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Thread: Gallien Kreuger 800rb troubles...

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    Gallien Kreuger 800rb troubles...

    Was given this amp being told it had a bad power supply board and would not fire up. Got it home, plugged it in and gave it a shot. It fired up, but immediately made smoke and sizzling noises. Turned it off. Saw it had fried 4 or 5 resistors on the power supply board. Found a salvaged board online for 60 bucks. put it in, fired it up, hum for 2 seconds then fuse blows, amp off. all resistors seem to be intact. All the reading I've been doing leads me to think I should start by replacing output transistors, would you agree? Im not 100% sure which are the output transistors though. Do the prefixes (mj, tip, mje...) give you a clue as to their role or stage placement?? Any info would be much appreciated. Hopefully ill have better luck here than the other forum. I am fully aware that there are professionals I can take it to. I enjoy this stuff and I am trying to learn more about my amps. I was going to attach schematics, but don't have permission. Thanks in advance!

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    Master Destroyer nosaj's Avatar
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    i would first check the power supply first disconnected from the preamp and power amp.
    Here is the service manual https://medias.audiofanzine.com/file...ual-472053.pdf
    Page 14 has the power supply voltages, if they are not right here nothing else will be either. Forget about the rest of the amp till you get through the power supply.

    An welcome aboard, this is a board where techs hang out and help everyone.

    nosaj

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    I'll add. Don't just start replacing parts. Troubleshoot the amp. Figure out what parts need to be replaced. Start by building a light bulb limiter so you don't keep blowing fuses and burning up parts. And, don't attach a speaker to the amp until you verify that it is working and there is no DC on the output(s).

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    Ha ha! Ive been holding off on getting a cab till I have a working head, so no problem there. And thanks for the tip Nosaj, Ill give that a shot. Just so I'm clear and on the same page, Im gonna put the black lead of my meter to the amps grounding post then test for voltages on the points labeled A-G?

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    Yep. That's a good start.

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    Ok, so I disconnected the power amp and preamp, built my light bulb limiter and got to work. short lived. The second I turned it on there was a pop and a blue spark that literally looked like it came from nowhere. it was between the two boards and in front of the transformer. Fuse still intact. With meter set at 200, I would get readings that would start at .5 or so then drop to .1 then fluctuate everywhere in-between, on all the test points(being the solder points from where the wires would go to the rest of the components. I even tried probing right at the transformer, same result. Does this make sense?Are these symptoms of a bad trans? Thanks!

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by SCStowaway View Post
    Ok, so I disconnected the power amp and preamp, built my light bulb limiter and got to work. short lived. The second I turned it on there was a pop and a blue spark that literally looked like it came from nowhere. it was between the two boards and in front of the transformer. Fuse still intact. With meter set at 200, I would get readings that would start at .5 or so then drop to .1 then fluctuate everywhere in-between, on all the test points(being the solder points from where the wires would go to the rest of the components. I even tried probing right at the transformer, same result. Does this make sense?Are these symptoms of a bad trans? Thanks!
    Well I can say the light buld limiter is not working correctly. If it was the light bulb would have lit up bright.https://antiqueradio.org/dimbulb.htm
    nosaj

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    Would it have stayed bright though? It may have lit up for an instant, it happened so fast. Im positive I wired the dim bulb tester correctly. frustrating

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    Quote Originally Posted by SCStowaway View Post
    Would it have stayed bright though? It may have lit up for an instant, it happened so fast. Im positive I wired the dim bulb tester correctly. frustrating
    yes unless the bulb blew, it the bulb works in a regular lamp, your limiter is wired wrong.

    See you can take a light bulb limiter and plug just a cord into it with the white and black wire twisted together and the limiter will glow bright because there is a short.
    In a normally functioning amp plugged into the limiter. the bulb will light full power for a sec or 2 then go dim once the caps have charged up.

    nosaj

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    Last edited by nosaj; 06-22-2019 at 07:45 PM. Reason: grammatic fluency
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    Ive plugged other items into the plugged in limiter and they get power. I did get a 200w bulb, i think cause the first video i watched on how to make one, that's what the guy recommended. Too high? Also, I can hear the transformer when plugged in and powered on, a very faint hum.

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    Yes it's too high. You want to be using a 50 or 60 watt bulb.

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    Quote Originally Posted by SCStowaway View Post
    Ive plugged other items into the plugged in limiter and they get power. I did get a 200w bulb, i think cause the first video i watched on how to make one, that's what the guy recommended. Too high? Also, I can hear the transformer when plugged in and powered on, a very faint hum.
    i typically use only a 40 watt bulb as a go no go test . Once it 's a go I take it off limiter, when I wire in something new I use bulb to test.
    nosaj

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    Awesome, Ill go get me a new bulb then. Thanks guys

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    Alright, I got me a 40 watt bulb, and a new multimeter and I got some actual results. The light started somewhat bright on start up, but quickly dimmed to just barely on, where it would stay for the remainder of the test. And for the first time, there were no sparks when I flipped the power on. All of the measurements I got were pretty close to what the schematics said, all were a tad under. where it should have been +-15, it was +-14, for +-65, it was about +-60, and for +-85, it was about +-78. Is that within tolerance? Whats next?!? Thanks!

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    Those voltages look good. They will be slightly low on LBL. For clarification sake: With the power supply disconnected from the rest of the amp, you blew a fuse without the light bulb limiter? That seems a bit odd since the voltages look ok on the limiter.

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    Incorrect, The fuse blew when everything was connected. I had a spark with power supply disconnected while using the 200w bulb, but fuse stayed intact. Today was the first time I've powered it up without a spark, fire or blown fuse since I've owned it.

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    Oh, and LBL?

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    LBL is light bulb limiter. The next thing I would do, since the power supply seems to be ok is leave it unhooked and powered off. Do a quick check to see if any of your output transistors are shorted. That would be the next most likely failure.

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by SCStowaway View Post
    Alright, I got me a 40 watt bulb, and a new multimeter and I got some actual results. The light started somewhat bright on start up, but quickly dimmed to just barely on, where it would stay for the remainder of the test. And for the first time, there were no sparks when I flipped the power on. All of the measurements I got were pretty close to what the schematics said, all were a tad under. where it should have been +-15, it was +-14, for +-65, it was about +-60, and for +-85, it was about +-78. Is that within tolerance? Whats next?!? Thanks!
    Leaving it on the light bulb I would hook up just the preamp and turn it on see what the bulb does. Then if ok hook up the poweramp with lightbulb still on if it's bright there. Test you output transistors.

    nosaj

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    ok cool. do the Output transistors need to be removed from the board in order to test? or am I just testing for a short?

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    Supporting Member nevetslab's Avatar
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    Quote Originally Posted by SCStowaway View Post
    ok cool. do the Output transistors need to be removed from the board in order to test? or am I just testing for a short?
    I've always left them in the board, soldered in and testing for shorts....CE, CB, EB. Also check the drivers, and current limiter xstrs.....check every semi, to be sure, and where faults are found, check the associated diodes & resistors, for both the LF amp AND the HF amp. I always check the bridge rectifiers and the discrete +/- 15V regulator circuits as well. There may still be issues on the preamp board, but need to get the basics sorted out first.

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    Gotcha. I’m not a hundred percent on which are which transistors. I’m assuming all the metal cap looking ones are output? Not sure which the drivers are. Is there a rule of thumb, or a type of transistor each type tends to be?? I’ll google it in the meantime, but any words of wisdom would be appreciated.

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    GK 800RB Service Manual, with parts placement guide for power amp, PS & Preamp PCB's

    Quote Originally Posted by SCStowaway View Post
    Gotcha. I’m not a hundred percent on which are which transistors. I’m assuming all the metal cap looking ones are output? Not sure which the drivers are. Is there a rule of thumb, or a type of transistor each type tends to be?? I’ll google it in the meantime, but any words of wisdom would be appreciated.

    gallien-krueger_800rb_sm.pdf

    Take a look at the parts placement guide in the 800RB SM2 document. 1st page shows the parts placement with schematic designation numbers, so you CAN identify each part in the circuit. On the LF amp, the outputs are Q19, Q20, Q23 & Q24, drivers are Q18 & Q22, Predrivers are Q17 & Q21. Current limiter stage is Q15 & Q16; bias xstrs are Q13 & Q14 (Q13 coupled to heat sink). Q12 is voltage gain stage, Q11 is level translator from the op amp.

    Similar relationships with the HF amp.

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    Last edited by nevetslab; 06-26-2019 at 08:19 AM.
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    Ah man, thanks for the info. The SM2 link doesn't seem to be working, But I do have that PDF already. Not sure if yours is different, but on mine it doesn't reference the part numbers on the power amp the way it does for the supply and pre. But the breakdown you gave me is very useful, much appreciated!

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    another q

    In looking at the amp, I don't see how I can hook up just the pre amp to the power supply and not the power amp. Maybe Im totally missing something, but it seems like the only wires going to the preamp come from the power amp, except for the speaker out jacks. Also, In testing transistors, am I testing for voltages or just for shorts? Amp powered on or no? thanks!

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    Quote Originally Posted by SCStowaway View Post
    In looking at the amp, I don't see how I can hook up just the pre amp to the power supply and not the power amp. Maybe Im totally missing something, but it seems like the only wires going to the preamp come from the power amp, except for the speaker out jacks. Also, In testing transistors, am I testing for voltages or just for shorts? Amp powered on or no? thanks!
    To just run the preamp from the internal power supply, you would actually have to disconnect the supply wires to both the LF amp and HF amp. I've always used an external bipolar lab supply in those cases. On the power supply diagram in the service manual, it calls out the wire colors between the power supply board's output to the power amp board below: BRN = +85VDC, RED = -85VDC, ORG = +60VDC, YEL = -60VDC. The +/- 15V supplies come down to the power amp board, then come over on one of those plug-in connectors. I thought I had some photos of the insides of the 800RB wiring, but couldn't find it.

    And, testing semiconductors with the DMM in Diode Test mode.......AMP is NOT powered up, and is DISCONNECTED from AC Mains! You're looking for shorts, as well as working B-E, B-C xstr junctions, no shorts across B-E, B-C, C-E xstr junctions, no shorts across diode A-K junctions, rather working A-K junctions (for both diodes and zener diodes.

    Look at the power amp schematics for a moment. With the parts all soldered into place, the LF Power Amp circuit has 3 pairs of NPN's and PNP's driven from the drivers, and they have 0.33 ohm emitter resistors that are tied to the output buss. When you measure for a good PN junction between the base and emitter, you're also measuring across the emitter resistors of the driver stage in parallel, which will give you a lower reading than a free-standing power xstr. It won't be the same as measuring between the base and collector in these cases. But, if you find one or more shorted B-E junctions, they will also influence the readings on non-shorted xstrs. In those cases, you'll have to remove the xstrs to confirm which ones ARE shorted and those that aren't.

    You won't be able to test zener voltage without limited current thru the part and appropriate voltage greater than the zener voltage rating. DMM's can't test for leakage. You're looking for the gross faults at this stage. Don't power up the amp until you know you've found all the failed semi's, as well as open resistors, shorted diodes & zener diodes, and of course remove/replace them. On the preamp, sometimes there are failed op amps that pull enough current to short the power supply rails.

    First check with Diode Test mode between the + and - pins on the op amp........which is looking at ALL of the op amps. Check in both directions. On individual op amps, you can check between output pin and the V+ and V- terminals. Though usually, unless the preamp board shorts out the bipolar power supply, I check the individual op amps with it powered up. Unless an op amp is being used for a DC function, in amplifier stage use, I'm looking for near-0V potentials at the op amp output terminal (on bipolar powering).

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    Nevetslab, thank you very much for all that info!! I appreciate you taking the time. So here's some results... Q17,18,19,20,22,23,24 all had shorts, in circuit. Q8,10,12,21 to the best of my knowledge tested good. I have only tested the bottle cap looking transistors so far. But I also found two bad diodes, D10 and D12. I believe this was due to when I was putting in the new power supply, I didn't realize a wire clipping and shot off and landed somewhere in their vicinity. upon power up there was a fire resulting in the death of R47, R51, and R53. I replaced them, but the diodes didn't look bad so I hoped for the best. oops. Anyway, Should I remove bad transistors and test again in hopes some may be ok, only testing bad cause of others? As for the diodes, Ill order new ones now. Side question, will bad solder joints just affect a transistors performance, or can they kill a them?? I ask cause when I got the amp, all of those transistors I tested had loose screws and had been desoldered. So I tightened the screws and soldered them in. Mind you this was before I got a new iron and was using a piece of junk, and on top of that my soldering skills are a work in progress.

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    Last edited by SCStowaway; 06-29-2019 at 05:56 AM.

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    Quote Originally Posted by SCStowaway View Post
    Nevetslab, thank you very much for all that info!! I appreciate you taking the time. So here's some results... Q17,18,19,20,22,23,24 all had shorts, in circuit. Q8,10,12,21 to the best of my knowledge tested good. I have only tested the bottle cap looking transistors so far. But I also found two bad diodes, D10 and D12. I believe this was due to when I was putting in the new power supply, I didn't realize a wire clipping and shot off and landed somewhere in their vicinity. upon power up there was a fire resulting in the death of R47, R51, and R53. I replaced them, but the diodes didn't look bad so I hoped for the best. oops. Anyway, Should I remove bad transistors and test again in hopes some may be ok, only testing bad cause of others? As for the diodes, Ill order new ones now. Side question, will bad solder joints just affect a transistors performance, or can they kill a them?? I ask cause when I got the amp, all of those transistors I tested had loose screws and had been desoldered. So I tightened the screws and soldered them in. Mind you this was before I got a new iron and was using a piece of junk, and on top of that my soldering skills are a work in progress.
    Yes, I would remove all the identified bad/shorted transistors found. I've found on a couple occasions the ones that measured shorted while soldered in/mounted on the GK 800RB amp board, that not every one WAS bad. This isn't uncommon when you have multiple parts on each side of the output buss (LF amp has 3 pairs. When you look between C-E, and also find the flyback diodes D11 & D12 shorted, you don't really know who IS shorted. I always double-check when pulling out the failed / suspected-failed parts.

    Will bad solder joints cause failures? It can.........having 3 pair to share the load current, and not all are firing, puts the strain on the one(s) having good connections. As a general rule, when I find questionable solder joints, and especially solder joint fractures, I first DE-SOLDER the connection, THEN apply fresh solder and RE-SOLDER them. With the TO-3 xstrs in this sort of design, ALL of the collector mtg screws need to be tight. Loose screws here is like bad solder joints.

    As you've already found loose hardware on the power amp board, you no doubt have similar issues thruout the amp. The bottom pair of output phone jacks (LF Amp Output) need to be tight, as do the HF jacks. All need to be tight as a general rule. Make sure the Grounding wire that's common to the sleeves on those output jacks are solid solder joints, and are solid connections at the other end.

    The top and bottom cover screws no doubt have many stripped holes. The original #6-32 FHMS cover screws strip out, particularly the end cover screws. I've been re-cutting the threads for #8-32 and using #8-32 x 5/16" 100 deg FHMS rather than sheet metal screws. Sometimes I have to use Keps Nuts on the end cover mtg screws, when they had already been buggered with #8 SMS or even #10 SMS. With the 800RB being in service for so many years, I see all sorts of various hardware forced into them. Once these amps have been put back into solid electrical working order, it would a crime to leave the mechanical issues undone.

    I also find the five front panel pushbutton switches needing replacement. I'll also solder in short wires to connect the unused side of the DPDT switches so you have redundancy. On the aged switches, where you don't have the replacements, spending the time exercising each switch, passing LF sine thru the amp lets you hear the grunge and grit that has built up over time, and often get a lot more use out of the aging switches. The pots are no longer available. I'll unsolder the gritty ones, pry off the cover tabs, open them up and brush in Caig DeOxit and exercise the resistance track/wipers so they're good for another long stretch of time.

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    Ok, so I got all the transistors in question out and I think most of them tested good. But I wanted to clarify a couple things. Now I know Im putting this in the most lame of laymans terms, but its my understanding that when meter is set to diode:

    A:Of the six possible contact combinations, I should only get 2 readings
    B: Those 2 readings should be roughly between 3xx-6xx
    C: If I get a 001, that =bad
    D: If I get a reading between 2 contacts and another reading between the same 2 contacts with + and - reversed, that's bad

    Please correct me if any or all of this is untrue. Thanks!!

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    Supporting Member The Dude's Avatar
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    That seems right. You're measuring the voltage drop, which should be around .6V. I'm assuming your meter displays 006. If that's the case, what you say above is correct. A reading of .1V or as your meter says 001 would be a short. You should only measure transistor junctions between base-emitter and base-collector if the transistor is good. The polarity will depend on if it's a PNP or NPN transistor. Output transistors are usually multiple transistors in parallel, so if one is shorted, the rest will also read shorted in circuit. When you remove them and re-test, some may be good.

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    Well, when good, my meter actually reads 576 for example, when bad, the number starts high, then quickly drops to 001. But I think we are on the same page. On the same ones that are bad, there are also continuity issues. thanks for the tips, the Dude abides, man.

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