1970 Delco Radio Repair - audio distorted

[Frankchie]

I'm trying to repair a 1970 GM am/fm car radio that plays pretty well on both bands, but the audio distorts at high volume levels. I have two questions:

1. Could this distortion be normal? It sounds pretty good at moderate volume but when volume is at the top say 15% of it's range it distorts. Using a 1khz tone my scope shows distortion beginning when the 10ohm speaker is driven above about 1.25 watts. Any guess as to how many watts this should be capable of with minimal distortion?

2. The 11.7v power supply driving the audio amp/output stage has high ripple when volume is high. Using the 1khz tone I can see a 1khz ripple of about .6 volts P-P. I can't find anything wrong with the PS and I have another similar radio that works better and that seems to have a similar ripple. Although that 2nd radio will also eventually distort at the very top of the volume control range. So, Is this ripple typical or acceptable?

3. I'll probably have to replace the entire audio stage with a modern audio module. I'm looking at a module based on the
TDA2030A chip, but I am open to any suggestions. I'd like to keep the radio original, but it may not be worth the effort.

Thanks
Frank

 

[duke37]

If the power supply has a large ripple, then a capacitor is faulty.

Look at the output waveform and see if it clips at high volumes. You will not get more output voltage than the PSU voltage. I think that more output could be obtained with a lower impedance speaker.

 

[Bluejets]

What is it like on a fully charged battery without any induced noise from a mains power supply?

After all, that's what it is designed for.

Probably never meant to be a HI-FI system in the 70's anyhow.

 

[kellys_eye]

Distortion levels of 10%+ are quite common but check that your PSU can deliver the required current before condemning it.

 

[73's de Edd]

a 1970 GM am/fm car radio . . . .

A more PRECISE model number and brand info needed if any researching of its present designed circuitry is to be analyzed .
Also second the use of the unlimited, and sag free power, as would be provided by a HOT car battery when testing its audio output at full power.

 

[shrtrnd]

Are the speakers as old as the radio?
If you haven't already, I'd be checking the speakers themselves before I went troubleshooting the radio.
Are the speakers the ohm-rating of the radio output?
I admit seeing distortion (I assume you have the test equipment used for that purpose) at higher volume is an
indication of a circuit problem, but my first test would be the speakers themselves.

 

[Frankchie]

If the power supply has a large ripple, then a capacitor is faulty.

Look at the output waveform and see if it clips at high volumes. You will not get more output voltage than the PSU voltage. I think that more output could be obtained with a lower impedance speaker.

If the power supply has a large ripple, then a capacitor is faulty.

Look at the output waveform and see if it clips at high volumes. You will not get more output voltage than the PSU voltage. I think that more output could be obtained with a lower impedance speaker.

Yes, it does clip at high volumes at about 6-7 volts, the output stage has a 11.7v supply. The filter cap measures 2300uf disconnected. The value markings are not visible. I have paralleled a 4000uf cap and the ripple reduced maybe 40%, but it didn't seem to help the distortion.

The volume seems actually quite adequate, but I'll keep your speaker suggestion in-mind.
Thanks for the reply

 

[Frankchie]

What is it like on a fully charged battery without any induced noise from a mains power supply?

After all, that's what it is designed for.

Probably never meant to be a HI-FI system in the 70's anyhow.

The bench power supply I am using is very stable, but at some point I'll probably have to try a car battery.
Thanks

 

[Frankchie]

Distortion levels of 10%+ are quite common but check that your PSU can deliver the required current before condemning it.

Thanks..like another poster said these were never claimed to be HIFI.

 

[Frankchie]

a 1970 GM am/fm car radio . . . .

A more PRECISE model number and brand info needed if any researching of its present designed circuitry is to be analyzed .
Also second the use of the unlimited, and sag free power, as would be provided by a HOT car battery when testing its audio output at full power.

The model number is not on the radio, probably a sticker has long been gone. I have a Sams schematic that's very close for a 1972 F-85 Olds. model 23afp1.

I'll have to give the car battery a try at some point.

Thanks

 

[Frankchie]

Are the speakers as old as the radio?
If you haven't already, I'd be checking the speakers themselves before I went troubleshooting the radio.
Are the speakers the ohm-rating of the radio output?
I admit seeing distortion (I assume you have the test equipment used for that purpose) at higher volume is an
indication of a circuit problem, but my first test would be the speakers themselves.

I have tried a couple of different speakers with no improvement. The schematic I have for another very similar radio specifies a 10 ohm speaker. If I can find a 10 ohm resistor I might try that as well.

I have injected a modulated carrier with a1khz sine wave via the antenna and a 1khz audio signal directly into the ist audio stage and both ways can see high volume distortion on my scope.

I think the output of the first stage shows some distortion and intend to dig into that area next.

Thanks

 

[Frankchie]

Quick update: I haven't spent much time but I think the problem may be that the negative feedback circuit is not providing enough feedback. This would explain why I seem to have plenty of volume and lot's of distortion at high volume. Again, I don't have the exact schematic, but I have one that's pretty close. First checks of the feedback circuit seems to give low resistance readings to ground. Seemingly much too low given the resistor values that I can see. I need to dig in further to investigate this.

More details: The first stage of this 3-stage direct coupled amplifier is a common emitter circuit with no emitter resistor, the emitter is tied directly to ground. This kind of circuit is limited to very small signal input given that the base bias is about .6v. At full volume the radio provides about a 0.2v P-P signal to the fist stage and that causes considerable distortion at the collector. I suspect that the input signal is too large because of weak negative feedback.

So, again, I need to dig into the feedback circuit. I'll post back with updates.

For now, I'm ignoring the P.S. high ripple .

 

[Frankchie]

Ouch... Old resistor colors (and old eyes) led me to think a 5.6 ohm resistor was 560 ohms. So the 5.6 ohm resistor explains my low reading to ground in the feedback circuit. Having an accurate schematic would really help, but I do not have one.

Worse yet, somehow the gain of the circuit seems to have an unexplained drop by about half during my troubleshooting activities. I may have somehow done some damage.

At this point, I'll probably wait for my on-order Chinese amplifier module that should replace the entire audio stage and probably perform much better than the original circuit.

 

[Cannonball]

The first stage that shows distortion is a class A amplifier. Without any input signal, measure the collector voltage.
It should read half of what your power supply shows.

I hope this helps.

 

[Frankchie]

The first stage that shows distortion is a class A amplifier. Without any input signal, measure the collector voltage.
It should read half of what your power supply shows.

I hope this helps.

Thanks for the tip...makes sense.

The collector has been reading about 7v with a 11.78 supply. So a little on the high side for your guideline. Although it's been that way since the beginning and I don't think it has changed with the recent drop in gain . I believe the "no-emitter-resistor" configuration makes collector voltage a little less stable since beta can vary with temperature, etc. and affect collector voltage. So 7v may be reasonable given that stability factor. BTW, there is what looks like a DC bias feedback circuit that may be intended to mitigate 1st stage beta drift.

 

[73's de Edd]

Sir Frankchie . . . . . . . .

Greetings . . . . . considering us having these initial forthcoming but limited info snippets to work with . . .or against.

"repair a 1970 GM am/fm car radio "
And will that be AM/FM and FM STEREO and is it also containing tape player provisions ?

"a 1972 F-85 Olds. model 23afp1 "
Got that . . . DELCO factory model coding . . . with 2 relating to a 1972 model . . . . 3 relating to the Oldsmobile division of Delco . . . . "afp" relating to feature variances and 1 as to circuitry revisions.
IF . . . . . . you have guessed correctly on your / that radios model.

"The schematic I have for another very similar radio specifies a 10 ohm speaker."
That is correct . . . if you do fudge in using an 8 ohm unit it will then mismatch in power capability a bit and that different speakers frequency reproduction response will
skew a bit in the respect of it boosting the treble and decreasing the bass.

"The first stage of this 3-stage direct coupled amplifier is a common emitter circuit with no emitter resistor, the emitter is tied directly to ground."
You will just have to compare your circuitry to this first supplied schematic to see how they compare . . . . or vary.

"This kind of circuit is limited to very small signal input given that the base bias is about .6v."

"At full volume the radio provides about a 0.2v P-P signal to the first stage."

" led me to think a 5.6 ohm resistor was 560 ohms."
I find no 56 variant value being used in this shematic . . . . . but we have a decimal 68 ohm con-fusible resistor on the AF output transistors emitter.
Where is that ? 56 ? resistor value of yours being located within the radios circuitry ?

" The collector has been reading about 7v with a 11.78 supply. "
Nowhere on THIS initial schematic will you find that 11.8 to 7V relationship being found. The closest 50/50 situation would be the Q4 driver stage, wherein, if you subtract the 2.58V emitter voltage
from the 10.69V collector voltage and get the effective voltage being across the complete C to E of that stage as 8.11V.
Then that 3.9V voltage at Q4's base will equate quite well with a 8.11/2 = 4.055V for a mid point biasing.

Using the two Samuels schematics that I see as possibly being your unit , one has a custom DELCO audio drive module being used to feed the output transistor . . . so there are no three DC direct coupled transistors.

The other schema is THIS one below , which fits your 3 DC coupled transistors criteria.
Two more Oldsmobile possibilities are viable, but I will have to pick up those two Samuels AUTO RADIO manuals when out at the mule barn Archival mass storage , this week end.

Then, if they are being 3 DC coupled transistor designs, I will Pee Dee Eff-er-ize them and submit.

On this submitted schema . . . . consider . . . . . with it being stee-re-oo, you already have a duplicate audio circuit "schematic" in the respect of having two LIKE channels, to compare each other against.

As HOT as auto interiors get, the YELLOW marked electrolytics are good candidates for time related capacitance decline.
Particularly consider the hi cap 250 ufd sections within the C1 triple section metal can.
The electrolytic interstage AF couplers have been known to drop down to multi tenths of a microfarad.

Be sure that the YELLOW -PINK autotransformer audio output transformers are MOUNTED in the radio and that your speakers are connecting to them.
A nephew brought me his Corvette radio to repair . . . sans speakers . . . . and I found that those audio auto transformers were with the speakers .

Confirm that . . . as they are about 2X2X2 inches . . . with E-I iron laminations in the cores.

Give us some found confirmations and further info on your comeback . . . . .

Le Petite SCHEMATIQUE . . . . .

73's de Edd

.

 

[Frankchie]

Here is the audio and PS section of the schematic that seems pretty close to my radio. I think it will answer many of your questions. BTW, it's not stereo and no tape player.

The 5.6 ohm resistor in my radio is R59 (2.7ohm) on the schematic.

Another difference is that my radio has the emitter of Q8 tied directly to ground and some of the resistors have different values.

As I mentioned, I'm planning to replace the audio section with a modern audio module, so any further work on the original audio section is just for curiosity.

I have included a schematic, hopefully I have done that correctly.

 

[73's de Edd]

Indeed . . . it is being a much cheaper mono FM radio.
The R59 resistor is part of a negative feedback voltage divider loop with its R58 companion. Higher value of R59 . . . more feedback along with lowered distortion, at a cost of overall system gain.
Also would want to be sure electrolytics are all up to full capacitance on the C5-C6-C7-C8 and C1(C)positions and that the bias pot is set to that specified spec.
A leaky 'ole "geranium" DS501 could also be at fault.

 

[Frankchie]

Right now the gain of the 1st stage seems to be about 1/2 what it was when I began troubleshooting. I must have done something stupid to cause this but nothing obvious is evident. It's a pretty simple circuit so I am disappointed that I haven't figured it out. Although if I do fix my apparent mistake, I'm still left with the original problem to repair.

Again, I think replacing the whole audio section with a modern audio module is the way to go because these radios never performed that well even when new. However, my curiosity may drive me to work on it some more if I get some time.

Thanks