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yes readings on Q3 were in ohms . I put Q3 back in so, I can't do diode test at this time,I will pull it again if need be . Q3 collector reading in circuit is -.580vdc . ACV mode test ,I ran line out of a portable CD player measured .070 to.085vac plugged into the input jack ,have the same reading at the tip, and all the way through to C6 . Then I get lost , you are light years ahead of my knowledge so I appreciate your patience and input. My reading at Q3 collector with this input is 1.8 to 2.0 vac. Q4 gate .187vac Am I on the right track or am I totally off.
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If you don't know where your going any road'll take you there : George Harrison -
I should have been a bit more inclusive verbalizing a junction of 5 parts, but you've got signal thru the discrete preamp stage, and the collector reading of Q3 is in the 'window' of what I'd expect. You're on your way. Now, you're just looking for the signal at each op amp stage thru the preamp.....you'll need to turn the volume control up to get the signal around 1VAC, as the next stages are all unity gain, if you have the controls at 12 o'clock (EQ controls), and the Presence & Contour fully CCW. You're looking at (in sequence) U2A pin 1, then U3A pin 1, then U4B pin 7, then U4A pin 1, then U5A pin 1, then your at the Send Jack. It hard-wires to the normal contact at the Return Jack, which, if anything is plugged in there, will disconnect the signal from U5A and before (all of your stages up to here). From the Return jack (still being fed from U5A pin 1, then U6B pin 7 (Boost Control adjusts this level at this stage, so turn it up to what you've been seeing up to this point), then U6A pin 1, then the signal is split into a crossover network. LF/full range passes thru U7A pin 1 and then thru U8A pin 1, then thru U8B pin 7. Backing up to the split, the HF range passes thru U7B pin 7, then thru U9B pin 7. This signal then goes to the Tweeter Volume control.Originally posted by shortcircuit View Post
Back down to the Woofer/Full range portion of the signal, where we stopped at U8B pin 7, it next goes thru Q4, where you'd see signal at U9A pin 1, assuming all is ok with Q4. Q4 is a weak link in the GK amps using this preamp circuit (GK800RB, 1001RB, 2001RB, and others, I believe). The Boost Control is ahead of this Q4 JFET, which tends to clip asymmetrically and doesn't have as much headroom as the op amp stages before and after it. But, it does pass signal just fine, below those limitations. Following U9A pin 1 is the Woofer Volume control. So, the Tweeter & Woofer outputs feed the tiny connectors via two wires each that plug into the left rear edge of the power amp PCB labeled Woofer and Tweeter.
Don't be too discouraged.....I also tend to dread pulling the covers off the 1001 & 2001RB's, as they're tedious to work on, and you're looking at the foil side of the preamp, so finding your way around is by reading the layout drawings, but translating them backwards to find pin 1 & 7 without shorting your probe to pin 8 & pin 7 at the same time, which will do damage (Pin 8 is +15V); the solder pads are too small, GK uses lead-free solder which on the high current parts breaks down over the years and stops conducting fully, and a recent 2001RB took 41 hrs to repair, after mis-connecting one of the 7-wire power supply connectors to one channel, which caused lots of damage. Only then to have the input jack go squirrelly on me when I returned it to inventory. I was beginning to look for a nearby brick wall to hurl it against at that point.Logic is an organized way of going wrong with confidenceComment
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Going through your steps in sequence here is what I came up with, all readings in VAC U2Ap1 6-8 U3Ap1 8-9 U4Bp7 6-8 U4Ap1 4-5 U5Ap1 3-4 U6Bp7 2-3(boost at full) U6Ap1 2.8(the only stable reading) U7Ap1 2-3 U8Ap1 9-10(eye opener) U8Bp7 2-3 U7Bp7 1 U9Ap1 5-6
On your side note , this was supposed to be a simple jack replace , finding out the solder is hard to work with . real kick is I can get a brand new pre amp board for $70 , holding out on that as a last resort. Can't thank you enough for your help , this forum has yet to let me down , a lot of talent hereLast edited by shortcircuit; 11-16-2018, 09:56 PM.If you don't know where your going any road'll take you there : George HarrisonComment
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The numbers you were reading......6-8; 8-9; etc.....this was using music as a signal source, I presume. What was the final outcome? Is the amp now passing signal ok again? I didn't know that a replacement preamp board was only $70 (plus shpg, of course), but, where's the fun in that? So many of us here are in for the chase. Anyway, hopefully you did find the culprit and have restored the amp to full operational status. And, a pleasure to be of service. Indeed, there IS a lot of talent here on the forum. It keeps me coming back too!!Originally posted by shortcircuit View PostLogic is an organized way of going wrong with confidenceComment
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yes music as signal source , still no resolution, what did I miss? , I'm here for the chase before replace, but I'm still driving a Model TIf you don't know where your going any road'll take you there : George HarrisonComment
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I don't think you've missed anything on the preamp.....I didn't see what the reading was on U9B pin 7, but, all the other readings indicate you're passing signal thru the HF path. That HF path is basically 5khz on up, so all this leads us to the more difficult monster.....the Power Amp PCB. You'll have to pull it up and out of the chassis to service it. As your problem is low distorted output, it may be 'as simple' as de-soldering & re-soldering the Secondary wiring connector, the 3 bridge rectifier terminals, the four filter caps, the AC mains IEC connector terminals, the black Thermistors that pass the AC mains current thru (in-rush current limiting devices), the AC mains fuse connections for that snap-in fuse.Originally posted by shortcircuit View Post
My 'avatar' image is what I use (surgical loupes, fiber optic headlight) to see all the nastiness in long-term solder joint degradation. It clearly reveals tiny radial solder fractures, oxidation barriers that have cropped up between the original flow-solder machine's soldering process and what you now have on component leads where high current flows. The 1001RB I just serviced this past week had that problem...it was intermittent in it's behavior. Others I've serviced remained low in output, or even no output. Yet, not blowing fuses, all the semiconductors would be ok, though failure of the HF Power Amp IC (LM3886T) often occurs. I thought I was dealing with that problem, but after checking again, and seeing very strange near-triangular waveform on the LF output...huge crossover distortion-looking signal, then nudging the bridge just behind the 7-pin AC connections from the transformer, it corrected itself to look like a perfectly fine sinewave......it was easy to see. Bright light and magnification reveals it.
Curing it is the PITA part. GK seems like they pride themselves on how small they can make solder pads for very large high current component leads, like those on the 4-terminal round-pin bridge rectifiers. They did a bit better on the obround pads on the 7-pin AC secondary wires header connector. The mating female connector is often overheated, and can literally fuse itself into position, with obvious burn evidence from the overheating of the poor connection over time. Hope you're not dealing with that. Some parts that overheat will scorch the PCB in that area....notably the ballast resistors across the HV filter caps (R70/R71).
Fortunately, they DID properly label the AC Primary PCB Terminals where all of the primary colored wires plug into, so following the 120V chart along that edge next to the power transformer, you can't make a mistake putting it back together.
You can power this module up NOT mounted in the chassis if you're careful. It might even be easier removing the Toroid Power Xfmr and plug it all back together outside the chassis, getting the thin black harness connectors from the preamp to re-connect to the power amp PCB. We can talk about that when we get there. First, you need to do bright-light inspection to see if THAT is where your problem lies.Logic is an organized way of going wrong with confidenceComment
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Solder joint fractures...images
Here are some examples of hairline radial solder joint fractures:
Logic is an organized way of going wrong with confidenceComment
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I talked to a guy that runs circuit boards.
I asked him WTF is wrong with the solder joints on production amplifiers.
I showed him some examples that I had.
The biggest issue he saw was the solder not flowing up into the top layer on double sided boards.
His answer to the issues was this: they are not preheating the boards long enough.
On a double sided board this is a killer.
Yeah, the solder is good on the bottom side but the top is not getting wetted enough.
In a vibrating environment (ie: a guitar amplifier) what will happen is the component will 'wiggle' and crack the top side.
Then, the only stiffness to the joint is the solder (which is soft) in the plated through hole.
Once that solder cracks, there is no longer a connection on the traces on the top side of the board.
Yummy.Comment
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One bright light inspection coming up . I do have a question , so by plugging into the return and sending to another amp and getting the same output results didn't isolate this to the pre amp board ??If you don't know where your going any road'll take you there : George HarrisonComment
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Originally posted by Jazz P Bass View PostIf you don't know where your going any road'll take you there : George HarrisonComment
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Plugging into the return, the only output path from there is the output of the power amp. If you plug into the INPUT of the amp, and go from the SEND to another amp, and you have the same problem, THEN yes, the problem IS in the preamp. Is THAT what you did?Originally posted by shortcircuit View PostLogic is an organized way of going wrong with confidenceComment
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My bad , we're good moving on to the power amp.Last edited by shortcircuit; 11-18-2018, 08:56 AM.If you don't know where your going any road'll take you there : George HarrisonComment
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I pulled the power amp did my inspection , major find LM3886 has been replaced ?? has definitely been removed at some point . I re-flowed any suspect joints , re-connected, no change. I'm now highly suspect of the LM3886 what should I read at the output pin 3 ?? I'm reading -8.8ohms I have my 40+ vdc at pins 1 & 5 or am I going about this the wrong wayLast edited by shortcircuit; 11-18-2018, 08:33 PM.If you don't know where your going any road'll take you there : George HarrisonComment
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On Pin 3 of the LM3886T, 0VDC is what you'd normally expect, +/-50mV. Not sure what you mean 'reading -8.8 ohms'? Reading that IC with power off and in Diode Test mode will lead you astray, as it's a complex circuit. Unless you read a short between Pin 3 and either of the supply pins....then you'd have a failed power amp IC again.Originally posted by shortcircuit View Post
Back to the previous question......plugging into the amp INPUT jack and patching the Send signal to another amp.....did you get normal behavior, or distorted signal. And, plugging into the Return jack and listening to the output of the amp (prior to removing the power amp from the chassis)...was that still low and distorted? Those two tests really direct our attention to where the problem is. I was led to believe you're getting sensible signal thru the preamp, though only going by the AC Voltage readings of music signal with your DMM. They did make sense, though would have been over-driving the stages and distorting, but, you got all the way thru the preamp, to where it feeds the power amp.Logic is an organized way of going wrong with confidenceComment
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Plugging into input patching send was much stronger and clean , plugging into return to amp out yielded same low distorted output . Had 0vdc at pin 3 of LM3886Originally posted by nevetslab View PostIf you don't know where your going any road'll take you there : George HarrisonComment
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