HP339A repair
Dear all,
May I hijack this thread?
I managed to connect the oscilator output of my HP339A to a 12axx7 tube anode, without blocking capacitor... putting 300V to the oscilator output
Yeh, that was not my finest moment ...
Anyways, 1] the oscilator output is dead, and the 2] rejection filter anly null about 20dB, but wont null-out full anymore.
Is it worth trying to repair? Replacing an opamp is no big deal, but should I expect much more work?
Could 1] be related to 2] even if the null-circuit is still OK? In other words: does the nulling circuit need a working oscilator, even when using a outboard sine-generator?
Dear all,
May I hijack this thread?
I managed to connect the oscilator output of my HP339A to a 12axx7 tube anode, without blocking capacitor... putting 300V to the oscilator output
Yeh, that was not my finest moment ...
Anyways, 1] the oscilator output is dead, and the 2] rejection filter anly null about 20dB, but wont null-out full anymore.
Is it worth trying to repair? Replacing an opamp is no big deal, but should I expect much more work?
Could 1] be related to 2] even if the null-circuit is still OK? In other words: does the nulling circuit need a working oscilator, even when using a outboard sine-generator?
Well you might be in lucky if all you did was to blow the tiny inline fuse in the attenuation block of the oscillator. Get a copy of the latest manual if you don't already have one. Locate the fuse and test it. There are also fuses in the analyzer section attenuator as well. The HP339a is supposed to be 300 volt tolerant.
Cheers,
Cheers,
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Didn't you guys just went through hundreds of pages on the Low-distortion Audio-range Oscillator thread upgrading the 339A? Wish someone would make a short list of easily replaceable parts...
I think the unit is perfectly fine for working on tube circuits with limited BW, but obviously not for you rocket scientists working of SOTA gear.Didn't you guys just went through hundreds of pages on the Low-distortion Audio-range Oscillator thread upgrading the 339A? Wish someone would make a short list of easily replaceable parts...
Yes but only because of it's age. In it's day 0.002% was pretty darn good and the oscillator is great.
It takes up too much room on the bench. That's what makes it a boat anchor.
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So far I haven't been able to find the oscilator's output fuse. I cann't find it on the PCB. The two fuses that I did find were still OK.
Meanwhile I have been looking over the schemo a bit, and I am happy to see two diode/zener pairs to ground and a 0.062A fuse on the oscilator's output. In case the zeners started to conduct, hopefully the fuse will blow. May need to replace the output buffer U3.
I am still a bit scared by the complex schematic and the fact that the Notch-auto-Nulling doesn't wotk with a external oscilator.....
PS
About 4 years ago, before the kids arrived, I noted down 0.006% THD with a marker on the 339A front panel...
grtz, Thijs
Meanwhile I have been looking over the schemo a bit, and I am happy to see two diode/zener pairs to ground and a 0.062A fuse on the oscilator's output. In case the zeners started to conduct, hopefully the fuse will blow. May need to replace the output buffer U3.
I am still a bit scared by the complex schematic and the fact that the Notch-auto-Nulling doesn't wotk with a external oscilator.....
PS
About 4 years ago, before the kids arrived, I noted down 0.006% THD with a marker on the 339A front panel...
grtz, Thijs
@tschrama -- the stock 339A, after a cal procedure, will read THD+N of it's own oscillator at about -100dB at 1kHz, or 0.001%. That's good enough for almost any gear, not just tubes. But SOTA type stuff does require more resolution.
I can't promise anything right away, but I'll make a few notes about mods and post here. RNMarsh and Davada did the most work, replacing opamps, etc. They will probably add some thoughts.
I found working on the 339 to be a real PITA. Note that the oscillator section is outstanding on it's own with THD+N well under -110dB. A few changes can add a few dB to that, converging on -120dB, 1 part per million, good by any standard.
I can't promise anything right away, but I'll make a few notes about mods and post here. RNMarsh and Davada did the most work, replacing opamps, etc. They will probably add some thoughts.
I found working on the 339 to be a real PITA. Note that the oscillator section is outstanding on it's own with THD+N well under -110dB. A few changes can add a few dB to that, converging on -120dB, 1 part per million, good by any standard.
Highly appriciated Richiem ! meanwhile I still have to figure out what was damaged in my 339A...
I would really like that also Dick. It think it would beneficial to many.
I have enjoyed your "greening" projects on your web site. Your projects are well documented and easy to understand. Maybe you could add this HP339 project to your web site.
I built your Twin T Notch Filter and now it is an important part of my bench equipment.
I have enjoyed your "greening" projects on your web site. Your projects are well documented and easy to understand. Maybe you could add this HP339 project to your web site.
I built your Twin T Notch Filter and now it is an important part of my bench equipment.
You have input damage. There is a string of diodes clamps to the power rails and a dual gate fet. If the fuse is open it's likely these diodes are shorted. That's what they are there for.
There is also the possibility a resistor is opened as well. Damage to the amplifier is possible but doubtful.
If you can input a signal then you can trace it with a scope and see where it stops or attenuates more than it should. Also look for open ground track.
Thank you so much for all those tips!
Update from last night:
Good news: Found the tiny little fuse on the oscilator output. It is not on the PCB, but point-to-point soldered from a tagboard to the output level switch. It was blown and saved the rest of the oscilator. I bridged the tiny blown fuse and ... Voila a nice sine appeared
Bad news: the Fundamental Rejection Circuit still wont attenuated more than 10dB. On the scope the monitor still shows a large fundamental which is a bit noisy..?
This is bad news, because the rejection circuit is pretty complicated, at least to me, and I doubt I can fix it.
Update from last night:
Good news: Found the tiny little fuse on the oscilator output. It is not on the PCB, but point-to-point soldered from a tagboard to the output level switch. It was blown and saved the rest of the oscilator. I bridged the tiny blown fuse and ... Voila a nice sine appeared
Bad news: the Fundamental Rejection Circuit still wont attenuated more than 10dB. On the scope the monitor still shows a large fundamental which is a bit noisy..?
This is bad news, because the rejection circuit is pretty complicated, at least to me, and I doubt I can fix it.
Those are very good numbers for those make/models and should be very useful for many projects.
have you made any mods to them - which/what?
-RNM
My 334A definitly has a sweet spot were you can get very respectable figures. Tuning the manual-frequency-dial a bit to the low side gives you best THD resolution: down to 0.004% at 10KHz, dominated heavely by 2nd harmonic. No Mods, just finding the sweet spot.
The HP 339A and KH4400 I bought because I got emthousiastic due to the 334A, but never used it that much... I managed to break it soon after I got it.
The HP 339A and KH4400 I bought because I got emthousiastic due to the 334A, but never used it that much... I managed to break it soon after I got it.
@tschrama -- lack of auto tuning in the notch filter, most likely caused by a bad amp or other component. This circuit looks complicated just due to the large number of components, but it is fairly simple. Two opto-resistors (LED and photocell in a sealed package) do the actual tuning adjustments -- they are known to be problematic. Just use a diode tester to check the LED section.
If that's good, then try measuring the resistance of the photocell -- it should be very high if the LED is off. If it's low that's a problem. Conversely, if the LED is OK, then put 1kohm in series with it and run it slowly up on a variable DC supply (make sure polarity is correct) and measure the photocell R -- it should go down as the LED intensity increases. But note other comments above, about the need to check the input circuits to be sure signal is actually getting to the notch filter and the tuning circuitry.
Good luck.
If that's good, then try measuring the resistance of the photocell -- it should be very high if the LED is off. If it's low that's a problem. Conversely, if the LED is OK, then put 1kohm in series with it and run it slowly up on a variable DC supply (make sure polarity is correct) and measure the photocell R -- it should go down as the LED intensity increases. But note other comments above, about the need to check the input circuits to be sure signal is actually getting to the notch filter and the tuning circuitry.
Good luck.
Once more: thanks for the tips! Will try to check out these 2 photocells.
Still working on the unit in very scarce evening hours. Taking out the whole vertical mounted PCB (I think balance and phase error detectors) actually make harly any differende to the Nulling.. -10dB in 3Volt input.. surprissingly lower input gives better Nulling , up to 30dB...?