Guitar Amp Marshall 75 reverb issue

[daigs]

Hi there, new fault hardware over my desk.

I'm talking of a Marshall Guitar Amp 75 Reverb.
Turning on the amp a "uuummmm" high level noise come out from the speaker. If I connect a source at the input nothing change.

Attached you can find the schematic.

I checked all the +/- voltages, 15, 24 and 45 and all are OK. The input fuse is not burned.

With the speaker connected or disconnected the R44 R45 R46 R47 are soon (10 seconds) very hot.
With the speaker disconnected I measure 45 volt DC on the speaker output terminals. So I checked the two darlington MJ3001 and MJ2501 disconnecting them and to me they seem OK. I heard that a positive multimeter check on a darlington doesn't assure it to be good.

Base-Emitter measurements in diode mode are:
0.6 V and 1.3 V PNP
0.6 V and 1.6 V NPN

Base-Emitter in resistence mode:
4.3 Kohm PNP
10 Kohm NPN


All other junctions are easier to check and are OK.

I also checked "in board" all the other BJTs and diodes, other than shorts on all electrolytic caps with no signs of fault. I haven't yet checked the values and ESRs on caps. But I think that 45 VDC on the output is a short on some component rather than a modified value.

How you think I could proceed with other measurements?

I'm a beginner in the world of audio amplifier repairing.

Thank you.

[alfatv]

Re: Guitar Amp Marshall 75 reverb issue

45V is obviously wrong, are are measuring it against the ground?
For transistor shorts check, measure collector to emitter.
Did you measure all DC voltages after rectifier?

[Devil_rider]

Re: Guitar Amp Marshall 75 reverb issue

The 45 v on the output is not ok and it is caused by an oscillation in your circuits could be a bad filtration or a coupling cap that cause this . My approach would be to identify the stage that oscillates by shorting the signal to ground with a 10uf cap in between each stage . After that check all the caps in that stage or if you want to kelp that amp for a long time just replace them all. I guess you already checked all semiconductors . Also replace all 4 power filter caps

[daigs]

Re: Guitar Amp Marshall 75 reverb issue

Thank you for your answers.
The measured voltages are after the capacitors and rectifier. Remember that we a have a mono amp stage here, not stereo.

Devil Rider, I changed only C48 and C49 capacitors. How this oscillation you are talking are created by the circuit? Can a filter cap (e.g. C40) causes the main rail voltage 45V to be across the speaker output?

The 45 DC volts are measured between the output wires speaker. There isn't a true ground but a +V 0 and -V.

Two extra questions:

-What voltage values should I expect across the remaining transistors?

-Can I check the remaining curcuit, power it on, assuming that the two darlington TR13 and TR12 are disconnected?


I talked with a technician and he told me that in some amplifier even if the darlington are ok with the multimiter they can be shorted when powered !


Thanks.

[R_J]

Re: Guitar Amp Marshall 75 reverb issue

Without the speaker connected you should have close to zero volts between 0 (common) and the connection between R67 and R68. If you measure +45volts, either TR12 is turned on or shorted or TR13 is not turned on or open. both transistors need to be biased so the resulting voltage to the speaker is near zero volts. Check that you don't have an open resistor in the driver or output circuit.
Did you check R67 and R68?

R44 R45 R46 R47 will get hot, they are dropping the voltage from +/- 45 v to 24v and 15v

[Devil_rider]

Re: Guitar Amp Marshall 75 reverb issue

You do have a 'ground' to use as reference for measurements , it is the 0v between the capacitors . The hummmm tells me that you do not have DC on speaker but ac from circuit oscillating. That is why I recommended to short the stages to ground. About the oscillation ... Any op Amp with a positive feedback can turn into an oscillator ( if you want to see the theory behind it search for opamp oscillator, it is one of the basic circuits that use op amps) . The oscillation can be caused by a bad coupling cap, a noisy DC voltage , an unballanced biasing circuit of the complementary transistors .....speaker should be at 0 v without signal. With 12 and 13 out of circuit you should not see anything on 8,10 and 11 but going back to 7 you should see +45 in e only.

[daigs]

Re: Guitar Amp Marshall 75 reverb issue

Lots of useful information, I appreciate that.

Devil_Rider, the sound is not a typical DC bump on the speaker, but as you said is more a sound like a ground but not a true ground sound. The "oscillation" cause you suggested could be true. Can you please explain me what stage I have short to ground with the cap, with reference to the schematic?

R_J I checked the R67 and R68 and to me they both measure 1.8 ohm each at my not professional multimeter (0.8 ohm is the internal probe resistance). I don't understand the true value from the schematic. .33 is 0.33 ohm? I attached a picture.

Can I check the circuit in your opinion without create damage, powering it on with the final stage disconnected (TR13 and TR12)?

[daigs]

Re: Guitar Amp Marshall 75 reverb issue

I confirm that I measure -45 volt DC between the 0 volt rail and the node between R67 and R68. The same absolute 45 V DC is measured between the speaker outptut.

TR12 and TR13 seem good measured with the multimeter!

Regards.

[R_J]

Re: Guitar Amp Marshall 75 reverb issue

If you measure negative (-) 45volts between 0 and R67/R68 that means either TR12 is biased OFF or TR13 is turned ON, so basically connecting the emitter and collector. Did you check ALL the resistor values between C36 (input cap) and the output transistors? All it takes is for one to be open or higher in value and the bias gets upset, Also check D10 and D11
It may work without TR12 and TR13 but it may not, sometimes you need to install about a 1K resistor between the emitter and base of Tr12 and TR13 with the transistors removed.

[petehall347]

Re: Guitar Amp Marshall 75 reverb issue

think its ok to remove the output transistors without adding resistors . there appears to be a feedback path without them so long as its intact .
saying that it won't harm anything to add them anyway .

[Devil_rider]

Re: Guitar Amp Marshall 75 reverb issue

Follow RJ's advice and take out the transistors. Check the voltage after and I am ready to bet it will be 0 . A brief description of the final stage is like this. Signal comes in in base of tr6. Tr6 and tr9 are a differential amplifier that is needed for the final stage feedback through r66 also sets the DC bias to 0 v in case the transistors are not paired right or circuit is drifting, and that is done through r61. Tr7 and tr8 are driver circuit for final transistors . If you measure the voltages on these transistors we might be able to help better. Also if you want to short the input like I said use a 10 uf cap from r51 to 0v line and check if the oscillation goes away. An oscillation depending of the stage where is happening can drive the output voltage one direction or other but not commonly to the point where you see full negative of positive DC . Did you happen to read the output voltage on ac scale?

[daigs]

Re: Guitar Amp Marshall 75 reverb issue

Can someone say me the .33 on R67 and R68 what mean? What value should I expect?

[budwich]

Re: Guitar Amp Marshall 75 reverb issue

those are .33 ohm resistors.... 5 watt rating

[daigs]

Re: Guitar Amp Marshall 75 reverb issue

Thanks budwich.

Some news here.

I powered up the amp with TR12 and TR13 disconnected. The -45V DC still present at the base trace of both TR12 and TR13 and some other points connected!
At the speaker output a voltage of about 9.5 V DC that after some seconds were 9.4, after 5 seconds 9.3... and so on.

Going backwards in the circuit I measured again TR7 (it was already measured at the first check) and noticed that the base emitter junction was open. I took out it from the circuit and all the junctions were open, so BC, BE and CE forward and reverse (I don't know if the heat during the desoldering opened the other junctions...). Another doubt I have is if TR7 was damaged by the amp working without TR12 and TR13, but I hope no.

I haven't yet powered up the amp without TR7, I'm not confortable with this amp with several stage. Is all a set of components in equilibrium and I'm not aware what can happen removing an active component.

By the way maybe TR7 is the cause of the fault. Without connection with the +45 DC (by means of TR7) the R59 and R56 were directly connected to -45 V DC via R60, R58, etc.

What do you think about?

Thank you.

[R_J]

Re: Guitar Amp Marshall 75 reverb issue

Replace TR7, it will likely fix it, I doubt soldering it caused it to go open.
without TR7 working, TR12 would likely not be biased on so Tr13 base would have been biased on causing the -45v to show up on the speaker

[daigs]

Re: Guitar Amp Marshall 75 reverb issue

Hard to find the BD370D BJT... I not even found a just match/equivalent online.

I found in a old laboratory a new BD370C that differ for the max collector emitter/base voltage. The "C" is 80V while the "D" is 100V.

In our opinion can I mount the "C"?

[alfatv]

Re: Guitar Amp Marshall 75 reverb issue

Hard to find the BD370D BJT... I not even found a just match/equivalent online.

I found in a old laboratory a new BD370C that differ for the max collector emitter/base voltage. The "C" is 80V while the "D" is 100V.

In our opinion can I mount the "C"?

There will be ~90V C-E worst case, so shouldn't use 80V rated.
You can at least try first another bjt 100V or more rated. Maybe not the best performance, but should at least function.

[R_J]

Re: Guitar Amp Marshall 75 reverb issue

The C version transistor should be ok just to see if that is going to fix the problem. If it does you can get the correct transistor.

[daigs]

Re: Guitar Amp Marshall 75 reverb issue

Ok, I will try.

What could be a correct replacement of BD370D?

[daigs]

Re: Guitar Amp Marshall 75 reverb issue

OK the Ampli work well with the new TR.

Now I have to change it with an equivalent BD370D. Any suggestion is appreciated.

Regards.