GK 2001RB Some repairs just don't last, I guess.

[nevetslab]

When I got in this morning, walking past the Repair pile on the main floor, I spotted a pair of GK amps. 2001RB's, both with a label on the top cover 'Ground Hum'. Fetched a hand truck to haul them into the shop, then, after moving each one up onto the check-out bench, recording their asset numbers to enter into my database, I connected each to the GK 410 bass cabinet in the shop, and powered it up off the variac. Once they came out of turn-on delay, there was a steady moderate level 120Hz pitch coming from the speaker. I had verified the pitch by checking against that of my B & K oscillator driving the same speaker. Power Supply issues. I checked my records. We only have two of these amps in our present west-coast inventory. One I had substantial failures on back in Aug 2018, then later in Dec 2018, and had posted this back then:

https://music-electronics-forum.com/...-repair/49322-

The other amp had a minor shut-down, when the Ch A Power Xfmr Secondary harness plug came loose (system shuts down if that comes undone...no LV supplies any more). Though it also had suffered contact problems with the harness plugs, and I believe I had replaced the Molex single-wipe terminals with Trifurcon terminals after this post:

https://music-electronics-forum.com/...15a-fuse-blows


Both of these amps sound the same connected to the test speaker, and dead-patching the Effects Return doesn't change anything. I won't know more until I pull the top cover and disconnect the two preamp connectors from the power amp assembly.

I really hate working on these amps. GK chops off all the component leads after flow-soldering nearly at the surface of the PCB with their lead-trimming machine, plus their solder mask exposes such a tiny amount of the copper laid down on the board.

So, I don' t know when each of these began exhibiting the current problem that got them sidelined, but....sure didn't last very long in service. Sigh......

[christarak]

Oh goody. Yesterday a GK800RB landed in "the Waiting Room" on the other side of the Pacific. No sign of life. It may be a couple of weeks before I get it on the bench, so I'll be watching your progress carefully. Good luck

[nevetslab]

GK 800RB's are much better, though they will often burn up the PCB tracks when their output stages fail. I've been keeping our fleet of 800RB's working for years. Much easier to service than these 11001/1002/550 Fusion's (which use the 1001RB power amp assembly). I've posted a number of threads on these, which can be found in our archives.....search under GK800RB...lots of threads from a lot of folks on that model.

[christarak]

GK 800RB's are much better, easier to service than these. search under GK800RB...lots of threads from a lot of folks on that model.

That's a very reassuring resource to have, nevetslab. Good luck with your repair. May it not give you any more grey hair.

[nevetslab]

That's a very reassuring resource to have, nevetslab. Good luck with your repair. May it not give you any more grey hair.

I WILL accept more white hair....I've come to like the 'wizard' look.

[nevetslab]

Finally back on these two GK 2001RB amps, both having the same residual HUM.....verified it was 120Hz earlier. I found if I disconnected P9 from the power amp PCB (input to both power amp channels), the hum disappeared. You can only get so far with interrupting harness connectors between the preamp PCB assy and the rear panel I/O board's connectors and that of the power amp. So, after verifying ground between chassis and the input connector, as well as the output connector's Ground and chassis, I noticed I had installed a brass grounding plate onto the input jack, picking up the sleeve contacts of that jack on the Front Panel. I've had trouble before with the Cliff phone jacks....those having tiny ground protrusions that extend past the plastic body of the jack, where that tiny metal probe is there to make contact with the mating panel surface. GK masks the metal panels at those locations prior to painting, so there's metal to metal contact. The input jack does NOT have that grounding protrusion. On this amp, I had installed a grounding plate. I opened the cover of the other amp with the same problem, and found I had done the same there as well.

So, just to verify this, I removed the two handles holding the front panel to the chassis, along with the two screws along the bottom of the panel, which then let the panel with the preamp separate from the chassis. Just to see if this was the problem, I moved the chassis close to the edge of the bench, and let the front panel assembly sit on the preamp PCB. It tipped forward, and the 5-pin connector J1 disconnected. I reattached it, and now with speaker connected, power connected, I turned it on. No hum. Good sign. powered down, then plugged in signal source to the input jack, and powered back up. No sound. Not a good sign. I had previously had a working amp, with hum, but now, nothing. I put the panel back onto the chassis, just using the two screws on the lower front edge. Getting no system noise with pots turned fully CW. Changed signal source, no difference. Patched into the Return jack on the rear, still nothing. Patched into the Chain Input, which connects directly to the power amps. Got signal, so the power amps are working. Checked to see if I had bipolar voltages on the dual op amps of the preamp, and had that....just no signal.

I set this aside, knowing I'll have to figure that mess out. This was the amp that flamed out before, with power supply failure that took out all of the preamp IC's, so every IC in the system has nice machined-pin sockets.

I moved the other amp up onto the bench, removed the front panel assy, then removed the preamp PCB from the panel, unsoldered that added grounding plate from the input jack, then reassembled everything. Powered it up, and no longer have the hum that was present before. Passes signal just fine.

Stupid is as stupid does, I guess! GK floats the input jack on the preamp, and takes signal ground all the way thru the system to the main Power Amp PCB, and grounds the system to the chassis by way of those stupid tiny metal protrusions on the output phone jacks. Those fail over time, leaving you with similar hum problems. Odd that I had verified the hum heard being 120Hz (power supply ripple), and NOT 60Hz.

So, success on one amp, while the applied logic on the first one tackled somehow has gone astray, leaving me with a witch hunt on the preamp PCB assy. Stupid jacks!

2001RB_Power_Amp.pdf
2001RB_Pre-Amp.pdf
2001RB_I-O_Board.pdf
2001RB_Switch_Board.pdf

[bsco]

I WILL accept more white hair....I've come to like the 'wizard' look.

Electronic repair has a habit of doing that to people..........

[bsco]

I will have to make a note of that GK repair issue you are having with the hum issue....learned something new here again that I did not know before.....
Cheers

[nevetslab]

Today, checking the +/- 35V supplies that arrive at the Preamp PCB from the power amp, I found those lower than normal....+/- 31VDC. Then I spotted burn residue around the Left Ch Tweeter Power Amp IC (LM3886T). So, had to pull the amp apart to address that, assuming I had more of those parts on hand. One left, thankfully....gotta order more of those. So, I removed the front panel assy, the shield wall between the Preamp and the power amp/power xfmr, the interface harness, then, unplugged the two fans and slipped the Wiha #1 Philips screwdriver thru the upper fan frame to access the four M3 x 35mm fan mtg screws, I removed the Left Ch Fan, and proceeded to do the same with the Right Ch fan. Left rear screw stopped turning, and instead the screwdriver was now skipping in the screw head. Pressed down to re-engage, with no results. Stupid screw locked up, with the screw head now damaged. I don't recall that from earlier. Nothing like having a damaged screw stop me dead in the water. I fetched my long-nose surgical pliers, hoping I could get enough grip on the screw head and make small angular twists to keep unthreading. Lucked out and got the damaged screw out.

Stopped to place an order with McMaster-Carr for some M3 x 35mm Socket Cap screws, 2.5mm drive. Resumed disassembly, and got to the damaged Tweeter Amp. I've already lost some solder pads on that Tweeter Amp hole pattern. Cut the forward leads off from the chip, bent them forward, then using the De-soldering iron, removed each lead one by one. Getting the back row out not so simple. I had to unsolder the top side pins, as pads on the back on that row have already been burned off. With the solder removed, I was able to chop off the body, then one by one, unsolder the remaining pins from the board, then had to clean the holes out. Installed the new IC, having to go in between two of the front row leads with a narrow soldering iron tip to top-solder that lead, then the rest was easy. I didn't see anything else out of place, so moved on to re-install the power amp PCB assy into the chassis.

Removed the preamp board from the front panel. While doing that, I discovered one of the IC's had come partially unplugged....IC located between the Send/Return Jacks and the preamp output. Re-aligned the leads and plugged it back in. Hopefully THAT was what stopped the signal flow on this amp. Unsoldered the brass grounding plate on the input jack and removed it, re-soldered the ground leads again. Put the preamp board back onto the panel, put the shield wall back into place, plugged the interface harness back in, and re-mounted the front panel assembly to the chassis, just holding it on with the lower pair of mtg screws, handles still off.

Powered the amp back up, GK 410 cabinet connected. Turned up the Woofer Master and the Gain control, and had residual noise again, and...No HUM. Plugged in my signal source, and had everything working like normal. So, it WAS that one IC having come dislodged. Installed the two handles. Now, I just have to wait for the UPS truck tomorrow for the replacement M3 x 35mm Fan Mtg screw to arrive to finish this.

[nevetslab]

Prolog. When I got in today, arriving a couple hours later then usual, having completed all of the current project work, I found one of the two GK 2001RB Bass Amps I had just serviced back for something else. The Tweeter Hi Cut button was sticking. Label was wrong. Button was stuck. So, I had to remove all of the front panel push-on knobs, loosen up all the pot mtg nuts, input jack nut, to see if I could free what was causing the button to be binding in the panel hole. No such luck. So, had to remove the top cover, then the handles.....one ferrule of which hit the concrete floor running...escaping towards the front of the bench underneath, where there's a panel that stops maybe 3/8" of an inch above the floor. Down on my hands and knees with a flashlight, to see a power cord laying along the floor on the other side of that restrictive panel, close to the panel, no chrome plated ferrule in sight. Sigh. Another 'football bounce' of essential hardware scores a victory.

So, removed the rest of the handle hardware, removed the two lower front panel screws (whose thread size still has me puzzled...measures like M3.5, but any M3.5 screws won't tighten in the pem nut. Not #6-32 nor #8-32. The original screws are gone, using two pan head screws that DO mate with the Pem Nuts). So, I got the PCB removed. The switch itself works fine, but, I found some black gooey substance on the button. Dipped a Kimwipe into my Denatured Alcohol dispenser and attempted wiping off the substance. That was a loosing battle, but applied fingernail behind the Kemwipe and got enough of it off to solve THAT contributing factor. Found a little bit of that same substance on a couple other buttons, though not as much as this offending switch button had. Cleaned all those up, put it back together, less the missing handle ferrule.

Went to the Guitar Dept's metal cabinet where they keep the stash of panel handles & hardware. Stacked in front of the cabinet were two SVT 810 cabinets, four guitar cases, a couple more road cases. Sure...why not. No reason access to that cabinet would be needed today. I complained, and labor was redirected to allow access to the cabinet, where I was certain, at this point, there wouldn't be any ferrules anyhow. Thankfully, there were plenty...just had to remove them from unused handles, and went on my merry way to complete the task of reassembling the amp. Ah well, added almost an hour of labor needed for this months labor in exchange for shop space.