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In Reply to: RE: HH Scott 340 power supply voltages? posted by halftime2020 on October 03, 2014 at 17:33:23
Hi Rick
Welcome to Audio Asylum..The early 340b receivers used a 1k and 750 ohm for R307 and R308.
Lets get you started here..It is very important in the Scott amps,preamps,and receivers put enough filtering on the filament cap supply to alleviate the hum on that line that feeds the preamp tubes and the negative bias supply for the 7591s, I would use a 220uf muse KZ in place of each 75uf@75v caps and you don't want to tie the grounds on those caps to the grounds on the high voltage power supply caps as it can become an issue.
For your radio issue you want to take your ohmmeter and make sure the cathode is grounded on first triode of the 6BQ7 and the second triode must have its control grid grounded...I have seen this four times now where those grounds may be lifted when filter caps are replaced and one of grounded tie points may be lifted...The RF amp is where many problems occur.
Honest amplification is better than excessive 2nd order distortion anytime.
Follow Ups:
Michael,
Thanks for your response!
I inherited this unit probably 15 years ago and packed it away. It was already in really good shape and I just cleaned it up, polished the knobs and face, replace the dial glass, dial background felt, added blue LED lights (I like blue!!), checked voltages, audio signals, ripple, grounds, replaced the power cord, replaced the output tubes and in general made sure it was safe to operate. The FM section was already dead before I even touched it, but I will proceed down the path you indicated.
The p.s. section, I replaced all of the cans except the 75uf/75v because as you already know, that can supplied a negative bias voltage and the can itself was positive (electrolytic). I did purchase another can at 50uf/350vand caps at 100uf/100v and was planning on installing a 100 uf arrangement. There is some ripple on the old 75uf/75v, but not that bad. That being said, that ripple probably contributes to the slight hum heard in my headphones. So, I will proceed with the 220uf caps that you suggested. If I read the schematic properly, the capacitors “+” are tied together and grounded (just like the 75uf/75v can.) I assume that is what you meant by not tying the grounds.....
I will post how I make out.
Rick
The grounds are tied together on the multi-section 75v can but remember you ground the positive lead of each of those caps.I just meant you shouldn't tie those positive grounds to a common point with the negative ground high voltage caps
Honest amplification is better than excessive 2nd order distortion anytime.
Finally updated the SCOTT with the 220uf caps and there is still is some low frequency buzzing (sounds like 120hz, but I have not measured it yet). The buzzing will get louder if the unit is not grounded (3 prong plug).
R307 in fact is 100 ohms and not 1K ohms. The resistor color bands in my unit could not be read because it was burned a bit from the heat generated by R308. It measured 200 ohms. I found an exact version of the 340B early and the resistor color code turned out to be 100 ohms in that working unit. Inserting a 1K 2W dropped the voltage more and proceeded to smoke that 1K resistor. I would need a 10W resistor at 1K. SO I pulled it and inserted a 100 ohm.
The FM section has the familiar FM noise, but does not get a signal and all the voltages seem correct and there is no floating or loose grounds. I believe that there are several tubes that are suspect including the 6U8A.
Anyway, the audio quality is quite remarkable. More later probably after the holidays.
Rick
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