GK 1001RB II output

[shortcircuit]

Hello hope everyone is having a great day !!
I took this amp in for what was supposed to be an input jack replacement (scratchy jack was the complaint ) When I plugged it in it powers up, LED goes from red to blue ,saying the amp is good to go , no output . Replaced the input jack , now I have a low distorted sound , plug into effects return ,same low distorted sound . Shut it off, as it shuts down low output sound gets louder and the clip light comes on briefly at the very end of shut down. No popped caps , nothing burnt . Should I be looking over the pre amp or the power amp ??

[The Dude]

Are you sure you installed the right jack? Maybe contact pins are different?
Was it distorted before the jack replacement?
Do you have it reassembled? Sometimes things ground via the chassis screws. If it's not reassembled, grounds are missing.

[shortcircuit]

Yes I am sure of proper jack replacement part, how ever, pulled 2 eyelets with old jack , now wait , had no signal before replacing jack so I thought these may have been broken from the traces and the answer to the problem. Did point to point to make sure everything was connected on replacement. I have the same results with pre amp installed or floating. remember I get the same results plugging into the effects return jack.

[The Dude]

The effects send/return on this amp are not directly in front of the power amp, so the problem could be in either preamp or power amp. You could narrow it down by inserting a signal at J4-1 for the woofer amp and J3-1 for the tweeter amp on the power amp schematic. I'd also do a quick check of power supply voltages.

P.S. You can also run the effects send into another amp to see if the problem is before or after the loop.

[nevetslab]

Yes I am sure of proper jack replacement part, how ever, pulled 2 eyelets with old jack , now wait , had no signal before replacing jack so I thought these may have been broken and the answer to the problem. Did point to point to make sure everything was connected on replacement. I have the same results with pre amp installed or floating. remember I get the same results plugging into the effects return jack.

Having just repaired one of these a couple days ago, I thought the issue was with the Tweeter Amp, having listened to the Woofer amp and Tweeter amp the other day. Before I pulled it apart, I fired it up again to have a look, now armed with the spec sheet for the LM3886T power amp IC. Instead, what I got was gross distortion on the woofer amp (which I didn't have the other day), and the Tweeter amp was fine. Severe crossover distortion, then, with a little probing with my chopstick, I moved the LV Bridge rectifier a touch, and it cured itself. Uh Huh. OK, time to pull the power amp PCB assy out and look at the AC mains & secondary wiring on the PCB.

Same age-old problem. Solder joints developing radial fractures with the lead-free solder, no longer making solid connection on the Secondary AC Wiring Header, the LV Bridge Rectifer pins, the four filter caps, and the AC mains IEC connector. De-soldered and Re-soldered all those, and then had solid output again. This happens often on those amps. AND, I curse the PCB layout person for using default pad stacks on the solder pads in all these areas.....NO PROPER SIZE used!!

[shortcircuit]

Thank You, I'll go do some home work, appreciate all your input. I still have a lot of faith in this forum .

[shortcircuit]

send to another amp yields the same results , narrows to the pre amp ?? I'll re examine input jack

[The Dude]

Check the + & - op amp supplies (pins 4 & 8 on the op amps).

Edit: Also make sure the replacement jack does not have the pins reversed- pins and switch contacts are on the same side of the jack as the original. They are available both ways.

[shortcircuit]

Check the + & - op amp supplies (pins 4 & 8 on the op amps).

Edit: Also make sure the replacement jack does not have the pins reversed- pins and switch contacts are on the same side of the jack as the original. They are available both ways.

The jack is correct , all op amps are consistent -15.5 vdc on pins 4 , +15.3 on pins 8. All caps are spot on uf rating tested in circuit . Q1= G .125vdc S .948vdc D 31.64vdc Q2= G .654vdc S .953vdc D 34.7vdc Q3= E 32.13vdc B 31.53vdc C .667vdc Q4= G 0vdc S 1.34vdc D 7.22vdc

[nevetslab]

The jack is correct , all op amps are consistent -15.5 vdc on pins 4 , +15.3 on pins 8. All caps are spot on uf rating tested in circuit . Q1= G .125vdc S .948vdc D 31.64vdc Q2= G .654vdc S .953vdc D 34.7vdc Q3= E 32.13vdc B 31.53vdc C .667vdc Q4= G 0vdc S 1.34vdc D 7.22vdc

The JFET voltages all seem reasonable. Do you have a scope to follow the applied signal thru the stages? If not, we can try it using your DMM in ACV mode. The voltages shown in a box on the schematic are AC voltages, and with 2mV input and 50mV output at Q3 collector, we have a gain of 25, or 28dB gain. There's enough headroom to inject greater than 2mV, if that's a problem. 100mV would yield 2.5V out at Q3 collector. You would want to turn the Volume pot down so you don't overdrive the rest of the circuits that follow, but this would allow tracking the signal to see that you're getting sensible AC readings or not. Setting the Presence and Contour controls fully CCW, and the four EQ controls at 12 o'clock, the response is flat, so you could use any frequency you want. 400Hz works fine. You could also drop the volume down so you have the same ACV readings as on the schematic, using that as a guide.

After you checked your supply voltages on the preamp, did you check to see what you had on pins 1 & 7 on the dual op amps? U2B is the exception, as it is a Clip LED driver and it's output will NOT be a 0V +/- 10mV DC.

[shortcircuit]

Sorry no scope , just went and checked pins 1&7 all are minimal 0 vdc , U2B has minimal at pin 1 and 14.7vdc on 7 . Are the instructions in post #10 for a scope or without, I'm taking it that they are with a scope yes ??

[nevetslab]

One of the other issues I've had with the GK 1001RB, 2001RB's input and output jacks is grounding. Looking close at their Output jacks, there is an extension pin that protrudes just beyond the mounting surface of the plastic body, which is supposed to engage the bare-metal panel to connect Analog Ground to the chassis. Those protrusions tend to get pushed back, no longer making proper ground. The Input jack on the 1001RB I just serviced did NOT have that pin, though the mold on the jack body was equipped with the slot for it. I ended up folding up a tiny piece of 7mil brass, cut to size, punched a 13/32" dia hole in it, folded and cut the extended flap to engage the two Ground Pins of the jack, installed it over the jack bushing and tack-soldered it into place. The front panel also has bare metal exposed around the input jack on the inside of the panel for that purpose. On jacks that have it, I've had to unsolder that ground terminal and push/force the grounding pin forward in the solder hole, so it makes contact with the front panel surface.

[shortcircuit]

I was under the understanding that the input jack was non-grounding and the output jack was grounding , thus the separate part #'s . I ran a jumper to the jack leg straight to chassis ground no difference,do appreciate all your help

[shortcircuit]

I removed the input jack , and re-installed it , no change. I pulled Q3 ,it's not shorted +E to -B 3.68M +C to -B 3.55M E to C O.L +B to -E O.L -C to +B O.L hfe 320 data sheet doesn't give me a max min is 100. hate to fail it's the easiest thing to do.

[nevetslab]

I removed the input jack , and re-installed it , no change. I pulled Q3 ,it's not shorted +E to -B 3.68M +C to -B 3.55M E to C O.L +B to -E O.L -C to +B O.L hfe 320 data sheet doesn't give me a max min is 100. hate to fail it's the easiest thing to do.

OK with the input jack. The readings you made were with the DMM in Ohms? If your DMM doesn't have a Diode Test mode, switch to 2k range. This range on all DMM's I've encountered provides a 1mA constant current source with a 2V potential behind it. You can turn on semiconductors in this mode...it's your Diode Test mode, which all DMM's have (forget those that only have a 1k setting). With this MPS A56 PNP xstr, Blk lead to base, Red lead to emitter and then to collector, you'd typically read 0.65V. Between C-E in either direction (xstr removed from circuit), it will read open circuit. Reverse the leads....Red lead to base, then it will read open circuit to Emitter and then Collector. That's the drill, if no Diode Test mode function.

What voltage did you read at the collector when Q3 was in circuit? I'd guess near 0V, though might be a couple volts in either direction. Should pass signal though.

In checking for signal using the ACV mode on your DMM, first check what your input signal is from your signal source, before plugging it into the amp. After plugging it into the amp, are you reading that same signal level at the phone jack Tip connection? Do you get the same reading at the junction of Q1 Gate, D6 cathode, R11 (1M), C5 & C6? Hopefully so. Then, the output thru this discrete input amp will be 25 times higher. On one of the schematics I have for the 1001RB, they show 2mV input, and 50mV output from Q3. I had suggested 100mV as input, which would produce 2.5V out at Q3 collector. You'd need to lower the Volume control to use this hot of a signal for all the rest of the stages, as it would eventually cause clipping when you get to Q4.