Fisher TA-600 with cracklies and general weirdness

[solidjackson]

I'm having an odd channel problem in my Fisher 600. One side is a little on the weak side and constantly hisses, snaps, crackles, pops when the volume is turned up. It's little audible in on the other side too, but only when I turn it all the way up.

The balance was really off when I got it, but replacing the small german electrolytics on the 12DW7's seems to have cleared that up. There's still a bunch of the things still in the amplifier though. I don't see any burnt out resistors and I've cleaned/lubed all the switches pots and sockets. I'd figure if it was one of the big electrolytics it would be in both channels. I've switched around all the tubes, and that doesn't make any difference either.

I've isolated a problem, though. When I turn the balance over to one side, and turn the volume up, I can hear the bass potentiometer scrape away when I turn it... but only on the left channel! Would that cause the intermittent noise?

 

[Tom Bavis]

A leaky capacitor putting DC on the control? If they coupling caps are original, they could be the problem. A resistor or two may have gone off-value - I often find higher values like 220K have gone even HIGHER (sometimes 2-3 times marked value!). Tubes can be swapped across channels to rule them out...

 

[solidjackson]

No luck. Hmm, looks like it's time to buy some new parts. This set seems to have some subtle changes from the schematic and pictures that I see online. Maybe it was a factory modification for the late models. This one has two orange drops coming off each 12DW7, whereas the ones I have seen online have three. The layout seems somewhat different too.

Also, this Fisher uses a electrolytic underneath with the value of 30/30/10/10. Would a 40/40/20/20 be a suitable replacement?

 

[audiodon]

The physical builds and schematics never line up exactly on Fisher stuff.

 

[dsndblm]

Check the tube lineup against the schematic. My TA-600 does not use any 12DW7s, only 12AX7s in the front end. The 12DW7 contains two dissimilar triodes.

It was a royal pain in the butt but I replaced all the mylar, small electrolytics and power supply electrolytics in my 600. I had lots of popping and hissing before the recap.

Tubesandmore.com will have all the multi section can caps that you need for the TA-600.

Check the grid resistors. My TA-600 schematic calls for 680K. All four of my 680K resistors were WAY off spec.

 

[solidjackson]

Yeah, it's stamped on the chassis, "7247". It's not a tube that I have extras for, so I'm not too happy about that. It also uses a bunch of 12AX7s for the phono/tone circuits.

 

[dsndblm]

Interesting. I didn't know that there were many variations of the TA-600. Do you have an schematic that you can post? It would be nice to see the difference between your model and mine.

 

[amphead4]

Here is a schematic and parts list. The component values are not printed in the schematic so you have to cross reference the parts list. Plus there are no voltages in the schematic but you can get the tube pin voltages from the little chart.