Faulty boost channel Marshall 5210

[Adrianbutt]

I wonder if anyone can help!?!

I have just had an old Marshall tranny amp come into my possession - it is the transistor version of the JCM800 (5210) and is meant to roar!  However, despite the clean channel having all the correct Marshall tones, the boost channel is thin and very quiet and appears to have some clean mixed into it - louder than the rather fizzy sounding distortion in the background...  I have cleaned the pots and switched jacks but no change...  I also took out all four op amps and replaced with sockets and new op amps of the correct type - that also made no change...  What would be the next move?

[Adrianbutt]

Circuit diagram is

http://www.drtube.com/schematics/marshall/5210-iss3.gif

Hope you all can help - this amp sounds great on YouTube!

[phatt]

Hi Adrianbutt, and in case I missed it,,a welcome. :tu:
Does the reverb switch on and off ok? 
My best guess is Tr 4,5 and 6 might be at fault they control the switching.
Better minds here will help just wait a while.
Phil.

[Adrianbutt]

Interesting - thank you for your quick reply.  I don't think it is to do with switching as without the footswitch plugged in you should be able to mix the clean and boost channels or turn one down completely to listen to the other.  My fault is consistent with or without the footswitch, and replicated with just the boost channel turned up and the normal channel off.  It also does not come and go depending on wiggling jacks, pots - or shock to the whole amp - the fault is consistent which is why I believe it to be at the component level and not mechanical...  But, I am open to all ideas!

[J M Fahey]

Interesting - thank you for your quick reply.  I don't think it is to do with switching as without the footswitch plugged in you should be able to mix the clean and boost channels or turn one down completely to listen to the other.  My fault is consistent with or without the footswitch, and replicated with just the boost channel turned up and the normal channel off.  It also does not come and go depending on wiggling jacks, pots - or shock to the whole amp - the fault is consistent which is why I believe it to be at the component level and not mechanical...  But, I am open to all ideas!

Nobody said it´s "mechanical"  , but that it was a *switching*  problem.
Which, as Phatt noted, includes the switches (both foot and panel) *and*  the switching transistors (Tr5/TR6) *and* the transistor which controls them TR7  .
Momentarily I leave Tr4 aside because the clean channel does work.

Basically there´s 2 main possibilities:
a) Tr5 or TR6 is shorted.
b) switching action is not working and they are fine, but muting fulltime.
plus a third:
c) that the distortion channel does not work.
Although in that case the clean channel should be muted if so selected.

Please measure voltage at collector and base of Tr7, same at TR2, post results.
You do not tell us whether the Led L3 works either, that´s important info you should have posted.

Does the reverb footswitch work?

[Adrianbutt]

Sorry - didn't mean to offend - really a beginner at this - although handy with a soldering iron!  The LED does work and does switch with the footswitch and the reverb switches in and out as it should...

[Adrianbutt]

The clean channel is muted when the boost channel is selected with the footswitch plugged in - I can hear a very distorted signal in the background (like the boost channel trying to work) with a clean but muted signal on top...  Hope this helps, Adrian.

[Adrianbutt]

Thought it might help if you can hear the problem so:

https://soundcloud.com/lucy-freud/marshall-5012mk2mp3

Let me know what you think!

Adrian.

[J M Fahey]

1)  no offense taken, don´t worry  :tu:

2) MEASURE those d*mn voltages !!!!  :lmao:

And in both positions of the footswitch, which you can call Led ON and LED OFF.

and that´s just the beginning

[Adrianbutt]

Very busy with work and gigging this week - but will try and get time to take it all apart again and get those voltages as soon as possible...

Thank you for your help, Adrian.