Peavey KB60

[lowell]

This amps PA is fried. Peavey charges $2.50 which I will pay if someone cannot help me w/ a freebee.

[Enzo]

Did you contact customer service at Peavey and ask for the schematic? Or did you just see the "$2.50 for a paper copy" message and stop there? Almost entirely, they will email the files to you free of charge.

Include your serial number as there may be more than one variation.

[lowell]

sweet! thanks i didn't think to call.

[lowell]

ok here's schem.. could someone please enlighten me on signal path here? This seems quite different from the amps I've worked on. I'm more used to the output being between the output transistors... but here it seems the output is elsewhere because the output are grounded in between the ballast resistors.

[Enzo]

This is very common in PV solid state amps, but also many other places. Most QSC amps for example.

This grounded emitter arrangement is really just like the "normal" circuit.

Th speaker is connected in between the ballast resistors and the power supply common like always. Only difference is they connected the emitter side of the speaker to ground instead of the power supply side. Note the filter caps and PT CT don't go to ground, they go to the speaker.

This is an example of "flying rail" circuits. The amp stage pulls the power supply rails up and down to the music. The voltage across each filter cap stays steady, but the voltage at the rail with respect to ground has the signal on it,.

[lowell]

great! that makes sense... as always thanks for your descriptive and informative illustrations Enzo.

[lowell]

Ok so I replaced the enitre power amp in this thing and it still blows fuses. Using my lightbulb limiter now. I disconnected R156 and R175 with no change. Disconnected C146 with no change. I then unplugged the amp and measured resistances to ground from the power rails and from all 4558 ICs power rails and all seem good. I also checked all the 4148 diodes that have all ICs pin 5 in between them - good. I even disconnect the power transformer secondary from the board and the light bulb did not glow at all so I think the PT is fine. Any thoughts?

[Enzo]

Amp blows fuses:
1. shorted output transistors.
2. shorted rectifiers
3. shorted filter caps.
4. shorted power transformer.

Not sure why we disconnected those two resistors. That cap was a good thought. If your outputs are not shorted, and your PT doesn;t blow fuses on its own, than I'd be looking for shorted main rectifier diodes or a shorted filter. And make sure none of your output transistor cases are shorted to the heat sink.

[lowell]

ok well I double checked the power transistors and they had been installed with the PNP on the positive rail side and vice versa for the NPN. Switched 'em w/ new ones and all good! Those heatsink mounted power transistors Q102 and Q104 are easy to get confused! thanks again Enzo.

[Enzo]

Whenever I pull power semis off their heat sinks, I always write their number next to them with a marker.


SO you might open some amp I have been in and see A1943 and C5200 written on the chassis. or whatever.

[tim]

I think techs have the market cornered on Sharpie pens, "ultra-fine" for me, thanks...

When I get done with an amp, there's no doubt that I've been in it , except vintage tube amps...then I just use a soft lead pencil, astutely aware that lead is graphite and hence is a conductor. Soft lead pencil is easy to erase with a gum eraser...

[lowell]

Word

[lowell]

Working on another one of these. I cannot find the tweeter connection on the schematic. Can you? The amp has 2 pairs on a 4pin connector that connect from the board to inside the cab. This is not shown on the schematic. I'm suspecting that the speaker or tweeter is blown in this amp. I may just take em out and check, but still curious if I'm missing it on the schematic.

[Enzo]

The schematic just shows a speaker. If there is a tweeter, it is likely a piezo, yes? In which case it has no crossover and is just wired parallel to the woofer. Woofer runs full range.

On the layout picture there are two speaker connectors, but looking at the tracework, there is one pair of pins simply jumped together, leaving ground and CT leads for the speaker itself. SO pop the woofer and see what the wires do down in the speaker vault.

[lowell]

you are correct sir. contacting peavey, the tweeter is bad... it blew the mains fuse.