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Thread: Gallien Krueger 800rb repair - No power

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    Stray Cap DrGonz78's Avatar
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    You have a multimeter right? Set it to diode mode and check the legs on the transistors. Transistors have diodes in them so one way it will read OL for open line and the other way it will show a voltage drop. Usually 450 - 650mv or something like that is normal. If it is short it will read 000mv or something like 019mv. Do a search on Google to really get so more info >>
    Testing transistors music-electronics-forum

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    Can they be tested with just the pins desoldered or do I need to completely remove each transistor from the PCB?

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    Supporting Member The Dude's Avatar
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    Test in circuit first. If you measure shorts, unsolder and check them out of circuit. Output transistors are generally paralleled, so if one is shorted all will measure shorted.

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    And I like to add, there is nothing in a circuit that can make a shorted transistor appear not shorted.

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    I’m getting a voltage drop measuring both ways - getting .375v

    Removed the transistors and outside the PCB they all test good: OL and .513v or so.

    If I measure the points on the PCB where the transistor pins connect I still get that .375v either way I measure it.

    Thoughts?

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    Last edited by beedoola; 10-20-2019 at 05:52 PM.

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    Anyone?

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    Supporting Member nevetslab's Avatar
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    Quote Originally Posted by beedoola View Post
    I’m getting a voltage drop measuring both ways - getting .375v

    Removed the transistors and outside the PCB they all test good: OL and .513v or so.

    If I measure the points on the PCB where the transistor pins connect I still get that .375v either way I measure it.

    Thoughts?
    You're getting this 0.375V using your DMM's Diode Test Function (or Ohmmeter in the 2k range, which is a 2V source and 1mA constant current source), connected between base & collector, base & emitter? First measure a known good xstr. NPN power xstr. B-E and B-C should measure about 0.48V to 0.53V one way only. C-E will be open, reversing the leads so BLK is on the base, RED is on the emitter or collector....that will measure open.

    I'm not sure where you are measuring on your circuit, as you didn't specify. On the output xstrs, measuring B-E, you'll be seeing a lower value, since there is an emitter resistor from the driver stage across the B-E of the outputs.

    We need to know WHERE you're finding this 0.375V in both directions with the xstr(s) out.

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    Quote Originally Posted by nevetslab View Post
    You're getting this 0.375V using your DMM's Diode Test Function (or Ohmmeter in the 2k range, which is a 2V source and 1mA constant current source), connected between base & collector, base & emitter? First measure a known good xstr. NPN power xstr. B-E and B-C should measure about 0.48V to 0.53V one way only. C-E will be open, reversing the leads so BLK is on the base, RED is on the emitter or collector....that will measure open.

    I'm not sure where you are measuring on your circuit, as you didn't specify. On the output xstrs, measuring B-E, you'll be seeing a lower value, since there is an emitter resistor from the driver stage across the B-E of the outputs.

    We need to know WHERE you're finding this 0.375V in both directions with the xstr(s) out.
    Yes, getting the .375v on the diode function.

    Testing each transistor (also in the diode testing mode on the DMM) out of the circuit all tests good - OL in one direction and .513 or a little above on every transistor.

    Using the DMM in the diode mode and touching the pads on the PCB where the transistor connect shows the .375v reading - regardless of the orientation of the probes.

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    Supporting Member nevetslab's Avatar
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    Quote Originally Posted by beedoola View Post
    Yes, getting the .375v on the diode function.

    Testing each transistor (also in the diode testing mode on the DMM) out of the circuit all tests good - OL in one direction and .513 or a little above on every transistor.

    Using the DMM in the diode mode and touching the pads on the PCB where the transistor connect shows the .375v reading - regardless of the orientation of the probes.
    Have you checked diodes D11, D12 (flyback diodes on the LF amp, D5, D6 (flyback diodes on the HF amp)? Also Limiter connect diodes D8 & D9 on LF amp, D2 & D3 limiter diodes on the HF amp, along with D10 in the emitter circuits of the drivers for LF amp, D4 in the emitter circuits of the drivers for HF amp.

    I've attached the PCB parts placement guide for the power amp below.

    800RB_SM2.pdf

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    Supporting Member The Dude's Avatar
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    That link is dead nevetslab. And, while we're at it, there is no valid schematic link in the thread and there are different versions of this amp. If you want accurate help, beedoola, it'd be best to post a schematic of your amp. Without it, we have no idea what you might be measuring.

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    Nick posted some schematics in post #4. Please advise which one is correct.

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  12. #47
    Supporting Member The Dude's Avatar
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    I tried those. I get a 404 server error.

    Edit: I have several, but no idea which amp we have here. As I said, there are different versions. I'll post one schematic. Let us know if this is the right one.

    gallien-krueger_800rb_sm.pdf

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    Last edited by The Dude; 10-24-2019 at 03:03 AM.
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  13. #48
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    Quote Originally Posted by beedoola View Post
    Yes, getting the .375v on the diode function.

    Testing each transistor (also in the diode testing mode on the DMM) out of the circuit all tests good - OL in one direction and .513 or a little above on every transistor.

    Using the DMM in the diode mode and touching the pads on the PCB where the transistor connect shows the .375v reading - regardless of the orientation of the probes.
    Not sure, but it doesn't sound like you're checking all of the transistor junctions for each transistor. You need to check B-E, B-C, and C-E for each. A transistor can be good (for instance) B-E, but shorted C-E.

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  14. #49
    Supporting Member nevetslab's Avatar
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    GK 800RB Power Amp PCB Parts Placement guide Rev C attached

    I ran that PCB Parts Placement guide thru my Acrobat Distiller, now this should appear. It always comes up in my computer, but, that's local. I didn't check the result yesterday after providing the link. This now comes up. Just the layout, not the rest of the Service manual.

    800RB_PwrAmp_PCB_Parts Placement_RevC.pdf

    Also, check the small signal xstrs in the circuit....the current limiter xstrs Q15, Q16, the bias xstrs Q13 & Q14, the Level Shift xstr Q11. Also check the 5W emitter resistors of the output xstrs, as well as the resistors R40 & R41 (2.7k 1W) Do likewise on the HF amp. At present, your reading of 0.375 in all directions isn't making sense, so since the large power xstrs are measuring ok out of circuit, something else is causing this. It's a relatively simple amplifier circuit. Opamp, level shifter xstr Q11, followed by a voltage gain stage xstr Q12, then it's all unity current gain stages (drivers Q17, Q21) and output xstrs Q18, Q19, Q20, Q22, Q23, Q24). We trust the power xstrs WERE installed correctly (NPN and PNP's being where they're supposed to be).

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    Quote Originally Posted by The Dude View Post
    Not sure, but it doesn't sound like you're checking all of the transistor junctions for each transistor. You need to check B-E, B-C, and C-E for each. A transistor can be good (for instance) B-E, but shorted C-E.

    Thanks. I did only check B-E on the output transistors initially - both in and out of circuit. I just tasted all the options with the collector as well, out of circuit. They all tested good.

    Quote Originally Posted by nevetslab View Post
    I ran that PCB Parts Placement guide thru my Acrobat Distiller, now this should appear. It always comes up in my computer, but, that's local. I didn't check the result yesterday after providing the link. This now comes up. Just the layout, not the rest of the Service manual.

    800RB_PwrAmp_PCB_Parts Placement_RevC.pdf

    Also, check the small signal xstrs in the circuit....the current limiter xstrs Q15, Q16, the bias xstrs Q13 & Q14, the Level Shift xstr Q11. Also check the 5W emitter resistors of the output xstrs, as well as the resistors R40 & R41 (2.7k 1W) Do likewise on the HF amp. At present, your reading of 0.375 in all directions isn't making sense, so since the large power xstrs are measuring ok out of circuit, something else is causing this. It's a relatively simple amplifier circuit. Opamp, level shifter xstr Q11, followed by a voltage gain stage xstr Q12, then it's all unity current gain stages (drivers Q17, Q21) and output xstrs Q18, Q19, Q20, Q22, Q23, Q24). We trust the power xstrs WERE installed correctly (NPN and PNP's being where they're supposed to be).
    Thank you, this was very informative!

    Tested Q-15, 16 and the others in circuit, some did not test correctly. Upon removing them from the circuit they all test good.

    R40 and R41 test good.

    Tested all the 5w resistors and they are good.
    I also tested some of the diodes as another poster suggested. Again, I got inconsistent readings in circuit but removing them and testing them they seem to be testing good.

    I also removed all the wiring from the power supply PCB to the transistor PCB just to see if the problem was on the transistor PCB. disconnecting the wires made no change to the readings I've mentioned before on the transistor pads nor did it stop the other transistors and diodes I mentioned from testing bad.

    I'm done for the tinkering today but I'm going to try and see what else I can do.

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    When you were getting the bright limiter bulb, did you have the amp unloaded?

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    Yes

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    Is there anything else that can be tested that would explain the aforementioned voltage across the transistor pads?

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    Quote Originally Posted by beedoola View Post
    Is there anything else that can be tested that would explain the aforementioned voltage across the transistor pads?
    Is this occurring on BOTH the LF and HF amps? Or just on the LF amp. Apart from the power supply levels, the basic circuit topology of the two amps is VERY similar. AND......the 0.375V reading you've been getting is the Diode Test reading with the power turned off, correct? I for one don't find an answer for what you've been reading. Apart from the device type used for Q2 and Q12 Voltage Gain Stage xstr of the two amps, and less outputs in the HF amp, the power amps ARE virtually identical. Same gain. Since your power xstrs measure ok out of circuit, have you put them back in, and, using your light bulb limiter, does the amp power back up without the bulb burning bright?

    Assuming your issue is with only the LF amp, and NOT the HF Amp, you have that HF amp as a reference for comparison to the LF amp in troubleshooting.

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    Yes. With the power turned off.

    With the amp On and the light bulb limiter it was showing a short. Amp kept blowing fuses.

    I gathered people thought that .375 was an anomaly. I pulled all the output transistors to test and ensure they weren't defective which they aren't.

    I'm kind of at a loss in terms of what else I can test.

    Can I turn the amp on without the leads from the capacitor board hooked up to see if the short is still present? Capping off the leads, of course.

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    Supporting Member The Dude's Avatar
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    I'll say it again (see posts#45 and #47). We need to know what amp you have. Please post a schematic or verify the one I posted is or isn't for your amp. Without some sort of road map, we can't tell you what to test.

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    Yup schematic time since flying blind is not very resourceful. When you post the schematic measure resistance from every output transistor leg to ground. Got make sure those aren't shorting to ground.

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    Supporting Member nevetslab's Avatar
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    Quote Originally Posted by beedoola View Post
    Yes. With the power turned off.

    With the amp On and the light bulb limiter it was showing a short. Amp kept blowing fuses.

    I gathered people thought that .375 was an anomaly. I pulled all the output transistors to test and ensure they weren't defective which they aren't.

    I'm kind of at a loss in terms of what else I can test.

    Can I turn the amp on without the leads from the capacitor board hooked up to see if the short is still present? Capping off the leads, of course.
    As you're using the light bulb limiter, I gather you're not equipped with a Variac and a power analyzer (or you would have been using them all along). What you CAN do, which seems like you're thinking towards that, is to disconnect the +/-80V supply leads from the upper power supply board to the power amp board below...cap the wire ends off so nothing shorts, and leave the +/- 60V supplies connected, and try powering up. If that's still brightly glowing, then disconnect the +/- 60V leads in the same fashion, and power up again, now having the power amp circuits disconnected from the supplies, leaving just the +/- 15V supplies connected to the front panel preamp board and what is used of that on the power amp circuits. If you're still glowing brightly, you have a fault in the power supply circuits. You can further isolate the lower voltage from the higher voltage by disconnecting the AC input to the bridges.....though at this point, a simple OHMMETER check in Diode Mode will identify a shorted bridge rectifier. Shorted supply caps can be found in Ohms mode. Mind that all the filters are in parallel (3 pairs in the /- 80V supply for LF, 2 pairs in the HF amp supply. Generally, fault in the +/- 15V supply won't take out the mains fuse, so normal troubleshooting of the discrete parts in that regulator circuit that comes off the +/- 60V supplies.

    If your supplies are all ok, I've often had to resort to using an external bipolar lab supply to bring up a power amp, even though I wouldn't be coming up to full normal supply potentials. It's often high enough to help isolate the problem, plus the lab supplies normally have current control so you can regulate the output for just idle conditions. A Variac also allows this sort of approach, now using the products internal power supplies. Good hunting.

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    Last edited by nevetslab; 11-08-2019 at 04:49 AM.
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    Quote Originally Posted by nevetslab View Post
    As you're using the light bulb limiter, I gather you're not equipped with a Variac and a power analyzer (or you would have been using them all along). What you CAN do, which seems like you're thinking towards that, is to disconnect the +/-80V supply leads from the upper power supply board to the power amp board below...cap the wire ends off so nothing shorts, and leave the +/- 60V supplies connected, and try powering up. If that's still brightly glowing, then disconnect the +/- 60V leads in the same fashion, and power up again, now having the power amp circuits disconnected from the supplies, leaving just the +/- 15V supplies connected to the front panel preamp board and what is used of that on the power amp circuits. If you're still glowing brightly, you have a fault in the power supply circuits. You can further isolate the lower voltage from the higher voltage by disconnecting the AC input to the bridges.....though at this point, a simple OHMMETER check in Diode Mode will identify a shorted bridge rectifier. Shorted supply caps can be found in Ohms mode. Mind that all the filters are in parallel (3 pairs in the /- 80V supply for LF, 2 pairs in the HF amp supply. Generally, fault in the +/- 15V supply won't take out the mains fuse, so normal troubleshooting of the discrete parts in that regulator circuit that comes off the +/- 60V supplies.

    If your supplies are all ok, I've often had to resort to using an external bipolar lab supply to bring up a power amp, even though I wouldn't be coming up to full normal supply potentials. It's often high enough to help isolate the problem, plus the lab supplies normally have current control so you can regulate the output for just idle conditions. A Variac also allows this sort of approach, now using the products internal power supplies. Good hunting.
    Power supply board is good and no shorts.

    Quote Originally Posted by The Dude View Post
    I'll say it again (see posts#45 and #47). We need to know what amp you have. Please post a schematic or verify the one I posted is or isn't for your amp. Without some sort of road map, we can't tell you what to test.
    Schematic links have been posted by others.

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    Pretty sure it has already been posted:

    https://medias.audiofanzine.com/file...ual-472053.pdf

    Quote Originally Posted by DrGonz78 View Post
    Yup schematic time since flying blind is not very resourceful. When you post the schematic measure resistance from every output transistor leg to ground. Got make sure those aren't shorting to ground.
    Yes, I had mentioned the readings in prior posts.

    Quote Originally Posted by nevetslab View Post
    As you're using the light bulb limiter, I gather you're not equipped with a Variac and a power analyzer (or you would have been using them all along). What you CAN do, which seems like you're thinking towards that, is to disconnect the +/-80V supply leads from the upper power supply board to the power amp board below...cap the wire ends off so nothing shorts, and leave the +/- 60V supplies connected, and try powering up. If that's still brightly glowing, then disconnect the +/- 60V leads in the same fashion, and power up again, now having the power amp circuits disconnected from the supplies, leaving just the +/- 15V supplies connected to the front panel preamp board and what is used of that on the power amp circuits. If you're still glowing brightly, you have a fault in the power supply circuits. You can further isolate the lower voltage from the higher voltage by disconnecting the AC input to the bridges.....though at this point, a simple OHMMETER check in Diode Mode will identify a shorted bridge rectifier. Shorted supply caps can be found in Ohms mode. Mind that all the filters are in parallel (3 pairs in the /- 80V supply for LF, 2 pairs in the HF amp supply. Generally, fault in the +/- 15V supply won't take out the mains fuse, so normal troubleshooting of the discrete parts in that regulator circuit that comes off the +/- 60V supplies.

    If your supplies are all ok, I've often had to resort to using an external bipolar lab supply to bring up a power amp, even though I wouldn't be coming up to full normal supply potentials. It's often high enough to help isolate the problem, plus the lab supplies normally have current control so you can regulate the output for just idle conditions. A Variac also allows this sort of approach, now using the products internal power supplies. Good hunting.
    I get the bulb glowing with the 60 and 85v connected. Disconnected the 85v first. Then the 60v - I get no light bulb glow with the 15v connected. Though should the indication LEDs be glowing at that point?

    I reconnected the 85v with the 60v disconnected just to see if the 60v was the culprit and the 85v was ok - that was not the case.

    I did the testing with all the components previous removed reinstalled.

    R3 and R4 on the power board blew and I'm not sure if I had the 15v connector on backwards. Replaced them and they haven't blown again.

    I'm going to take apart the transistor board again and take the metal mounting plates off to make sure some conductive material didn't somehow get in there and is what is causing a short somewhere.

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    Last edited by beedoola; 11-26-2019 at 05:21 PM.

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    I've retested every output transistor out of circuit and they all test ok.

    I've removed the other transistors mentioned: Q9, Q7, Q2. Q15, Q16. etc and they all test good OUT of the circuit. But they don't in the circuit. This is with the board not connected to the PS board or anything else. Should that be the case?

    I removed the metal transistor mounting plates and cleaned them.

    I had also read about an issue with the 800RBs shutting off do to an issue with the effects loop and inserting a patch cable can fix the issue. Could that be the culprit here?

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    Transistors can test OK with a DMM or tester and fail in circuit, as the testers use very small voltages (maybe 5v). The saying I've seen is that a tester can tell you if a part is bad, but not that it is good.

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  28. #63
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    I think you are talking about testing the transistors with diode check in circuit but unpowered.
    If so, you can not expect them to test the same as out of circuit.

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    Quote Originally Posted by g1 View Post
    I think you are talking about testing the transistors with diode check in circuit but unpowered.
    If so, you can not expect them to test the same as out of circuit.
    I removed each output transistor from the circuit board and tested it with the diode check that way.

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    Yes. But when you put them back into the board, they will no longer give the same readings on diode check. That is normal.

    To see what they are doing in circuit, you really need to power up the board and do DC voltage measurements.

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    What DC voltages should I see if the transistors are good? Do I need to do the same combination of measurements as in diode check?

    That is: B to E, B to C, E to B, etc.

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    For the voltage measurements you measure each pin to ground. So leave black probe connected to ground and check at E,B, and C.

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    Quote Originally Posted by g1 View Post
    For the voltage measurements you measure each pin to ground. So leave black probe connected to ground and check at E,B, and C.

    Thanks. Yes, I understand that, but what voltages should I see on the DMM that would indicate normal or abnormal voltages?

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    That will be different for each application. That's why it's easiest to post them so they can be analyzed here.
    The only general statement I can make about 'normal' is that there will be around .5 to .7VDC between emitter and base for a transistor that is turned on. Polarity will depend on whether the transistor is NPN or PNP. And for darlingtons it will be double that value.

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    I had new transistors I bought and waited to install. Still getting a short - power board caps are shorted when everything is connected.

    Can someone explain to me how this amp is supposed to work with the Collector of the output transistors being tied to ground and the Positive and Negative power rails also being in contact with some of the Collectors? It all connects together with the mounting plate for the chassis.

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