QUAD FM3

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22-Jan-2020 04:54 PM

LOWTHERMAN62

LOWTHERMAN62

Posts: 77

Hi All, I would be grateful for some help in fixing a n Fm3. Early model no555

I have replaced the 2x1000uf caps and the 470uf cap  (note this is c113 and was originally 300uf)and I am getting the following readings

Rectifier +20.89 and -23.09vdc

yellow cable -3.45vdc

red cable +20.72vdc

black cable 3.19vdc

Diodes mr101 and mr102have been removed and checked and both approx 12 volts tested good. This is an early model the two trimmers rv102 and rv103 make no difference to the readings when adjusted. Stereo light is on constantly. Your help would be greatly appreciated. 

Regards .

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22-Jan-2020 05:47 PM

LOWTHERMAN62

LOWTHERMAN62

Posts: 77

Some additional information to add to my previous message. I have removed rv102 and rv103 and they measure 63k and 5k respectively. I think these should both be 5k. Okease let me know if there are any other issues I should look at.

Regards

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23-Jan-2020 02:43 PM

LOWTHERMAN62

LOWTHERMAN62

Posts: 77

Further to my previous 2 posts and after copious checking I have found TR101 faulty. Currently this is a BC184k . I have some Bc237 left over from a 303 repair. Would these be a suitable replacement and if so should I replace TR105 as well as it is also a BC184k. The other 3 resistors on this board all measure good.

Regards

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23-Jan-2020 07:01 PM

crayhow

crayhow

Posts: 7


If I was you I would get the circuit of the Fm3 and it will no doubt tell you all what you want know.

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23-Jan-2020 07:34 PM

LOWTHERMAN62

LOWTHERMAN62

Posts: 77

Thank you for you advise but

 I already have the circuit and lots of information and think I have found all the answers but this does not cover topics such as substitute transistors and the like, Also I was really looking for some input from the experts. If you know the answer to my third post or are able to contribute to the first two posts  would be be most grateful.

Regards

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02-Feb-2020 04:08 AM

EJP

EJP

Posts: 1712

BC237 has lower gain than BC184, 110 versus 240. I think I would try a good old BC549. Tr105 is only the mixer for the mono output on pin 1 which you won't be using unless you have the correct 3-core cable, so it hardly matters at all.

I can't make head or tail of your voltages. The black wire cannot but be negative, and the pink wire should be zero. (Your reading on the pink wire indicates a severe mistuning of L5.) The only possible explanation for the asymmetry between red and black is the incorrect pot values, and it appears you've fixed those, so the problem should have disappeared.

Check the polarity of the main PSU capacitors. They point in opposite directions.

It is possible to upgrade this PSU to the later one >= 5885, by simply removing the resistor/pot string, changing the zeners to 15V, and grounding the bases of the inner transistors. I think I would do that in this case, as it is simpler and more stable.

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02-Feb-2020 11:31 AM

LOWTHERMAN62

LOWTHERMAN62

Posts: 77

Thanks for the reply EJP. The upgrade to the psu sounds interesting. Is it possible that you could give me a step by step. ie what to remove /bridge/ replace. Bearing in mind it is serial number 555. I have some bc550 and some bc184 which are best for the transistor replacements?

Thanks so much

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02-Feb-2020 12:51 PM

EJP

EJP

Posts: 1712

What part of it didn't you understand?
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02-Feb-2020 02:28 PM

LOWTHERMAN62

LOWTHERMAN62

Posts: 77

Hi EJP,

PSU capacitors are correctly orientated.

Correct me if I am wrong on the following:-

Remove two no 5k trimmer. Do I need to bridge anything here?

Not sure which resistors to remove, again do I need to bridge anything here?

Not sure which transistors to ground base, also best way to do this?

Any other things I need to do?

BC184 or BC550 I have both?

Kind regards




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02-Feb-2020 08:56 PM

LOWTHERMAN62

LOWTHERMAN62

Posts: 77

Hi EJP,

I think I have found the answer on your blog but clarification required on a couple of items.

1 Remove the two trimmers rv102/rv103 and short out. (I take it to be soldering a link from wiper to track? )

2 Remove R106/R107?

3 Replace the two 12 volt zeners M102/M103 with 15V?

4 Replace C116/C117 with 470nF polyprop as an option.

You also mentioned grounding transistor base legs. Can you tell me which transistors and best method? 

Regards




 

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03-Feb-2020 02:24 AM

EJP

EJP

Posts: 1712

The blog is correct. I had an early FM3 on the bench when I wrote it. I don't at the moment, so I can't improve on it. Shorting out the wiper connection to the ground connection of each pot with a link accomplishes grounding the bases of the inner transistors.

EJP

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03-Feb-2020 09:17 PM

LOWTHERMAN62

LOWTHERMAN62

Posts: 77

Thanks EJP I will do some works ,awaiting parts. Interesting I removed 4 axial caps c108,c116/117/118 as they looked very black and goey at one end and measured. The measurements as follows but not in any order 

1 1821nF esr 44 ohms

2 1308nF esr 30 ohms

3 1630nF esr 28 ohms 

4 1839nF esr 47 ohms

These according to the service notes should all be 640nF. and markings on the caps indicate 0.68uF I have ordered some miniature kenmet 680nF to replace. Any ideas why these would be so far out of tolerance?

I will continue checking all the other caps on this board. Note I am not touching anything on the other board.

Regards 

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03-Feb-2020 11:43 PM

LOWTHERMAN62

LOWTHERMAN62

Posts: 77

Hi EJP. Ref my previous post and after a little research I found that these BAD ! caps are mullard c426 electrolytics. Do I need to replace with electrolytics or would film caps be a better idea, On circuit board marked +.? If electrolytics what value? as I have ordered some 680nF film caps of a suitable size.

Regards 

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04-Feb-2020 02:19 AM

EJP

EJP

Posts: 1712

C108 is described as 'decoupling for the doubler'. It won't be critical in value. The others are all coupling capacitors and their value isn't critical either. Bigger the better actually. I would use film. 680nF is fine.
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