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?

[25 CPS]

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.

Come to think of it, it might be worth lifting one side of CR1 or removing it completely for two reasons. One, to test it to make sure it isn't shorted or leaky which would mess with the multivibrator, and two, it would allow you to see if the multivibrator circuit runs freely by itself without anything due to possible CR1 or 6BQ7A issues affecting the multivibrator. 6BQ7A shows up in many HH Scott FM tuner front ends and they've been pretty reliable in my experience. It's the 6U8 next to them that seem to have reliability problems, particularly with H-K shorts introducing hum from the filaments.

[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.

Come to think of it, it might be worth lifting one side of CR1 or removing it completely for two reasons. One, to test it to make sure it isn't shorted or leaky which would mess with the multivibrator, and two, it would allow you to see if the multivibrator circuit runs freely by itself without anything due to possible CR1 or 6BQ7A issues affecting the multivibrator. 6BQ7A shows up in many HH Scott FM tuner front ends and they've been pretty reliable in my experience. It's the 6U8 next to them that seem to have reliability problems, particularly with H-K shorts introducing hum from the filaments.

That's already on my list. The replacement V5 (6C4) should arrive this Friday. I also have some additional capacitors coming from Mouser which should also arrive Friday. Game plan is thusly...

Replace V5. No fix?

Lift CR1 to isolate the trigger. No fix?

Mass replace the capacitors on the range switch including the disc capacitors on the board except for the micas on the range switch. Have you priced mica capacitors lately? Holy shit. No fix?

Throw unit out 3rd floor window and have a beer.

[Zenith]

Mass replace the capacitors on the range switch including the disc capacitors on the board except for the micas on the range switch. Have you priced mica capacitors lately? Holy shit. No fix?

Is the problem the same on all ranges? If so it would be possible all those capacitors have conspired to act up in the same way, but it's unlikely.

Mica capacitors, in my experience, rarely act up, even when they have good reason to. I have a collection I've snapped up over the years. I haven't been forced to dip into it very often.

Throw unit out 3rd floor window and have a beer.

It might hit someone walking past, and then you'd have problems.

I had a Racal RA17, which seemed to be taunting me. I have a 25lb sledge hammer in the garage, a thing that Thor would not be ashamed of. It crossed my mind a couple of times, to take the set round the back and fettle it with the big hammer, making sure the gremlin infesting it got its just deserts. I didn't. Rage subsided and peace and love returned. I recall the problem was a dry soldered joint.

With this it's hard to say without seeing it in the flesh. I have an idea that 6C4 might be acting up. It might be something more obscure, such as crud across a valve base causing problems.

[MED6753]

Mass replace the capacitors on the range switch including the disc capacitors on the board except for the micas on the range switch. Have you priced mica capacitors lately? Holy shit. No fix?

Is the problem the same on all ranges? If so it would be possible all those capacitors have conspired to act up in the same way, but it's unlikely.

Mica capacitors, in my experience, rarely act up, even when they have good reason to. I have a collection I've snapped up over the years. I haven't been forced to dip into it very often.

Throw unit out 3rd floor window and have a beer.

It might hit someone walking past, and then you'd have problems.

I had a Racal RA17, which seemed to be taunting me. I have a 25lb sledge hammer in the garage, a thing that Thor would not be ashamed of. It crossed my mind a couple of times, to take the set round the back and fettle it with the big hammer, making sure the gremlin infesting it got its just deserts. I didn't. Rage subsided and peace and love returned. I recall the problem was a dry soldered joint.

With this it's hard to say without seeing it in the flesh. I have an idea that 6C4 might be acting up. It might be something more obscure, such as crud across a valve base causing problems.

As I go up in frequency the square wave looks better and better to where it's almost 60/40 on the highest range. On the upper ranges the frequency is determined by the mica capacitors and I agree that they rarely cause issues.

[tggzzz]

As I go up in frequency the square wave looks better and better to where it's almost 60/40 on the highest range. On the upper ranges the frequency is determined by the mica capacitors and I agree that they rarely cause issues.

Might that mean the problem is in the half that isn't changing as much as it should?

Or maybe the signal conditioning after the multivibrator shows some asymmetry.