Help me get this hellish blues Deville out of my shop

[Steelwitch]

Hello! Today I come to the forum with a plea to help me along this truly unenjoyable amp repair experience. The reason it is as bad as it is, is my own fault. While doing some unrelated work on this amp, one of the filter caps shorted to ground through the standoff.. I totally forgot the early Devilles have metal standoffs which can short the board to ground.. duh! Anyway, what makes it worse is that there was a 30A car fuse inside, and the connections for pins 2 and 3 for V5 lit on fire! On top of it all, the ribbon connectors at V1 we’re so brittle and shitty, the connections on the board were compromised.

I have finished handwiring the ribbon connector back, with solid core wire, repairing the damage to V5, but I am not getting any of the low voltage supply. The channel switching is not functional at all and I have no output. All of the B+ supply looks normal, so we can rule that out! I’m starting to think there was damage to the board and is no longer conducting to that part of the circuit. Help! Thank you!

[nickb]

My worry would be that since there was such a heavy load on the power transformer that it might be compromised...

Anyway break out your DVM and check for ACV on the brown lead from the PT to CP17. If that is OK check for DCV on each side of R85 and R86 with respect to OV.

[Steelwitch]

I will check, but I don’t think that’s the case since all of the filters are supplying the correct voltages

[Steelwitch]

Actually you may be right, I checked the low voltage supply and it is 0v

[Steelwitch]

I didn’t want to believe! Thanks Nick B for helping me get this amp out of my life forever

[Steelwitch]

So I replaced the power transformer but I'm still getting no voltage on the low supply, and only 220v coming from the B+. is there something else in the power supply that could be causing this? It seems unlikely to me that another transformer would go right away.

[52 Bill]

What ac voltages are you getting directly from the power transformer?

[Steelwitch]

I’m getting 120vAC on the primary side, and 170vAC on each side of the secondary B+, 25vAC on the low supply... so I guess the rectifier isn’t rectifying?

[Steelwitch]

I just noticed this. Attached is the image. Should there be a connection at CR14? Is it really just missing a diode?

https://imgshare.io/image/NZI2q

[52 Bill]

I just noticed this. Attached is the image. Should there be a connection at CR14? Is it really just missing a diode?

https://imgshare.io/image/NZI2q

On the original transformers, there is no center tap on the HV secondary. They use a full wave bridge to get the final dc voltage.

Normally there are 4 diodes in the high voltage power supply bridge. Your photo shows two and the other two missing with white wires connected to the cut ends of the old diodes. Where do those wires go?

Did this have the original power transformer?

[DrGonz78]

I just noticed this. Attached is the image. Should there be a connection at CR14? Is it really just missing a diode?

Post the schematic so we don't have to go find it please

[shortcircuit]

[TomCarlos]

I am looking at a diagram of the Circuit Board layout for a Blues Deville. I cannot see where these diodes would be located. Steelwitch, can you please take another photo but this time, zoom out just a tad?

[nickb]

On the original transformers, there is no center tap on the HV secondary. They use a full wave bridge to get the final dc voltage.

Normally there are 4 diodes in the high voltage power supply bridge. Your photo shows two and the other two missing with white wires connected to the cut ends of the old diodes. Where do those wires go?

Did this have the original power transformer?

To add to this, the correct transformer does not have a center tap on any winding. This makes me think you might have bought the wrong one. Can you provide a link to it?

[Steelwitch]

On the original transformers, there is no center tap on the HV secondary. They use a full wave bridge to get the final dc voltage.

Normally there are 4 diodes in the high voltage power supply bridge. Your photo shows two and the other two missing with white wires connected to the cut ends of the old diodes. Where do those wires go?

Did this have the original power transformer?

Initially it did until I put in the new one. It has one of those reactive load boxes that's connected to where the rest of the rectifier should be