Tubes can talk - Fisher x-100

[oldman55]

That's so cheap you have to wonder if the audio signals are buffered at all. If I was doing this, I would probably just throw together a very simple Darlington emitter follower for each channel and bias it warm enough to handle four 600R loads. The input side could even be DC-coupled if you provide positive and negative supply rails (maybe two wall-warts). I recall doing something like this as a headphone amplifier several years ago with a Sziklai pair instead of a Darlington pair. The Sziklai arrangement has lower distortion, but it's prone to RF instability with some transistors when configured for unity gain. If you can check this with a scope, then go for it, because the stability issue is usually easy to fix.

Hot 'lytics can blow up in your face. Suggest you fix this very soon, one way or another.

Thanks for advice on splitting input but it is all way over my head. But if you think the cheapo splitter will keep my gear safe I may just go that route as I am still using the tape out except for on the Fisher. It is indeed a different animal.
I do know, however, that hot lytics can do substantial damage. I was fortunate on my encounter.
Is there a downside to only stuffing one of the cans and leaving the other as is?
And if anybody knows a source for a stock drop-in replacement please let me know.

 

[gadget73]

Hayseed Hamfest can make you custom rebuild caps if you want to go that route. They don't quite look like the old ones, but they're not bad looking and they can build them to whatever values you need.

 

[BinaryMike]

Thanks for advice on splitting input but it is all way over my head.

Here's a quick tutorial on buffer stages: http://sound.whsites.net/articles/followers.html

 

[oldman55]

Getting a quote from Hamfest to replace the one hot can.

Mike; started reading the article until it zipped over my head but I will continue on.

UPS dropped off my cheapo splitter. First order of business is always to void the warranty/return by cracking it open to see what you got.

Any idea what this thing is doing?
My impression is that its pretty dainty looking in there but giving it a try. Hope lack of video inputs doesnt matter. Whats the worse that could happen? I'll have to anchor it down as the weight of the cables far exceed this thing.

 

[BinaryMike]

All I can say for sure is that it's built with discrete transistors instead of ICs. You might try shorting one of the audio outputs while listening to another, to discover if the outputs are properly isolated from one another. It probably performs best for audio when the video section is left unconnected.

 

[oldman55]

No clicks or pops when other channel shorted.

Review:
Pros: works and only 14.99 to the door
Cons: more wire clutter and it wants to go where the cables came from-weighs only a few ounces

But hasn't passed the test of time.
and it takes up one more plug on the strip. Probably should check how much power I am sucking on the outlet. Have a Killawatt EZ but I never could find out how to turn the beeping off.

 

[oldman55]

Installed new cans from Hayseed Hamfest. But now it has a nasty hum. Triple checked against before photos and schematic. Replaced tubes.
Did replace a few out of tolerance resistors, double checked values.

Adding a signal source quiets it a bit. Tried different inputs also.

It is also completely gone from one channel when balance attenuates it. And when I remove the 7427s. Cans were built exactly like original with correct values in same location as old.

BTW, HV b+ is on the money as well as bias voltage.

Would problem likely be in preamp section?

 

[arts]

[QUOTE="larryderouin, post: 10412030, member: 42316"

Of course smoke from any of them is a definate sign that it needs replacement. You can't put the smoke back in it. It takes a shop with specialized smoke tools to do this.


Larry[/QUOTE]

We work on a lot of British around here,and this is our usual remedy.Also available in negative ground for those who can't ''go with the flow''

 

[oldman55]

In researching the 60 hz hum, I came across a post on this site that referred to checking the rectifier diodes on the x-100. Since my x-100 doesnt have any, was this a mod or maybe there are different submodels to this amp?

Still cant find the cause of the hum but wouldnt it likely not be ground related if it's 60hz?

 

[BinaryMike]

X-100-B and X-100-C both use a silicon diode voltage doubler setup for B+.

So the hum persists in one channel only with volume at minimum or 7247 absent?

 

[oldman55]

X-100-B and X-100-C both use a silicon diode voltage doubler setup for B+.

So the hum persists in one channel only with volume at minimum or 7247 absent?

So I apparently have a x-100-a? Hum is in both channels and increases with volume. No hum w/o the 7247s.

 

[BinaryMike]

Just X-100. Fisher didn't append an A suffix to every new model, like HP did.

For hum diagnosis, try isolating it to the preceding stage by switching to tape monitor and shorting the MON jacks.

 

[larryderouin]

Actually there WAS a X-100-A. It was the last of the X-100 line, built in 1966. It was also known as the X-100-2 S.M. available here....

 

[oldman55]

Thanks for the info Larry, will be interesting to see the difference. Mine most closely matched the x-100 serial nos. 30,000 to 39,999 with a schematic labeled P781 Schematic. It is serial number 12xxx.

In regards to hum, on monitor; no appreciable hum with no source, light hum when rca cables plugged in and Loud hum when shorted.

 

[gadget73]

what are you using for a source? Just noticing a trend that multiple amps you have seem to have hum issues. Curious if its a source issue rather than an amp issue.

 

[BinaryMike]

In regards to hum, on monitor; no appreciable hum with no source, light hum when rca cables plugged in and Loud hum when shorted.

Shorted where? Tuner/Aux or Tape Monitor inputs (when selected), or both? Are you using known-good shorting plugs for this test?

Have you measured AC ripple voltages in the B+ filter chain? Do they reduce dramatically at each stage as they should?

 

[oldman55]

what are you using for a source? Just noticing a trend that multiple amps you have seem to have hum issues. Curious if its a source issue rather than an amp issue.

On the bench just mp3 files off of my phone through a variable gain SS preamp.
Using primarily a dac on my main system. While I have three "in progress" amps with hum issues, I have several others that are dead quiet. I'll go ahead and move this over to the better source and see if anything changes.

BTW, I still havent found a good way to play four amps from one source. The cheap splitter ended up going out on me.

Shorted where? Tuner/Aux or Tape Monitor inputs (when selected), or both? Are you using known-good shorting plugs for this test?
Have you measured AC ripple voltages in the B+ filter chain? Do they reduce dramatically at each stage as they should?

I just manually shorted the tape monitor inputs with monitor switch on. Not sure if I have the ability to measure AC ripple voltages? If its just a matter of measuring the AC, I did that and voltages are almost dead on with 117v input.

 

[BinaryMike]

Just switch your DMM to measure AC voltage. It will then ignore DC voltage on the filter caps. This is the next best thing to a scope for confirming that the B+ filter chain is working as designed.

It's extremely unusual for hum to increase when an input is shorted to ground. This test probably should be repeated with great care to be sure the result is real. If it's real, then I would be very suspicious of bad ground connections.

 

[gadget73]

Try maybe an mp3 player direct into the amp, no extra preamp stage? Half wondering if its some sort of weird ground loop thing. Dunno, just trying to eliminate possible sources of issue. I've chased my tail with things like this before, only to find that it was something dumb and not at all related to the piece I was working on.

 

[oldman55]

Mike, I stand corrected. When input goes to chassis the hum subsides. Touching the inputs while shorting made the noise.
Gadget, yes it was the source. When put on the dac and decent speakers, it puts out clean crisp music. I recently added the preamp to the bench to bring up the line level of my phone and it is likely the cause of the other amp hum I have.

Thanks to all for the help. This one is going to remain in the listening room for now. Amazing how those small opts put out the sound. Hopefully the new cans wont heat up.