Finally Time for some Sherwood S8000 Updates

woodhawk1

Montana59457
Happy New Year - and with the new year it is time to dust off the Sherwood S8000. I picked up the receiver back in 2015 with hopes of restoring it over the holidays. I did go in and re-stuff a couple cans with the help of gadget73 and jaymanaa. Thanks again for the guidance.
http://audiokarma.org/forums/index.php?threads/another-sherwood-s8000-rebuild-question.645624/

And then it went on the shelf. Now a few years have passed, and it is time to get back to work.

A few images of the S8000 are attached. Faceplate and chassis are both in good shape (I really love the sleek Sherwood look). I tested the tubes on my Jackson 658A, and all pass (some are marginal). The 12AT7 was dead, and so at present a 12AX7 holds its spot (new tube is on its way).

All switches and bulbs work. Aux is clear and strong in both mono and stereo (sounds great!). FM mono is clear and strong too. FM mx is very weak (barely comes in), but what comes is in stereo. There is a minor hum that becomes audible at about 1/2 way up on the volume, and it increases as I turn up the volume. It is present on all settings (aux, phono, FM).

My goals are to:
1) get FM stereo up and running.
2) find the source of that hum and eliminate it.
3) add 10 ohm ¼ watt resistors so I can measure current on the output tubes. Right now the bias is -17.2 vdc (I don’t think there is any way at present to adjust this – my S5500ii has a pot to adjust all at once).
4) replace resistors and capacitors as needed (or as suggested).
5) add cl-80 thermistor (maybe after I determine the fate of the two 120 uf cans above - space could be an issue).

And so – first things first - later today I am hoping to add the 10 ohm resistors but I must admit the connection points are less than obvious to me at the moment. Based on the recent great rebuild thread by Dave 451 on his S8000 IV and my own experiences with my S5500ii I am guessing pin 3 is the correct connection point for the resistors, but there are wires heading off from pin 3 that cause me to pause (see attachments). One shielded wire (red arrow) heads off to the power transformer. At the opposite end of the output tubes another thin brown wire heads off to where I have yet to determine (perhaps just ground? - two wires are connected to the pin marked by the red arrow – one is connected to pin 3 on the adjacent tube – the other ???). I am also searching for a good spot to bring the resistors to ground. There is little room and no obvious great choices.

Thanks for now!
 

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Why guess? check a tube manual for the pinout of the 7591 tubes. If your amp has fixed bias then the grounded cathode may also be a ground for other components. That means you must create another ground system to accommodate those other parts as well.
 
Right. Pin 3 is the cathode on a 7868. Good point about having to find alternative grounds for the attached components. It could prove interesting given space limitations, but it can be done.
 
Well I did some testing last evening with the multimeter, and some of the voltages are off in some of the "front end" FM circuits (and curiously the 6BS8 tube doesn't "light up" at all and stays cool, although it glows bright and tests "good" in the tube tester). And so, without better test equipment (and a better grasp of the schematic), the best plan is probably to pay a visit to the local HiFi repair shop here in the Twin Cities where more capable hands can have at it. I will update if good/interesting things come to pass...
 
Woodhawk,
I had a similar problem in the front end of my S8000 IV with the front end tube filaments. The heater wires are rather fine gauge with small chokes between tubes and one of these wires was parted. I had to repair it, then the front end tube filaments started to work. Might be worth a look at yours. I took my work further and installed a bias/balance circuit for the output tubes in mine so I could individually bias the 7868's. I also installed ten ohm cathode sensing resistors as well as 100 ohm screen protection resistors. The S8000 series are a bit of a beast to work on, being pretty crowded, but they are well built and engineered and sound very good.

The latest (I hope last) issue is that the neon stereo indicator bulb is going bad and is making noise in the pre-amp (tape/phono) circuit. I have a replacement on order, then I hope everything is in order with this stylish receiver.
 
The 12AT7 should be in the FM multiplexer, so that not having the proper tube can't be helping. The stereo might sort itself once you get that replaced. Looks like half of it runs the neon lamp and the other half is the 19kc pilot tone amp.

The 6BS8 is the RF amp. The whole stage goes dead if that doesn't light. Most RF front ends have the chokes to keep the tuner isolated from the rest of the circuit, and they are a bit fragile. My own S-7000 had a broken solder joint on one of the RF coils under the tuner deck. All the tubes lit, but the tuner was dead unless I pushed on the chassis just so. Not all tuner work is voodoo and black art, broken connections are more of an eyeball and maybe ohm meter diagnosis.
 
Thank you both for the input. I just spent many minutes with a magnifying glass exploring the many thin filaments and all seemed OK (connected at both ends). I am a little concerned about the scrappy looking inductor (0.39 uh) attached to the socket that hosts the 6BS8 (photo attached), although I understand that it may be OK despite its appearance. I again checked the 6BS8 tube and it remains cool to the touch (and dark) even after the receiver has been on for ~20 minutes. The FM mono works well with strong reception (for my basement). Only the FM stereo (mx) is giving me problems.
 

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if you get reception, the RF amp is working. The tuner would go totally dead if you had no heater feed to that socket. Not all tubes get particularly hot.

Wait till you get the 12AT7 replaced and see what that does for the MX performance.
 
Agree--if that tube wasn't working you wouldn't get anything through the tuner. +1 on the 12AT7--you need that tube for the multiplex decoder to work.
 
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