Marshall Short Part II (old subject new thread)

[sweatyk]

Will do tomorrow. Eyes are starting to cross so no more playing with electricity tonight......

[sweatyk]

OK I measured from the emitter on TR8 to the emitter of TR10. It started at .014V and slowly climbed. After about 10 to 15 seconds it was ay .027V and climbing. Is this to be expected? Should I have waited to see where it would level out? And it was plugged into the limiter with the 75 watt bulb in.

[g1]

Yes, see where it levels out or when the bulb gets brighter. .02V across .66ohms is only 3mA current, no big deal.
If there is no DC at the output, and the bulb isn't getting bright, you should be ok, but see where that reading levels off.

[sweatyk]

May have to recap on my last post. I posted the voltage reading as +. I guess I wasn't paying attention. However this time I let it run till it seemed to level out and went from -.014v to -.173v. The bulb gradually got brighter but not to full illumination. Kinda like a dimmer set to low. So what does this mean? I will be glad to take pics of the readings, connections, and the bulb if you would like and post them.

[DrGonz78]

Those readings seems very normal, perhaps a bit higher than Juan had posted. Perhaps the bias trim pot is up high and needs to be turned down a bit?

When you turn on the amp, with bulb limiter inline, the light many times will get really bright and then it goes dim. They also call the light bulb limiter the dim bulb tester, so all that is very normal. What the light bulb does is limiter current and will get reallly bright if there are shorts in the amp that draw excessive current.

[sweatyk]

Those readings seems very normal, perhaps a bit higher than Juan had posted. Perhaps the bias trim pot is up high and needs to be turned down a bit?

There is no bias trim pot on this amp.

The bulb did not get bright when I first turned it on to measure this voltage. It was barely lit and increased as the voltage increased. Leveling out at .173v which I'm guessing is equivalent to 17mv. This was after it being one for around one minute. Should I leave it on longer to see if something starts breaking down and draws more current?

[sweatyk]

OK I looked up a v to mv converter and it's looking like it leveled out at 173mv which is way too high from what Juan said.....( 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.)

It started at 14mv and it is negative voltage which was not indicated in his quote.

Am I confusing anyone?

[g1]

TR9 is thermally tracking the output stage, so it needs proper mounting and thermal grease at the heatsink. Also check that R73 and R74 are the correct value.
Also, when the outputs stabilize and you have that 173mV between the emitters, what is the voltage from TR8 base to TR10 base?

[sweatyk]

TR9 is thermally tracking the output stage, so it needs proper mounting and thermal grease at the heatsink. Also check that R73 and R74 are the correct value.
Also, when the outputs stabilize and you have that 173mV between the emitters, what is the voltage from TR8 base to TR10 base?

TR9 is properly mounted and has sufficient thermal grease. Voltage reading from base of TR8 to base of TR10 is -2.43v. Value of R73 and R74 were correct. Read them out of circuit.

[DrGonz78]

Sorry about my inaccurate post earlier and the bias trim pot comment. I was rushing off to work and did not have the schematic in front of me. I forgot this is a fixed bias amp.

Anyhow earlier in post#13 you measured 14mv(0.014v) DC on the output. How long did you leave the amp on while taking that measurement? Perhaps if you leave it on longer then you will get 173mv on the output? The polarity changes in voltages is quite common and could be caused by your power in your house even. For example, you might observe 14mv or -14mv on the output that amp and it will work just fine. Oh and the light bulb on the limiter does not ALWAYS light up when first turned on. The lighting up of the bulb upon turning on the amp is in repect to the caps charging up(drawing).

So you are measuring the voltage between the bases of TR8 & TR10. That voltage is carried up to the bases by way of the bias circuit R74>TR9>R73. Please measure for DC voltage across R73 and post that reading too. As far as I can tell is that the amp is probably working fine(assumption) right now but the bias is not where it needs to be. Now there could be a problem with the Opamp IC8B or TR4-7 all need to be working correctly as well.

Now my main point on this post is in relation to a related thread that is up on the MEF site. It is here http://music-electronics-forum.com/t5913/

The idea here is that there could be a bit of difference between the having TIP142 & TIP147 or BDV64/65. Honestly it's a small problem if this is all that is wrong. Read Enzo's post#4 and WMK's post#5. This might be the answer to your problem, maybe...?

[DrGonz78]

It seems on that other thread that he left in the BDV64/65 transistors and changed R74 in the bias circuit. So maybe this is not a problem with the TIP142/147 but with the way this amp behaves. It is really important to stress that you either test and better yet replace all the TR4-7 transistors if you have not yet. At that point it could be that IC8B opamp too. If after all that then maybe the best way to go is to adjust the bias circuit manually by adding different rated resistor(s) or adding a trim pot. Seems these amps are just prone to this problem from what I can tell.

P.S. run the amp for about 10mins and while observing the DC offset on the output.

[sweatyk]

Before I go any further I need to clarify something that has confused me. In post #15 I it was recommended that I adhere to the steps laid out by Juan. " 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." When in fact once it settled I was at 173mv. Please help me understand that it is OK when Juan said it shouldn't be over 20mv. Am I missing something here? I know you said it was OK but I would like to know why if it's possible to explain. Not only do I want to repair this amp I also want to know why so that I can learn from the experience or once it's diagnosed it will all come together and I will have an ah-ha moment?

OK now that I got all of that out. The voltage across R73 is -.63v

[DrGonz78]

At this point on this amp we are confused by Juan's numbers on his post on the other thread, regarding reading across tr8 & tr10 Emitters. I am sorry I only could use other people's examples for that part of the troubleshoot. We can hope that Juan checks into the thread to answer that question. Maybe someone else here can confirm why he said 20mv or less across the emitters. I am not sure myself to be honest. I too want to know the solution just as much as you. Take it slow like you have and it usually leads to an ahhah!! moment. Perhaps G-One can help us understand that part together?

I also wanted you to reconfirm the DC offset on the output to make sure it is not sitting at 14mv and is actually going up to 173mv when heated up. Honestly, if the amp is sitting there with 173mv on the output with 75watt bulb, then that is the sign of health for the amp. What is the voltage (referenced to ground) of the each emitter of TR8 & TR10? TR9 is doing it's job and at full power(no light bulb limiter) we would see about 0.7vDC across R73 and about 2.55v measure from C to E of TR9. So 2.43v at the bases of TR8 and TR10 is pretty normal with lower voltages.

[DrGonz78]

One last question is do you have the preamp tube in position? I know you had it out at the very begining and just wondering if you have it back in now. It would be good to know if that has an impact on the DC offset. Even IC's leading into the output of the amp can have some change in the DC offset to a small degree too. So check for DC voltages on the pins of IC8 report those readings too. There will be the main power rails, but we need to see if the offset could be caused by the IC too.

[g1]

Dr.Gonz: It's between the eimtters of TR8 & 10 that has the 173mV, not at the output. That seems high to me, as it did to JM Fahey in the other thread when he mentioned 20mV there. 173mV across .66 ohms is about 260mA current through the output devices, (130mA each), which is high for idle, 5 to 10mA each is often seen as a bias current spec.

I lift every resistor out of the circuit and checked their value. All but one were within tolerance.R118 which is a .33 was reading .7…….just wondering if that is enough imbalance to cause the problem.

If you didn't replace the resistor, this may be what is causing the high reading between TR8 & 10 emitters.

Beyond this, I don't know if changing the value of R73 is a viable fix, or just a band-aid that does not deal with an underlying issue.
As the bias is stabilizing at 173mV, perhaps it is not a fault and just a matter of needing a bias adjustment. However, in my limited experience with these valvestates (maybe a dozen or 2) I never had to alter a bias circuit.