Smoking bridge rectifier in Valve Jr filament PS (link to schematic)

Hi all,

I bought a non-working Valve Jr head cheap. The seller advertised as non-working condition, but the price was still right. I have some PTs, OTs, etc on hand to get this thing up and running regardless.

Overall, it looks stock - I see no evidence of mods or repairs.

For reference, the schematic is here:

VJschematicVer3.gif gif by SomeoneYouKnew | Photobucket

I looked it over and saw no signs of fire or major catastrophes. Inside and out, the condition looked clean and sound. I tested the 12ax7 and 6bq5, and they both tested good. I pulled out the chassis. I pulled the transformer spade connections off the PCB, and I got reasonable resistance on all windings of the PT and OT.

So, I removed the tubes, put a non-inductive 8ohm load on the OT, and turned on the power.

It was on for 10 seconds, and I could see and smell smoke, so I shut it down. The 2A fuse never popped. The smoke was coming off the pc-mount bridge rectifier for the filament DC. That's the part labeled B1 on the schematic linked above.

FYI, the actual part is labeled KBU 10A (KBU=package type, and 10A I assume means 10 amp).

I found a similar part at Mouser: their number = 905-KBU10005. I'll order a few shortly.

My questions: Why the smoke? What might have caused this failure? Nothing else on the board looks bad. Should I replace that 4700uf cap too (C12)? Any other suspects? Anything else I should look at before dropping in a replacement and firing it up again?

Finally, with the PT secondaries still disconnected from the PCB, I checked AC on all the windings - they all read on spec.

Any thoughts on why that bridge rectifier is toast? Any suggestions on other things to check/replace before, during, or after popping in a new one?

Thanks!
 
......... I pulled the transformer spade connections off the PCB, and I got reasonable resistance on all windings of the PT and OT.

So, I removed the tubes, put a non-inductive 8ohm load on the OT, and turned on the power.

It was on for 10 seconds, and I could see and smell smoke, so I shut it down. The 2A fuse never popped. The smoke was coming off the pc-mount bridge rectifier for the filament DC. That's the part labeled B1 on the schematic linked above..................

...............

Finally, with the PT secondaries still disconnected from the PCB, I checked AC on all the windings - they all read on spec.

Any thoughts on why that bridge rectifier is toast? Any suggestions on other things to check/replace before, during, or after popping in a new one?

Thanks!

Is your description of the troubleshooting sequence accurate? It sounds like the bridge rectifier is smoking with all the power disconnected (all PT secondaries disconnected), but that doesn't make sense.

Certainly a bad bridge would be high on my list.
I bought a couple of VJs years ago which were sold as 'dead, for parts' and the fix was pretty obvious - one had a too-tight PT wire that had pulled off the spade, the other had a bad tube. Too bad you weren't as lucky!

I'd go over the PCB with a good light and magnifier and look for solder bridges.
Pulling that rectifier DC cap (as already suggested) would also be on my list.
 
Hi,
If I understood what you explained that with the filament wires disconnected from the board and when power it is ON the bridge still smoked then I agreed with VictoriaGuy that you may have a solder bridge maybe from the high voltage to the filament voltage. If this is the case them read the voltage at the bridge to see if you have voltage at the bridge when power it is ON. You should not have any voltage at the bridge with the filament wire disconnected.
 
Thanks for the replies so far.

Enzo, I read a voltage drop of just under 0.5V across all sides of the bridge when I checked. By the way - I see you're in Lansing. I was born near Traverse City, currently live near Ann Arbor.

Tauro, I removed the 4700uf cap, and checked it for shorts. It reads open / no shorts. I'll check for other shorts in the way you suggested when I next have a chance.

VictoriaGuy, you asked if my description of the troubleshooting sequence was accurate, and whether the PT secondaries were disconnected when the rectifier started smoking. I guess that's two questions. The sequence was described accurately in terms of order, but I did leave out some steps, like the one you are getting at. So yes: before powering up and seeing the smoke, I had re-connected the PT secondaries to the PCB. Sorry for any confusion.

I have a new bridge rectifier on order. I'll post an update when I next have a chance.

Meanwhile all further ideas are welcome. Thanks again.
 
If the bridge got hot enough to smoke, then replace it even if it tests OK. If it were shorted enough to smoke, it would measure so. So that means to me it is having its output loaded by a likely short.

Instead of removing parts one by one, measure across the power supply the bridge makes, is it shorted? Have the tubes removed, or their low resistance will confuse the readings. In other words, measure where the cap was. The cap was not shorted, so leave it there, it won;t hide a short elsewhere.

This circuit is all on one board, right, so there are no connectors in the path of this supply. So look at the tube sockets closely, especially the power tube, it has the heater pins next to each other, any chance they are shorted together? A solder blob or they are bent together? The supply appears to be +6v and ground, so follow that +6 trace from th bridge and cap over to the sockets, looking carefully all along its length, any chance it touches something to either side on the way?