Marshall Short Part II (old subject new thread)

[sweatyk]

OK g-one, I checked R118 and R96 out of circuit and both read .3 ohms. I have recently discovered that my DMM leads have reached an age that they seem to have built up some resistance. I adjusted my findings accordingly.

DrGonz per your suggestion I read the emitters on both TR8 & 10 with respect to ground. I found the results on TR8 a little strange.
TR8 at start up read .024V I left it on for about a minute and it leveled off at .090V but then it started to descend and after about 15 seconds I removed the probe and it was at .084 and still going down.............strange.
Tr10 at start up was +.007V and after a minute had climbed to -.119V and was still climbing.
IC8 (1) +.01V
(2) +.005V
(3) 0V
(4) -14.98V
(5) +.015V
(6) +.020V
(7) +.226V
(8) +15.33V

All of this was tested without the preamp tube installed.

[sweatyk]

No one?........well that's ok because I believe that I need to replace my meter leads at this point before going any further. They're about 25 years old and it has become obvious that they have built up some resistance and that throws off true readings. I tested them last night on a different meter and the same result. I will get some soon and then back track and retest all of the voltages. Thanks and I will chime back in sometime next week.

[DrGonz78]

Can you post all voltages for TR4-7 and TR11-12? Right now we need to see all that data to pinpoint anything that is not balanced in the circuit. Even try your measurements again with tube installed, but that probably won't matter very much. There is a tiny bit of DC offset on pin7 of IC8B, but that is probably not a big deal. Even replacing those emitter resistors can be helpful. However, your meter will pickup about 0.4ohms of resistance from the test leads or something in that ballpark. So measuring 0.7ohms on those emitters seems good and it's not like they are open. Good luck.

[J M Fahey]

1) 173 mV across emitter fresistors is high, the amp is overbiased.
So it *will* work for a few minutes , but very probably start overheating even more and eventually burning.
Try to lower biasing.

*Now* with the biasing resistor values shown (R74 1K8 , R73 680r) you should nominally have 2.37V bias base to base, TR8/Tr10 .
So far you have 2.43V which is within a few percent, perfect.

Problem is, that value seems to be very much for that particular pair of TIP142/147 , which does not mean they are bad, simply that Marshall *should* have spent a few extra cents in a trimmer pot.

You can still adjust that bias , of course.

First, just to check, measure voltage base to base TR8/TR10 and voltage across R118 and 196 .

First with amp cold and then after, say, 5 and 10 minutes, so we check whether it's stable or it raises.

Then, turn amp off, and solder a 10 K resistor in parallel with R74 (1K8), WAIT 10/15 MINUTES , and repeat measurements.

Why wait?
Because you have just heated TR9 big time when soldering, you want everything at ambient temperature.
That's a very common mistake, people solder different resistors to adjust bias, gets what they wanted ..... and next day find everything out of whack .

I expect you will have lower base to base voltage, say 2.2V or thereabouts, but mainly you will have much lower voltage across 0r33 emitter resistors.
I wouldn't be surprised for it to be close to 0, go figure.
Post results.

[g1]

Thanks JM, makes sense that it may just require bias adjustment.
When I mentioned above that I had never needed to do a bias adjustment on these amps, I had forgotten that we always used BDV64 & 65, never the TIP subs.

[J M Fahey]

FWIW I *slightly* underbias my amps.
99% don't complain ( or at least not in my face ) , the 1% which does always happens to be bedroom warriors, not sure why they bought a 100W amp and a 24"x24"x12"cabinet to begin with
I tell those to open the chassis and cut a specific resistor (what here would be akin to pulling the added 10K resistor).

[sweatyk]

Gentlemen I just acquired a new DMM today.....YEAH! However the weekend is too busy so next week I'll be getting into what JM suggested and report back. Hope yall have a great weekend!

[sweatyk]

OK JM here are the results. All testing was done with limiter w/75watt bulb.

COLD testing without 10K resistor parallel on R74:
TR8 & TR10 base to base = +2.43V
across R118 = +5v and increased up to +20v before I stopped
across R96 = +5v and increased up to +20V before I stopped

HOT testing without 10K resistor parallel on R74:
TR8 & TR10 base to base = -2.32V
across R118 = +91V and very slowly creeping up
across R96 = +92V and very slowly creeping up

COLD testing WITH 10K resistor in parallel on R74:
TR8 & TR10 base to base = -2.14V
R118 = -2.2V
R96 = -2.2V

HOT testing WITH 10K resistor in parallel on R74
TR8 & TR10 base to base = -2.18V
R118 = -002.4mv
R96 = -002.4mv

[sweatyk]

So does this bring it down to what is considered the ideal voltage here? What step is next?

[DrGonz78]

COLD testing without 10K resistor parallel on R74:
TR8 & TR10 base to base = +2.43V
across R118 = +5v and increased up to +20v before I stopped
across R96 = +5v and increased up to +20V before I stopped

HOT testing without 10K resistor parallel on R74:
TR8 & TR10 base to base = -2.32V
across R118 = +91V and very slowly creeping up
across R96 = +92V and very slowly creeping up

COLD testing WITH 10K resistor in parallel on R74:
TR8 & TR10 base to base = -2.14V
R118 = -2.2V
R96 = -2.2V

HOT testing WITH 10K resistor in parallel on R74
TR8 & TR10 base to base = -2.18V
R118 = -002.4mv
R96 = -002.4mv

Please correct your post before continuing... I am thinking typo and you meant mv and not volts. Confirm your readings as to continue correctly. If the underlined parts of your other post are in mv's then it is looking great!

[sweatyk]

Please correct your post before continuing... I am thinking typo and you meant mv and not volts. Confirm your readings as to continue correctly. If the underlined parts of your other post are in mv's then it is looking great!

Getting use to my new DMM. I will recheck and adjust accordingly. Thanks for catching that.

[sweatyk]

My bad on the typo in the previous post. I have corrected and here are the results. Please advise.


OK JM here are the results. All testing was done with limiter w/75watt bulb.

COLD testing without 10K resistor parallel on R74:
TR8 & TR10 base to base = +2.43V
across R118 = +5v and increased up to +20v before I stopped
across R96 = +5v and increased up to +20V before I stopped

HOT testing without 10K resistor parallel on R74:
TR8 & TR10 base to base = -2.32V
across R118 = +91V and very slowly creeping up
across R96 = +92V and very slowly creeping up

COLD testing WITH 10K resistor in parallel on R74:
TR8 & TR10 base to base = -2.14V
R118 = -2.2V
R96 = -2.2V

HOT testing WITH 10K resistor in parallel on R74
TR8 & TR10 base to base = -2.18V
R118 = -002.4v
R96 = -002.4v

[DrGonz78]

So your meter has an auto range function right? Are you sure those aren't mv's? If those readings are in Volts (exception for TR8 to TR10 base to base) then we are in trouble. What's the DC offset readings on the output?

I mean think about it the highest voltage is B+ which would be just lower than 40volts(with limiter). How could we have 91v across the emitters?

[sweatyk]

I am reposting the results with typos corrected properly this time. Sorry for the amateur mistake. I had some urgent family business come up and had to drop everything. But here it is.............


OK JM here are the results. All testing was done with limiter w/75watt bulb.

COLD testing without 10K resistor parallel on R74:
TR8 & TR10 base to base = +2.43V
across R118 = +5v and increased up to +20v before I stopped
across R96 = +5v and increased up to +20V before I stopped

HOT testing without 10K resistor parallel on R74:
TR8 & TR10 base to base = -2.32V
across R118 = +91V and very slowly creeping up
across R96 = +92V and very slowly creeping up

COLD testing WITH 10K resistor in parallel on R74:
TR8 & TR10 base to base = -2.14V
R118 = +2.2mv
R96 = +2.2mv
HOT testing WITH 10K resistor in parallel on R74
TR8 & TR10 base to base = -2.18V
R118 = +2.4mv
R96 = +2.4mv

Please advise...........thanks guys

[DrGonz78]

COLD testing without 10K resistor parallel on R74:
TR8 & TR10 base to base = +2.43V
across R118 = +5v and increased up to +20v before I stopped
across R96 = +5v and increased up to +20V before I stopped

HOT testing without 10K resistor parallel on R74:
TR8 & TR10 base to base = -2.32V
across R118 = +91V and very slowly creeping up
across R96 = +92V and very slowly creeping up

COLD testing WITH 10K resistor in parallel on R74:
TR8 & TR10 base to base = -2.14V
R118 = +2.2mv
R96 = +2.2mv
HOT testing WITH 10K resistor in parallel on R74
TR8 & TR10 base to base = -2.18V
R118 = +2.4mv
R96 = +2.4mv

Please advise...........thanks guys

I am thinking you got the measurements all straightened out on the readings with the 10k resistor in parallel. However, still confused by these numbers that I have underlined up above. I am thinking that they are really mv's. The measurements with the 10k are looking great btw.

Edit: I am guessing the auto range feature on your new meter was different than older meter. So when taking these readings your instinct to read the meter as Volts. Let us know if that be the case.