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More transistor radio repair - faultfinding prior to audio stage

de_light

Oct 15, 2020
12
Joined
Oct 15, 2020
Messages
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Well, thanks for all your help for my last attempt at fixing a transistor radio. It's something I'm trying to teach myself and I'm following along with Lemon's Transistor Radio Repair Course (which is great). The last radio I fixed with your help just had a speaker fault but this seems to have a bit more (and I have a service manual, which is great!)

Service manual: https://drive.google.com/file/d/1MZlnBU4tm5tqtN6veWuyA1ZD2W8-gj4E/view?usp=sharing

It is an old National Panasonic R-238R which is capable of receiving MW and SW. I've been powering it with my DC PSU at 4.5V and it draws a current of 18mA when idling.

At present, I can hear a quiet static when it is on and from my research, this means the audio stages are good. I can use a 1Khz audio signal generated from my computer and inject it at pin 3 of M1 and still get an audible output from my speaker (and it is very loud/amplified).

I've traced the voltages for each of the RF amplifier, converter and IF transistors and attached them below: to my eye, they seem OK?

table.png

If anyone has any suggestions on where to look next, or if you need any information, please let me know and I will do my best. I am only using an audible signal generator off my laptop for the minute - a proper function generator is in the post and will be a few weeks. Thanks.

Edit: should add I have held a working AM radio next to this one and can hear an oscillator.
 
Last edited:

73's de Edd

Aug 21, 2015
3,555
Joined
Aug 21, 2015
Messages
3,555
Sir de_light . . . . .


Well it looks like you have moved up from the last sets less than desirable germanium transistors and now graduating into more modern SILLY -CONE transistors.
I do see a lackluster wide band frontal RF amp stage that is not even using a tuned circuit in its base input circuitry. ( Don't even ask . . . " If it aint's there . . . you not gonna puts one in there " . . . considering it needs to cover 3 bands.

Your choice of M1 pin 3 AF for signal injection IS covering all of your audio stages.
I would only consider testing the radio on standard broadcast band initially.
With programing available on internet, some RF Short Wave radio frequencies have now become as quiet as a mouse peeing on a cotton ball.
Work through ALL of your positions of your band switch m a a a a a a a a a ny times, in case of contact oxide build up on a dormant unit for decades of non use..
With NO RF generator test source available yet, you might get a known good operating TV infrared remote hand unit and place it up against and kissing the antenna staff. Start pushing its buttons to see if any bursts of ~ 40 Khz UPPER harmonics pass thru the sets IF stages and thereby will make resultant tone bursts heard in the speaker..
FIO . . .
It is also possible to bring a brush motor powered electric shaver up next to the staff and see if the arcing of its then created wide band damped wave "static" passes thru the sets RF mixer and IF stages.
The TV remote is the preferred alternate.


If that reception is experienced, but you still can not pick up broadcast stations when tuning across the band, the sets local oscillator might not be working.
Confirming that, would be Chapter 2 materiel.

You now go . . . . . makee . . . . . .testee-testee . . . . . . .

73's de Edd . . . . .

At my age . . . . . I'm still hot. . . . . it just happens to come and go in flashes.


.
 

de_light

Oct 15, 2020
12
Joined
Oct 15, 2020
Messages
12
Sir de_light . . . . .


Well it looks like you have moved up from the last sets less than desirable germanium transistors and now graduating into more modern SILLY -CONE transistors.
I do see a lackluster wide band frontal RF amp stage that is not even using a tuned circuit in its base input circuitry. ( Don't even ask . . . " If it aint's there . . . you not gonna puts one in there " . . . considering it needs to cover 3 bands.

Your choice of M1 pin 3 AF for signal injection IS covering all of your audio stages.
I would only consider testing the radio on standard broadcast band initially.
With programing available on internet, some RF Short Wave radio frequencies have now become as quiet as a mouse peeing on a cotton ball.
Work through ALL of your positions of your band switch m a a a a a a a a a ny times, in case of contact oxide build up on a dormant unit for decades of non use..
With NO RF generator test source available yet, you might get a known good operating TV infrared remote hand unit and place it up against and kissing the antenna staff. Start pushing its buttons to see if any bursts of ~ 40 Khz UPPER harmonics pass thru the sets IF stages and thereby will make resultant tone bursts heard in the speaker..
FIO . . .
It is also possible to bring a brush motor powered electric shaver up next to the staff and see if the arcing of its then created wide band damped wave "static" passes thru the sets RF mixer and IF stages.
The TV remote is the preferred alternate.


If that reception is experienced, but you still can not pick up broadcast stations when tuning across the band, the sets local oscillator might not be working.
Confirming that, would be Chapter 2 materiel.

You now go . . . . . makee . . . . . .testee-testee . . . . . . .

73's de Edd . . . . .

At my age . . . . . I'm still hot. . . . . it just happens to come and go in flashes.


.

Thanks, I'll get to work! I've attached a recording of me moving the tuning dial of the radio to be repaired next to a working AM radio - is this an functioning local oscillator I'm hearing?

https://drive.google.com/file/d/11w1Q9-FOsU1NN8_uWRZD5pocxAYlf2Fk/view?usp=drivesdk
 

73's de Edd

Aug 21, 2015
3,555
Joined
Aug 21, 2015
Messages
3,555
That is EXACTLY correct . . . . . so it seems that the to be repaired Punysonics local oscillator IS functioning properly.
So . . . . . can you now pick up the harmonics of an IR remote transmitter that will / can extend on up into the tested radio receivers IF spectrum ?
Standing by . . . . and . . . . .
EatingPopcorn.jpg


73's de Edd . . . . .
 

de_light

Oct 15, 2020
12
Joined
Oct 15, 2020
Messages
12
That is EXACTLY correct . . . . . so it seems that the to be repaired Punysonics local oscillator IS functioning properly.
So . . . . . can you now pick up the harmonics of an IR remote transmitter that will / can extend on up into the tested radio receivers IF spectrum ?
Standing by . . . . and . . . . .
EatingPopcorn.jpg


73's de Edd . . . . .

I'm one of those strange people who don't have a TV so I'm looking for an IR remote. I tried using an IR blaster app with my phone (it has an IR blaster capability), set the freq to 60 KHz and held it to the staff but heard nothing.

I don't know if this is relevant or not, but I have a small handheld amateur radio transceiver that I touched aerial to the radio staff and transmitted. I got a burst of sound through the set and the below waveform at the speaker which matched up to the transmitter frequency.


NewFile7.png
 
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