GK 2001RB going into Protect Mode Bridge Mono

[nevetslab]

A GK 2001RB arrived at my shop yesterday morning from The Voice. The complaint was in Bridge Mono mode, driving an 810 4-ohm cabinet, it's going into Protect mode (flashing Orange LED). They already have one of our 2001RB's as back-up, which I'd serviced recently. So, sure, I'll dig into this one and see what this one wants. This is a Rev B2 main power amp PCB, though I couldn't find any date codes on exposed components.

I wasn't able to drive it into protect mode, though not using a speaker cabinet (all I have in the shop is a 4 ohm Ampeg 15" cab and an 8 ohm Hartke 15" cab), so I used my high wattage dummy loads. Started by verifying all the power supply voltages were correct, no DC offset voltages on the output. Under drive, looked first in Dual Mono mode, 1 ch at a time to see I had sufficient output (clips around 40VAC out/4 ohms). Then, set it into Bridge Mono mode, stacked up the loads for 4 ohm/2kW, as it is capable of 1kW into 4 ohms. I ran it with 50Hz 1/3 Oct Pink Noise, which is really variable-amplitude 50Hz sine, set it for periodic clipping, while average output level was around 30VAC. I was seeing periodic current draw greater than 12A. I let it run for quite a while, with thermocouple on the heat sinks. I got the fans to full speed quickly, and, interestingly, the Right Ch was running around 78 deg C on the heat sink, while the Left Ch only 56 deg C. Temp on the surfaces of the three bridges of each channel....those on the Right Ch were 10 deg hotter than the Left Ch. I didn't stop to monitor each output channel under this condition, since it was random amplitude.

It never did go into protect. So, I pulled the amp apart, knowing I was probably going to be dealing with solder joints in the power supply area. GK is NOT one of my favorite amps to service with regards to dealing with poor solder joints, particularly in the power supply. The are an excellent example of how NOT to do solder masks nor providing adequate solder pad sizes for high current components!!




This is the Left Ch power supply, top and bottom. Right Ch is no better. While there is current flow, and it hasn't coughed up on the bench just yet, I'm suspicious of the voids I see on the top side of the three bridge rectifier leads, and see similar voids on two of the leads on the bottom side of the middle low voltage bridge. Same on the right ch.

I just have NO idea WHY GK would produce their PCB's with the tiniest solder mask openings where you need good solder wicking onto the high current component leads. They didn't do so bad on the power supply caps, though they're not a great example of how to do it there either, but........geeze! Just to get AT the top side terminals to un-solder then re-solder the three bridge leads, you've got to remove the filter caps. There is enough room to get in at the middle LV bridge, but once again......look at the size of the exposed plated copper!

I haven't yet pulled the Protect board out for inspection. That's next, and will have a look at it before I dive into address what I see on the two power supply areas.

2001RB_Power_Amp.pdf
2001RB_Switch_Board.pdf

[The Dude]

I think you're correct about solder/connection issues. I've seen it before on these amps. I suspect the reason you're unable to duplicate the symptom in the shop is that you don't have the head sitting directly on top of a vibrating bass cabinet.

[nevetslab]

I think you're correct about solder/connection issues. I've seen it before on these amps. I suspect the reason you're unable to duplicate the symptom in the shop is that you don't have the head sitting directly on top of a vibrating bass cabinet.

There's another project I'll have to tackle. A suitable box loaded with one of the Drum Stool shaker motors that I can drive to couple substantial vibration into any amp set upon it. For the time being, after having this 2001RB apart, I repaired a number of gamey solder joints on the AC Mains/Protect PCB assy, as well as desoldering bridge rectifier terminals, scraping away solder mask to there was ample copper to tin and re-solder, now having proper solder pads and no voids on the two channels of power supplies.



There were gamey connections on the AC Mains/Protect PCB's power transformer pins, the little solder-blob 'jumper' used to strap that small xfmr for 120VAC was a goob of cold solder, big void on one of the leads of the in-rush Thermistor (which ends up getting bypassed by the relays on this board.

In lieu of a shaker motor platform to vibrate the chassis while driving it, I've set the 2001RB atop a 10" woofer, driving that at the best resonance I found (38Hz) while sweeping the generator to drive the woofer, so there's decent vibration coupled into the 2001RB chassis. So far, still haven't seen it go into protect, and hopefully won't again in use, as I've had these horrible solder connections made to the tiny annular ring pads separate from the diode and connector leads over time, resulting in the amp becoming intermittent. The amp is again being driven with 50Hz 1/3 Oct Pink Noise into 8 ohms bridge mono, output set for regular clipping while the signal is really random variable-amplitude 50Hz Sine, having over 12dB min/max level variance.

So, we'll see. Better than it was.

[nosaj]

There's another project I'll have to tackle. A suitable box loaded with one of the Drum Stool shaker motors that I can drive to couple substantial vibration into any amp set upon it. For the time being, after having this 2001RB apart, I repaired a number of gamey solder joints on the AC Mains/Protect PCB assy, as well as desoldering bridge rectifier terminals, scraping away solder mask to there was ample copper to tin and re-solder, now having proper solder pads and no voids on the two channels of power supplies.

[ATTACH=CONFIG]51964[/ATTACH] [ATTACH=CONFIG]51965[/ATTACH] [ATTACH=CONFIG]51966[/ATTACH] [ATTACH=CONFIG]51967[/ATTACH] [ATTACH=CONFIG]51968[/ATTACH] [ATTACH=CONFIG]51969[/ATTACH][ATTACH=CONFIG]51970[/ATTACH]

There were gamey connections on the AC Mains/Protect PCB's power transformer pins, the little solder-blob 'jumper' used to strap that small xfmr for 120VAC was a goob of cold solder, big void on one of the leads of the in-rush Thermistor (which ends up getting bypassed by the relays on this board.

In lieu of a shaker motor platform to vibrate the chassis while driving it, I've set the 2001RB atop a 10" woofer, driving that at the best resonance I found (38Hz) while sweeping the generator to drive the woofer, so there's decent vibration coupled into the 2001RB chassis. So far, still haven't seen it go into protect, and hopefully won't again in use, as I've had these horrible solder connections made to the tiny annular ring pads separate from the diode and connector leads over time, resulting in the amp becoming intermittent. The amp is again being driven with 50Hz 1/3 Oct Pink Noise into 8 ohms bridge mono, output set for regular clipping while the signal is really random variable-amplitude 50Hz Sine, having over 12dB min/max level variance.

So, we'll see. Better than it was.

An Electronic Enzo Whacker machine... I like it......

nosaj

[nevetslab]

This morning, after running the amp again after repairs made to gamey solder joints on the power amp bridge terminals, AC mains connections on the Protect/AC Mains PCB assy, I checked the bias, which for some reason I hadn't done before. Both were set the same, about 6.8mV, where the nominal spec is 10mV. The amp had been running for a while anyway, so I bumped both channels back up to 10mV.

While it's been running, I checked the surface temperature on the two heat sinks, on a thick edge nearest the middle of the PCB, as well as the temp on the surfaces of the six bridge rectifiers. Before, I was seeing a 20 deg C difference in heat sink temperature under Bridge Mono drive. That's now down to 10 deg at the worst, 6 deg difference at the best similar positions. And, all the bridge rectifiers were at the same temperature, where before I attacked the solder joints/copper pads for 'proper' solder pads, I was seeing more than 10 deg C difference. I never did see any difference in output waveform under drive before, both Dual Mono and Bridge Mono. I will take the change as a positive sign.

Still haven't gotten the amp to go into protect. I put a power amp between the 'shaker speaker' oscillator's power amp output so I could increase the coupled vibration.