Marshall short

[Jazz P Bass]

measuring TR7 , it doesnt match the other small transistors , it has +32v / +32v/ +1v on its legs--does that look odd ?

Tr7 is a PNP transistor.
It takes a difference of voltage at the Base compared to the Emitter to turn it on.
I think if you measure From Base to Emitter you will find that difference. (it is not showing in your Base /Emitter readings. Retake them if you like)
All of the static voltages look good.
Remove the lamp.

[J M Fahey]

Your output transistor measuremets look *perfect*
yet I would not pull the lamp limiter until I make these tests:

1) re-re-measure DC voltage (no speaker connected) at the speaker out rail.
Expect less than 100mV , in fact less than 50 V , any polarity there. This tells me DC balance is fine.

2) remeasure from TR8 emitter to TR10 emitter (thus across R118 and R96), I expect less than 20 mV.
This tells the amp is not overbiased.

3) without turning it off, reconnect speaker.
Remeasure 1 and 2 , then apply some music.
You shoukd be able to get a few Watts into that speaker, 4 or 5W are quite loud, and bulb should blink to the rhythm of the music.

4) only then connect straight to wall power, no limiter.

5) rise volume and annoy neighbours.
Heatsink should only get moderately warm to the touch, definitely niot unbearably hot.

As you see, same as JPB suggested, only slightly more paranoid

[Valvehead]

ok--im also replacing the parts Enzo mentioned first

[Valvehead]

ok---- i just replaced =
TR8/9/10

TR4/5/6/7

R75

with amp on and limiter in place i get =

r101/102 still getting pretty hot

dc on speaker jack is 240mv dc ! ( my good amp has 25 mv)

at each end of R96 / r118 i get 250mv dc to ground

r118 has 250mv dc to ground , each end

seems high from what was posted above ??


EDIT--- i actually just picked up another amp EXACTLY like this one tonight !! Sure enough, the same resistors ive been saying are HOT....they are hot in the good amp too

[g1]

Yes, you can ignore R101,102,112, & 113, they are supposed to run hot (that's why they use 5watt).
For your R96/R118 readings, please specify whether + or - DC volts. Measure at the speaker jack, and at the emitter of TR8 and TR10.
Also please provide readings for the bases of TR8 and TR10 (and specify +or-).

[Valvehead]

ok.... in case someone missed it -- tr8/tr9/tr10 are brand new and the amp is powered up with a limiter .

neg probe of meter to ground on all the following...

r96 has +248mvdc / +238 mvdc

r118 +258 mvdc / + 244 mvdc

my good amp has about half of those readings
a reminder-- i have an identical marshall here so i can compare numbers !

TR8
neg probe to grnd.
E = +262 mvdc
B = +1.3 vdc

TR10
E = +256 mvdc
B = +1 vdc

the emitter numbers are about 100 mv higher than my good amp.

Speaker jack pos to grnd = + 253 mvdc
the good amp has 25 mvdc here


Another interesting measurement ---

at the 2 zeners , ZD1 / ZD2

Bad amp=
zd1 pos is 0vdc / zd1 neg is -13vdc
zd2 pos is +12.9vdc / zd2 neg is 0v


Good amp=

zd1 pos is 0v / zd1 neg is -4vdc

zd2 pos is +7vdc / zd2 neg 0v

So, could it be the 2 zeners are bad , causing the higher voltage? I do have some new ones on the way in case

[g1]

Those voltages at ZD1 and ZD2 are shown on the schematic. There should be plus and minus 15V. Check both amps again. You should be checking where ZD1 meets R101 and where ZD2 meets R102.
Back in post #9, you said:

measuring across ZD1 i have -15v / 0v
across ZD2 is +15v , 0v

Those voltages were correct. Has something changed?

Edit: you are reading +/-13V with the bulb limiter so that sounds right, but you should get the same with your "good" amp or else it is not good.

[Valvehead]

i had to change the bulb in my limiter from 20w to a 75w.

i get -14.8 vdc at r101

and + 14.9 vdc at r102

thats with the limiter

[g1]

Those voltages are correct. You should have those voltages in both your amps.

[Valvehead]

today i replaced TR11 , TR12. No change.

If you look at that scem, it seems i have the high voltage ( + 350mv dc ) right on that long center line , that a lot of parts are connected to , it ends at the + speaker jack.

im going to test some more tonight---checking all the resistors in the output area. IC8 seems good, it has -14v/+14v

[Valvehead]

update--Monday night=

on page 2 of the schematic, that shows the output section, i tested EVERY resistor on that page. If any tested off, i clipped one end and tested again. ALL resistors check out ok.

So...thats everything checked / or replaced except the caps....I guess ill start on those, unless anyone has any other ideas ??

[Jazz P Bass]

What dc voltages do you read at the bases of TR5 & 6?
Have you swapped out IC8?

[g1]

IC8 seems good, it has -14v/+14v

This is not a valid indication that the IC is any good.

Have you swapped out IC8?

That is how to verify if IC8 was the problem.

[Valvehead]

What dc voltages do you read at the bases of TR5 & 6?
Have you swapped out IC8?

they are both new
tr5 base is -1.5vdc
tr6 is +1.5vdc

on IC8....its a tl072 . the only chips i have here are 4558 , tl071 , tl082 . will any of those be a good replacement ?

[Jazz P Bass]

The only IC in your batch that is pin to pin compatable is the 4558.
It is not a jfet input opamp.
That said, it may help to sort out the problem by temporaily installing it.
If it does help, you really should have the correct part.