New X-101-B owner

gtxtom

Active Member
Just picked up an X-101-B off of craigslist and started checking it out. I had been PMing Larry prior to picking it up getting his opinions, but I figured I'd post here. It looks like it has at least been serviced on 2/8/77 as per the yellow tag and it looks like some work has been done, but I'll be doing the usual maintenance/upgrades. I do have a few questions at this point now that it's in my possession.

It's missing the fuse and fuse cap. Does the X-101-B share that with any other Fisher?

The coupling caps are going to be replaced. I read about replacing all the dog turd caps. Are these cylindrical brown mylar caps the dog turds? It looks like one (C34) got replaced at some point, but it's not the right value ...

When installing the CL80 thermistor, do I install it before or after the fuse? Does it matter? It looks much easier to install after the fuse.

The square orange cap(?) - that's bad, correct? It's pretty bulge-y. Anyone know what it is and what values? It reads "1346145 PC50 187-2" on it

Finally, the 5AR4 tube has a chip in the glass. I assume that it's no longer safe to use?

Thanks in advance, guys

IMG_20180215_183435.jpg IMG_20180215_183530.jpg IMG_20180215_193150.jpg IMG_20180215_195123.jpg IMG_20180215_201739.jpg IMG_20180215_202854.jpg
 
Just picked up an X-101-B off of craigslist and started checking it out. I had been PMing Larry prior to picking it up getting his opinions, but I figured I'd post here. It looks like it has at least been serviced on 2/8/77 as per the yellow tag and it looks like some work has been done, but I'll be doing the usual maintenance/upgrades. I do have a few questions at this point now that it's in my possession.

It's missing the fuse and fuse cap. Does the X-101-B share that with any other Fisher?

Virtually all the Fisher instruments's I've seen have the same fuse holder. You can scrounge up a replacement cap or, better, just pick up a similar assembly for cheap and replace what's there.


The coupling caps are going to be replaced. I read about replacing all the dog turd caps. Are these cylindrical brown mylar caps the dog turds? It looks like one (C34) got replaced at some point, but it's not the right value ...

Yeah, that's them. And if you wanna sleep right replace the mylars as well although they are probably OK. I would replace the cap you identify as C-34 for the correct value but that's just me. The existing cap is probably functional if not a bit off spec. Check your schematic to make sure.


When installing the CL80 thermistor, do I install it before or after the fuse? Does it matter? It looks much easier to install after the fuse.

I insert it after the fuse but paying major attention to mounting. You want it solidly placed with room around it so that its heat doesn't unduly affect the surrounding components.

The square orange cap(?) - that's bad, correct? It's pretty bulge-y. Anyone know what it is and what values? It reads "1346145 PC50 187-2" on it

That's not a cap, or rather, it's not a "discreet" cap. It's a "packaged electronic component" or PEC that is a combination of resistors and capacitors. If you look on the schematic it is designated as a broken line box surrounding all the component values involved. They usually don't go bad so I'd leave well enuf alone unless you can trace a fault to it.

I can't tell from the pic re the rectifier so I'll hold any comment.
 
If the rectifier tube has been fractured, you can usually tell by looking for any whitening of the mirroring inside the envelope. Can't see where the damage is, due to blurry, out of focus pic, at least as viewed on my phone. Put it in & try it, most that can happen is it burns up the filament. Just be ready to shut 'er down.
 
If that 5ar4 envelope has a chip in it, DO NOT USE! PERIOD. It WILL fracture on the initial warmup and die on you, and there is the possibility that the glass could implode on you when you put it in or pick it up from the tube socket. Be proactive and get another 5ar4. Don't ask me how I know that!

I can't add anything to what George(NotDigital) has written. I will however advise to get the SMALLEST PHYSICAL SIZED coupling caps for the outputs as they are in the FRONT LEFT CORNER of the chassis (unit upside down and you facing the front). There is very little space and the chassis is VERY SHALLOW so you can't use a long lead and hang them from the sockets. See inside Red Rectangle. Cornell Dublier DME radials are quite small and fit nicely, and they sound nice too. The Green resistor inside the Blue Rectangle Center Left should be replaced with a higher wattage unit. If it's 5w go to 7w or 10W. Disregard the resistor in the Right hand rectangle. I thought it was a FRAKO cap at 1st and couldn't UNDO the rectangle.

In addition to the green resistor, pull all 5 of the ORANGE resistors and check them for value. I can't remember if I replaced mine, but they do have a tendency to run hot and get heat stressed. Look for discoloration near the ends like on the green one. If the smaller ones are 7W bump them up to 10W. R43-44 are 125ohm 7W(1st off the Recifier BUMP TO 10W each), R102-103 are 24ohm 7W(Either side of the speaker switch), R89 is 920ohm 10W(probably the biggest of the 5). R89 I'd bump to 15W if possible. This is the big bias resistor in parallel to the heaters in the bias circuit.
 
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Larry, did you mean to post a picture? I can't see anything. Thanks for the tips. I'm going to be looking for parts today!

Also, on the 7591, what pin do I add the 10 ohm resistor? And what wattage is sufficient for these?
 
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gtx - The 10 ohm goes from pin 5 to ground. There is a ground right below pin 5 on my 500-C. 1/2 watt is what I used. On some units there is a bare wire already there. Just remove it before you insert the resistors. In addition to taking the brunt of a short in the tube, you can use the resistor to measure the voltage generated there which tells you how much current each tube is drawing. Thorne
 
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Also, on the 7591, what pin do I add the 10 ohm resistor? And what wattage is sufficient for these?

I like to use 1/4w 1% metal films in that position. Accurate as hell for measurement purposes and small enuf to go "pop" if a fault develops.
 
Yeah I did. Forgot to add it.

That 1/4w will pop like a small M-80. It's very distinct and you'll know what it is the 1st time one goes. A metal film pops the best. Carbon Film will just melt and stop when it opens.

Here's the pic. Use the instr from Post #4. I added some info about the BIG ORANGE Resistors When you get around to it, post the Serial # please.
X-101-B diagram..JPG
 
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Is there any way to tell if the attached power tube pin 5 image is of a metal or carbon resistor and if it's 1/2 or 1/4 watt? They were installed by a previous owner which is why I don't know. Thorne
 

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The closest I could find for the 920ohm 10W resistor (upgraded to 15W) on mouser.com was a 910ohm 15W. Maybe I'm missing something?
 
It depends on what type of resistor you select. Some types don't come in a 920 ohm value. Try selecting type of "Film Resistors" and then "Metal Film resistors".
 

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You're doing something wrong. See image I attached. What are all the options you're selecting?
 

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Ah OK I see now. If you choose 931 or 910 ohms you get more results. But if you have to have 920 ohms, yeah, mouser is lacking stock. Have you tried digikey.com or radioshack.com?
 
5W and above are going to be WIREWOUND & Thru Hole. No metal film or Carbon films. These are POWER RESISTORS and have to Dissipate anywhere from 50% of their rating up to 80-90% Rating.

You might have to double up on Resistors to get 920. 910 is probably close enough as they have a tolerance of 10-20%. So a 910 is going to raise the bias voltage maybe a volt. You can adjust for that with the bias pot. 10% is 91 ohms so your probably within 1%. Go with the 910ohm 15W and adjust it out withthe pot. 71-CW010910R0JE12HE 910ohm 10W 5% tolerance. This will work. OR this one is a 15W 910. 280-CR15-910-RC Either will work.
 
Wow, I never thought of that, Larry. 2 * 460 ohms in series = 920. What does doubling up like that do to the wattage and tolerances if anything? Thorne
 
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