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Old 26th July 2011, 04:44 PM   #31
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Iam looking at my schematic, and remembering problems in servicing these. I have read ( but may not remember all that has been said so far...so please excuse).

Questions:
are the 5ar4 tubes new? Both? Same brand? Have a tube tester and the both test good, close sides, no shorts or leakage?

First cap (20 at 450), what is the AC ripple? More than 1% or 4 volts?
Second power cap (20@450) what is is the AC ripple there? millivolts?

Have you checked the preamp tubes for short/leagage? Are any EI or Seimens or RFT?

This hum that appeared after a few weeks ( correct? ), how did it come on...slowly, all at once...is it there as soon as you turn on now or does it take a few minutes to appear?

Have you checked or rechecked C202 in the bias chain? Do you have 48ish volts at the common point for these caps....>>>>>(and these good) the cathode caps C127 and 128, 27 and 28?.

Have you double checked ( power off all caps discharged) all your grounds from one common point?

That you had this appear after some time without any warning, tells me it is not your transformer. You can check your windings ( unsolder the leads) against ground and or other windings if you are in doubt.
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Old 26th July 2011, 04:50 PM   #32
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also...use your didital meter and see if you have the hum hz at the tape out. If you don't have a frequency function, the ac should be near zero if not zero after a few seconds.

If you need to check stages, disconnect the lead out from the loudness and try to input it to a different amp ( it is cap protected at that point so no DC will be sent to the second amp).
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Old 26th July 2011, 05:35 PM   #33
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The HH Scott amps have chassis ground wires looped between terminal strips and through the common connections of the electrolytics. The wires provide a good ground connection that doesn't rely on the terminal strip rivets to provide stable connections to the chassis.
Is it possible that you have broken the ground wire circuit when you disconnected the original electrolytics?
Perhaps one of the riveted ground terminals has developed some resistance to the chassis.

Since you tested the amplifier using a 5U4 tube and observed the same hum, I doubt that the problem is the 5AR4 tube.

Whatever. It must be power supply related since you have hum in both channels.

I'm still a bit concerned that the B+ voltage is low. The old 6BQ5 tubes might be drawing excessive current. You may have a power supply problem that has become audible because one or more of the output tubes is drawing too much current.
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Old 26th July 2011, 08:28 PM   #34
anatech is offline anatech  Canada
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I attempted to post the following last night, but Microsoft deemed I wasn't worthy. I did manage to save my post, so here it is. Please excuse material already covered from then by other members ...

Quote:
Hi Ken,
If your transformer ran for three hours without warming up, I'd be a bit suspicious that the primary wasn't receiving power. All transformers have some energy lost through eddy currents in the core, so expect them to become a little warm when left running with no load on the secondaries. Just a little warm is normal for that test. Also, when running that test, measure the secondaries to the chassis again and write down the voltages please.

If a 6BL8 were to inject noise, I would expect it to be in the form of a 60 cycle sine wave. If you are getting a saw tooth at 120 Hz, that tells me you have a problem in a rectified power supply. I'd be more comfortable if you just followed the clues rather than jumped to conclusions. Let's forget about the 6BL8 for now and follow the clues.

From the looks of the schematic, your heaters are firmly grounded. The small signal tube heaters appear to be supplied by the cathode current from the four output tubes. This is supposed to supply an almost noise-free DC current. Therefore, no playing around with DC heater bias.

Two things come to mind now. Your low plate voltages can be caused by resistors that have gone up in value, or an output stage that is drawing more current than normal. Either can cause low plate voltage, and yours isn't extremely low anyway. Have you checked the resistances of the dropping resistors in the power supply yet? It's also a good idea to measure the voltage drop across each of the resistors and write everything down on paper as you make the measurements.

A bad output tube, either low or high plate current that doesn't match the other of the pair will create a hum on that channel. Make sure the various balance controls are near their center positions to start. Then perform the balancing adjustments per the manual. Once you have finished the various adjustments, listen for hum again.

-Chris
How's that? I put it in a quote to differentiate it from right now.

Hi Frank,
You're saying pretty much what I am - so I think you're on the right track! I still would like to consider that the older dropping resistors have shifted value. Something I see very commonly along with all the others (excepting ww types).

Hi Ken,
I guess luke-warm is ambient then, and your xformer is receiving power just fine. Your equal AC voltages pretty much clear your transformer. Please do check the resistances of your HV dropping resistors, then go back with all the tubes installed and measure your actual voltage drops please. It isn't easy to measure cathode current in this amp, so one idea may be to install 1R0 resistors in series with each output tube cathode, then measure the drops. These will not interfere with normal operation too much and allow you to easily determine what each output tube is conducting. You can also get a really good idea of balance between each pair as well.

-Chris
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Old 26th July 2011, 11:06 PM   #35
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Thanks for all these great suggestions fellas. There's a lot to digest. Please allow me a few days to try them out, then I'll report the results.

btw, a friend brought in a Scott 222D today that needs restoring, so I'm glad you're here to help.

Ken
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Old 27th July 2011, 04:15 PM   #36
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I removed the off topic post about the Pilot SP215 schematic and created a new thread for it..
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Old 18th August 2011, 03:07 PM   #37
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All; The apparent cause of the 120Hz hum was poor grounding. The exisiting ground connections were using light gauge wire, so I beefed up the grounds throughout the chassis and the humming disappeared. This 50 year old amplifier has be operating daily for 2 weeks now, without any issues. Thanks for all your great advice.
--Ken
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