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Scott 299B, blown transformer?

V

valvestart

Member
Hello,

I'm trying to make a Scott 299B works but powering it with my variac it reaches 1-2 Amperes with very few volts. Some ceramic caps were alredy changed by the old owner but probably some eletrolytics are not working at 100%. I get the same problem powering it without tubes.
It looks like it gets a short circuit, can it only be a transformer fault or there are other usual causes?

Thanks to All,

Rob
 
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Is the rectifier good?

Can caps changed?

Some experts will probably chime in with more pertinent diagnosis infos (ohming power tansfo but I don't know what to expect fromm a good working transfo, sorry)...

Patrice
 
You could have a shorted bias rectifier (solid state), so I'd be sure this was out of circuit for the test.
 
If it does end up being a transformer, Heybour in Michigan makes quite a few replacements.
(good chance they'll have 1 that's been cloned and upgraded)
 
bktheking said:
Transformers rarely go bad, it happens but not often- i'm with buzzK , check the rectifier first.
Click to expand...

Yeah, I was thinking the same thing. I only hear about blown transformers when somebody makes a major oops. Doubt that's the case hear, but I'm no tech. Tubes amps rely on well-designed transformers, and the old ones had VERY-well-designed transformers (ESPECIALLY that Scott).
 
I'm working on a scott right now, the rectifier is on my list to change. The transformers were indeed decent in most tube gear from back in the day , believe it or not I had a bad one (output) in my 500-C.

Howdy Kegg, long time no chat.
 
Hello,

Thanks for your responses, I substitute the bridge rectfier and I power it up without tube but I got the same problem. I also disconnected the first resistor from the tube rectifier socket, the wires from the transformer to the 6BL8 and 7189 and to the bridge rectfier with no improvements.

I took the test of the link suggested by Patrice and I got the following readings:

- Primary resistance test across power cord plug: 4.2 Ohm as per specifications
- Secondary resistances: 1 ohm across rectfier pin8 and pin2 and 14 Ohm across the bridge rectfier as per specifications but only 13.7 ohm across rectfier pin4 and 6 while is required a value between 40 and 200 ohm.

The resistance of a my working Scott 222B across rectfier pin4 and 6 is about 100 ohm while the other readings are similar.

Would that explain the problem at all? Is it possible to fix it?

Thanks to everyone,

Rob
 
The tube rectifier (5AR4 I guess) is removed isn't it? You said tubes removed but did you mean all of them?

Patrice
 
If you like call me and I'll walk you through how to determine if the transformers are good. If its bad I will give you the part description you need to order a replacement directly from the supplier. Sorry to say I'm just too busy right now to post all the questions and responses to test this....much quicker and easier to just do it on the phone.

810-287-6135
 
Opening the Trafo

Hi,

I opened the transformer and I found it rusty with the paper that holds the wires very dry breaking easily. I attached here some images of it.
That amp came from California, probably the heat and the sea humidity did the damage. At first I just separate the contact between the wires to pin 4 and 6 of rectifier but powering it up the problem remained. Afterwards I cleaned the rusty winding edges with a cutter and used a vacuum cleaner for the remaining dust.
This time it worked! I was very happy to see that I could turn up the voltage knob to 120V with the ampere needle near to zero.

Now I would substitute the wires and do a clean job to hold them on the winding top.
I have no experience in that so any suggestion for fixing and where to buy vintage style wires would be appreciated.

Thanks to all and NOSValves for his disposability.

Rob
 

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valvestart said:
Hi,

I opened the transformer and I found it rusty with the paper that holds the wires very dry breaking easily. I attached here some images of it.
That amp came from California, probably the heat and the sea humidity did the damage. At first I just separate the contact between the wires to pin 4 and 6 of rectifier but powering it up the problem remained. Afterwards I cleaned the rusty winding edges with a cutter and used a vacuum cleaner for the remaining dust.
This time it worked! I was very happy to see that I could turn up the voltage knob to 120V with the ampere needle near to zero.

Now I would substitute the wires and do a clean job to hold them on the winding top.
I have no experience in that so any suggestion for fixing and where to buy vintage style wires would be appreciated.

Thanks to all and NOSValves for his disposability.

Rob
Click to expand...

Looking at the picture - your only option is to dump this thing and buy a new transformer, unless you want ticking bomb in your room and have fire extinguisher ready.

I suspect that output transformers can be in the same poor shape. I would check them too.
 
jaymanaa said:
Does this mean we can throw his ass away? :banana:

Just kidding mang.:D
Click to expand...

How 'bout we thank him for his INdisposability instead? (Well, actually, YOU guys can thank him... I only just joined this thread a minute ago, so I'm a bit out of my element)

Seems kinda' blasphemic to use the word "disposable" when talking to a tube-amp guy. Maybe it's just me... :D
 
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