Marshall Valvestate 8080 has only 12V + and - on 15 volt rails and it has a loud hum

[nevetslab]

Ok, so now I have replaced the 2200uF caps, TR11 (our version of the BC212 is a 2N3906, pinout is way different, european), Replaced both HT caps the 10uF and 33uF. Powered up and still have half voltage on R101. Here's an interesting thing though. I noticed that the top of C40 was puffed up, just touched the top with my DMM lead and it spewed oil which means it's getting hot. So it is definitely blown. Replaced the C40 cap and now I have -14.7 on the negative but the positive which was 14.7 has now dropped to 12.2VDC. Looking at the schematic, I got curious about TR6. At R83 (1.8K) I have +12.2VDC going in but only +0.2VDC coming out which goes to the base of TR6. This doesn't appear correct to me but I'm not good with formulas and haven't done this in a while so I checked R84(1.2K), it has 38.9 going in and 34.1 coming out about 22% voltage drop. The difference in resistor size is only about .667%, So i figure with 12.2 going in I should have around 9.6 coming out I only have 0.3VDC coming out, going to see what happens when I change TR6. Let me know if this is not the right way to think about this. I have also noticed if I switch channels the voltage changes from 12.2 to 13.7 but I still have +4.7VDC on the positve lead of the speaker, I think causes the speaker to push out when connected. I believe there should only be AC on the speaker. Correct? Is it possible that I have a bad IC? How do I check for a bad IC?

First, disconnect the speaker, so it doesn't get damaged. We don't need it as a load, since this is a solid state amp, works fine unloaded.

Also, disconnect limiter coupling diodes D5 & D4 so the limiter circuit is out of operation of the main power amp circuit.

Did you replace TR5? it was shorted, based on the junction readings you made. With the 12V drop across R83 that biases the base of TR6, that's not normal. If you didn't replace TR5, then that 6.7mA current is probably flowing thru D2, D1, R80 and thru the shorted B-C junction of TR5, and also pulling similar current thru R78. If you did replace TR5, then there's way too much current flowing into TR6 base. There's already 4mA flowing thru R84, its' emitter resistor, so there's a lot of excess current flowing thru this level-shift stage in the amp. With 4.7VDC on the output, the circuit is out of balance, and I'm not yet sure what we have.

Make a full-page print-out of just the power amp schematic (with the power supply shown), and measure / mark the following voltages at: TR6 B, E, C; TR5 B, E, C; TR7 E, C; TR4 C, E; TR8 B, E; TR10 B, E, IC8B Pin 7, and the junction of R82 & R79. From all that, we can measure the current flow and see what's happening with the circuit.

Can IC 8 be bad? Yes, though it would normally go high or low, meaning it's output at pin 7 would be sitting near 12.2VDC or -14.7VDC with the current bipolar supply voltages.

[rpape2601]

OK so here are the readings as requested, DMM black kead on ground
B C E
TR6 0.2 34 1.5

TR5 -2.5 -2 -30.8

TR7 X(34) 5.7 31.5

TR4 X (-30.8) -31.1 3.5

TR8 5.8 X(38.4) 4.7

TR10 3.6 X(38.3) 4.5
PIN 7
IC8 10.4

All readings are taken with DMM black on ground. The readings in brackets weren't asked for but I included them just in case it would be helpful. These are taken with D4 & D5 removed from the circuit and yes both TR5 and 6 have been replaced at the time I took these readings.
Out of curiosity, the items that look like resistors that are labeled "L1 thru L20", are they links or jumpers? I can't find them on the schematic and they do not have any real resistance, like 0.3 ohms.
Out of curiosity (again) I measured the voltage on all the pins on IC8. 1 thru 4 - 0 volts, pin 5 - 4.5v, pin 6 - 0.9v, pin 7 - 10.4v, pin 8 - 12.1v. Looking like IC8 is suspect.

[g1]

Yes, L = link here (jumper) if they are the white or beige guys with the one black stripe. Sometimes L is used for inductors, which would also measure very low resistance, but those would be shown on the schematic, and would have colour code bands on them.

Where is your negative supply for pin4 of IC8? Are you really measuring 0 volts there? If so, IC8 should not be suspected, it is missing one of it's supplies.

Also, please do not use - as a dash. When posting voltages, only use - to indicate negative voltage. Use colon : rather than dash if needed for listing numbers.
The numbers you gave for IC8 are confusing for this reason.

[nevetslab]

OK so here are the readings as requested, DMM black kead on ground
B C E
TR6 0.2 34 1.5

TR5 -2.5 -2 -30.8

TR7 X(34) 5.7 31.5

TR4 X (-30.8) -31.1 3.5

TR8 5.8 X(38.4) 4.7

TR10 3.6 X(38.3) 4.5
PIN 7
IC8 10.4

All readings are taken with DMM black on ground. The readings in brackets weren't asked for but I included them just in case it would be helpful. These are taken with D4 & D5 removed from the circuit and yes both TR5 and 6 have been replaced at the time I took these readings.
Out of curiosity, the items that look like resistors that are labeled "L1 thru L20", are they links or jumpers? I can't find them on the schematic and they do not have any real resistance, like 0.3 ohms.
Out of curiosity (again) I measured the voltage on all the pins on IC8. 1 thru 4 - 0 volts, pin 5 - 4.5v, pin 6 - 0.9v, pin 7 - 10.4v, pin 8 - 12.1v. Looking like IC8 is suspect.

I don't have a photo of the PCB to see the parts labeled L1 thru L20, but I'm guessing the are zero ohm resistors (jumpers). Usually when you find those parts, it suggests the parts are machine-placed, which wants to see a component body and not a piece of folded wire. There would be a single black stripe on the middle of the body.

I extracted the power amp schematic, cropped it and moved it into Photoshop to label it with the voltages you provided.....with some interpretations...one being the collector of TR10.

I agree with 'g1' regarding the 0V reading on pin 4 of the IC. That would indeed force a level shift. TR6 looks suspect, with the Emitter being more positive than the base. We'd expect -0.4V there. Also, TR7 looks suspect, where the Vbe is 2.5V. What we would expect to see there is 33.4V.

There appears to be about 32mA flowing thru the voltage gain stage of TR7, TR9 & TR4, and the bias voltage between the bases of TR8 & TR10 look about right. I just question the reading of TR6 collector/TR7 base being 34V.

So, check again on the Pin 4 of the IC. If it's instead -14.7V, then that half of IC8 is suspect. IC8A, 0V on pins 1, 2 & 3 would be ok.

[rpape2601]

I'll remember to use a colon instead of a dash when posting voltages. On IC8 pin 4 has negative 14.4, same as the power supply.

[nevetslab]

I'll remember to use a colon instead of a dash when posting voltages. On IC8 pin 4 has negative 14.4, same as the power supply.

Looking at the schematic I posted in the earlier thread this afternoon, see what happens if you lift up the IC end of R81, and connect that end of the resistor to gnd with a clip lead. Does the output go to 0VDC, or remain shifted to 4.7V or so? I'm still suspicious of the collector and emitter readings of TR6, as they don't make sense....and with 34V on TR6 Collector/TR7 base, it doesn't make sense with TR7, since I'm seeing around 32mA flowing thru TR7/TR9/TR4, which does make sense.

It may be that IC 8 is bad, but, it may be trying to correct the overall loop around the amp due to TR6 being bad. Lifting R81 out does open the DC loop, though it's companion cap C48 is still closing the loop in it's frequency range.

[rpape2601]

I'll recheck the voltages tonight. It was late when I checked them so I may have messed up on one or 2.

[rpape2601]

Changed TR6 and IC 8, didn't improve anything. Then I did it again, hand slipped checking voltages on TR10, blew TR 8 again, changed it. TR8 was a TIP142, I bought the BVD65 that the drawing spec'd, put it in, now I'm showing -52.4V B-C, .6V B-E, 51.4V C-E. Had a really busy day moving furniture and i guess my hands are a little tired and i slipped with a lead....again. Crap.... Xfer output is now 29V + & -. DC before the resistors 101, 102 is 31vdc + & -. 101 is now 11.9vdc &102 is 9.1. Should have quit while I was ahead, checking the BDV65 slipped and took something else out. Dead short somewhere that's taking incoming down to 35VAC, crap.... That's enough for tonight.








7

[nevetslab]

Changed TR6 and IC 8, didn't improve anything. Then I did it again, hand slipped checking voltages on TR10, blew TR 8 again, changed it. TR8 was a TIP142, I bought the BVD65 that the drawing spec'd, put it in, now I'm showing -52.4V B-C, .6V B-E, 51.4V C-E. Had a really busy day moving furniture and i guess my hands are a little tired and i slipped with a lead....again. Crap.... Xfer output is now 29V + & -. DC before the resistors 101, 102 is 31vdc + & -. 101 is now 11.9vdc &102 is 9.1. Should have quit while I was ahead, checking the BDV65 slipped and took something else out. Dead short somewhere that's taking incoming down to 35VAC, crap.... That's enough for tonight.7

So sorry to hear that sequence of calamities! I've had days like that too. I just downloaded the TIP 142 data sheet.....On Semi, shows the TIP 140, 141, 142 and the compliments. TIp 142 & Tip 147 being suitable replacements for the BVD65 & BDV64.

So, TR6 being changed still showed the same B-C-E voltages as before? That points to TR7, except the voltages you had stated before showed around 33mA flowing thru TR7/TR9/TR4. Sigh...........

You might want to add some shrink tubing to limit how much of your voltage probes are exposed. Mine have a shroud that can move into place for just those occasions. Hang in there.

[rpape2601]

That's the first thing I'm going to do is to add a sleeve to each lead so I don't fat finger it again. Makes you feel pretty stupid when you do that......twice...in one night, LOL. The question I have though is, I'm reading around 50 volts on the C-E of both TR8 and TR10. I am assuming that this is the sum of the positive and negative and that I am running straight through TR 8 & 10, that's why I'm getting that 50 volt reading which means these 2 are shorted. Ordered some more replacements cuz this will probably happen again before I'm done. In the mean time, I'll change TR7 and when the parts arrive I'll install them and see how it looks.

[nevetslab]

Now, if the power supply is +/- 50VDC, and the output is no longer offset as it was, you 'd see virtually 50VDC C-E on TR8 & 5V E-C on TR10. I had to stop to find the edited schematic, which wasn't on my shop computer, so copied it off the earlier post to see what the voltages were. OK.....+/- 38.4VDC. Sounds like the supplies are dragged down by one of the two outputs being shorted C-E.

Disconnect the power, discharge the supply caps and look with the DMM in Diode Test mode to find what has shorted.

Back to the earlier concerns, which are shown with the schematic I labeled having your voltage readings, your replacing TR6 and getting the same results...something is wrong, as you won't have that kind of B-E voltage on TR7 unless it wasn't read correctly, as well as the B-E voltages on TR6 is wrong. It suggests TR6 is NOT conducting current. And, since you had 4.7VDC offset, there may be resistors that are no longer correct. But, the voltages on TR7 (disregarding the base voltage reading) seems to suggest normal current flow thru this TR7/TR9/TR4 stage, as well as the output stage TR8/TR10. It suggests the problem is at the voltage gain stage TR6/TR5 and the output voltage at the op amp Pin 7.

You'll need to get back to working parts again. And, check ALL of the resistors in this circuit to make sure we don't have open resistors or some that are way off. You may have to lift one end to verify, as semicondctors and adjacent parts on circuit nodes affect the readings.

The inquiry on lifting the op amp side of R81 was to see if the output returned to 0VDC.

[rpape2601]

Well, I replaced TR8, TR9 & TR10, plus TR7. I have a short somewhere. Current limiter light glowing quite bright so something has smoked. Almost feel like bypassing the light to see where the smoke will come from. Feels like I'm chasing my tail. But I think the offset is ok now, but something is pulling down the transformer. With the xfer connected only about 35vac on the incoming, disconnected it goes back to 120vac and I have 36vac on the secondary, so it looks like it survived at least. I guess I'm back at square one again, maybe a little worse because the dc ain't so good now. Only about 1vdc. Going to pull the bridge rectifier tomorrow to see if I smoked it. Damn.....

[nevetslab]

Well, I replaced TR8, TR9 & TR10, plus TR7. I have a short somewhere. Current limiter light glowing quite bright so something has smoked. Almost feel like bypassing the light to see where the smoke will come from. Feels like I'm chasing my tail. But I think the offset is ok now, but something is pulling down the transformer. With the xfer connected only about 35vac on the incoming, disconnected it goes back to 120vac and I have 36vac on the secondary, so it looks like it survived at least. I guess I'm back at square one again, maybe a little worse because the dc ain't so good now. Only about 1vdc. Going to pull the bridge rectifier tomorrow to see if I smoked it. Damn.....

I'll get back with you on this later today....gotta finish up the shop work and get to my billing to keep afloat. The 35VAC incoming....is that the secondary side feeding the bridge? And the ~1VDC.....where it that? Not the output from your bridge?

[rpape2601]

That's the secondary side of the cafeteria. With the leads disconnected. The 1Vdc is after R101 & R102 with the xfer connected. I checked the zener diodes in the circuit and they conducted in both directions. I'm going to take them out and check them again.

[nevetslab]

That's the secondary side of the cafeteria. With the leads disconnected. The 1Vdc is after R101 & R102 with the xfer connected. I checked the zener diodes in the circuit and they conducted in both directions. I'm going to take them out and check them again.

Ah, that's with the light bulb limiter ahead of the amp. OK. Yeah, the zeners are definitely toast....3W zeners gone down. Rats. Are all the IC's socket-mounted, or soldered into place? I'd lift one end of R101 & R102 up, remove the two zeners so you can measure across the +/- LV supplies and see if it's really low ohms. If the IC's were socketed, you could remove them one at a time until finding what failed and shorted out the zeners....assuming it was an IC that killed your zeners. The load presented by the power amp circuit, looking into R83 & R78 wouldn't take them out, even if there was a short between the amp circuit end of those two resistors.

I gotta give this some more thought.