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Old 11th Jun 2020, 2:06 pm   #21
mbyarm2000
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Default Re: Guidance on 1970 blaupunkt MW/LW radio

Hi Peter. I have replaced V101 & V102 (details earlier in thread) but voltages are all over so I am looking to test with TIP31&32 as per your advice and waiting for them to arrive. I think it's time to pause, cup of tea, and start afresh. I think I'm going to wire across to a breadboard for all transistors to avoid further damage to the PCB - it's shed a couple of pads already.

It was also felt that V103 could be faulty (AD161; substituted for AD157 before I got radio) and luckily I managed to find one on Ebay at a reasonable price. I should have enough bits in the post to move onto the next step of the problem.

One other thing I need to check is that the AD156/7 were substituted correctly (right way round) with the AD161/2. The 'lower' number of one pair matches the 'higher' number of the other. I might have already checked this but it came to mind as I was ordering the AD161.

All measurements have been taken with a speaker in the circuit.
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Old 11th Jun 2020, 4:19 pm   #22
orbanp1
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Default Re: Guidance on 1970 blaupunkt MW/LW radio

Hi Micheal,

Building up the circuit on a separate board is a sound idea!
Probably some basic principle that you would expect to work always, like "wire conducts electricity" is not working (there could be a tiny break in a trace that you just do not see, or similar).
Building up the circuit again will eliminate such things.
Double check everything, measure every resistor, including the speaker, also that caps are not shorted, check the transistors again, the E-B and C-B diodes, and that there is no short in C-E, and Hfe if you can, those you can check with a DMM.
If you test first with the TIP31/32, you do not need to worry about the the correct idle bias current, just make sure that you can get the half supply voltage correctly at the output.

You will get there, it is probably some small silly thing!
This happens all the time (at least to me)!

Pads on a PCB can be repaired with small eyelets. That radio is old enough that there is enough room to put those in. Then you can run wires to the other end of the trace. I use for this teflon insulated wire-wrapping wire, that is thin, and you do not burn the insulation.

Regards, Peter

Last edited by orbanp1; 11th Jun 2020 at 4:25 pm.
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Old 11th Jun 2020, 4:28 pm   #23
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Default Re: Guidance on 1970 blaupunkt MW/LW radio

Before doing anything else though, triple check that you have identified the transistor leadouts and have wired them correctly. It's so easy to get this wrong when changing transistors. If you make a wiring error, you will obviously see strange circuit voltages.
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Old 14th Jun 2020, 8:14 pm   #24
mbyarm2000
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Default Re: Guidance on 1970 Blaupunkt MW/LW radio

Hi Paul, I will heed your advice - it's easy to slip-up. I have the radio PCB ready with jumper wire for 4 x transistors to breadboard for when my new transistors arrive. I will make sure all are correctly wired. Thanks for all your help to date. Michael
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Old 16th Jun 2020, 8:48 pm   #25
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Default Re: Guidance on 1970 Blaupunkt MW/LW radio

An update for everybody who's helped with this one.

I have wired out V101,V102, V103 & V104 to breadboard to help with swapping out and measurement - it really makes things easier.

Initial measurements showed strange voltages at V101 & V102 so I replaced them with fresh versions of the modern general purpose transistors I had put in (numbers earlier in post).
I then replaced V103 with the new AD161 I bought. Voltages generally now looked much better, but the emitter junction of AD161 & AD162 was low based on what Peter had said (6v with 14v in).

Up until now I hadn't been too concerned in trying to tune the set but with this setup, the volume had increased and the set seemed more lively. I was so happy when I picked up a station and got good volume but a lot of crackle/interference.

I then swapped the AD161 back for the original, and the set continued to play in the same way so the original must be ok.

I'm not sure what the original problem has been, but I will continue to work on this. I haven't yet tried adjusting R106 to lift the emitter voltage as Peter suggested but I will.

Thanks so much to everybody who has given input on this - I was so determined to get the radio working (for nothing but the challenge) and now I've reached a milestone I've come in for a beer!

The crackling seems to be coming from an 6-way extension that has some sort of spike suppression/filter in it. I had tried two power supplies with the same results (both plugged into the same extension) when I noticed the crackle seemed to be in sync with the flashing neon 'power' indicator on the extension. Switching to a car battery got rid of the problem.

A little bit more work and it will be ready to build back up again. Thanks to everybody.
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Old 18th Jun 2020, 9:07 am   #26
mbyarm2000
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Default Re: Guidance on 1970 Blaupunkt MW/LW radio

To Peter (orbanp1) - the resistor needed to bring the emitter voltage to half input was 22k. Thanks for assisting with this and taking the time to provide the simulation - most helpful. The radio is now sounding good and ready to reassemble.

Also, the some of the crackling I referred to earlier came back even when powered by battery - confused initially, but then realised it was coming from my temp-controlled soldering iron! I have worked on several radios with this setup without any problems so I guess this era of radio is more susceptible to interference.

Michael
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Old 18th Jun 2020, 11:09 am   #27
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Default Re: Guidance on 1970 Blaupunkt MW/LW radio

Congratulations on getting it sorted out.
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Old 24th Jun 2020, 10:59 am   #28
mbyarm2000
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Default Re: Guidance on 1970 Blaupunkt MW/LW radio

I got everything re-assembled and it still works fine. I have included a picture of the finished item (no restoration on case,etc). Hard to see in picture but it's about 2/3 scale of modern DIN size car radio.

It actually picks up MW better than a 1980's/90's Blaupunkt.

Michael.
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Old 24th Jun 2020, 11:51 am   #29
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Default Re: Guidance on 1970 Blaupunkt MW/LW radio

Hi Micheal,

Congrats on the repair!
Nice looking radio!
I am sure there are some faceplates that would allow for mounting that 2/3 size DIN radio in a regular slot! You could probably even make one yourself!
But, I would be missing FM, and not sure that LW would be of any use, at least not here in NA.

Regards, Peter
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Old 24th Jun 2020, 2:51 pm   #30
mbyarm2000
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Default Re: Guidance on 1970 Blaupunkt MW/LW radio

Thanks Peter. It's not of much interest to me due to the lack of FM - I think it's one to move on to somebody with a 1969/70 car. I bought it because I was curious as it was 'Made in Japan' which is quite unusual for Blaupunkt. I'll probably move on to my next radio in the pile (I'm a sucker for buying 'spares or repair' and taking a gamble - makes it fun though). I have a modern radio set aside that is getting very hot around a surface mount IC on the CD board - the challenge on this one is the repairs to surface mount PCB which I have dabbled with but want to take further. The low-melt solder and liquid flux really helps keep a pool of molten solder that allows you to pick the components off and replace quite neatly.

Hope you're all ok over there - hearing some nasty things about increasing infection rates in certain US states this week.

Michael
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Old 24th Jun 2020, 7:35 pm   #31
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Default Re: Guidance on 1970 Blaupunkt MW/LW radio

I had one of those years ago. No special memories, but it did work well.
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