scott 121-C - No tubes and DBT show short

havols

Well-Known Member
Hi AK members
Today my preamp Scott 121-C began to make a noise like shhrhrhrhhrhrrr in the power transformer.
I took out all the tubes and it still made the noise
So I plugged in the Dim bulb Test, and it shows a short, flickering slightly. The power transformer is getting hot quickly.
Am I missing the transformer?
Is there any way to test the transformer?
thanks for help

Regards
 
You probably have a shorted electrolytic in the power supply. Check them all with sa capacitor tester at the rated voltage.
 
tube rectifier? If thats out, its not an HV cap shorted. Possible short in the bias supply if it has one, or in the heater circuit though.

If nothing seems obvious, disconnect all of the secondary wires on the transformer. If it still flunks the dim bulb test, the transformer is almost certainly shorted internally. Thats basically the test.
 
Thanks for reply.
I will check and post results.
I hope that problem isn't in power TAP.....
Regards
 
Bad news
I suspected the problem was in the power transformer
Well. I disconected all the secondary wires (4 heater circuit and 2 B+), and power on with DBT.
The Shorts still. Cheat.
I will need your help now. I never worked with transformer before. All the rest services on amp and preamp tube sections i did before. But power and output taps never.
Well. Is time to work with it.

I think the short is in the secondary section of the heat filaments.
Wish i shoult take out the transformer from the preamp?
Can i open it, to try to find the short problem?
Regards
 
Unlikely that it's the heater winding as they are generally only a single layer, and adjacent turns have only a few tenths of a volt between them. Though the lead-out wires could short together. Primary or HV secondary are more likely since they have overlapping layers with much more voltage between them. And if it's intermittent, you may not be able to find it with an ohmmeter, as it may not short at such a low voltage.

I have the Sams folder for 121B & C, will scan it for you.
 
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Thanks for reply Tom.
It means that i need replace the power transformer?
Thank you for the 121B & C folder. I really need then.
Regards
 
Of course you'll need to take out the transformer from the preamp but if there's an internal short in the windings there's nothing you can do to fix it. As there are no compatible "drop in" transformers currently available on the market it will need to be rebuilt by a transformer winding company. Just send them the old transformer and ask to rewind it according to the following specs (for the 121C):

Primary: 117VAC
Total Power: appx. 35VA
Secondary #1: 510 VAC C.T (255-0-255) @ 0.03A
Secondary #2: 6.3 VAC @ 1.5A
Secondary #3: 25 VAC @ 0.5A

Old power transformers sometimes fails without any specific reasons but I would strongly recommend checking if there are no shorts or defective parts in your preamplifier who might have caused the transformer overload and subsequent failure. If not already done, you must at least replace the low voltage (DC heater) selenium rectifier with a modern sillicon bridge. Aging selenium rectifiers usually have high reverse current which overloads the filter caps and ultimately may cause overheating/destruction of the power transformer. After fitting the new transformer and rectifier it might be necessary to adapt (increase) the value of the (2.7 ohms) series resistors at the bridge output to get the correct voltage (18 VDC) across your tubes heater string.
 
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Thanks Tubologic.
6 months ago i replaced the selenium rectifier by silicon diode bridge.
Two days ago I used this preamp by 2 hours, and sounds fine. Today, when i power on, i listened the noise from power tap.
I really don't know why does it occurs. I will check.
Thank you for help.
 
Thanks for help
I ordered a new power transformer to me.
I think it will take about 30 days to have it on my hand.
regards
 
If your transformer test shows a short, it won't hurt anything to open it open and double check to see if a wire was not pinched against the transformer shield or something like that. It may not fix your tranny (if you are lucky it may) but at least you will have the peace of mind knowing where the fault was.
 
If your transformer test shows a short, it won't hurt anything to open it open and double check to see if a wire was not pinched against the transformer shield or something like that. It may not fix your tranny (if you are lucky it may) but at least you will have the peace of mind knowing where the fault was.
I didi the test... unfortunately the short is internal
Thank you for the idea
Regards
 
Hi guys
I am here again. I buy a new one Power Tap to my scott 121-C
I received the transformer today
I tested, and the voltages is this:
With 116V on the wall, i have:

Primary: 116VAC
Secondary #1: 250-0-250VAC @ 0.03A
Secondary #2: 6.45 VAC @ 1.5A
Secondary #3: 25 VAC @ 0.5A

The secondary #2 goes to 6x4 heaters.
Is there any problem with this high 6.45VAC voltage in the heaters that should receive 6.3VAC?
I imagine in some moments it will work with 6.5 or 6.55 VAC, depending of line oscillation.
Maybe the best voltage is 6.2 or 6.3VAC in heaters of 6x4

Is it safe to operate it with 6.45VAC up to 6.55VAC on heaters of only 6x4 rectifier tube?
Is there any way to reduce this voltage?
Thank you very much.

greetings
 
With no load the voltage will read high. It should drop once you connect it to a tube heater. How many tubes are you powering from that secondary? 1.5 amps isn't very much. I hope there is another transformer or winding to power the other heaters.

Not sure what you mean about powering it without a plug-in connector.
 
Thanks Gadget
I will instal the Power Tap on preamp, and take the voltages.
After, i will post results here

"powering it without a plug-in connector" was my mistake
Sorry

Regards
 
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