Fisher soft on one side

[jwrosenthal]

So I had my Fisher 500c rebuilt last year. It is the staple of my living room system, and runs around 15-20 hours a week. When I had it rebuilt, my tech replaced a dozen caps or so, and swapped out the two phono tubes with JJ's as one of the originals was microphonic (he gave me back the stock Tele's). At that time, he said I would eventually need to replace my output tubes but they were fine for now, but didn't mention why.

The other day I was sitting closer to the right speaker than the left, and noticed that the highs were a little soft. I flipped to mono, and balanced back and forth, and the right was still soft. I swapped my speakers with another set I had, thinking I had blown a tweeter, but the other speakers exhibited the same sound. I immediatly thought it was the JJ's and was prepared to replace the 12ax7's in my phono stage again...but last night I was playing FM, and noticed again that the right channel was soft. It's not losing it's output as the volume is the same on both speakers, but the right channel is just lacking the sparkle of the left. It's not horribly noticable unless you get close to the speaker, but it's obvious that something is not right.

Is this a symptom of a failing output tube? I don't have any other 7591 tubes around to swap (they are all the original RCA "The Fisher" tubes), so before I go shopping for vintage tubes, I want to make sure I'm hunting the right problem down.

James R.

[sloober]

Hey- I just finished restoring my 800c. Try swapping the output tubes to see if the problem moves to the other side. Mark them a-d and then swap a&b with c&d. If so, then I suspect your tubes are failing. If you have a tester, see what they show. If you do need new output tubes I recommend the EH 7591a from Electroharmonix. I had to replace all my 7591 tubes and am very happy with the EH tubes. They are a little fatter and taller than originals, but they sound wonderful.

John

[terryfox3]

I just noticed a similar problem to yours.Can you wiggle the phase inverters near the output transformers or the 3 that are v 14 to 16.Anyways the 3 near the tuning knob.I have fixed this problem that way.
I'm going to buy a new quad of eh's myself soon.I replaced my sherwood s-7700's power tubes and got double the power.Really a astonishing difference.The eh do sound good too.
My problem is off and on i get one side a bit low.I can fix it with balance but I'm pretty sure its a tube or pin problem.

[drewbolce]

Don't be so sure that the volume is the same on both sides. I have played with this and by ear you can be way off.

These units almost always have tracking problems with the volume pots. This is because of the volume and on/off combination. Usually it is concentrated in the first one third of the rotation. Also a failing carbon pot can alter the sound without being scratchy.

Here is a fairly simple way to check the volume pot tracking with your computer soundcard and a few cables and adapters.

Get the RMAA free software: http://audio.rightmark.org/download.shtml

Install the software and set it up for your soundcard using a loopback from the line out to line in. This will usually be a 1/8" to 1/8" stereo cable. After you are sure you have it set up correctly hook up your Fisher.

Use the headphone jack for the output. I use a 1/4" to 1/8" adapter and the cable used above for the input. For the soundcard output I use a 1/8" to RCA cable and input to the AUX input. Turn the volume down on the Fisher, turn it on and let it warm up.

In this case we are only interested in the test signal screen. You would have already seen it in the loopback setup procedure. Start cranking up the volume control. I usually turn off the speakers since the test tone is so obnoxious. The headphone output has load resistors so this is permissible.

The test signal screen has both numeric and graphic indicators for each channel. I usually watch the db numeric indicator as I turn the volume up. Be prepared for a surprise. Most of vintage units with this switch arrangement are out 6db or more at low volume.

[terryfox3]

Interesting i never knew that good post!

[jwrosenthal]

Interesting....I have noticed that the volume knob had felt "funny" over the past few months. It clicks on, then is fairly free with a slight feeling of grit until about 9 o'clock, then the resistance it used to have is there (tactile resistance of the knob turning something mechanical).....is this the kind of thing your talking about? I was all prepared to order new tubes (which I may anyway), but condsidering I have to have them re-biased anyway (which I will just allow my tech to do as I have no equipment for that whatsoever), I may have him inspect the volume pot.

James R.