Sunday, January 02, 2011

Next Peavey up is a Mark IV 400BH solid state bass head.

Initial inspection indicates this unit has been harvested for knobs, or a shade tree technician removed the knobs on disassembly and didn't return them to the device when put back together. The result is the same: the knobs are gone forever, perhaps floating in a rift in the space/time continuum hanging out with pre-war change and 45 rounds in Elvis Presley's pocket; or maybe resisting decay somewhere on a garbage barge. It's good practice to completely reassemble something if it's going to face any length of time away from the bench.

I digress, plucked knobs says "parts unit" to me, lending weight to the notion that this amp is a goner. Goody!

Inspection also indicates a bent slider shaft, which proved easy enough to bend back into shape, and a stressed power cable.

The power cable is cinched tightly around the heat sink, which doesn't do this failing cable insulation at the chassis clamp any good. I don't see any frayed wires though, so let's plug it in and see what happens!

Amplifier powers on and stays on.

DC voltage reading at speaker output is in the 200 millivolt range. Not exceptional, but it also hasn't had a chance to temperature stabilize; the point is there isn't a DC voltage in the speaker cooking range, yet there's obviously power of some sort.

Next step, since I already have the meter plugged into the power output jack and my o-scope is not at hand, is to set the meter at the low end of AC range and look at floor noise (that occurs within the time base window of my DMM). I naturally assume that this will not read hiss and thermal noise, but will read the frequency range of hum and buzz coming from the power supply. Meter indicates low millivolt range of AC voltage on output.

Since I'm doing this in the relative warmth and comfort of the apartment, I'm not at liberty to plug in and ramp up 200 watts of power into a cabinet, and the scope and dummy loads are elsewhere. So, the next step just confirms signal path and certain elements of operation, it is also a solid state only procedure, as the mega-ohm range on the input of the DMM basically amounts to no load at all...

With voltmeter plugged into the speaker output jack and set to an AC voltage range that supports 30 VAC, I plugged an instrument cable in the high gain jack and touched the tip without grounding myself to the shaft. ~10 volts AC appeared at the output. The low gain jack produces roughly 25% of output voltage that the high gain jack does.

While producing signal in this manner, I turn the volume controls, they seem to operate as one would expect. The limiter indicator also fires once output voltage hits 27 volts AC, though output voltage ranged to around the 33 VAC mark.

All of these steps occurred well within the space of 5 minutes. So, without having actually tested the amplifier section under load the amp passes signal and does not produce copious DC on the output, so the next step is to plug it in and observe its operation in actual use. That's for another day.

Fix list so far on this unit: locate suitable knobs (knurled 0.185" diameter) and release the power cable to pull it into the chassis by about half an inch.

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