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Fisher X-101-B need help!

S

sheeba

New Member
I'm hoping some of you knowledgeable folks can help me while I still have some hair left. I am primarily a guitar amp tech but decided to help out a friend with his stereo. I have recapped, retubed and replaced a few resistors that were out of tolerance. I also replaced the coupling caps on the phase inverter.

I've had some crazy/ frustrating balance issues but I believe the volume pot is to blame. The two portions are not very close in resistance. My questions are these.

1. When I turn off the unit I get a "pop" with a loud surge of audio through the speakers that slowly dissipates.

2. How can I set the A/C balance pots without a distortion analyzer?

Thank you for your help!
 
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I don't know about #1, but from everything I've read about #2, you have to have a distortion analyzer as well as other devices such as a wave analyzer. Sorry. Hopefully someone else has a better answer.
 
Sheeba -- Welcome to the AK Fisher group! I built plenty of guitar amps as a kid and played in a pretty decent band, so I know from where you have come. Servicing stereo/hifi gear involves similar principles, but are often used in different ways, so there will be a learning curve for you. Here's a couple of tests that will help you track down your issues:

1. Feed a mono source into a high level (Aux) inputs of both channels -- but it must be a source that has its own volume control. Turn the source volume full down, and turn on the Fisher and turn its volume control up full. This effectively eliminates the Fisher's volume control from the equation. If there is still unbalanced sound, then the cause is within other circuits of the Fisher. However, if the channels are now equal, but not when the Fisher's volume control is set to its normal operating range, then the control is the cause of the unbalance.

2. Operate the Fisher normally with a source playing and the Fisher's volume control in its normal range as well. Now turn the Fisher off by unplugging it (not by the power switch). If the sound fades away normally, then the loud pop and surge of audio when turning the unit off by the Fisher's AC power switch is caused by the volume control as well.

Finally, look in the "stickies" section for a way to adjust the phase inverter control. It involves a procedure I gave to another AKer privately, who thought it valuable enough to publish for other AKer's to use.

Good luck with your X-101B -- I hope this helps!

Dave
 
I'll try this tonight to confirm imbalance! I have disconnected power at variac and there is no pop or loud audio. It's most certainly the switch. Should I try a cap across the contacts or maybe an X2 across the mains? Thanks!
 
Ok I was able to confirm the Volume pot is causing the balance issue. When turned all the way up the balance is perfect. I was also able to set the AC Balance by scoping each set of power tubes. I adjusted the pot until was able to get a flat line. I repeated this for both sets of power tubes. I retested with a cd player and it sounds incredible. I realize this isn't the accepted way to do it but it made a shocking improvement in the sound. It still has the pop issue but it seems many do. I will instruct the user on the correct power down sequence and the speaker switch. Now I have an X100B and 2 tuners to tackle. Thanks!
 
Glad you've made progress! A cap across the switch will likely help -- but it only acting to cover up the real problem, being that when the volume control is turned to off, the carbon track is likely worn in the off position (particularly given the other problems you've indicated with it), causing the ground terminal of the track to become ungrounded -- allowing the wiper element to pick up any radiated EMF when the switch opens, and the sound to go loud before fading away. Ultimately, the control looks to be the source of all the problems you've been chasing.

Dave
 
Thanks Dave. It's better but not 100%. I still think it could be better. I wish I had a distortion analyzer to balance it properly. The balance issue changes at different volume positions. Its pretty frustrating. I'll give the owner some options. Thanks for the support!
 
The channel imbalance due to volume control tracking issues is a pretty common problem in vintage equipment. Even when new, it was not perfect. That's but one of the reasons that these units all included a balance control. Manufacturers advertised the presence of a balance control as a feature being provided to address any imbalance issues with the early stereo sources (which was in fact an issue at times back in the day), but the control was also present to help make the two channels within the amplifier present an identical gain characteristic as well to compensate for things like slight volume control mis-tracking. Today, it's aggravated by the use of a hot source signal (CD players routinely have a 2.0 volt output signal) that cause the volume control to operate "down in the weeds" where tracking issues are usually most prevalent -- and now made even worse today due to wear in the carbon tracks -- wear promoted by rotating the control every time the unit is turned on or off. As well, over the decades, moisture finds its way into the carbon tracks where they are worn, so that today, volume control tracking issues are often more prevalent than not.

There are new controls available on the auction site that make for a pretty good replacement, but they are not inexpensive. You might check that option out for your owner as well.

Dave
 
Dave I did see the new volume pots. $90. I'll certainly let him know. I was listening last night and I noticed some distortion as things got louder. Is this common with a standard CD player? It does seem like a hot signal. I started thinking maybe I didn't let the amp warm up completely before I did the balancing.
 
Dave can you send me a link for the AC balance procedure you recommend? The one I see in the "stickies is for the 400C. Is this the one?
 
Looking over the 400c instructions it appears this method strictly balances each phase inverter and not the components after that. Would this provide a better result that scoping pin 6 (or Pin 3) of each power tube and adjusting for balance?
 
That procedure was devised to help those with a phase inverter control well out of adjustment, but only possess a DVM. With a dual channel scope however, one strategy that works well would be to create a cathode circuit "buss" and then install 10Ω cathode sampling resistors between each cathode terminal, and the buss. With the resistors installed, you can then attach each channel of the scope to a cathode terminal, and adjust the AC Balance control for equal signal strength at that point. This process then takes into account the characteristics and condition of the output tubes, allowing you to adjust for equal current flow under dynamic conditions. Of course, a distortion test set is still the very best approach when possible, as even the approach I just laid out depends on an OPT primary winding that is perfectly balanced -- which is not always the case. Still, any such imbalance is always small, so that balancing by equal current flow still produces very good results.

Dave
 
Dave - Am I understanding you to say that I can hook up 2 channels of a scope to the cathode pins on a pair of output tubes and use the scope to adjust the AC Balance until the 2 waves on the scope have the same amplitude? I'm unsure what scope measurements should be used.

Or can one use a DVM to measure current coming into the cathode pins? (Both cases require the 10 ohm cathode resistor on the cathode pin) Thorne
 
Technically you can use either. But you also need a generator and load resistor. With the meter you can adjust for minimal difference between the two cathode connections. With a scope you also adjust for equal amplitude, but you can also get an idea of any distortion present.

Dave
 
If the CD source is extremely hot, just pad it down with a voltage divider so the volume control can be operated at a higher setting. That would also allow for more consistent volume when changing sources.
 
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