Transistor radio repair - Philips RR-230

[de_light]

I'm looking to get some help in troubleshooting and repairing an old transistor radio. I'm interesting in learning about what is going wrong here so I can develop my skills and fix more stuff - I love these old radios and on a mission to save them from landfill!

I've got a beginners level understanding of electronics but actively learning. Got a little home lab with a CV-CC PSU, Rigol DS1054Z DSO, multimeters, basic parts etc. I can solder reasonably well. I've been reading a great old book from the 1970s, 'Transistor Radio Servicing Course' by Lemons. It is pretty awesome. And finally, off to get my amateur radio licence in a fortnight!


So: I can't find a schematic for this radio, annoyingly. I can tune an FM radio frequency (so I can assume that the IF and AF amplifier sections are all working okay). The problem is distortion that occurs at medium to high volumes. At low volumes, there is no distortion. The distortion seems to appear on the 'punchier' parts of the song. It is present running off mains, or using my DC PSU attached to the battery clips and the board. I have tried shunting a 470uF cap across the large 2000 uF/10V cap and the prominent 470uF cap on the main PCB with no change. I have recapped the audio PCB (the one with the 7 transistors and the variable capacitor/ferrite rod). Here is a clip going through the volume control.

https://drive.google.com/file/d/10wx47PR8QQB767PIkoGX3WgyQduKMblA/view?usp=sharing


My instinct tells me this might be related to some worn electrolytics because this seems to be the most common fault, but just a guess. Could this be 'clipping'? Again, interested to learn why this is occurring, rather than a quick fix!


Few images:


Original caps are grey and blue, any replacements of mine are brown or black/white.

 

[Audioguru]

Any radio produces distortion above a certain output power. The radio is designed so that normal levels have the volume control set a little above halfway so that low level inputs can be turned up. But then normal input levels can be turned up too high where the output cannot go that high and produces clipping distortion.

An amplifier output produces clipping when its output voltage swing cannot go any higher than its power supply voltage because the input level is too high.

The circuit looks like it uses germanium output transistors with little metal cases held in clips that are soldered to the small audio transformer for cooling. Then the maximum undistorted output power might be only 1W that is not loud. The little speaker is stamped 1.5W.

The extremely old circuit has audio transformers that were last used in transistor amplifiers about 58 years ago.

 

[de_light]

Any radio produces distortion above a certain output power. The radio is designed so that normal levels have the volume control set a little above halfway so that low level inputs can be turned up. But then normal input levels can be turned up too high where the output cannot go that high and produces clipping distortion.

An amplifier output produces clipping when its output voltage swing cannot go any higher than its power supply voltage because the input level is too high.

The circuit looks like it uses germanium output transistors with little metal cases held in clips that are soldered to the small audio transformer for cooling. Then the maximum undistorted output power might be only 1W that is not loud. The little speaker is stamped 1.5W.

The extremely old circuit has audio transformers that were last used in transistor amplifiers about 58 years ago.

Thank you very much for replying.

Just to clarify:

1. Is that distortion heard in the audio sample an example of clipping, so I know how to identify it in future?
2. Would you be able to point out which are the output transistors? Is that at the top of the first attached image with the small transformer with blue windings just next to the fuse and bridge rectifiers?

Apart from experience, how do you know what part is what on a circuit if you don't have any schematics? For example: how did you know that those are output transistors? Could you point out some of the other important components on that board (like, can you tell which are the IF transistors etc?)

3. Where could I probe with my oscilloscope on this circuit to see the actual clipping occurring?

 

[Audioguru]

Your Google Drive is not an ordinary audio player so I could not hear your distortion.
In Google I entered, "audio example of amplifier clipping" and got a few video explanations and demos of ampflier clipping.

The parts in a cheap old radio are obvious.
A 'scope will show clipping at the speaker output of the amplifier.

 

[Bluejets]

how do you know what part is what on a circuit if you don't have any schematics?

It's a thing called experience and usually gained over many years.
Today's radios tend to be a 1 chip affair.

For the output, start at the centre pin on the volume control and work towards the speaker .
If you Google old transistor radio schematics you will end up with heaps of examples.
Most will follow a basic outline, enough to get one through.
I doubt any will have scope examples, those tend to be displayed more in TV schematics etc.

 

[de_light]

https://drive.google.com/file/d/116ky9eC8Gwt-zsqzN3VtcIMuaZ-MF5vI/view?usp=drivesdk

Sorry about that. Here's the audio.

 

[Bluejets]

Distortion there seems almost over the entire range....some of the crackling and popping might be from a scratchy volume control pot.

 

[de_light]

Distortion there seems almost over the entire range....some of the crackling and popping might be from a scratchy volume control pot.

Would that still cause distortion even when the pot wasn't being moved?

 

[Audioguru]

The sound is extremely distorted even when you turned down the volume and it also sounds tinny because the low frequencies are missing.
Listening to it I realize that in my life and my job as a sound systems engineer I have NEVER listened to an amplifier clipping.
I do not think your amplifier is clipping, instead a part in it has failed. Maybe a transistor or its speaker is bad.

 

[Bluejets]

My opinion also....

 

[de_light]

Thanks guys. Here's another audio clip.
https://drive.google.com/file/d/11OizbIrg_j3SEQ7d9jFt-QZZ52JJVioD/view?usp=drivesdk

Speaker is reading 1 Ohm impedance across the terminals. I think it is a speaker fault though! When I plug headphones in, it sounds fine.

 

[Bluejets]

Speaker is reading 1 Ohm impedance across the terminals. I think it is a speaker fault though! When I plug headphones in, it sounds fine.

You cannot measure impedance with a normal everyday multimeter.
Only indication, if moving coil type meter, is a click from the speaker and some resemblence of a circuit in the speaker coil.
Getting good sound from the earpiece just means you are listening to the un-amplified audio.
Fault still in the amp section somewhere.
In the days gone by, a quick check was to use freeze spray on the output transistors as sometimes this would show faulty components.

 

[de_light]

You cannot measure impedance with a normal everyday multimeter.
Only indication, if moving coil type meter, is a click from the speaker and some resemblence of a circuit in the speaker coil.
Getting good sound from the earpiece just means you are listening to the un-amplified audio.
Fault still in the amp section somewhere.
In the days gone by, a quick check was to use freeze spray on the output transistors as sometimes this would show faulty components.

Thanks - that's really helpful actually. So, I'll try tracing from the volume slider pot to the speaker and see if I can find any faulty components in the amplifier section.

 

[Bluejets]

You might want to build yourself a simple audio signal tracer to make life easier.

Plenty of simple how to's out there in Google.

 

[ramussons]

Thanks guys. Here's another audio clip.
https://drive.google.com/file/d/11OizbIrg_j3SEQ7d9jFt-QZZ52JJVioD/view?usp=drivesdk

Speaker is reading 1 Ohm impedance across the terminals. I think it is a speaker fault though! When I plug headphones in, it sounds fine.

Maybe you have found the problem. Why not try out another similar speaker?

 

[Audioguru]

The radio looks very very old so i think it has an output transformer.
I think you are measuring the normal resistance of the output transformer. Disconnect the speaker then measure only the speaker.

 

[73's de Edd]

Sir de_light . . . . .


After hearing that, I would also like to see you either have the speaker out of its present circuit and test it with a like power level of low impedance audio from another source.
BTW . . . . what is the impedance of the test heaphones being used . . . .low like the speaker . . . .OR high . . . where you would not even be loading the amp ?
OR . . . connect another speaker into the units speaker output where you lifted the original speaker. ( A mismatch of a higher impedance speaker is also permissible . . . considering there will be the experiencing of lower volume level and reduced bass / a more tinnier sound. . . .but none of that distortion now being present .
THEN . . . . . considering the speaker not being at fault . . . .circuit wise . . . . that sounds like a transistors base bias point being skewing as being introduced by a DC leakage from a just frontal electroytic coupling capacitor.
Next considertion would be from a transistors emitter bypass electrolytic having capacitance DECLINE.

Thaaaaaaaaaaassssssssit . . . . .waiting for the speaker status . . .and I'm poking around for that Philips schematic.

Re. . . .
The radio looks very very old [late 60'ish] so i think it has an output transformer.
Using the photos . . . then find the transformer with the 2 "winged' wrap around heatsinks going to
the 2 companion audio power output transistors . . .thats it .


73's de Edd . . . . .


I started out with nothing . . . . . . . and with due stinginess and frugality . . . . . . I still have most of it left !



.

 

[de_light]

Thanks for replying! I've ordered a new speaker and when it arrives, I'll let you know.

 

[de_light]

Thanks for replying! I've ordered a new speaker and when it arrives, I'll let you know.

Seems to have been the speaker! Thanks guys. If anyone comes across the service manual for this, I'd be grateful. I seem to have undone the tuner pointer string thing.

 

[Bluejets]

I seem to have undone the tuner pointer string thing.

Known as dial cord if you're looking for new.....