X-202 Question???

[larryderouin]

Starting back up working on Naptown Rob's X-202, and I ran into a problem.

Take a look at the page below from the sam's. I circled the problem on the schematic as I see it. Look like a connection between the - side and the + side of the Bridge in the PS, powering the tube filaments. Is this correct or did Sams Goof. If someone has the X-202 (not the "B" or "C" model) manual from FISHER, I'd appreciate it. Fisher/yahoo group DOES NOT have this one.

All the grounds are daisy chained around the unit on one wire, including the case of the DUAL 1000uf can cap.(The case is insulated from the chassis, with the neg side tying in to the grounds and also the heaters for V4 and V2.)

I haven't chased down the other 4 heaters yet, but am going back to check.

Larry

 

[Tom Bavis]

No, that's correct, as far as I can see. The plus side of bridge is ground (through 15 Ohm dropping resistor). Minus side is -19V to heaters and bias pot, also minus side of caps. It IS arranged a little differently than I'm used to seeing.

 

[larryderouin]

Thanks Tom. I hand over hand traced it all out and it all jives with the schematic. It just looked funny to me to have the DC sides connected. It works, but it's got a wicked hum on one channel I've tracked down to the 7247's area. But specifically I haven't found it yet. It's been recapped, and new tubes in it. Couple of items missing from it vs the schematic. Doesn't look like they were ever put in. So far 2 resistors and 2 caps between the Balance and the input to the 7247's. both sides. Wonder if they are needed at all as it was running before without them and no indication they were even installed at the factory.

 

[Dadbar]

If it weren't connected, the caps would prevent current flow. Looks like it's a connection on paper for sure.

The -19v off the bridge is a bit weird, though.........9.5 volts across a 12v heater?

 

[larryderouin]

Larry,
You just dont give up, do you Here are some of my before pics, maybe they will help. If you find area of concern I can look to see if I have better shot and maybe email to you for better res

Dave; This thing will probably be the death of me!! :tears: Can you check your's between the balance pot and Pin 7 on V5 and V10. Should be a 47K resistor, .1uf/250V cap, and a 820K resistor(see part #'s above if you have the SAM's). The 820k is between the .1uf and pin7 with one leg to ground. This one doesn't have the cap and 820K resistor. Just wondering if they intentionally left them out or what. No note that some models didn't have them. also can you provide your serial #. Want to compare with this one. If it's earlier than yours then they probably goofed on this one. If later it could be a revision, or a goof.

Larry

 

[larryderouin]

Tonight's ok. Thanks Dave.

Larry

 

[audiodon]

I think -19 would be conservative. It's more likely be -22 to -24.

 

[larryderouin]

I fired it up today with the switch bypassed at the fuse to rule out shielding problems due to hum on V10. NOW NO HUM! So I resoldered the conduit shield to the power switch cover, the cover to the volume pot and the conduit to the clamps(2). Reconnected the switch. STILL NO HUM. HOT DAMN! Still have balance problem (been troubleshooting for 3 or more months off and on with some distortion on the"B" channel.) Disconnected the Dimension control. No difference. Had a flexing intermittent which I finally located. Solder blob contacting the chassis from a terminal strip. Fixed that. I can even out the A-B balance with the balance pot knob pointing @ 9:00. Anything closer to 12:00 and the "B" channel gets very loud.....2-3db difference. And it sounds grainy. It's either the volume pot(which is a weird one with 2 loudness taps, or the balance pot itself which has a tap for the dimension control.)

Perchdog. This one is a 32xxx series, so it's not much older than yours but has been extensively recapped.

I've rotated tubes side to side, no change in grainyness. I'm thining the one side is being over driven. Source is a FM tuner, connected to Tuner or Aux.

Larry

 

[larryderouin]

the 47K's got replaced, after I matched them to under 1% back in March. I couldn't find any hint of the 820K or .1uf caps. I saw where they are on the SAMS, but no joy on this one. I checked everything from the balance pots all the way to the outputs. Nothing. Dave Gillespie says they are part of a high bypass filter. maybe i should go ahead and install them just for the hell of it.

Larry