K
number 4: https://vocaroo.com/i/s01mvQlvi9iA
number 5: https://vocaroo.com/i/s0xrtrPRUI57
number 6: https://vocaroo.com/i/s1wtSwU8LFk7

You can see the progressive distortion. Even with a good audio present at number 2, it still distorts it very bad (number 5). that's the paradox that I'm talking about
If I insert music ANYWHERE, it sounds excellent.
The distortion could be due to variation of the power supply
The power supply are actually 6 D cells batteries (they are huge), providing the necessary 9v pure dc current to the radio.
I think that all the distortion is caused by overdriving the amp?
Why so? I have 2 proofs:
1st: If I insert music ANYWHERE I get excellent audio. But I insert the music by connecting the signal where it's needed and adjusting the volume (of the phone) so I get a clear and strong audio at max volume (on the radio).
Even though I get good audio on FM at numbers 1 and 2, and the original amp is still distorting, but on a separate amp it sounds good, that may be because the amps are designed for different input signals and between the signals and the external amplifier are 200kohms resistance.
2nd: The current consumption:
With the radio distorting like hell, the current consumption is over 150ma!, at that current, the light bulbs start flickering and the voltage at the batteries starts dropping and oscillating.
With the audio disconnected and music inserted, I get a maximum of 100mA, with extremely loud and clear volume!
Also, I've seen that if I increase the input signal, the current consumption will raise in concordance too,
That means-- too much input-- the amplifier overdrives and distorts the signal--- it consumes much more current doing these--
The problem can be a transistor that amplifies the early stages of the AM and FM signals (it must be in a shared circuit). Or even a capacitor or resistance that messes up its gain factor.
You could use a medium wave oscillator modulated with a pure audio tone to look at the AM output
If it's possible to use a phone to generate a modulated am sine and somehow insert it into the radio, where it is detected preamplified and amplified, it can be much easier to trace, because we already know at which level the voltage should be, so the point of overamplification can be easily seen.
Thanks!
 

duke37

Jan 9, 2011
5,364
Joined
Jan 9, 2011
Messages
5,364
The quiescent current consumption will be about 10 or 20mA. The current will rise dramatically when the amplifier is driving the speaker. The power must come from somewhere.

The distortion on number 6 (AM) sounds just like the signal from radio Luxemburg which used to be received in the UK. i think it was due to multipath phase distortion.

Number 1 sounds as if you have two signals, Are AM and FM running together?

If you have a signal generator which can produce RF at the appropriate frequencies then a simple loop may provide sufficient field strength for the radio to pick up.

I do not think I can help much further
 

Richard9025

May 24, 2016
205
Joined
May 24, 2016
Messages
205
Number 1 sounds as if you have two signals, Are AM and FM running together?
Number one represents the only strong station that I can receive.
Because the VHF works in 64-73MHz eastern European band, it only receives harmonics of the stations in the 88-108MHz band.
The radio is also receiving another station on that strong harmonic, but very faintly... The test is just for audio quality.
If you have a signal generator which can produce RF
The only signal generator that I have is the phone that can produce, using its soundboard, audio signals (sine square and triangular). not RF modulated signals.
I don't think that I can find signal generators in that deposit, but I'll take a look Monday.
In the meantime, I checked the preamps of the FM and AM circuits and I think I found a problem:
8RR0x2 - Copy - Copy (2).jpg

21mV output on FM! Isn't that a lot? (with an input of 5,4mV)
Gain on FM: 3,8
Gain on AM: 1,5

I also checked the transistors, they're all ok:
2 transistors are not in the circuit, strange.
///TRANSISTOR NO.////BASE-COLLECTOR////BASE-EMITTER///
T11=703-724
T12=726-730
T9=718-747
T2=761-764
T6=760-766
T4=667-668 (out of the circuit: 723-736)
T8=760-765
T1=732-758
T3=770-782
T10=708-712
T5= Nowhere to be found
T7= Nowhere to be found

I've pm'ed @73's de Edd , but it doesn't look like Edd has checked this thread recently.
No problem if you think you can't help further, I'll post all my progress here.
Thanks so much!
 
Last edited:

73's de Edd

Aug 21, 2015
3,613
Joined
Aug 21, 2015
Messages
3,613
Sir Richard9025 . . . . .

Let's just see if the following info, can create in your " minds eye ", just exactly what is occuring in your audio output stage.
ALL without the use of your non available oscillosope or even your then having the full experince in using one.
Consider the use of these vector displays being shown below in depicting a pure sine waveforms gradual transitioning . . . . at progressively increasing audio input levels.

A PURE SINE WAVES ANALYSIS . . . . .

sine_to_reduce_II.png


Shown, at the very top row, is the nice, pure sine waveform in the BLUE outlining.
Its further color enhanced markups, depict the pitfalls that may befall it at higher amplitude audio input levels.
As the waveforms upper + node approaces your power supplys maximum + voltage threshold capabiliy, the very top tip of the waveform starts progressively compressing and crunching down into a more rounded waveshape, as is being shown in the YELLOW marked in outlines.
Meanwhile, its lower - node waveform tip is also approaching ground, and it just can't go BELOW ground . Expect the - node to be taking on the same compression and gradual transition into the compressed and rounded YELLOW - node depiction.
Now with there being an even further increase in audio input level . . . . the YELLOW rounding of the sines + and - nodes start a transitioning of the node tiplets into a flattening.
We now have ourselves a waveform transition into a quasi squarewave, and that result will not sound good at all. It even sounds progressively worse at higher input levels.

The next cluster of sine depictions are split up into three audio input levels as is being marked as Performance 1-2 and 3 levels at the right side of the plots..
The left and right halves of each level present the effects upon the + node of the sine on the left and the negative node on the left.

Finally . . . . . for making an Instant Visual Evaluation . . . . . all of the sine portions are being color coded as per the decline of quality of the waveform.

GREEN's are basically ALL of the units being unaffected, and staying pure sine waveforms.
YELLOW is being a noticable degradation of the pure sine waveform.
ORANGE is being a severe degradation of the pure sine waveform.

Now lets see how the three performance levels relate to your unit, by initially observing the relevant portion of the schematic now placed just below.

AUDIO OUTPUT SECTION SCHEMATIC . . . . .

VEF_260_2_AF_Amp_Snippet.jpg


Initial point of interest will be the three YELLOW marked up resistors that make up a voltage divider bridge that halves the supply voltage with about a 10% swing above and below 1/2 of the 9VDC supply voltage.
This is used for setting the base bias on the VT2 transistor which is the first in a sequence of 5 direct DC coupled transistors, to the very output of the amp, where it finally transistions to using AC coupling into the single speaker.

SINE PLOTTINGS . . . . REFERENCING AGAIN . . . .

Now in referencing back to Sine Waveforms Performance 1 Level it is split into the left half in the effect upon the + sine portion of the waveform and the right half relates to the - node portion of the sine waveform.
it is showing the handling of a low level input waveform that is of such low level that, no distortion will be noticable.
Take particular attention of the fact that the left half illustration is showing a sine wave form that is imposed upon a trio of +4.5 Vdc . . . .a +4.8 Vdc and a +5.1Vdc levels as are obtained by adjusting the R18 trim pot.
Then the trim pot is adjusted in the other direction, such that the same effect is noticed on the -node of the sine waveform as is being shown on the right half of the picture.
Now in either situatiuon of the extreme adjustments of the control, both the + and - nodes show no distortion . . .yet.
That situation is being reserved for the Performance 2 Level plots which are just below.
In that situation the audio input has been gradually increased . . .such that . . . . .
In the first plot, the bias is set at one half of the supply voltage and the + and - nodes will then just fill in their amplitude capabilities.
Run the bias on up to ~+4.8 and the + sine waveform starts taking on a rounding compression of its + node.
With the bias up to +5.1 and the sine waveform takes on a harsh transition into a harsh quasi square wave waveform . . . . .that's being one unpleasant sound.
Look to the right half of that Performance 2 Level plot and you can see the same effect upon the - portion of the sine wave, with the -4.2 and -3.9V bias settings of R18 control..

The last Performance 3 Level plot is where the bias is set at +4.5 Vdc and the audio input level is applied to such a high level that the sine waveform is even showing the start of tip clipping at that level.
Then as the different possibilities of bias settings are made, it only gets worse in all of the settings.

Look at all of the GREEN positive and negative color coded waveforms, and it reveals that R18 should be adjusted such that when the pot is set at ~ one half the 9Vdc supply voltage that the max level of input AC waveform should not show any effect on its + and - node tips.
Figure that as your max input level that you would want coming into this amp input. That does not seem to be the case on the level of audio which is coming into the audio output amp from your post amplifiers after the AM and FM detector stages. That is being even after the somewhat high losses of passing thru the loudness compensation and tone control networks located just before it.
Let me later, show how to incrementally reduce that excess audio input coming into the audio input stage, on down to that level that the amp stage will handle without overloading.

CROSSOVER DISTORTION . . . . .

A last point to consider on this unit is the fact that the low level transition of a sine wave from O volts amplitude on up to 700 millivolts will be cut out due to the turn on need of a silicon transistor in requiring that level of input voltage for its base to initially start reacting to it in creating any progressive collector conductance.
700 MV on both the + and - node is quite a click-click " dead space" to account for.
Soooo . . . . seems like this circuit design has set up a series biasing voltage loop thru the output transistors, so that they will always be slightly conducting, which THEREBY leaves a pre conducting path for that very low level audio to ride upon and share that very same DC current loop path .
In now referring back to the SCHEMATIC snippet, one will be seeing the marked in series of GREEN resistors, which make a current loop conductive path, it starting from the +9 DC supply line and then going down thru R30-R34-R37-R38 pot-R39-R41 and the conducting transistors to ground at the emitter of VT10.
AGAIN . . . . just as was done with the R18 symmetry pot . . . . . and in this case, R38 pot, they have minumized the adjustment range of those pots by assigning their resistance change variance, to have but a minimum of range adjustment.
Basically, its having just enough swing to bring upon correction of any associated manufacturers parts variances and aging with time.
If you figure out the current loop resistance values, they permit right at 1 milliampere of current passage. That then gets multiplied on up by the gain of the 2 output transistors and their 2 driver transistors. Those are being configured, not as darlingtons, but instead, they are using a very high current gain Sziklai ***** pair circuit configuration.

Sziklai ***** . . . . .
https://image.ibb.co/dHWSxS/Sziklai_explanation.png


THE POWER AMPS SITUATION . . . .

It seems to be operating normally, with the limiting range effect of both the R18 symmetry pot and the R38 two trim posts making it impossible to GROSSLY misadjust them, by virtue of their only having a very limited range of adjustment / effect.


AN INSERTION . . . . .

Just after having seen your posting of scope waveforms.
That display of waveform peak nodes just confirms the symmetrical clipping of both of the waveform, with too much audio input.
That scope just reminds me of Telequipment of the '70s which was a cheaper knock off of the Tek scopes. Of course, your unit might even happen to be a Russkie wannabe of those units.
You seem to be in AC coupling mode into the scope which will settle the waveform to the center of the Y axis trace pattern.
I think that the option for DC coupling mode, which would ADDITIONALLY let you see any combined DC element , with the AC waveform . . . . . is being a 3 position slide switch at the very bottom of the front control panel.
I think that it is marked as DC---GND---AC as its two modes. With the GND position letting you place the sweep X trace position either at mid screen graticule line or at the very bottom graticule line.

YOUR GAIN SITUATION . . . . .

You now neeed to refer to the very bottoms schematic snippet, which depicts the FM and AM detector outputs and their following, single post ampolifiers:

Using the Reference 1 left half . . . . .

You will see that they have selected beta options of both a VT11 NPN and VT12 NPN transistors that match the required needed gain of the higher level FM detectors audio output, versus the lower output AM detectors audio output.
PLUS . . . they are using a R52 COMMON SHARED collector resistor, with the actual audio input being selected by the switching off of power to the respective AM or FM RF stages.
The GREEN ARROW path shows the AM signal flow, while the RED ARROW path shows the FM signal path flow. They combine at the C61 coupling capacitor to then flow to the very lossy tone and volume circuitry just prior to to C14 at the input of the power amp section.

Now move over to Reference 2 right half . . . . .

Lift R52 and sub in a 5K trim pot and take its center tap to C61 where you have since lifted it from the collector connnection of VT11.
Extreme CCW terminal of trim pot connects to B+ supply junction of R49 & R48, with extreme CW terminal of trim pot to shared collectors of VT11 and VT12.
You should then be able to power up and use the trim pot to bring your excessive audio levels down to the input level that does not have clipping until the max adjustment of the Radios MAIN volume control.

DETECTOR SCHEMATIC REFERENCING . . . . .

VEF_260_2_AF_Detectors_Snippet.jpg



Thasssssssit . . . . .


73's de Edd
.....
 
Last edited:

Richard9025

May 24, 2016
205
Joined
May 24, 2016
Messages
205
Edd strikes again!
I'm thankful for this great reply of yours!
The first part of the reply really Edd-ucated me! ;)
THE POWER AMPS SITUATION . . . .

It seems to be operating normally
Good, so no more fiddling with the power amp.
a very high current gain Sziklai
Just read about the Darlington configuration versus the Sziklai one, very interesting!
That scope just reminds me of Telequipment of the '70s which was a cheaper knock off of the Tek scopes. Of course, your unit might even happen to be a Russkie wannabe of those units.
It actually is a fully (except the CRT) build Romanian oscilloscope. If it is a copy of some Russian wannabes, most probably. It's 1970's technology, even though it is made in '86.. still works, that matters...
I think that it is marked as DC---GND---AC as its two modes. With the GND position letting you place the sweep X trace position either at mid screen graticule line or at the very bottom graticule line.
Exactly.
In all the oscilloscope tests, it was set to AC coupling mode.

Now move over to Reference 2 right half . . . . .

Lift R52 and sub in a 5K trim pot and take its center tap to C61 where you have since lifted it from the collector connnection of VT11.
Extreme CCW terminal of trim pot connects to B+ supply junction of R49 & R48, with extreme CW terminal of trim pot to shared collectors of VT11 and VT12.
You should then be able to power up and use the trim pot to bring your excessive audio levels down to the input level that does not have clipping until the max adjustment of the Radios MAIN volume control.

For this test, I'll explain step by step what I did:
I should start with the fact that I couldn't find a 5k pot. The closest one was a 8,1k pot...
Firstly, I disconnected the leads of C61 and R52.
Then I "tackled" the necessary crocodile leads and the pot.
More exactly:
It is shown where each coloured lead connect to.
IMG_20180418_191407.jpg
Now the pot:
IMG_20180418_191407.jpg

Good, now the actual measurements.
For the initial test, I made a recording in which I turn the pot in the following way:
IMG_20180418_191407 - Copy (2).jpg
From the leftmost peak to the rightmost peak, I gradually turned clockwise the pot (resistance between the left-most peak and the center lead increasing)
https://vocaroo.com/i/s0LXcYut5RtN
(The radio's volume is at ~50% if you're wondering)
You notice how from weak audio it goes up to extremely bad audio:
IMG_20180418_191407 - Copy (3).jpg

Adjusting the pot, I found a point in which the audio is not distorted and extremely loud!!
I measured that position, it is at 1,09kohms between the left lead and the centre lead, and 8 kohms between the centre lead and the rightmost lead)
But I also noticed something:
The audio quality is pretty bad, take a listen:
(the radio's volume is at 100%, before this, I couldn't even understand anything)
https://vocaroo.com/i/s05N3JclFtXq
And at ~80%
https://vocaroo.com/i/s14InSx76HQj
At 80% and 100% the output is extremely loud, the sound, that is still intelligible, being heard even outside the house.

I don't think that the audio is distorted, it doesn't clip and the speaker is acting like normal.
But the audio quality is just very poor.

To verify, I turned the pot in its leftmost point (where the audio is the best and the maximum volume is the lowest in comparison with higher settings) (marked in
BLUE in the drawing above)
https://vocaroo.com/i/s0n7Nt76rqNW
It certainly sounds better, but still, it is a poor quality sound (it sounds better in the recording than in real-life)
The recording is playing like it's loud, but it isn't, at max volume, it is mediocre, even quiet audio.

To capitulate this test.
These modifications surely change the sound in the better, stopping that annoying bass effect and making the output intelligible, even if the loudness is the same as before !!! But the audio is very poor regarding the sound quality...


This test was taken with the radio on AM mode.

I suggest you take a look at post #84 where I took the signals present in those numbers and amplified them using another radio.
The audio after the FM stage is very good, but if coupled to the original preamp and amp, it sounds exactly as bad as the AM
The audio after the AM stage is bad, coupled with the preamp and amp, it sounds even worse.
Very strange problems...
I say that we can do the same test, but for the FM stage (I think we need a different set-up), because on FM, the signal present at the preamp is good, unlike on AM... check post #84
I think that both the audio levels present at the input of the different preamps are too high, and on AM ALSO too poor...

I can't say all the thanks that you deserve, you deserve too much!! That like button is too little..:)
 
Last edited:

73's de Edd

Aug 21, 2015
3,613
Joined
Aug 21, 2015
Messages
3,613
You need to get that pot value on down closer to that original collector load resistance. That should enhance the audio quality appreciably. I just thought that you were more likely to have a 5 K pot salvaged from your junque pile of salvaged / pulled parts.
 

Richard9025

May 24, 2016
205
Joined
May 24, 2016
Messages
205
You need to get that pot value on down closer to that original collector load resistance. That should enhance the audio quality appreciably.
The only pot that is close to 3,6k is a 3k one.
With it in the exact same configuration as before (the pots look almost the same), I get much much better audio at the same loudness level!
With the pot adjusted in a position in which the audio is loud and clear, I get the following audio:
At 100% volume:
Speech:https://vocaroo.com/i/s1SJgK4tXIN9
Folk music:https://vocaroo.com/i/s012KkuNwN1W

The audio is very loud (and clear!). But on other receivers, at max volume, the sound is a little bit louder. I think that if I insert audio from the phone, I can get louder audio without distortion.

Still, the difference is HUGE between the audio that I get with this test and the audio that the receiver normally produces.

I consider this test as a success!

I don't really know what this pot does? lower the gain of the preamp transistor?
I don't want to "tackle" the pot there. I want to find the bad component that does this.
Oh, I forgot to measure the resistance across the 3 leads of the "new" 3k pot in its "perfect" position. If you need the values, just say! Thanks!
 

Richard9025

May 24, 2016
205
Joined
May 24, 2016
Messages
205
Measured the resistance of the pot (in it's "perfect" position):
IMG_20180418_191407 - Copy (2).jpg

Isn't it strange? that the BLUE resistance is lower than the PINK resistance? Should't it be the opposite?
The actual pot:
IMG_20180421_143049.jpg
IMG_20180421_143103.jpg

It is marked as a 2,5k pot but the highest resistance is 3k. That pot is from '85
 

73's de Edd

Aug 21, 2015
3,613
Joined
Aug 21, 2015
Messages
3,613
With all of time that you have spent painstakingly testing all of the associated transistors and components in the audio stages, I don't think that the EXACT cause of the problem will be found. Loss of gain is the NORMAL, this condition of having excess gain, after the detector stages is an indeed rarity.

After finding those resistance values created by two values of the wiper setting of the pot, I would insert those two values into the holes in the PCB where the former load resistor was, and then connect the resistors two free center leadsds together and run a wire from that joint over to the audio input capacitor.

That will make minimal room being needed for this connection technique.

SINCE I have never seen a radio that did not break up and overload at the last 10% or so or its max volume position. Therefore, I would probably recheck the pot setting and rethink those two resistor values, to create that little extra amount of volume and the ensuing last bit of max volume distortion.
 

Richard9025

May 24, 2016
205
Joined
May 24, 2016
Messages
205
this condition of having excess gain, after the detector stages is an indeed rarity.
Indeed, we're having TOO MUCH of something...
Looking back at the voltages, I see one area where we have too much DC voltage:
VEF 260-2 White Schema Full   Size - Copy - Copy (2).jpg

Around T8 that is. I've read on other forums that some people had problems with components around T8, but not experiencing my symptoms...
You can also see that T5 and T7 are nowhere to be found, that would mean lower gain, but we have too much of it...

I fiddled with R29, and it just lowers the volume a little bit, still, it doesn't change much. If I turn it the other way, it distorts like hell then goes silent. I just readjusted it to its factory position

"jumping" the audio after the detection to C61, I get the exact same distortion provoked by overloading of the amp...: The preamp clearly isn't a problem.
jump.jpg

On some russian forums, they said that the transistors test good but in circuit are bad. A solution to this is freeze spray, which I shall get this week. Maybe the transistors act funny and overamplify the signal?

How did the russian engineers manage to reassure the gain needed without T5 and T7?

Looking at the actual transistors in the radio section (A4), they look very unusual.
Some TO92 packages are in a military-like brown package.
KT368BM%20Bipolar%20Transistor%20PNP%20(800MHz,%2015V)%20(1).jpg

Other trannies, are in a strange top-hat package with gold plated leads, they're marked 2T306B.
They look exactly like this:
2t306b.jpg

If needed, I can photo each transistor and document them.

I've also specified in this thread that I had an exact older model of this radio, just the circuitry was a little different.
Now it is completely torn apart, for components. But it had the exact same problem, not in the amplifier, but in the radio unit. The radio unit has all the components in it, I could pull out all of the components and test them to see which are bad. If I find something, I'll double check in my current unit.

I would insert those two values into the holes in the PCB where the former load resistor was
This is a solution to this. We can keep looking for the problem or I just do what you said here.
I say that we continue to search for the culprit this week. If we do not make any progress, I just tackle a pot, adjust it and leave it be.

Thanks!
 

73's de Edd

Aug 21, 2015
3,613
Joined
Aug 21, 2015
Messages
3,613
"jumping" the audio after the detection to C61
What was the volume level produced just above, since you were being without the AM post amplifier stage ?

Looking back at the voltages, I see one area where we have too much DC voltage:

That voltage is being related to the FM section IF amplifier strip ONLY, and on FM you are even having to use an FM subharmonic to be able to EVEN get any " psuedo " FM reception at all, since this old receiver operates at 65.8---74 Mhz.

I fiddled with R29, and it just lowers the volume a little bit, still, it doesn't change much. If I turn it the other way it distorts like hell then goes silent. I just readjusted it to its factory position.

That relates to that prior stages base voltage and all that I can make out for sure on its translation from Russkie is the "adjustment" aspect , but it is being the FM AGC control.

Looking at the actual transistors in the radio section (A4), they look very unusual.
Some TO92 packages are in a military-like brown package.

The shown transistor with its gold flashed bottom base case and leads, is being a PWEMIUM BWEWED Russkie unit.



 
Last edited:

Richard9025

May 24, 2016
205
Joined
May 24, 2016
Messages
205
What was the volume level produced just above, since you were being without the AM post amplifier stage ?
Without the AM post amplifier: (Gradually increasing)
https://vocaroo.com/i/s1bYQNKXqsHX
The difference between the sound with the preamp and without the preamp is that, without the preamp, the sound at lower volumes is not distorted and it starts distorting later (at ~40%) instead of the sound with the preamp, that starts distorting at ~15-20% volume.
That relates to that prior stages base voltage and all that I can make out for sure on its translation from Russkie is the "adjustment" aspect , but it is being the FM AGC control.
In the English version of the schematic (poor quality) it is marked as MODE SETTING:
PncrG.png

is being a PWEMIUM BWEWED Russkie unit.
The Russians had a weird habit of combining consumer grade units with military stuff...
Just look at this video in which someone analyses a Russian TV:

In the meantime. I'll search for some replacement transistors, even though I doubt that the present transistors are bad...
Thanks!
 

Richard9025

May 24, 2016
205
Joined
May 24, 2016
Messages
205
Looking at the inside of the radio unit, I found T5 and T7!
IMG_20180426_200741.jpg

T7's junctions measured good, (720-730mV)
T5's measured kinda bad out of circuit (650-660mV)
I replaced T5 with an exact same model of transistor from another radio, that measured good.
With T5 replaced, NOTHING changes, the results are the same.

Ahh, I can't understand why it distorts on AM and on FM... There must be something shared between them, they can't be equally as wrong...
 

Richard9025

May 24, 2016
205
Joined
May 24, 2016
Messages
205
I checked many more components including:
ALL resistors (only R48 measures 38k instead of 36k)
ALL ceramic capacitors (known for bad reliability, all measure good)
ALL electrolytics
Some other components
Take a look:
8p9cMo.jpg
 

duke37

Jan 9, 2011
5,364
Joined
Jan 9, 2011
Messages
5,364
You loaded five snippets of sound a long way back. I only listened to two of them.

The first one was the FM radio which I believe does not receive the broadcasts in Romania so that the radio is responing to spurious responses. If the deviation is extra large due to multiplication, this will give distortion.

The last one was AM and the distortion was due to poor radio propagation.

All radios will distort if the volume is too high. This is basically a battery set so normal maximum will be quite low. The absolute volume control position is a bit irrelevant.

I think you are looking for faults which do not exist.
 

Richard9025

May 24, 2016
205
Joined
May 24, 2016
Messages
205
Today I've replaced most of the transistors in the radio unit.
And checked almost all of the capacitors.
My capacitance meter will make lower picofarad values seem higher, it isn't made for values under 30pf.
measurements.jpg

Replaced T5, T7, T4, T2, T6, and T8
After all of these replacements, NOTHING changes.

Then I looked at the remaining transistors, T3, T1 and T9
T1 and T9 are the gold plated ones, they look beautiful shiny and golden!. They are some beauties, they measure good. I didn't replace them.

T3, on the other hand, is a low gain (45) high-frequency transistor (400MHz)
The only low gain PNP transistor that I found in my junk box, was an EFT323 germanium transistor, but it had a frequency of only 1MHz! It was marked with blue, so it had a gain between 50 and 60.

I replaced T3 with an EFT323.
With that germanium installed, it doesn't pick anything, buuuttt the static was sounding very good, not distorted. (The radio distorts so bad the sound with the original T3, that even the static gets distorted). So I guess, that with a good replacement, it should sound good (the level of the sound after the detection will be lower, thus the amplifier not overdriving and distorting).

More detailed: The loudness of the static at max volume on this radio was almost, if not, the same as my other radios, so I guess it is in parameters regarding the audio levels.

To check if the T3 may be bad, I replaced that EFT323 with a BC556B (medium frequency, high gain)
BC556B is a high gain transistor, 240 MINIMUM gain, while the original T3 was MINIMUM 45 gain.
With BC556B installed, EXACT SAME results, no different than the original T3, even though they are completely different transistors...

Looking at the original T3 datasheet, the gain is between 45 and 160

I also looked at the FM stage.
On FM, no change with the new transistors.
But listening a bit at the sound, the distortion is not exactly the same as on AM.
The type of distortion is the same, they sound the same, but the level at which the amplifier distorts, its different.
On AM, it starts distorting even at 10-15%, while on FM it starts distorting at 30%...
This leads me to think that the problems are separate...
The only stage in the FM circuit which I didn't check, is the screened VHF block.
The former owner's only change to this radio was to adjust the variable inductor cans there, I think to receive the stations in the 88-108mhz band...

If the deviation is extra large due to multiplication, this will give distortion.
I have another radio, with the same FM band (65-74MHz) and it receives stations, and they sound very good!

due to poor radio propagation.
It's doing this to every station that it receives, even the static (including RRA, the station that is only 3km from my location, that is received by almost everything, even poorly screened amplifiers)...
I don't think that this may be a problem, as any of my radio can receive these stations and sound very good!

I think you are looking for faults which do not exist.
I've thought about this, but measurements and comparisons with any radio that I choose show that this unit is bad...
I'll look for a replacement for T3

For @73's de Edd
Can't we "tackle" a pot or some resistors to reduce the gain of T3, as we did with the preamp? To check if T3 is overamplifying the signal?
T1 and T8 are only for FM or in a shared circuit with the AM stage?

Thanks!
 
Top