SHFA1 sounds terrible, one channel low.

This Amplifier has been recapped, most resistors checked and within tolerance, tubes checked and good, one pair of new EH 6973's, (doesn't make a difference swapping them around either) all four transistors replaced with new NTC102's, new volume control, double checked to make sure it was wired in correctly.

Channel 1 has good volume but is really fuzzy and distorted, Channel 2 is very weak and is also fuzzy.
I subbed an iPod for the input to rule out a bad cartridge or Stylus, it sounded pretty much the same.

Speakers seem to sound fine using the test tone position, a nice loud hum both sides, speaker crossover network has been recapped also.

Can those flat integrated Resistor networks on the Treble control go bad and stop the signal from getting through from the pre amp?
A couple of the ones on channel 2 have green corrosion on the lead going into the network and the Treble control seems to have little effect on the sound on either channel, the Bass control works OK however.

I haven't checked the resistance on them yet since I don't want to take them off if I don't need to.

By the way, this amp started out with a dead channel, I think it was channel 2.

BTW, I am running it with the Muting connector unplugged, the contacts are probably out of adjustment and it kills the sound completely if it's plugged in, been that way since I got it, I don't know if that will affect the diagnosis or not?

Tried it both with the 6BJ6 AVC tubes plugged in and without, originally (before capping) I had to remove the AVC tube to get any volume out of the good channel, I also seem to recall it sounding better than it does now so hopefully I didn't wire something in wrong, It'd have to affect both channels if I did.

Eric,
Fix the mech contacts first--or jump the orange wire, on the large mute plug, to ground when testing.
Check power supply voltages and see if something is "off" there--
Ron Rich

Okay Ron, I had a partial success. I plugged in the Mute connector and worked on the contacts until I figured out my mistake.
Contacts ML and MR were closed during play, killing the signal from the pick up, I adjusted them to 1/64 (more or less) while playing and the sound came back, but still only on one channel, (Ch 1)
The good news is that that channel now sounds like it's clear and crisp, Channel 2 is still dead however.

Channel 2 originally had decent volume after I recapped it but it was intermittent, turned out I had broken a 470k resistor between the standoff and the socket for the 12AX7 that's next to the 6973 Output tubes.
I replaced that resistor by partially removing the socket and standoff, I disconnected a half dozen wires and caps in the process, (I should have gone the easy rout and just tacked a new on in on the outside of the standoff) I have double and triple checked the correctness of these connections and everything appears to be right, I even checked them against some Photos I took before I started and everything seems to be in the correct location.

I do think however that the problem has something to do with me replacing that resistor since it was working prior to that though the sound was fuzzy same as Channel 1, apparently the Mute plug being disconnected has some effect on the sound also since Ch 1 now sounds OK?
Is there a Bias voltage supplied by it or something?

If a straight wire with gain is the goal of Amp designers they failed with this one, it's more like a Gordian Knot.

Eric,
Sorry--but I can't "see" what you have done--however, both of the 12AX7 "stand-off" sockets are "the same"--so check your work on the one by looking at the other one--- Ron Rich

Ron, I did compare it to the other side, they are wired the same on the standoffs. I can touch one of the pins on the standoff (either of them) and get a nice hum, wouldn't that indicate at least the outputs are working OK?

I had very little time to work on it last night, hopefully tonight I'll have more.

Now that the Mute problem is solved I'll take some voltages and see what's up.

Eric,
Yes--if you can get "hum" that way, I would assume the output is working--how well? is the question---
Ron Rich

I made a little more progress.
I started at the Transistors and started measuring voltages, all was OK up to V109, the second 12AX7 in Ch 2.
Pin 1 had 15 volts, it was supposed to have 175, resistor R195 was getting a little warm too, pin 6 on the other end of the resistor had 220v of B+ on it..

I don't exactly know what I did to fix it, I was poking around in that area, clipped the cap #C156, tested the tube (OK) but when I put it back together it worked.
Ch 2 has sound, it's lower and more muffled than Ch 1 but the output tubes aren't the same side to side (old & tired RCA's in Ch 2) so that could account for some of it.
It still sounds a little fuzzy but my In Laws were sleeping so I couldn't crank it up to really see how it sounds.

Eric,
I hate when "THAT" happens---There are some that you never know about--I fixed a 35 year old amp the other day that I know had been "worked-on" many times, for the same "problem". I too got "lucky", as I just happened to turn my head just right and see a "spark". There was a jumper installed by the factory that was not soldered--thus being intermittent, and causing a partial/intermittent, failure of one channel, when it lost contact.
Good luck-- Ron Rich

OK, got a chance to mess with this thing again.

First thing I did was hook up a component Turntable with a good Magnetic cartridge to rule out a bad Cartridge or Stylus, while this did result in an improvement in clarity the overall sound was the same.

Channel 1 sounds pretty good, has highs but somewhat lacking on Bass, Channel 2 sounds muffled but has good Bass, the overall effect sounds pretty decent but not as good as I think it should.
Before I spend any more time on it I would like to know is this how this was designed to work or should both channels sound the same?

If this is normal then it's pretty disappointing, especially compared to the 10 years older 100B which shames this box as it stands now.

I have a good Magnavox Stereo Amp with 4 6V6 outputs, it'd take a little re-engineering but I'm seriously considering the swap.

Eric,
Yes--there actually is two amps in that chassis--both are identical, and should sound exactly the same--especially if playing a mono record.
The only problem in changing that to a "home amp" is the mute circuits. I have never seen that done "successfully". BTW, the "speaker test" is just that--a speaker test ! The "noise" you hear is just the AC filament voltage interjected to the speakers, and indicates nothing about the amp functions. Ron Rich

Hi Ron,

I put it back together and let it play for a while last night and it sounds pretty decent.
No doubt some new styli would help, other than that I'm pretty sure that channel 2 is the biggest problem, it's really muffled sounding, I tried an external speaker on both channels with the same results.

I'm going to check those integrated resistor modules on the Treble control, there's some greenish corrosion on some of the leads on the Ch2 side particularly, I know at least in old TV's of that vintage those can cause problems.

Here's a short video of it playing a Mono Ella Fitzgerald record, you can hear it has full sound but it's because one channel has bass the other treble.

http://www.youtube.com/watch?v=PTAC_X8A4go

Eric,
Green grounge is bad--but I have never seen one of those resistor packs fail--yet. Usually when the grounge is present on the pack, there is more of it down on the switch wafer contacts. Once removed, it seems to be OK-- Ron Rich