HP 211A

[Zenith]

I was looking over things today and noticed something. R29 does not look very healthy. Burnt I'd say. It's a 1.5K, 1W, 1%. I don't see any obvious reason why it would burn unless V5 is drawing excessive cathode current. When I get a chance I'll replace R29 and see what happens. It just so happens that this is the area that controls the symmetry of the square wave.

By the look of the schematic, it should be possible to test R29 and R30, R31 and R32 in circuit. That's no more than a guide, because old resistors can do strange things as they warm up. Check the voltages in that part of the circuit. With a halfway decent DMM with a 10M input resistance, you can check grid voltages directly. It's possible V5 has gone gassy.

[MED6753]

I haven't had the chance to pull R29 but I did measure across it with an ohm meter and it's 1.5K. I measured the voltages and it's -195V which is the PSU voltage and -175V which is the correct cathode voltage for V5. V5 does not appear to be gassy.

When I do get a chance to pull R29 I'll check it's resistance again but it appears it's correct as measured in circuit. I may replace it anyway because I think at one time it was overloaded.

Stay tuned.

[MED6753]

Decided (for now) to leave those black beauties in place. Getting to them is gonna be a bitch unless I pull the entire range switch. So as long as the ranges calibrate properly they will stay in place.



So the re-cap is done. Next step is replace R29 then troubleshoot the symmetry issue. But not today. Other priorities.

[MED6753]

I don't have a 1.5K/1w/1% resistor on hand but I do have an assortment of Chinese resistors that are supposedly 1w/1%. So 1K plus 470 ohm plus 22 ohm and we have 1493 ohms. Close enough.



Pulled the old R29 and as I expected measured 1499 ohms. I know this looks like a bodge but it works. Been powered up for a while and voltages are OK and there's no smoke. So that's a win.

[MED6753]

So started troubleshooting the symmetry issue. First I checked all the resistors in the multivibrator circuit and they are OK. The manual states for gross symmetry defects the issue should be V2 or V3. I changed both with no change. Changed V4 and still no change. I don't have V1 (6BQ7A) and V5 (6C4) in stock so to rule them out I placed an order on the money draining site. Until they arrive this is on hold.

[MED6753]

Question for you smart guys. Is that circuit an Astable, Monostable, or Bistable multivibrator circuit? Depending upon the correct answer it may be functioning properly and the problem lies further down the circuit path.

[25 CPS]

Question for you smart guys. Is that circuit an Astable, Monostable, or Bistable multivibrator circuit? Depending upon the correct answer it may be functioning properly and the problem lies further down the circuit path.

From my quick read of the theory of operation in the manual, it sounds like it's an astable multivibrator that's been wired so that it can be force triggered by the sync input through CR1 when the plate of the second section of V1 goes negative (! - I wasn't kidding about common positive tube circuits messing with me) and causes the grid of V3 to go negative and cut the tube off, forcing V2 on which kicks off the next cycle of the multivibrator. Absent a sync input it freeruns on its own, so it's an astable...that can be armtwisted by the sync input.

[Zenith]

It looks like an astable. I assume the trigger forces it to start at a particular point in the square wave and it free runs until the next trigger. The manual might clear that up.

What are V6 and V7? Presumably the output. What happens if you remove them and see what is happening with a scope?

[MED6753]

If it is indeed an Astable Multivibrator then it definitely has an issue because the output should be a square wave and depending on the frequency selected it's a pulse which becomes more of a square wave at the highest frequencies. I'll wait until I get a new 6C4 and see what happens. I doubt the 6BQ7A in the Schmitt Trigger is the issue but I want to have a spare on hand.

[tggzzz]

What happens if you swap V3 and V4?

[MED6753]

It looks like an astable. I assume the trigger forces it to start at a particular point in the square wave and it free runs until the next trigger. The manual might clear that up.

What are V6 and V7? Presumably the output. What happens if you remove them and see what is happening with a scope?

This is the balance of the circuit. The output of V2/V3 is coupled to V6/V7 thru C17 and C18.

[MED6753]

What happens if you swap V3 and V4?

V2, V3, and V4 were all swapped out with new tubes. No change.

[Zenith]

C17 and C18 are not by any chance the infamous Black Beauties are they? If so, change them.

They are to be compared with Red Hunts and Hunts MoldSeal, which I assume was so called because the mold was sealed in.

I shall continue to follow with interest.

[MED6753]

C17 and C18 are not by any chance the infamous Black Beauties are they? If so, change them.

They are to be compared with Red Hunts and Hunts MoldSeal, which I assume was so called because the mold was sealed in.

I shall continue to follow with interest.

C17 and C18 were these bad boys and they were changed.

[tggzzz]

Have you cleaned the switch contacts?

Have you probed all the gate and anode voltages, looking for differences between the LHS and RHS?