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Old 21st October 2013, 10:05 AM   #21
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Default 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?
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Old 21st October 2013, 02:12 PM   #22
davada is offline davada  Canada
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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,
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Last edited by davada; 21st October 2013 at 02:15 PM.
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Old 21st October 2013, 07:06 PM   #23
RNMarsh is offline RNMarsh  United States
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that fuse in series with the output is close to the output and can be easily seen with the cover(s) off. Might replace the output buffer/driver IC as well. beyond that, you will definitely need a manual and schematic and test equipment to trouble-shoot it.

-RM
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Old 21st October 2013, 07:25 PM   #24
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Great, thanks for the tips! Will post how it turned out.....
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Old 21st October 2013, 07:56 PM   #25
richiem is offline richiem  United States
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It is possible that one of the power supplies is now wonky. Definitely check the plus and minus 15V supplies.
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Old 22nd October 2013, 07:36 AM   #26
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Quote:
Originally Posted by davada View Post

The 339A analyzer is a boat anchor.

Cheers,
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.
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Old 22nd October 2013, 01:22 PM   #27
davada is offline davada  Canada
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Quote:
Originally Posted by jazbo8 View Post
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.
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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Last edited by davada; 22nd October 2013 at 01:25 PM.
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Old 22nd October 2013, 02:55 PM   #28
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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
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Old 22nd October 2013, 08:21 PM   #29
richiem is offline richiem  United States
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@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.
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Old 22nd October 2013, 08:33 PM   #30
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Originally Posted by richiem View Post
@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.
Highly appriciated Richiem ! meanwhile I still have to figure out what was damaged in my 339A...
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