1988_Peavey_KB100_Mystery

[hewo]

1. Sound was bad from infamous switching 1/4" receptacles so disassembled extracting mainboard/output_pcb. Retensioning and contacts DeOxit cleaning achieved zero ohms reliability of normally closed switches. Retensioning involves removal of barbed soldertails switching contacts, bending and reinstall.
2. Also epoxy filled replica stem on reverb pot (sheared off), required removal and reinstall of pot, for gravity fill into cylindrical well created by blue painter's masking tape.
3. Used Molex pin extractor tool to increase tension female pins (downsize hollow). Used carbide needle pin to tension male pins (upsize nozzle). This imparted undue force onto motherboard pin solder joints causing cold joints. Jumper harness(s) molex pins exempt. When motherboard reassembled and tested, no speaker fiducial background noise (nil dc, nil ac, speaker spades, continuity good back to mainboard soldered molex pins). Mainboard then removed and found cold solder aforementioned, and repaired. Ground of AC power amp rails was lifted (XFMR CT) during initial energized trial. Could this have blown the TO3's (limitation voltage differential exceeded, base to emmiter)? I ask because post cold solder correction, still, speaker no fiducial background noise (nil ac, nil dc, across speaker spades). Speaker (and capacitored horn) is good.
4. 1988 schematic (all opamps index point towards rear channel heatsink) specifies different TO3's (SJ6392, SJ6505) then actuals in place (both Motorola with Motorola numbers, factory originals Peavey). All plus minus DC rails proper (46.5, 25, 16), there is no heat (very mild heat on sandblocks DC rail reduction), and DC rail is present where tracings distribute.
5. Confused. Send audio signal into front end input and confirm presence at preamp out?
6. Thanks in advance. Performing musician gave it to me and I want to give it operating, to my church worship group.

[Enzo]

SJ6392 is old number for 70483100, they are Pevey numbers for 2N3055A in older gear, then more modern replacement is MJ15015.

SJ6505 is old number for 70473100, they are Peavey numbers for 2N6228 in older gear, then more modern replacement is MJ15016.

Those two are complements.

OK, so the problem is "no output" to the speakers?

WHy do we think the TO3s are bad? It doesn;t blow fuses, does it?

Plug a signal into the power amp in jack. That tests just the power amp.

But before even doing that, look at the headphones jack. The output of the amp runs over to the jack board, through the phones jack, then on to the speakers. If that jack is compromised, or if the connections on that little board are bad, no sound. Also, for tests, you could unplug the output wires from the main power amap board, and just connect the speaker wires directly there. Sound that way?

[hewo]

fiducial_background_noise_speaker_absent_altogether

1. there is no speaker spades voltage both ac/dc.
2. dc rails are present in board circuitry
3. there should be something, at least millivolts dc / ac for background circuitry influence into speaker spades
4. but there is none whatsoever
5. this means what?
6. all switching (normally closed) 1/4" receptacles perfect 1 ohm closed circut as measured on underside switch solder landings (repaired restored correctly)
7. next step, unsolder all three legged semiconductors (fet dump, To92 complementary pair, To226 complementary pair, To3 complementary pair, and test on transistor tester? To3(70474140/W8936 & 70484140/W8932), To226(R921/7036 & R926/5331), T092(MPS6534 & MPS6530), Fet(2N5462/T936)
8. or just order these from peavey and swap out?
9. what of the exceedance mentioned of base to emmitter voltage? truth or moot?
10. Thanks in advance

SJ6392 is old number for 70483100, they are Pevey numbers for 2N3055A in older gear, then more modern replacement is MJ15015.

SJ6505 is old number for 70473100, they are Peavey numbers for 2N6228 in older gear, then more modern replacement is MJ15016.

Those two are complements.

OK, so the problem is "no output" to the speakers?

WHy do we think the TO3s are bad? It doesn;t blow fuses, does it?

Plug a signal into the power amp in jack. That tests just the power amp.

But before even doing that, look at the headphones jack. The output of the amp runs over to the jack board, through the phones jack, then on to the speakers. If that jack is compromised, or if the connections on that little board are bad, no sound. Also, for tests, you could unplug the output wires from the main power amap board, and just connect the speaker wires directly there. Sound that way?

[Enzo]

Really, do not assume you have a bad transistor. Replace a transistor if it tests bad or you have other positive evidence of its failure. Otherwise you are just throwing parts at the amp hoping to hit the problem with one. A broken connection is just as much a problem to an amp as a bad part. And such problems are very common.

Did you try connecting the speakers directly to the power amp?

Have you verified the speaker itself works? And test the speaker for continuity through the wires, not right at the speaker terminals. We need to know that the speaker and its wiring are good from the connector all the way to the speaker.

It may be reasonable to expect a few millivolts of background sound, but that isn;t how I look for it. The amp is not putting DC on the output, so connect the speaker. You can hear noise easier than you can measure it.

When you flip the power switch, does the speaker make any sort of thump or pop noise? If it does not, that points again at a speaker problem.

Assuming the speaker is OK, and is connected strasight to the amp, trace a signal through the amp. PLug a signal into the power amp in jack. Use a scope, a signal tracer or an AC voltmeter to follow the signal. Is it on both sides of C132? Is it on pin 1 of U3A? Q104 is a power-on mute. You can measure resistance across it - pin 3 to ground of U3A - WHILE THE AMP IS POWERED. Measure that with power off, and you should get a low resistance, but with power on, that Q104 should turn off and go to a higher resistance. If in doubt, just remove Q104 and see if the amp wakes up.

U6 is the compressor chip, little 8-leg 87478 chip. It could have failed, simple remove it and see if the amp wakes up. We would want to replace a bad one, but the amp will work without it.

If you have signal at pin 1 of U3A, then do you have it at pin 1 of U7A too?

I hope that helps, I'll be gone for a week now.

[hewo]

KB100 _Mystery

Thanks Enzo,
1. To3 aluminum channel heatsink is earthed and both To3 complement transistors are thermal grease mica anchored to it yet To3 casing earths casing regardless (10x32 machine screws bite casing as well as channel heatsink) . Such is peavey’s intelligent design using complementary pair.
2. Must lift rail smoothing caps to isolate and test transistors. Caps are heat glued down to fiberglass, vibration proofing peavey intelligent design of massive weights under speaker vibration preventing cold solder failure.
3. Broken tracing and or cold solder joints meticulously scanned metered and affirmed not present. During 1st mainboard removal out of tray, did torque torex 10x32 machine screws anchoring both To3’s and noted torex heads galvanic corrosion. It may be possible the added torque disrupted continuity somehow? Casing is collector.
4. Switching ¼” receptacles all test good on normally closed contacts, three input channels, preamp equalizer courtesy input, four rear outputs courtesy (pre_out/pwr_in/rvb_sw/ph_out).
5. Continuity affirmed from spkr spades into mainboard solder pins molex via normal designed lineup, this proves ¼” receptacles switching functional.
6. Spkr known good, penlight battery test on voice coil generates audible transient bark, horn is capacitored no reply. Hand reciprocated paper cone inwards outwards affirmed no scrape. Spkr well broken in, battery test barking not as sensitive as lower wattage spkrs e.g. single ended vacuum tube el84 eight inch spkr’s. usually barks are extremely sensitive but kb100 spkr is 18” involving lots of paper mass.
7. I fluke79 measured .122_AC_mA on spkr spades energized. This is definitely not correct. DC is absolute nil here.
8. Look on the rear chassis output pcb layout diagram: I inadvertently mixed up the positions of molex jumpering connectors. There are two molex headers on this pcb. One says power amp out, the other says speaker. I inadvertently connected the two opposite of what the peavey layout diagram specifies. But no harm, right? The phones is not ground planed right? Therefore the pcb hdr for speaker bridges by phone receptacle normally closed switches into the pcb hdr for poweramp out. Or am I wrong? By incorrectly attaching the mainboard’s output into the output pcb’s spkr hdr (P201,P202) would the mainboard’s power output be earthed completely? And that’s why no paper spkr background fiduciary circuitry noise? Or am I wrong? Look at the jpeg closeup of the involved output pcb’s section and notice the headphone receptacle has no “G.P.” meaning earthed sleeving (ground plane).

[hewo]

error_correction_kb100_mystery

1. the To3's are insulated off the heatsink because the casing are the rails, plus minus 46.5DC
2. identified floating ground of the plus minus rails 46.5DC with respect to chassis lug (mains grounding green wire power cord) - these rails are not referenced back to chassis lug. is this a problem? shouldn't both pre and power amplification be referenced back to the same ground, chassis ground?
3. checked preamp circuit ground and yes it is same as chassis lug (mains grounding green wire power cord)
4. thanks in advance

Thanks Enzo,
1. To3 aluminum channel heatsink is earthed and both To3 complement transistors are thermal grease mica anchored to it yet To3 casing earths casing regardless (10x32 machine screws bite casing as well as channel heatsink) . Such is peavey’s intelligent design using complementary pair.
2. Must lift rail smoothing caps to isolate and test transistors. Caps are heat glued down to fiberglass, vibration proofing peavey intelligent design of massive weights under speaker vibration preventing cold solder failure.
3. Broken tracing and or cold solder joints meticulously scanned metered and affirmed not present. During 1st mainboard removal out of tray, did torque torex 10x32 machine screws anchoring both To3’s and noted torex heads galvanic corrosion. It may be possible the added torque disrupted continuity somehow? Casing is collector.
4. Switching ¼” receptacles all test good on normally closed contacts, three input channels, preamp equalizer courtesy input, four rear outputs courtesy (pre_out/pwr_in/rvb_sw/ph_out).
5. Continuity affirmed from spkr spades into mainboard solder pins molex via normal designed lineup, this proves ¼” receptacles switching functional.
6. Spkr known good, penlight battery test on voice coil generates audible transient bark, horn is capacitored no reply. Hand reciprocated paper cone inwards outwards affirmed no scrape. Spkr well broken in, battery test barking not as sensitive as lower wattage spkrs e.g. single ended vacuum tube el84 eight inch spkr’s. usually barks are extremely sensitive but kb100 spkr is 18” involving lots of paper mass.
7. I fluke79 measured .122_AC_mA on spkr spades energized. This is definitely not correct. DC is absolute nil here.
8. Look on the rear chassis output pcb layout diagram: I inadvertently mixed up the positions of molex jumpering connectors. There are two molex headers on this pcb. One says power amp out, the other says speaker. I inadvertently connected the two opposite of what the peavey layout diagram specifies. But no harm, right? The phones is not ground planed right? Therefore the pcb hdr for speaker bridges by phone receptacle normally closed switches into the pcb hdr for poweramp out. Or am I wrong? By incorrectly attaching the mainboard’s output into the output pcb’s spkr hdr (P201,P202) would the mainboard’s power output be earthed completely? And that’s why no paper spkr background fiduciary circuitry noise? Or am I wrong? Look at the jpeg closeup of the involved output pcb’s section and notice the headphone receptacle has no “G.P.” meaning earthed sleeving (ground plane).

[Jazz P Bass]

I am confused.
What is wrong with the amp?
There are three separate & distinct grounds on this amp.

[hewo]

I succeeded. the ground side molex mainboard swaged pin for poweramp output (for jumpering wire harness out to rear of chassis separate output pcb) was lifted so i bridged its soldertail connection with a shank of resistor lead to the next landing station on that bussing. next problem will be the thermal heat of the poweramp DC rails ballast resistors (pair of 2K 5watt). the proximity (1.5mm) heat will eventually compromise the 105C rated 4700uFx63wvdc axial pair condensers. best option is to elevate sandblocks higher on standoffs (height off of mainboard) but alas, this introduces vibration susceptibility of cold solder joint fracture. maybe directly aluminum finning each sandblock, as is, would dissipate waste heat faster and reduce proximity temperature?