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Marshall VS100R transistors

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

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Check that T7 and T8 and their diodes are ok. Check all 4 of the 0R33 5W resistors.
Power up on the bulb with no load connected.
Measure DC volts at e,b,c, of T4, T6, T9, T12.


All diodes and .33 emitter resistors test good.

T4 E -35.7v
   B -35.3v
   C 31.5v

T6 E 31.6v
   B 31.9v
   C 32.1v

T9 E 30.9v
   B 32.2v
   C 32.1v

T12 E 30.9v
    B 31.8v
    C 30.8v


The collector on T12 should not be +30.8v, it is connected to the negative rail.
Are you keeping the common (Black) lead of your meter on 0V (ground) all the time ?
It is very important to clearly indicate the positive and negative readings so the experts (not me) can work out what is wrong.
Maybe worth re-measuring the voltages ?

There are no stupid questions.
There are only stupid mistakes.


Assuming that the T12 collector voltage TV mentioned is just a typo, T4 voltages do not seem right.  E-B voltage indicates it should be turned on, but collector voltage is not pulling down.


T12 E 30.8
    B 33.8
    C -38.3

So I did have the collector wrong of T12.


Remeasured T4 and voltages listed are correct. Originally had a TIP29CH replaced with TIP29C.


The T4 voltages indicate either T3 is bad and not letting T4 turn on fully, T4 is bad, or something is wrong elsewhere in it's string (shown in red).

You cannot view this attachment.


That helps, will report back with my findings..


Everything in that string is good. Retested T4 and T3 are good. Replaced T6. Made sure pinouts are all correct. Still showing voltage at the output.


Can you post E,B,C voltages for TR4 and TR6 ?


Had a few other projects to tend to, but finally got around to getting those voltages.

T6 MJF122
E  34.6V
B  35.5v
C  35.1v

T4 TIP29
E  -39.5v
B  -39.2v
C  35.2v


Do you have those reversed or is it a typo?  T4 should be the TIP29.


I corrected that above, yes it was just a typo. Voltages are correct for the given transistors.


The E to B voltage of T4 (0.3V) seems to not be enough to turn it on.  Maybe checking directly with probes at E and B will give a more accurate reading than checking voltage to ground.
The voltage to turn it on should be developed across R5.  That is created because of current through T1.  What are E,B,C voltages for T1 and T2 ?


T4 E to B  0.3V

T1 E 0.6v
   B 0v
   C -35.8v

T2 E 0.6V
   B 15.1v
   C -36.5v

R5 671 ohms

D1 checks good

So looks like T1 is bad?