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July 20, 2024, 11:12:37 AM

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Marshall VS100R transistors

Started by markorock37, March 01, 2024, 12:34:07 AM

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g1

Yes it would seem T1 is bad as you verified R5 and D1.  But you also need to check their solder connections.
As T1 and T2 form a differential amplifier, it is best to replace them as a pair.

markorock37

I don't see 2SA872 on Mouser, is KSA992 a good sub?

g1

Quote from: markorock37 on April 13, 2024, 02:57:58 PMI don't see 2SA872 on Mouser, is KSA992 a good sub?
Yes, those should work.

markorock37

Well, I've replaced T1 and T2 as well as D1. Still reading pretty much the same voltages. At this point, all transistors have been replaced in the amplifier portion. I hate to deem this unit a loss but I am leaning that way as it was quite a mess with the traces from prior techs. Im 99% sure I have all the traces repaired correctly.

saturated

Hang in there bud  8)

Don't give up before the miracle  :P

If anybody can get your amp running these guys can.

Maybe post up some pics of the boards

 :)
I ask stupid questions
and make stupid mistakes

criticism, critique, derision, flaming, verbal abuse welcome

g1

Yes, pictures should help.  Especially of any trace repairs that have been done.
Also, e,b,c voltages for T3 and T5.

markorock37

T3 E -40.1v
   B -40.6v
   C -40.6v

T5 E 0.6v
   B 39.0v
   C 31.4v

I'll get some pics up when I can, I'm out of town for a couple days. Had time to snag some voltages before i left.

g1

Again, voltages that are impossible for e,b,c locations.
Is it another mistake, or is it possible you have some transistors in backwards?

markorock37

Well shoot. I was assuming the same pinouts as T1 and T2. Here it is corrected.

T3 E -40.6v
   B -40.6v
   C -40.1v

T5 E 0.6v
   B 39.0v
   C 31.4v

T3 pins 1,2,3=C,B,E
T5 pins 1,2,3=E,B,C


g1

Quote from: markorock37 on May 02, 2024, 07:49:32 PMT3 pins 1,2,3=C,B,E
T5 pins 1,2,3=E,B,C
As far as I know, there is no such thing as CBE pinout for these.  But, apparently there are 2 versions of each, and could be either EBC or ECB.
Can you post a link to the exact ones that you bought?



markorock37

Actually looks like both have CBE pinouts
So T3
c -40.1v
b -40.6v
e -40.6v

T5
c 31.4v
b 39.0v
e 0.6v

Is it possible the new ones dont have the same pinout as the originals?

g1

Quote from: markorock37 on May 07, 2024, 12:59:57 AMIs it possible the new ones dont have the same pinout as the originals?
Yes it is possible.  Measure resistance from D4 cathode to T5.  One of the T5 pins should read zero ohms to D4 cathode.  Which pin is it, pin 1,2, or 3?
Which T5 pin reads short to R42?

markorock37

D4 cathode goes to pin 2 of T5
R42 goes to pin 3 of T5
That checks out according to the schematic.