Freeing siezed vernier on vintage HP oscillator

I just received an early HP 201C oscillator, looks to be a 1957 unit based on the tube date codes. It needs the usual rehab (caps, cleaning, etc.) but otherwise appears to be in solid shape.

One issue is that the vernier shaft is siezed up; it won't turn, and it's stuck in the disengaged position. I tried to saturate it with PB Blaster overnight, but that didn't free it up.

I'm not totally familiar with this mechanism, so I'm not sure which way to try to "persuade" it. It's stuck with the knob flush against the faceplate, and the flange not in contact with the main dial; am I correct in assuming that it should actually be sitting forward of that? I mean, if I remove the main knob and dial, I should be able to tap the back (interior) side of the vernier shaft toward the front to free it up - is that right?
 
A photo would be useful...
Try WD40, leave it 24 hours before trying anything, or a similar penetrating lube. Use the nozzle to reach the hard to reach areas.

Be gentle.
The hard case scenario would be to soak the shaft in kerosene, diesel or brake oil, though brake oil was suggested by a member here, I have not tried that ever...
 
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Remove the knob and metal disc, and see if the shaft will turn by itself.
Maybe the disc is scraping on the front panel.

The tuning capacitor is unlikely to be frozen itself. But a little deoxit could help
if it is frozen. Don't get any spray on the plates of the capacitor.
 
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PRR

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Joined 2003
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Yes, whatever lube they used in the dial-shaft turned to super-glue in 50 years. (Not the cap shaft. Not the dial edge.)

I remember a complete disassembly and a week of soaking in noxious solvents, and even then it did not slide out but had to be forced. No other audio electronics ever fought me like this.

I hope somebody who KNOWS has a better plan.
 
Well, good news and bad news.

The good news is that I was able to free it up by removing the knob, grabbing the shaft with vise grips, and slowly applying as little torque as I could get away with. Which happened to be a lot, but somehow I managed to avoid chewing up the shaft.

Then I discovered that someone had assembled it incorrectly. It should go knob, spring, outer ring, inner ring, but it was knob, outer ring, spring, inner ring. And both rings were seized to the shaft. I found a deep socket just wider than the shaft, set it inside, and started tapping with a screwdriver handle to free it.

Both rings shattered almost immediately. The plastic is super brittle, and unbeknownst to me, the inner ring was molded to a hidden knurled section of the shaft!

I did my best to find all the bits and superglue then back together, but when all was said and done it just didn't work right to turn the main dial, and made a bothersome scraping sound, so I ended up putting it back the way it had been to start.

I don't really need it; if I need the kind of precision that the vernier affords, I'll be using a modern function generator anyway. But at least now I know how the dang thing goes together!
 
Yes, whatever lube they used in the dial-shaft turned to super-glue in 50 years. (Not the cap shaft. Not the dial edge.)

The main dial itself turns just fine, it was just the vernier that was seized. Turned out to be rust, fwiw. No rust anywhere else, though.

It's not a basket case or anything, but this unit has a few ... let's say "challenges". The fuse holder on the front is broken on the inside, and instead of fixing/replacing that, someone added a new fuse holder on the back and rewired the power cord. That one's missing the cap, though. The power switch is cracked. Minor stuff like that.

The only replacement tube is the rectifier, dated 1965.

I discovered I'm out of 100nf caps, so I'm waiting to start repairs for a few more days.
 
Completely recapped the unit last night (save the couple mica caps). The as-built circuit doesn't match the schematic in the manual completely, even after accounting for the backdating changes.

Early units like mine use a 6SJ7 and 6K6 in the oscillator, whereas the schematic shows a 6AU6 and 6AQ5. This is accounted for in backdating change #7, however there are some discrepancies:

  • R12 (V1 plate resistor) - schematic says 120k, change 7 says 82k, circuit has 68k.
  • R14 (cathode resistor for V2) - schematic says 680R, change 7 says 1k, circuit has 1.5k.
  • C7 (cathode bypass for V2) is missing from the change 7 schematic.
  • R28 (B+ dropping resistor for V1) - schematic and change 7 say 4.7k, circuit has 10k
  • C12A (filter cap for V1 node) - schematic and change 7 say 20uf, circuit has 40-50uf (measured out at 52uf, replaced with 47uf).

Every time I get into something this old, I'm always amazed at how physically large some of these components used to be. There's an 0.5uf/200V coupling cap that's the size of half a hot dog.

Anyway, I got it fired up and it works! Needs alignment (literally - I need to align the main frequency dial), and the X100 frequency setting doesn't give any output, but it's nice to see that the PT, OT, and tubes are all in good shape.
 
Did yours have wax caps in it? I've got an old Eico that I've been neglecting to recap as its all point to point soldering...

I also have the eico dual trace module for it. Apparently, it can be made into a tube amp if you disable the last section of the trace amplifier. Can't remember where I saw it at, but someone had the modified schematics on how to do it.
 
Did yours have wax caps in it? I've got an old Eico that I've been neglecting to recap as its all point to point soldering...

No wax caps, no. A couple micas, a few electrolytics, and the rest were plastic encapsulated paper.

It's all PTP. And it's a pain to work on because HP built this stuff to last. You can suck every last bit of solder from a joint and still need to get in there and wrestle the leads with pliers to free it up.

I also have the eico dual trace module for it. Apparently, it can be made into a tube amp if you disable the last section of the trace amplifier. Can't remember where I saw it at, but someone had the modified schematics on how to do it.

The 201C has a push-pull 6K6/6V6 power amp capable of 3W into 600R. At least one person has modified theirs to be a monoblock. Mine is definitely out of spec on the power amp; the manual says maximum 50V into an open load, but I'm seeing over 75V. It's biased pretty cold, too - the schematic calls for an oddly mismatched 15V/19V on the power tube cathodes, but I'm seeing 23V. Just another bit of oddness to explore.

Here are the ironically named "Tiny Chief" coupling cap from the oscillator section - weighs close to two ounces! - and a 100nF 600V coupling cap from the power amp.
 

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