GK 1001RB II output

[nevetslab]

I'm not feeling lucky on this one Q11 is shorted and under the sink , R22 , R26 , R27 , D5 all toast, R29(3.92k) is reading 1095 ohm in circuit , getting high readings on some of the emitter resistors under the sink . I'm here for the knowledge,education(if I remember anything) a nod to Enzo's quote , and to gain Logic (: if I was in it for $$$ I'd be much better off collecting returnable bottles. Added a pic , note R26 is not soldered in.
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I get claustrophobic just looking at that photo. Unkind words escape my mouth when I've been at that same state of assessment on these compacted GK amps.

What I've had to resort to doing on Q11 is to clip the leads off near the top side of the solder joint, remove the screw from the bottom on that part, loosen the adjacent screws around it that are all involved in clamping force being applied to all. Then, if you're lucky enough to have a pair of needle-nose pliers that can navigate around the vertical obstacles blocking access to the part...bend the leads up enough to grab hold of the part and wiggle it free and extract it. You'll have to remove R25 as well as D5 & R26 to make way for the xstr body to come out. I just hope Thermoset hasn't taken place from the years of pressure being applied. Removing the left-over leads will likewise be tricky. I have really small tools that can reach in there and grab hold of the lead and lift it out while a small dia soldering tip heats up the joint....or drive it out thru the board to be removed from the back side. It may take more than one iteration with loosening screws in the neighborhood to allow Q11 to come loose.

When you do finally get Q11 free....look closely at the metal tab on the body that was face-up, in contact with the heat sink insulation material. If there is torn particles of that insulator, you'll need a Case 77 size Mica washer and thin layer of grease on each side when replacing the part.

Remove R27, and also Q12. With both of those out of circuit, you can now assess R29, as there no longer is a circuit closed around it. Lift one end of R30, so you can re-measure Q13, which is the bias xstr mounted right next to Q11, under the heat sink..a larger TO-220 body part. Hopefully it is ok. We need all of the bias circuit working, so if Q12 is bad, obviously replace it, as well as any of these parts.

I gather you didn't find any fault with the output stage parts, which would be good news.

Yes....I too indulge in all of these pursuits for the knowledge & education....nothing like being close to the fire, if you can keep from being burned. I have one not so dissimilar in the degree of effort and tedium repairing and then restoring an Ampeg SVT6-Pro, where my hastiness turned the wrong trim pot and killed an output stage....different thread I posted yesterday.

I'll catch you on the rebound on this one. Good luck!

[shortcircuit]

Work in progress , Q11 came out clean, insulator still in place and looks good what little I can see of it . Q12 is no good , R29 is fine , Q13 is good. D9 came in at 3vdc , but will replace it regardless . along with Q8 , D4 . I 'm still not sure on readings from emitter resistors under the sink , sure they are being effected by other components. I am still making a parts list , as usual I can't get everything from one source . Thought I'd keep the thread fresh with as many views as it's got , maybe this bit of info will help another novice like me understand a bit better.

[nevetslab]

Work in progress , Q11 came out clean, insulator still in place and looks good what little I can see of it . Q12 is no good , R29 is fine , Q13 is good. D9 came in at 3vdc , but will replace it regardless . along with Q8 , D4 . I 'm still not sure on readings from emitter resistors under the sink , sure they are being effected by other components. I am still making a parts list , as usual I can't get everything from one source . Thought I'd keep the thread fresh with as many views as it's got , maybe this bit of info will help another novice like me understand a bit better.
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Installing a new Q11 is a bit tricky. I had to do a similar lead-bending/shaping procedure on a voltage regulator on a GK2001B, similar parts density. I had to bend and fold the 3 leads up then back down, so once the body got into place, I was able to guide the leads into the holes, then poke them in, grabbing hold of the lead ends as I got them thru the holes. I did a dry run with a similar TO-220 part to practice the exercise, before committing to the actual replacement part. Still a PITA, since they're all installed before the heat sink goes down onto them all. Good the bias xstrs Q12 is ok, as well as R29. Yeah, I'd replace D9 while replacing D5.

Earlier, you had found the same 30VDC potential at both collectors of Q11 & Q14. Common to those circuit nodes is electrolytic capacitor C19. It's an AC bypass around the level shift of the two zeners and the bias circuit for signal flow thru this voltage gain stage of Q14 & Q11. I wonder if that part is shorted? Also, you had expressed concern about Q16. Likewise Q7. Both are the driver xstrs to the lower +/- output drive stage. Q17 & Q3 are the upper driver xstrs to the upper +/- output stages. Those xstrs are luckily on the same edge, to the left and right of the bias & lower gain stage xstr Q11.

[shortcircuit]

Just a note Q12 is bad , Q26 & Q27 are both shorted and buried under the sink , going to have to back up and re check pretty much everything

[nevetslab]

Just a note Q12 is bad , Q26 & Q27 are both shorted and buried under the sink , going to have to back up and re check pretty much everything

Ah, that's sad news. Though, not unexpected. It does force the issue of having to remove the heat sink, once all the transistor mounting screws have been removed. You'll have to carefully pry it up, which on first attempt, will probably want to lift a good portion of the transistors up with it. You'll have to slid in a think long knife blade in to break the bond between each of the transistors and the insulator sheet. And, in the re-build, once all the failed transistors have been found & removed, and the new parts' leads have been bent so the screws align with the heat sink holes, you'll have to use mica washers and thermal grease on both sides to re-mount the heat sink. It's a bit tricky....you may have to use some tooth picks or similar to serve as guide pins as you lower the heat sink onto the prepared xstrs with their greased mica insulator face up.

When you find shorted transistors like Q26 & Q27, often their associated emitter resistors fail open or change in value substantially, and will also have to be replaced. These two parts being shorted explains the level shift found at Q14 collector (30VDC) noted earlier. At least at this point, replacing Q11 will now be easy, and won't have to deal with the tricky part of folding the leads in a way to get them into the solder pad holes. Let us know what you find in assessing the full extent of the output stage failure.

[shortcircuit]

Ah, that's sad news. Though, not unexpected. It does force the issue of having to remove the heat sink, once all the transistor mounting screws have been removed. You'll have to carefully pry it up, which on first attempt, will probably want to lift a good portion of the transistors up with it. You'll have to slid in a think long knife blade in to break the bond between each of the transistors and the insulator sheet. And, in the re-build, once all the failed transistors have been found & removed, and the new parts' leads have been bent so the screws align with the heat sink holes, you'll have to use mica washers and thermal grease on both sides to re-mount the heat sink. It's a bit tricky....you may have to use some tooth picks or similar to serve as guide pins as you lower the heat sink onto the prepared xstrs with their greased mica insulator face up.

When you find shorted transistors like Q26 & Q27, often their associated emitter resistors fail open or change in value substantially, and will also have to be replaced. These two parts being shorted explains the level shift found at Q14 collector (30VDC) noted earlier. At least at this point, replacing Q11 will now be easy, and won't have to deal with the tricky part of folding the leads in a way to get them into the solder pad holes. Let us know what you find in assessing the full extent of the output stage failure.

Here is a pic under the sink , R61 is cooked , as well as most of the emitter resistors , I have now deemed this not trust worthy. The biggest deterrent being getting reliable replacement parts, a lot of these are obsolete from any reputable source, before you all put me in the MEF stockade, I will continue to weed out damaged components to diagnose total damage .My question now is, with a new replacement pre-amp and power amp board installed what is the best way to initially turn this on ?? Variac?? Limiter ?? cross my fingers and push the button ??

[nevetslab]

Here is a pic under the sink , R61 is cooked , as well as most of the emitter resistors , I have now deemed this not trust worthy. The biggest deterrent being getting reliable replacement parts, a lot of these are obsolete from any reputable source, before you all put me in the MEF stockade, I will continue to weed out damaged components to diagnose total damage .My question now is, with a new replacement pre-amp and power amp board installed what is the best way to initially turn this on ?? Variac?? Limiter ?? cross my fingers and push the button ??
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The semiconductors and resistors ARE available. Q32 looks like its' base lead has burned open, or buggered. If you're going the route of a new power amp assembly (don't need the new preamp, as we've verified it's working, right?)....powering up for the first time, if you have access to a variac and AC ammeter (power analyzer), that's the way to go. The light bulb limiter will do. I've always got a DMM monitoring the output connector looking for any DC offset, which is a dead giveaway.

[drewl]

I just assumed he clipped out Q32

[shortcircuit]

I just assumed he clipped out Q32

assuming is against the law here, or so I've been told , Q32 & Q34 are both victims of "ITD" Inexperienced Tech Disaster (: oops !! They will have to be replaced ,but don't factor into the true diagnosis. Thanks for chiming in I was starting to wonder what I have done to alienate the forum. And if I have I apologize !!!!!! G & K doesn't use individual insulators , thought that was different, cost effective in manufacturing process I'm sure.

[nevetslab]

assuming is against the law here, or so I've been told , Q32 & Q34 are both victims of "ITD" Inexperienced Tech Disaster (: oops !! They will have to be replaced ,but don't factor into the true diagnosis. Thanks for chiming in I was starting to wonder what I have done to alienate the forum. And if I have I apologize !!!!!! G & K doesn't use individual insulators , thought that was different, cost effective in manufacturing process I'm sure.
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When you finally got all the heat sink mtg hardware removed, allowing the next step....lifting off the heat sink from the still-mounted xstrs....did it lift right off, or did it need to be pried up in stages? If the latter, look real close at each of the power transistor's mtg base that mates with the single-sheet thermal insulating sheet, to see if there's particles of that insulator on them. Usually, they are NOT reliable as an insulator once breakdown of the mating surface to the xtrs has occured. Over time, thermoset takes place that kinda works like an adhesive, as it gets to that state over the long course of thermal conduction...expands, contracts cyclically from the xstrs and the heat sink. Sheets of that size in manufacturing typically cost around $3 or more in 1000pc lots. GK still inventories them, as this is a current product. If the replacement cost is $10 or so + shpg, I'd replace it, just to be certain you have full DC isolation between the collector potentials and the ground potential of the heat sink. At BGW Systems, our purchase qty was much lower, and a similar sq in size we used was closer to $9 ea from Bergquist (K-6 material).

Ah, Q32 was clinging onto it's security blanket for dear life, eh?

[shortcircuit]

When you finally got all the heat sink mtg hardware removed, allowing the next step....lifting off the heat sink from the still-mounted xstrs....did it lift right off, or did it need to be pried up in stages? If the latter, look real close at each of the power transistor's mtg base that mates with the single-sheet thermal insulating sheet, to see if there's particles of that insulator on them. Usually, they are NOT reliable as an insulator once breakdown of the mating surface to the xtrs has occured. Over time, thermoset takes place that kinda works like an adhesive, as it gets to that state over the long course of thermal conduction...expands, contracts cyclically from the xstrs and the heat sink. Sheets of that size in manufacturing typically cost around $3 or more in 1000pc lots. GK still inventories them, as this is a current product. If the replacement cost is $10 or so + shpg, I'd replace it, just to be certain you have full DC isolation between the collector potentials and the ground potential of the heat sink. At BGW Systems, our purchase qty was much lower, and a similar sq in size we used was closer to $9 ea from Bergquist (K-6 material).

Ah, Q32 was clinging onto it's security blanket for dear life, eh?

Definitely took some prying , I tried to be careful , used wooden door shims cut into mini shims , worked fairly well rip Q32 & Q34 . Transistors look pretty clean but I wouldn't chance re-using insulator , I'll contact G&K but they might not deal with me, being Joe nobody,they never answered any calls or emails before.

[nevetslab]

Definitely took some prying , I tried to be careful , used wooden door shims cut into mini shims , worked fairly well rip Q32 & Q34 . Transistors look pretty clean but I wouldn't chance re-using insulator , I'll contact G&K but they might not deal with me, being Joe nobody,they never answered any calls or emails before.

Let me know on that, as we have a good relationship with GK, as they own a lot of the GK amps in our rental inventory....lots of high end clients pass thru here using our rehearsal studios, so a lot of amp companies pick up business here. Also have Fender, Roland, Paul Reed Smith, Zildjain and other's Artist Relations industry offices here. Most of the service parts for the GK amps come in no charge.

[The Dude]

......Thanks for chiming in I was starting to wonder what I have done to alienate the forum. And if I have I apologize !!!!!!......

Oh no! Don't think that. I'm following along. Just nothing to add. nevetslab has it well under control. Good luck!

[shortcircuit]

Sorry this thread went into hibernation , the owner opted to go new board(s) ,yee of little faith . Thanks to all for your input, special thanks and apologies to nevetslab He put on a Solid State 101 course or better, learned a lot. It wasn't all for nothing , I now have a scope(that I barely know how to use effectively ), built a signal tracer, found a free frequency generator website , and acquired a soldering station with suction, most of the work was done with a radio shack station and a hand pump , a tough act with G&K's titanium/Roswell solder and micro pads . Made it this far, again Thanks

GK Project.pdf

[nevetslab]

Sorry this thread went into hibernation , the owner opted to go new board(s) ,yee of little faith . Thanks to all for your input, special thanks and apologies to nevetslab He put on a Solid State 101 course or better, learned a lot. It wasn't all for nothing , I now have a scope(that I barely know how to use effectively ), built a signal tracer, found a free frequency generator website , and acquired a soldering station with suction, most of the work was done with a radio shack station and a hand pump , a tough act with G&K's titanium/Roswell solder and micro pads . Made it this far, again Thanks

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Well, we all have learned a lot from this project, and it was a rewarding daily adventure chasing electrons and seeing what havoc they wrought when things go wrong. Always glad to be of service!