classic 30, no sound, funny voltages

[cooldude666]

Heyo mis amigos. A friend gave me a classic 30 he picked up at a garage sale for $30 to see if I could make worth his investment. It doesn't make any sound other than a very troublesome electrical sounding buzz (not from the speaker mind you- best I can tell is it comes from somewhere near where the power transformer secondaries meet the board. It goes away when they're disconnected and sometimes when I'm taking readings I'm not sure that this is any sort of issue but it's definitely not nice sounding. edit: i typed this before i had replaced the caps- this stopped after replacing the caps.).

My voltages from the rectifier are pretty low. 250s vs the 330s the schematic claims they should be. I took readings from the power transformer secondaries and they look ok. about 280vac and 27vac. Power here is a little higher than 120 normally so they seem about right. I replaced the 47uF caps and the 400R5W from the Plate to the screen on the power supply rail. With that 400R lifted the voltage are more like 380, which makes sense given the 280vac unloaded, but what could be causing that drop when the rest of the supply is hooked in? Can ya'll give me some help debugging this?

[Mick Bailey]

Looks like the voltage is getting half-wave rectified instead of full wave. A rectifier diode may be out.

Edit; Factoring in your higher supply voltage, your B+ is roughly 0.7 what it should be. You can also get this if your 1st node cap is open - RMS vs the peak you get with charging a cap. You have two caps in parallel so this may not be the case, but I'd double check anyhow. You'd expect a lot of hum either way.

[Enzo]

His 400 ohm lifted allows B+ to return to normal. My first impression is something beyond the 400 ohm is loading B+ down, like a bad filter downstream. And of course on this model you have all those bare wire jumpers between the boards, they can break and cause issues.

[Mick Bailey]

Ah yes - so that rules out the 1st node caps. You'd think that pulling the B+ down by that much would get something pretty hot.

Best to pull the output tubes to see if excessive screen current is the problem.

[cooldude666]

pulled the power tubes, voltage shoots up to about 380V. Is this indicative of excessive screen current being drawn?

[daz]

And of course on this model you have all those bare wire jumpers between the boards, they can break and cause issues.

Man o man, thats the truth ! I had 2 C30's and those jumpers fractured and caused many issues from no sound to you name it many time. Worse design ever. And those jumpers fracturing is uber common, or at least it was with mine. I think if used to gig, the vibration during transport fractures those solid core jumpers in time, and theres probably nearly 100 of em. If used on a single board solid core jumpers are fine. But from one board to the other is a insanely stupid design due to the fact they can vibrate separately causing constant strain. I eventually paralleled a small gauge stranded wire over every one of them to keep mine from breaking down at gigs.

[Chuck H]

Heyo mis amigos. A friend gave me a classic 30 he picked up at a garage sale for $30 to see if I could make worth his investment. It doesn't make any sound other than a very troublesome electrical sounding buzz (not from the speaker mind you- best I can tell is it comes from somewhere near where the power transformer secondaries meet the board. It goes away when they're disconnected and sometimes when I'm taking readings I'm not sure that this is any sort of issue but it's definitely not nice sounding. edit: i typed this before i had replaced the caps- this stopped after replacing the caps.).

My voltages from the rectifier are pretty low. 250s vs the 330s the schematic claims they should be. I took readings from the power transformer secondaries and they look ok. about 280vac and 27vac. Power here is a little higher than 120 normally so they seem about right. I replaced the 47uF caps and the 400R5W from the Plate to the screen on the power supply rail. With that 400R lifted the voltage are more like 380, which makes sense given the 280vac unloaded, but what could be causing that drop when the rest of the supply is hooked in? Can ya'll give me some help debugging this?

Keep the tubes out for now. Do you have filament voltage? It's possible a power tube shorted plate to heater and that has caused a failure with the filament circuit. The shorted power tube would explain the current draw with the tubes in and the failed filament circuit would explain no sound from the speaker.