1967 Fender Bassman Question - Power Supply Issues

[keithb7]

I have a stock circuit AB165 1967 Fender Bassman on hand today. I local hobby musician asked if I could look at it. It immediately blows uses when powered up. I thought about the B+ caps so I pulled the dog house to take a look. Earlier last year the owner had paid someone to perform a cap job. They did a poor job. One lead of the two main power caps in parallel was not soldered in place well. It lifted right up and was possibly contacting the dog house cover (ground). I quickly fixed this and powered the amp back up. I used light bulb limiter and it was very bright. I could tell the circuit was still drawing all the amps that the limiter would allow. The 2 amp fuse remained in tact of course, due to the light bulb limiter. I decided to remove the limiter to confirm. I powered it up and pop. The fuse blew immediately again.

I don't think I have the right tools to test a power transformer properly. Is there something I can do with a DMM, to test the PT or a few other tricks to nail the problem down? Owner tells me the amp worked for a couple of minutes then gave up. I suspect the transformer fried when the 450V B+ from the PT went to ground long enough to cook the PT. Does this seem possible? I would rather order a new PT on a hunch. Any tips to confirm?

Thanks. Keith

[The Dude]

First, pull the tubes and see if the limiter is still bright.

[keithb7]

Tubes pulled. Put in a 10A fuse that I have kicking around. Again bright bulb when using the limiter. Removed light bulb limiter and plugged into the wall...Tranny buzzed for 1.5 seconds and the 10A fuse blew.

[The Dude]

Next, I'd check the rectifier diodes for shorts.

[nosaj]

Tubes pulled. Put in a 10A fuse that I have kicking around. Again bright bulb when using the limiter. Removed light bulb limiter and plugged into the wall...Tranny buzzed for 1.5 seconds and the 10A fuse blew.

I'd lift the PT secondaries then put it on light bulb limiter, if it's still bright PT is toast.

nosaj

[keithb7]

Will pull PT secondaries and try again and report back.

1 point I forgot to mention, no pilot lamp at all during testing. Likely indicating we have no AC heater power either.

[olddawg]

Man.. that’s some really rock hammer testing you are doing. If that PT isn’t shorted you are surely stressing the hell out of it. Float the secondary wires and see if your light bulb still glows. NEVER over fuse a power supply. Since you don’t have a variac with an amp meter you need to divide the circuit in smaller pieces. Isolate the transformer secondaries. If it’s still lighting up your bulb limiter it’s probably shorted. If it isn’t and it’s blowing fuses and lighting up the limiter with the tubes out.. check for a shorted rectifier diode... or another short to ground after the secondary.

[The Dude]

There are several ways to skin a cat, but it takes seconds to check for a shorted rectifier. I'd check that before I went to unsoldering all the secondary wires. And, DON'T PUT 10A FUSES IN THERE!

[keithb7]

Ok agreed. I wanted to move further on with testing. Only fuse I had was a 10. Will go buy some more proper 2A fuses before proceeding.

[The Dude]

Not trying to be a "Richard Noggin", but you could have started with a $.03 shorted diode. Then,... power the thing up with a 10A fuse and you short your $150 PT. It's not worth the cost of a fuse.

[Randall]

Damn son, a 10A fuse? Off the bulb limiter to see what would happen? I fail to see the logic here.

[Justin Thomas]

My 67 AB165 came with a 20A fuse. Luckily, that was the ONLY thing wrong with it when I bought it. Now it's the first thing I always check.

Justin

[keithb7]

My apologies. I get it. Its all I had! I'd like to move on from the fuse when you folks are ready.
I'd like to report back with next steps and my findings.

[Randall]

"My apologies. I get it. Its all I had!"

Understood, but this is important. Regardless of what you had on hand, THIS WAS THE WRONG THING TO DO.

Respond when you get why you understand this.

[pdf64]

My apologies. I get it. Its all I had! I'd like to move on from the fuse when you folks are ready.
I'd like to report back with next steps and my findings.

That's fine, we've all done stupid stuff, but you need to learn from this and accept that your thought process (to take gear off the light bulb limiter when it's obviously drawing fault current, then compound things by using a way too big fuse) was totally wrong.
The point of the limiter is to use it until the fault current indication has been resolved.
The pilot light not illuminating may just have been a bad bulb or oxidised contact in the holder.
Not wanting to belittle you, just that in future your keenness to proceed should be tempered somewhat.