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Marshall mosfet 100 amphead, power amp stage signal loss

Started by telecasterock, January 04, 2016, 03:30:32 PM

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telecasterock

I'm a newbie to this forum.  I just finished a successful build of an old 5E3 tube amp, now I've taken on my nephews Marshall Mosfet 100 amp repair.  I got a schematic off the web.  All the voltages appear correct.  I followed the signal path to the power amp stage, which begins with a couple of A06 transistors.  The transistors in the pwr amp stage seem to pass the simple transistor test.  Is there a way to test the power output transisitors?  Any ideas on how to find why the signal disappears in this part of the curcuit.  Thanks for any help.
schematic      http://www.drtube.com/schematics/marshall/3210-iss7.gif

Enzo

Just so you know, those transistors are MPSA06. "A06" is just shorthand.

PAth "to" the power amp, but how about THROUGH the power amp?  Apply a signal to the amp, got signal at the base of TR6?  How about the collector.  And at the collector of TR7, an MPSA93?  If it got that far, it ought to appear on the gates of the output MOSFETs, wwhich APPEAR to be a pink and a green wire.  Is it?  You say voltages are OK, but does 35v appear right on the MOSFETs, looks like yellow and white wires?  And the red wire appears to be the output there at the junction of the two 0.33 ohm resistors, got signal there?   That should go to the speaker hot on the jacks, and make sure the speaker cold is indeed grounded via the black wire.

And just for grins, make sure the speaker and cord are working by connecting them to some other amp, a working 5E3 for example.

The key to fixing anything is to isolate the problem.  Once you find out WHERE the signal goes away, it is a lot easier to figure out WHY.

telecasterock

thanks for your response.  I've done a little more checking around the output stage, measuring static component values and found that the mosfet 2SK134 had a short from the yellow wire to the blue.  I 've ordered a buz 901 to replace it with.  I hope that will be a 'drop in' replacement.  From what I could tell all the other components seem to be okay.

Vitrolin

you should get the complementary buz906 as well

always use complentaries, so when switching to another type change all transistors,

Loudthud

Is the Buz 901 part a lateral or vertical FET? The difference being the Source is connected to the case on a lateral (2SK134) and the Drain connected to the case on a vertical. Biasing is completely different too. Unless you can find some laterals, circuit mods will be required.

J M Fahey

BUZ are Lateral/Audio , not Vertical/Switching so that is covered, BUT pinout might be different.

Easy to check, download both datasheets and see, then look again  ;)

That said, I think today most, if not all, freshly made Laterals come from Exicon, who supply the dies characterized by parameters, other Factories buy them raw, encapsulate and print whatever label they wish or they legally own.

Meaning "probably they are all the same under the skin".

*Personally* , if you have no MosFets in parallel but "one upstairs - one downstairs* I wouldn't worry *that*  much, provided that the "partner" works properly.
But hey!!, that's me, I respect different opinions.

Of course, if some are used in parallel, I would use all same batch or carefully measure one by one.

g1

  Before throwing away those priceless old Fet's, I'm not completely convinced the 134 is bad.  Fet's can be difficult to measure, and will sometimes show shorts depending on how they are tested.

telecasterock

I replaced the 134 with the Buz 901, the amp is working now.  The output is controlled by the volume switch VR9, however the 'gain' pots VR 1 and 2 don't do anything.  It seems that part of the circuit isn't working.  There's good clean sound coming out of the speaker with lots of volume.  I'll poke around the 'Gain' section to see what isn't working there.   Any ideas on how to approach that? 
I feel lucky that I only had to replace the 134.  I thought I'd have to do both (I have both), but can't argue with success.
Thanks for all the help. 

Enzo

Vr1 &2 only function when you are in the dirt channel.  Vr4 only works when you in clean channel.  Vr9 is the master volume that affects all channels.  Various outputs from IC2 ground off the signal in the channels to "select" the other channel.  With channels A and B, if we ground off A we are left with B, and vice versa.

Are your channels shifting?

telecasterock

I think it was operator error.  i didn't understand how the controls worked.  I did get everything to work once I saw how the channels (or signals) were being routed.  It sounds great.  Dirtied up the signal and got a pretty rich tone.  I have all fender amps, this is my nephews Marshall.  I want to spend some more time with it and enjoy 'that' sound.  I have a friend that designed and built his own amp.  One channel has as Marshall preamp design and the other a Fender (Tremulux).  That's a great sounding amp - mix, single channel, lots of possibilities.
thanks again for all the help.