Dean Markley CD30 blown OT? Sanity check

[xtian]

Dean Markley CD30 on the bench, nasty distortion at output, but not from preamp. Using sine wave and o-scope, I can see clean signal at pins 5 of the power tubes, but signal at output jack is badly distorted and jumpy. Not a loose connection.

Testing DC from OT primary CT to each plate, I expect a few volts, but I'm seeing varying voltages, from ~100mV to 4vDC.

Any other tests I can use to rule out or in the OT?

[xtian]

Same result with two sets of old tubes and one set of brand new tubes.

[Leo_Gnardo]

Any other tests I can use to rule out or in the OT?

Substitution, that's about it. For a test it doesn't have to be the same exact transformer, more or less anything will do. If it passes good power and no more jumpy galumpy then find a compatible replacement.

[xtian]

Good plan. Amp has spade connectors, so swapping in another OT won't take much time.

[xtian]

Replacement OT shows same distortion! Made me think I need to look at the reservoir and screen supplies, and BANGZOOM, there is 150vAC at B+ (at the reservoir cap). That's one heck of a lot of ripple, and yes, I noticed an elevated level of hum during take-in procedure.

[Leo_Gnardo]

Replacement OT shows same distortion! Made me think I need to look at the reservoir and screen supplies, and BANGZOOM, there is 150vAC at B+ (at the reservoir cap). That's one heck of a lot of ripple, and yes, I noticed an elevated level of hum during take-in procedure.

So, a bad main filter cap. Or disconnected somehow. It's amazing how much power supply ripple can cancel in a push pull output, this amp apparently tested that to the max. Glad you found the problem, plus it's sure cheaper than an OT to fix.

[xtian]

I tacked in a new 100uF/350v on the bad reservoir cap, and amp is better. Plate voltage jumped from 330v to 440v, reservoir ripple dropped to 2.5vAC, and output distortion vanished. Seeing 32 watts clean.

[Leo_Gnardo]

I tacked in a new 100uF/350v on the bad reservoir cap, and amp is better. Plate voltage jumped from 330v to 440v, reservoir ripple dropped to 2.5vAC, and output distortion vanished. Seeing 32 watts clean.

Yay xtian wins again. I'm sure you'll replace both 100 uF series caps. If one went bad can the other be far behind?

[xtian]

Thx, Leo. Yes, I will replace the e-caps as a set--I had to place an order.