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Post subject: Radio Craftsmen 500 monoblock Weird Short
Posted: Tue Jan 31, 2017 8:34 pm
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OK Retro, this ones got me buffaloed.
I got in an old mono block amp from the 1950s.
It had a honking 25 watt wirewound resistor cobbed into the rectifier tube to limit current. Obviously a mod
Its not shown in the schematic.
It seems to be a modification to allow the amp to operate(more or less)without blowing fuses.
So I remove the resistor and wire it directly from the 5AR4 rectifier to the 20uF/600 Volt can.
As soon as the rectifier tube starts conducting, the current rises and blows the fuse.
I disconnected all the circuitry from the rectifier and it doesn't blow the fuse.
I connected a new 47/500 to the rectifier output with no load. It blows the fuse when the rectifier warms up.
I connected the transformer wires to another porcelain socket, a new rectifier tube..and a new filter capacitor. It blows the fuse on warmup.
If I remove the capacitor the current is nominal. .42amps AC 120
When I monitor AC current it in runaway mode, it will rise up to 10amps over 5-10 seconds and blows the fuse in the Variac.
It seems to only be a problem when any appreciable current is flowing, then it avalanches.
I suspect a bad PT. Theres really nothing else left in the circuit.
But I've never seen an amp act this way?????


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Post subject: Re: Radio Craftsmen 500 monoblock Weird Short
Posted: Tue Jan 31, 2017 9:00 pm
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Geez, the power tranny is about the only thing left. Have you checked it?

How about any errant wiring shorts within the P/S, especially in and around the recto socket?

And how about that socket -- did you inspect it for any evidence of arcing between the contacts?

Weird!

Arjay

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Post subject: Re: Radio Craftsmen 500 monoblock Weird Short
Posted: Wed Feb 01, 2017 9:42 am
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I know I'm a little wet behind the ears but don't overlook replacing the fuse assembly. Then all that's left is the PT.


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Post subject: Re: Radio Craftsmen 500 monoblock Weird Short
Posted: Thu Feb 02, 2017 12:17 am
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So with the 25-watt (?-ohm) resistor inline and the rectifier and first stage cap intact --- the amp does not blow a fuse?

Does the amp blow a fuse with no rectifier in the socket? If it does... it's your PT (provided the fuse is before the primary side of the PT).


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Post subject: Re: Radio Craftsmen 500 monoblock Weird Short
Posted: Thu Feb 02, 2017 1:04 am
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We *really* need to see a schematic.

Arjay

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Post subject: Re: Radio Craftsmen 500 monoblock Weird Short
Posted: Thu Feb 02, 2017 2:05 am
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Found this off Tube Asylum site. Seems pretty conventional type of amp --- except for the adjustable cathode bias, in the output stage.

Wonder if this inline resistor in question is the 22k-ohm one in the heater line?

I'd also check those chokes & 8mfd caps in the PSU rail for shorts. Are the capacitors being carried in a multi-section metal cap can? You may need to replace the entire can. 40/40/20/20 should be fine.

Image


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Post subject: Re: Radio Craftsmen 500 monoblock Weird Short
Posted: Thu Feb 02, 2017 2:46 am
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BMW2002Ti wrote:
Seems pretty conventional type of amp --- except for the adjustable cathode bias, in the output stage.

Wonder if this inline resistor in question is the 22k-ohm one in the heater line?


That is a weird cathode-biasing configuration.

I'm wondering if there's an issue with that center tap for the heater line. If that .22 mfd cap was shot and the 22 KΩ resistor (or the huge wire-wound type that a previous owner had substituted) was removed it might very well cause the symptoms described.

Arjay

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Post subject: Re: Radio Craftsmen 500 monoblock Weird Short
Posted: Thu Feb 02, 2017 3:20 am
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Why you don't use the 100 bulb lamp in series between amp and wall outlet, this will save fuse.
Yes look the OT, you can test it to be sure.


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Post subject: Re: Radio Craftsmen 500 monoblock Weird Short
Posted: Thu Feb 02, 2017 8:50 am
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Retroverbial wrote:
That is a weird cathode-biasing configuration.

I'm wondering if there's an issue with that center tap for the heater line. If that .22 mfd cap was shot and the 22 KΩ resistor (or the huge wire-wound type that a previous owner had substituted) was removed it might very well cause the symptoms described.

Arjay


What we now need are some photos of the current setup... hint, hint... nudge, nudge. :)

I have seen other older amps with that BALANCE (hum) + BIAS (rheostat) Class "A," cathode-bias arrangement. IIRC, this setup seems to get noisy with time and age. Most ppl replace the rheostat with a fixed resistor.


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Post subject: Re: Radio Craftsmen 500 monoblock Weird Short
Posted: Thu Feb 02, 2017 10:56 am
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Sorry I haven't gotten back to y'all. I had to set aside. It has lowly "customer for eventual sale project" status. I've been tied up with "gig by Friday stuff. I'll get some pics soon.
That schematic is the one I'm working from.
When I set it aside, it had a new socket and tube and filter cap. There is no circuitry connected as a load. The filaments are still connected to the amp.
When you turn it on, the current rises as the rectifier starts conducting after warmup. It will shortly blow the fuse.
If the filter capacitor is removed, the amp idles normally at .4 amps.
The difference is that DC current is flowing when the capacitor is connected. This will cause windings in the transformer to develop a voltage potential between them that doesn't exist when no rectified current is flowing. Or I'm just blowing pink smoke out my a**
In any case, my suspicion is that there is some kind of failure in the transformer. There's nothing else it could be, Right?
I'm used to seeing them go in a puff of smoke or a slow smelly burn.
But I'm gonna do an autopsy on this thing and see if I can find anything out. I may be able to saturate it with laquer and seal the shorts. Or I have a tranny for a Hammond organ that should be a fair replacement.
Thanks for your help here. I'm thought I was missing something obvious to others


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Post subject: Re: Radio Craftsmen 500 monoblock Weird Short
Posted: Thu Feb 02, 2017 11:11 am
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" If the filter capacitor is removed, the amp idles normally at .4 amps. "

I'd say you have a shorted filter cap (or cap can). Check the voltage off the rectifier, when it's running without the PSU rail attached. I'll bet it's within specs. Which means the PT is ok. At least the primary and HV secondary wiring.

It takes a little while to blow the fuse after startup, because of the slow-startup nature of that 5AR4 rectifier. Once the rectifier start conducting, the faulty cap(s) is activated. And starts sucking current. Too much current ---> BOOM! Popped fuse.

That's my WAG. ;)


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Post subject: Re: Radio Craftsmen 500 monoblock Weird Short
Posted: Thu Feb 02, 2017 2:21 pm
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BMW2002Ti wrote:
" If the filter capacitor is removed, the amp idles normally at .4 amps. "

I'd say you have a shorted filter cap (or cap can). Check the voltage off the rectifier, when it's running without the PSU rail attached. I'll bet it's within specs. Which means the PT is ok. At least the primary and HV secondary wiring.

It takes a little while to blow the fuse after startup, because of the slow-startup nature of that 5AR4 rectifier. Once the rectifier start conducting, the faulty cap(s) is activated. And starts sucking current. Too much current ---> BOOM! Popped fuse.

That's my WAG. ;)


I concur. It takes a *lot* to kill a power tranny. By contrast, filter caps have a relatively low mortality threshold. Age and thermal exposure quickly take their toll.

Arjay

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Post subject: Re: Radio Craftsmen 500 monoblock Weird Short
Posted: Fri Feb 03, 2017 7:13 pm
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OK, my week is done and I can git sum pics together.
This is what I’m reduced to. A new socket, a new tube and a new cap. Yes, tried several caps, two different tubes
There are no loads connected to the HV, filaments are still connected to the PT

Image

This first setup shows my current across the fuseholder at .37 amps. The current limiter bulb is dark. The 47/500 is ungrounded

Image
This next photo shows the capacitor grounded with a white clip. The current is in runaway mode and the current limiting lamp is bright
Image
I’m not tryin to fool anyone here. I’m still mystified why the PT doesn’t go up in a shower of sparks.
Well, next week I'll tear it down and see if there's any arcing inside


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Post subject: Re: Radio Craftsmen 500 monoblock Weird Short
Posted: Fri Feb 03, 2017 7:28 pm
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Tim, if the PT & rectifier run fine, by themselves --- would indicate that the PT is okay. I think you may need a resistance load, as well as the cap (a pi-filter). It has something to do with how the rectifier charges with the AC cycling of the PT.

I usually setup the power rail (in situ) with all components as the OEM setup. Just disconnect the B+ to output tranny connection and pull all gain stage, PI, input, etc tubes --- to test the PSU rail for problems. Measure the tap points on the rail. They should have the relative drop in voltage, like the schematic shows. Though the absolute numbers will be higher, due to no OPT and tube loads.

It's the relative drop in voltage at each PSU node that is important. Should drop same manner as you move away from the rectifier.


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