Carvin DCM1000 amp repairs

[BenPlucken]

I have a DCM1000 (rev C) that recently blew one of the DC rectifying caps (10000uf 80V) when the unit was powered on. I have since replaced the caps (both, with OEM) and checked out the power board - great DC power +/-60V, +/-15V.

But now the unit is outputting a loud AM-radio static and crackling (like bacon frying). With speaker loads there are no clip lights lit (but the clip lights flicker if there are no loads).

Any thoughts on which circuitry to look next?

[Jazz P Bass]

Can you post a schematic?

[BenPlucken]

schematic attached...

s01000H-B.pdf

[BenPlucken]

I changed what are labeled C1 and C2 (lower left corner - on the DCM1000 they are 80v not 63v) and have verified all the components on that card - with signals coming out on the H1A and H2A connectors from that card - all good with the -24v signal (from the transformer) being a little "hot" at -28v but otherwise excellent.

Thanks in advance for any help or wisdom!

[Jazz P Bass]

And both channels are acting up?

[BenPlucken]

yes - both identical in the noise

[Jazz P Bass]

Then the cap failing took out identical parts of both channels?
I would look at what is common to both amplifiers.
The power supply.
Scope or measure all of the rails looking for any Volts ac ripple or hash riding on any of the rails.

[BenPlucken]

Jazz,

I am on-board with the unlikely chance that the cap failure took out both channels' components equally and identically - but rather it is more likely that the problem is still in the power supply board which is common to both.

I failed to look at the AC ripple on the DC signals once I got everything soldered together - I am just too close to the problem I guess...and I think I am onto something as a result of going back and checking the AC. When the amp is connected as the factory sent it, I get +/-60VDC being sent out - but the AC measures a whopping 20volts! (I am measuring with a Fluke Multimeter as I don't have access to a scope so I know it's like Fred Flintstone doing an engine overhaul on a Ferrari - so 20v may also be related to the waveshape and how my meter handles it). When I disconnect the amp from the power supply board (no load on the power supply), the AC drops to ~4.5VAC (which still seems like a lot).

Yet, almost all of the AC is coming across C1 (only 0.5VAC across C2) despite the fact that they are a matched pair for splitting the DC in half. Further C2 holds a charge with power off (for hours) but C1 is discharging immediately when I power off...yet there is no drain path when the amp is disconnected?! So something is wrong on the circuit.

I changed out C1 with the exact same results.

I pulled both C1 and C2 (with the power supply still disconnected from the amp) and got a 98VDC signal out of the rectifier with the same 4.5VAC (i.e. the capacitors aren't filtering it out). Note that, with the caps out the center point is supposedly not connected to anything and "floats" at 50VDC with 4.1VAC on each half.

I say "supposedly" as I discovered that the case of the rectifier/diode bridge is tied to the center ground through the heat sink/case ground system. That seems somewhat relevant because three of the diodes of the bridge measure fine with the Fluke's diode checker (block one way, 0.5 ohm the other polarity) but the 4th measured open (OL) with the capacitors in...I thought I had the culprit (bad rectifier diode) until I unsoldered it and rechecked it alone on a bench (where it was fine). It seems the volts on C2 are not letting the Fluke test it as with no capacitors in place, it tests fine on the board. Yet I can't explain this (the diode between DC- out and AC-in pin H7J on the schematic)

I can't measure any resistance between the case of the bridge and any diode of the bridge - but I cannot explain where else C1's voltage is discharging (I could better explain C2's voltage discharging through the 15 volt circuits if something where bad there - but that's not the case -- C2 is not discharging and both 15VDC supplies are measuring millivolts of AC).

So with a circuit of only one rectifier bridge and two capacitors (one of which blew in the original problem), I am considering the potential that the rectifier may be my remaining issue. It will take me days to get one here. Does this seem logical? Am I missing the forest for the trees?

[BenPlucken]

Thanks for the help...problem solved. It was indeed in the Power Supply...good advice. The printed circuit board apparently had a through-hole connection that came out when the capacitor was removed. Without it, the top of the pcb was missing a connection to the bottom of the pcb. I couldn't figure the strange voltages and draining capacitors because I was seeing only half the circuit. Once I installed a wire jumper to replace the through-grommet, things are quiet (in a good way).

[Jazz P Bass]

Yeah, the plated trough hole connects the top & bottom trace (if there is one)
Mighty fine sleuthing.