Dean Markley CD 60 PS short

[Randall]

This is the amp that had a flame out and charred the pcb around pin 3 of one of the 6L6s. I cut away the char, and rebuilt the traces and connections, looks good. But when I put it on the light bulb limiter with no tubes to test it out, the bulb is pretty bright, I'm only seeing 12 - 15v on B+. +/- 1v on the rails, and about 1 ac on the filaments. I lifted the OT and choke, no change. Standby switch open, no change. Lifted the +/- rail supply winding as well as the filament winding, no change to B+.

In reverse with secondary winding lifted and the rest connected I get +/- 18v on rails, 6.8 vac on the filaments, and 400 vac from the disconnected secondary winding. With secondary still d/c I read around 132 ohms to ground at one of the secondary connectors, and 279K ohms on the other. PT passed the neon lamp test. The four rectifier diodes look good in circuit, as does the two 100/350 filter caps and their 100K 1W pals.

So it seems I have a short early on in the PS, but the problem is I don't find the right schematic for this late model CD60. The best I can find is this CD120 which seems close so far, but it can't be trusted because I read 425 ohms to ground at the junction of the two diodes and the standby switch, with the switch open. I just don't see the path. It's a double sided board so I will have to rip it out again to try to trace this out, but it's late now.

I wonder what I may be missing?

http://music-electronics-forum.com/a...120-page-4.jpg

[The Dude]

I'd contact Dean Markley and see if you can get the right schematic so you're not chasing your tail. Assuming the one posted is close, my best guess would be a shorted or leaky rectifier since the standby switch is open for your tests, but that is referencing a schematic that may or may not be correct for this amp. You say the diodes "look good in circuit". Is that using the diode check function, or how did you test them?

[Randall]

Diode check function in circuit. I don't think Dean Markley has support on amps these days, at least not on their site. I am going to look at those diodes more closely, because they did not all check the same in circuit, two were around .5v and two were about half of that I think. Easy enough to lift and check once the board is out again.

[Randall]

Yep. It was diodes, two 'leaky diodes'. I'm not sure I have ever come across a diode that failed in this way, let alone two. I usually think of them as being full open or shorted. Anyway, the amp is purring right along now. Good sounding amp as well. Customer will be very pleased as initially he thought it was toast, and he really likes the sound of it.

[Enzo]

In a bridge or a center tapped winding with a pair of rectifiers, look at it this way. If one diode fails, and they usually fail shorted, then that leaves the other diode as right across the winding. SO every half cycle the diode is a dead short across it. That will cook the diode pretty quick, So whenever one side of your rectifier shorts, replace the other one anyway, even if your meter says it is OK.

[Randall]

"If one diode fails, and they usually fail shorted, then that leaves the other diode as right across the winding. SO every half cycle the diode is a dead short across it."

Yes, but these aren't dead shorted, they have a resistance from + to - the same in either direction. One is 325 ohms and the other is 145 ohms. That's what I thought odd. But I do take your point about replacing them in pairs. Heck I should probably have replaced all four I suppose knowing this, but I only had two on hand and that board is a real bear to get in and out, so it's too late now.

[Enzo]

Oh, "shorted" doesn't necessarily mean zero ohms, it just means not good anymore because it doesn't control the flow of current any more. A good one is a one way gate.