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

  1. #36
    Senior Member nevetslab's Avatar
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    Quote Originally Posted by rpape2601 View Post
    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.

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

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    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.








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  4. #39
    Senior Member nevetslab's Avatar
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    Quote Originally Posted by rpape2601 View Post
    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.

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    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.

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    Senior Member nevetslab's Avatar
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    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.

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    Last edited by nevetslab; 11-26-2018 at 10:05 PM.
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    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.....

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    Senior Member nevetslab's Avatar
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    Quote Originally Posted by rpape2601 View Post
    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?

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    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.

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  10. #45
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    Quote Originally Posted by rpape2601 View Post
    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.

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    Xtrs 8, 9, & 10 are new. So is TR7. I'm going to try that tomorrow night.

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    I check the zeners out of the board and they check ok, but I'll replace them anyway.

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    Zeners are replaced, all the transistors TR8, TR9 & TR10 replaced. Did a "my bad". Had 8 & 10 reversed of course it would short...duhhh. Anyway they are in the correct position now and the offset is back. I have R101 and R102 lifted and I am getting -49.0vdc on R101 with the resistor lifted and 48.9 on R102 with the resistor lifted. I am getting 36.3 vac on both sides of the transformer now, just not sure the 49 vdc +/- is correct with the resistors lifted. Do the zeners act as a voltage drain to bring the voltage down to 15 volts?

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    Hooked the resistors R101 and R102 back in and I have -47vdc on R101 and +23.5 on R102. Don't know why I have this voltage now when it should be 15 +/-. Tube is out so it's nothing to do with that. This is all really annoying and I am just about done with this amp.

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    Looking at the drawing, the only thing I can figure is the 2 caps are shorted from that mishap and are causing the - & + to lose the separation and sum the voltages to give the 49vdc +/-. I get 49vdc +/- off the bridge too. Really frustrating.

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  16. #51
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    When you talk about measurements at R101 and R102, you need to say which side you are measuring.
    Should be around +/-48V at the bridge ends, and around +/-15V at the zener ends of R101 & R102.
    If the +23V you are measuring is at the bridge end of R102, switch your meter over to AC volts and see what you get coming off the + end of the bridge. (this will be the filter cap ripple voltage)

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    The funny thing is i'm getting 48vdc on the zener side of the resistor

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    Are you sure the zeners are the right way around? (they go opposite of what you might expect)

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    Without studying the whole thread, a common mistake I see is people assuming the negative lead of the filtar caps always go to ground. They don't. On the negative supplies the positive terminal is grounded. Make sure we did not install filter caps with all negative legs grounded.

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    Going back to 12/08/18 Post # 4, your secondary voltage of 36.3VAC....is that measured from GND to either of the ~ bridge terminals (AC input)? This would be loaded by the power amp circuit, and with R101 & R102 lifted. Measuring from both ~ terminals of your bridge (AC input), you'd get 72.6VAC or close to that, correct? Earlier, we had the DC supply readings of +/- 38.3VDC, and +12.1/-14.7V on the zeners.

    If you are indeed now getting +/- 49VDC on the output of the bridge at the caps C62/C61, that sounds reasonable. But, if you're also getting +23.5V at C41/R102/ZD2 and -47VDC at C40/R101/ZD1, the preamp's IC's have now failed, ZD1 appears to now be open circuit for some reason, and ZD2 has also changed, as neither zeners are regulating for your +/- 15VDC supplies.

    Enzo mentioned the capacitor polarity markings for the supply filters, where mistakes have been made by some who didn't understand that the (+) terminal would be connected to ground on the negative side of the bipolar supplies (such as C40, C61).

    I fully appreciate the agony you're going thru, being on the learning end of these amplifier circuits, and contending with parts that failed to begin with, and not yet finding all else that failed. I just looked at the preamp schematic, finding 10 IC's in there...three of the M5201's which are not so common, TL072's, TL071's and a 1448...the last three being common parts. If these are socket-mounted, I'd remove them while trying to get the power amp circuit to work. If indeed you now have +23/-47V for the bipolar supplies to run these IC's.....they will all need to be replaced. The M5201 can be replaced with a New Japan Radio P/N NJM2120.

    Not having one of these amps on the bench, I dug a bit on the internet to find the following photos.....some of these may be a different Valvestate model, but let us know. I'm looking to see if there's a simple way to isolate the preamp circuits power supply connections from that for the power amp. Sometimes those of us who have to isolate sections of the amp from a single-PCB construction like this, it requires cutting traces, which later would have to be jumpered back together by scraping off the green solder mask to expose the copper foil underneath, allowing jumpers to be soldered to re-connect the supplies.

    Click image for larger version. 

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    Are these images what we're working on?

    Earlier, when I was trying to find out why you had 4.7VDC on the output bus, I had suggested lifting R81 from the IC8 Pin 7 end, to see if that 4.7VDC shifted. That removes the DC feedback loop. I'd also lift the cap C48 from Pin 7 end as well, so the feedback is fully open. This allows working on the power amp circuit to re-establish DC stability, once all the faulty transistors, diodes and resistors have been replaced. And, IC8 also would have been damaged by that +23V/-47V supply potential.

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    Last edited by nevetslab; 12-10-2018 at 08:13 PM. Reason: Adding photos
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