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

[nevetslab]

I had read in another thread somewhere that the person had removed TR and then powered up the amp, when I did that The resistor R82 fried, not sure if this is indicative of anything but it was another annoying incident. Going thru the circuit board, I noticed that C44 was changed to a resistor by someone other than me. Could this cause some of the issues?
Seriously thinking of changing all the xstrs. I'm a little worried about the ICs, they can be a little delicate when the voltages go nuts.

C44 isn't doing much, and the nominal voltage on the output of the op amp stage ahead of C44 would be very low mV (under 10mV typically). The stage following C44, where three op amps are summed together to provide enough current drive to feed the input to the reverb tank...that stage has no DC gain by virtue of C56 in the feedback shunt network. So, that's not any problem. I'd expect maybe a 100 ohm resistor in place of C44.

Now, with R82 fried, THAT IS a problem. The signal from the op amp IC8B passes thru to both TR5 & TR6 by way of those emitter resistors R79 & R82. If R82 burned open, you'd have no signal thru TR8, since TR6 wouldn't be conducting, TR7 would turn off, and no current would be flowing thru the bias string, so the output stage would turn off, and under signal drive, be very low and distorted.

The IC's in the amp are fairly rugged. I'd only worry about them if either of the zener diodes failed OPEN, allowing much higher voltage to be across the IC's.

What was the outcome with your power transformer? Last we read you weren't getting voltage from the power xfmr.

[rpape2601]

On the transformer with the output lead off, I get full voltage, with them reconnected, about 5-1/2 volts. And that's with the R101 & R102 disconnected from the circuit. I pulled the tube to see if that changes anything, it didn't. I do see other devices that are prior to the R102 & R101. What is the likely hood that something there caught a power spike?

[nevetslab]

Are there wires to/from the bridge rectifier, or does it just solder into place (as a 4-terminal device)? Power down, disconnect the AC mains, and check the four diodes within the bridge by: Blk lead on (+) output (Cathodes), Red lead applied to each of the AC inputs (Anodes). Then Red lead on the (-) output (Anodes), and the Blk lead to the AC inputs (Cathodes). If you get a short in any of the four positions, you have a shorted bridge, which needs to be replaced.

If not that, then you still have a short in your power amp. Did you remove TR8? You indicated having your test lead slip and damaged it. Was R82 already burnt as described previously, or did that happen when you slipped with your probe.

You need to complete checking for semiconductor shorts before going forward, as that's no doubt where the drain on the transformer is occurring.

[rpape2601]

It solders in place. I made a bridge out of 4 4001 diodes just to check how it reads because when the bridge was soldered in, I was getting shorts on all 4 kegs, so I removed the bridge just to be sure I wasn't reading something else in the circuit. With the bridge out, the DC + & - don't conduct which was the same as my 4001 bridge. I put a new TR8 in so I wouldn't smoke that R82 again. I don't think it should have smoked, something strange there. Also with the bridge in place and TR8 back in, I'm getting 1.5VDC on the speaker leads, so some dc is getting through, a cap maybe? When I get home tonight I'm going to do a check on all the xstrs wjth my meter in diode check mode (power off) and log all the readings then post then for you to see. Is there a way of checking caps? I did replace both 1000uF filter caps because one was puffed out at the top but the problem still existed after that. That's when I disconnected R101 & R102 thinking I might try to isolate the issue. And that's where I'm at right now.

[nevetslab]

With your DMM in OHMS setting, 20k range, connecting the Red (+) and Blk (-) leads across a given electrolytic cap, same polarity, you would see an initial low reading, then it quickly increase to open circuit (per your 20k range). Larger capacitance values take longer to charge up than smaller ones, like a 10uF. A 10uF might not even show a reading, unless you went to 200K or 2M. You're mostly looking for a cap that remains low or 0 ohms (shorted). Film or ceramic caps will normally show open circuit as their RC timing to this test is way to fast to get any sort of reading.

I've never had the 8080 on my test bench. I just looked to see what old Marshall chassis was on my floor covered in dust....a 5150. It's power amp board had a square bridge soldered in, with the short snap-in filter caps soldered in, so if it's built like that, all on one long narrow board running most of the chassis, you can't separate the power supply output from the power amp circuit. Obviously can with regards to the lower bipolar supplies by lifting out R101/R102. Best you can do is to remove TR8 & TR10, those two being the parts that can pull the supply voltage down if shorted (C-E shorts). The rest of the circuit is relatively low current draw (15-30mA thru the voltage gain stage TR7/TR4 with the bias xstrs between them TR9.) and around 3-4mA thru the level-shift stage TR6/TR5. Current limiter circuit stage TR12/TR11 are out of circuit unless they've shorted. Then, they just pull current away from the base of their associated power xstr. Obviously all that are found shorted need to be replaced. Make sure bias xstr TR9 isn't bad. There is no bias adjustment on this part of the circuit, so the resistors around it R73/R74 have been set to establish a voltage potential between the two bases of TR8/TR10 to be about 0.8 to 1.0V....close to putting those power xstrs into conduction, but not quite. If TR9 was open circuit, TR8/TR10 would turn on hard, as their bases would be pulled high & low by TR7 & TR4.

[rpape2601]

I made a mistake on the resistor that fried, it was R85. I looked at the schematic and I don't see why it fried. Boy my eyes aren't what they used to be.

[rpape2601]

Cap Volt Rating Position Reading @ 20K setting
2200uF 50V C62 1.70K
2200uF 50V C61 5.18K
1100uF 16V C41 1.40K NEW
1100uF 16V C40 5.50K NEW
10uF 50v C55 7.54K
100uF 25v C20 6.17K
10uF 50V C17 NO READING
10uF 50V C42 9.92K
10uF 50V C16 NO READING
10uF 50V C42 9.91K
22uF 25V C26 1.47K
22uF 25V C24 5.56K
22uF 25V C56 NO READING
10uF 350V C53 NO READING
33uF 350V C59 NO READING

XTR B-E B-C C-E
TR7 756 728 773
TR6 655 652 729
TR12 668 234 711
TR11 0 163 0
TR5 0 0 659
TR4 631 622 0
TR9 557 605 906
TR8 715 659 534
TR10 644 645 SHORTED

[rpape2601]

XTR B-E B-C C-E
TR7 756 728 773
TR6 655 652 729
TR12 668 234 711
TR11 0 163 0
TR5 0 0 659
TR4 631 622 0
TR9 557 605 906
TR8 715 659 534
TR10 644 645 SHORTED
These were taken with DMM on continuity and following whether the xtr was PNP or NPN

Cap Volt Rating Position Reading @ 20K setting
2200uF 50V C62 1.70K
2200uF 50V C61 5.18K
1100uF 16V C41 1.40K NEW
1100uF 16V C40 5.50K NEW
10uF 50v C55 7.54K
100uF 25v C20 6.17K
10uF 50V C17 NO READING
10uF 50V C42 9.92K
10uF 50V C16 NO READING
10uF 50V C42 9.91K
22uF 25V C26 1.47K
22uF 25V C24 5.56K
22uF 25V C56 NO READING
10uF 350V C53 NO READING
33uF 350V C59 NO READING
These are taken with DMM at 20K

[rpape2601]

On R101 I get 5.0VDC and it drops to 3.5VDC and when its at the 3.5VDC I'm getting 7.7VDC on the speaker leads, does this help with the issue?

[nevetslab]

Well, TR11 current limiter looks shorted, C-E, as well as B-E. TR5 looks shorted B-E & B-C, TR4's C-E short may be seeing the short of TR10 C-E.

I would suggest you unsolder these parts and verify their readings. In reading across C-E, when they're in circuit, and you look in both directions, often you're going to read the semiconductor junction of the next stage, or reading a resistor that's connected. Normally when the B-E and B-C are good, the C-E is good, though not the case on TR10. I'd say TR10 is the main reason you're not getting 'normal' power supply voltage with all the bad devices connected.

You'll also want to measure the diodes....D1, D2, D4 & D5. I don't see a D3 anywhere.

At any rate, you're getting at the problem.....negative half of the power amp stage.

When you measured the caps....did you observe the reading start low and increase, or just read that way right off? Take a look at any Electrolytic cap you have not in circuit, such as a 470uF/25V part. What you should see is the reading start low, steadily increase and go open (above 20kohms). Reverse the leads, and it would decrease to and pass thru zero and increase again to 'open' (off scale). That's the drill in checking caps. If they're bad, they'll read low and remain low. Then, you need to see if there's a resistor across it causing the low reading, such as a cap across a cathode resistor on a tube triode stage.

A general question for all of you following this......as there's no driver or pre-driver stage ahead of the single complimentary pair of output xstrs, which will be driving a loudspeaker (not even sure what the power output level of this Marshall Valvestate 8080 is), what would you guess the current thru the voltage gain stage TR7/TR4 with driver TR9 between them be? It would be sourcing the base current for the output xstrs, so my first guess of around 15-30mA seems quite low. The only clue now that I could see would be to work backwards from the output stage, working off of the power output, and what that current would be thru the outputs, and what base current would be needed, etc.

Ah....got the schematic I could read. Outputs are BDV65 & BDV64 Darlington Power Xstrs. hfe around 1000, so with 80W into 4 ohms, that's 17.9V RMS, or 25.3V pk, and 6.32A pk current flowing thru them and thru the load. Base current needed would be 6.32mA, so I guess 15-30mA thru the voltage gain stage isn't a bad guess? The power supply voltage isn't stated. What would you think.....+/- 35V to +/- 40V (at idle)?

[rpape2601]

When I tested the caps, the ones that gave a reading, started low and increased in the reading. I kept the DMM leads on until the number stabilized. Some, I did not get any reading on at all. Not sure why that is, I checked my leads and tried on both directions just to be sure. When I check the xfer output, it's around 38V +/- give or take a 10th or 2.

[nevetslab]

Low value caps take your meter's current charge so fast, your reading rate of 2-4 readings/sec would never catch it. No matter. We're just looking for shorts in this case.

The power supply amp voltage sounds about right, in the middle of what I guessed at.

You bought spare BDV64's and BDV65's? Good man. I saw one of them was shown as an obolete part on the Mouser website, No Stock as a result. When I went looking for info on the amp last night, I saw an inside photo of your amp's PCB's, enough to show the two power xstrs and the small bias xstr between them on the heat sink. Do those three parts use greased mica insulators, or a 'rubberised' insulator sheet mounted between the bottom of the xstrs and the heat sink? If not greased mica, and you had to 'pry' the parts up to get them off the heat sink, you'll want to replace the insulators. While they are more friendly to install, they can be damaged if long-term thermoset occurs, and prying them back off the heat sink will usually tear the surface of the insulator, and no more HV insulation. I use greased mica usually...better thermal conductivity anyway, though messy.

[rpape2601]

I bought all 3 power xtrs at Digikey for $6 plus shipping so why not. I have TR11 out of the PCB and it checks ok. The power xtrs use thermal grease and mica plates, I bought extra plates just in case. Now that I have a better understand on how to check caps, I rechecked C61 and C62. C61 charges to infinite without any issue. C62 charges only to 2.78 so I think there's a problem there. I'm wondering if C62 is pulling current to grd and sucking my negative with it. I'm going to buy a pair of 2200uF caps and replace them both and see what happens. I'm also going to buy some BC212 and BC182 xtrs and replace them as well. This guy has had this amp sitting in his basement for a couple of years. He didn't want to take to an amp cuz it would be too expensive, its only a $200 amp. I don't mind poking around in it because I'm learning a lot of stuff from all of you. I really appreciate the help, thank you.

[nevetslab]

I bought all 3 power xtrs at Digikey for $6 plus shipping so why not. I have TR11 out of the PCB and it checks ok. The power xtrs use thermal grease and mica plates, I bought extra plates just in case. Now that I have a better understand on how to check caps, I rechecked C61 and C62. C61 charges to infinite without any issue. C62 charges only to 2.78 so I think there's a problem there. I'm wondering if C62 is pulling current to grd and sucking my negative with it. I'm going to buy a pair of 2200uF caps and replace them both and see what happens. I'm also going to buy some BC212 and BC182 xtrs and replace them as well. This guy has had this amp sitting in his basement for a couple of years. He didn't want to take to an amp cuz it would be too expensive, its only a $200 amp. I don't mind poking around in it because I'm learning a lot of stuff from all of you. I really appreciate the help, thank you.

I don't see any simple way to check C61 & C62, short of removing them from the PCB. As you're planning on replacing them, that would be a good time to re-check them. They will normally charge up to the extent of the voltage source supplying the current in Ohms mode, and register open circuit. If you used 200k range or 2M range, it would take a long time to charge them up since the current source is so much lower than the 2k range (1mA). Out of curiosity, measuring C62 in both directions, does it settle to the same / near same reading? (2.78k)?

At any rate, looks like you're on a roll here....let us know how this comes out.

[rpape2601]

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?