Author Topic: Help with HP 334a  (Read 382 times)

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Offline dave the sound guy

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Help with HP 334a
« on: May 30, 2020, 11:08:41 pm »
Hi all,
I just picked up a clean 334a distortion analyzer from a local guy for a nice price as it did not function.  Upon diagnosis I found the input fuse was open and the selector switch stacks were plagued with intermittent connections.  I also discovered the 1R 3w resistor (R9) between the chassis ground and earth conductor of the power entry was open and cooked well-done, which makes me wonder if a high power amp (or straight line AC) was fed into the input ground post before the instrument was shelved.  After replacing the fuse and resistor, reflowing the wires soldered to the switches and a generous dose of deoxit all around, the analyzer works and behaves as I would expect it to.

I'm now going through the calibration procedure and have hit a bit of a hiccup adjusting bias pot A3R16 and measuring at TP2 on the rejection amp.  The manual calls for -20.2v +/-0.3v at TP2.  On my unit, I am able to get down to -20.1v, but only with A3R16 turned all the way ccw.  Technically it's within spec, but I don't think calibration trim pots should ever have to be pegged one way or the other.  Following the troubleshooting guidance on p. 5-13  I checked C7 (47nf/200v mylar) which measured good at 50nf and 14 ohms ESR, and I tested Q4-Q6 in circuit and found no faults.  The manual also suggests increasing R17 as a solution for insufficient bias adjustment.

Is there any reason I shouldn't go forward with raising R17 to 2.7 or 3.3k to achieve a larger adjustment range?  From my calculations it looks like the change would have minimal impact on impedance/phase of the R17-C8 filter.

Or should I just call -20.1v good enough?

Manual here:
http://www.jamminpower.com/PDF/HP/HP%20333A-334A%2009.68.pdf

Thanks in advance.
DtSG
« Last Edit: May 31, 2020, 12:12:22 am by dave the sound guy »
 

Offline dave the sound guy

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Re: Help with HP 334a
« Reply #1 on: May 31, 2020, 03:43:55 pm »
I didn't notice earlier, but It appears JFET Q4 (manual indicates a TI SFB 2696) was replaced with a Siliconix part with "5-006 7340" on it.  I can't find any data on either the TI or the Siliconix but I suspect the part change is what's throwing the range A3R16 off.
 

Offline fbetti9

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Re: Help with HP 334a
« Reply #2 on: September 22, 2020, 02:15:31 am »
Are you still using your HP 334A? I just spent a few hours refurbishing a unit manufactured around 1972, and I suspect yours is about the same vintage. According to Change #14 in the available Backdating Information, voltage limits and regulation values for A3TP2 were changed from -20.2 V ± 0.3 V to -19.5 V ± 0.5 V on subsequent production runs. Mine still uses A3R37 = 47 kΩ so I had to stick to the older values (-20.2 V ± 0.3 V). Newer production units use A3R37 = 330 Ω and the updated bias information (-19.5 V ± 0.5 V).
I had some serious problems during some bias adjustments until I found out that there were some very leaky electrolytics capacitors upsetting the bias balance in this A3 board. A2 board also had some defective electrolytics but they were basically open so both failure modes (open and short) are expected to be found in such an old piece of equipment. on the other hand, it is surprising that only a few of them have failed, and the remaining still show good (low) ESR as I tested some of them on a sample basis (100% on the A2 board which is the most critical, I think).
If you still interested in such a followup, let me ask a couple of questions regarding the noise level as I am unable to reach anywhere close to the nominal 80 dB fundamental rejection. It looks that the equipment is somewhat plagued by extraneous noise (both mains frequency and high frequency harsh) through unexpected paths. One of them is the shafts (they are concentric) of S2 and S2R8 (Sensitivity and Sensitivity Vernier) which are seemingly purposedly left floating but unlike S1 (Function) and S3 (Meter Range) which have a insulating composite shaft protuding to their knobs, S2 does use a metal shaft which conducts all the way through the wafers of the switch, coupling the noise from your hand directly into where it shouldn't be. This can only be felt during low distortion measurements, of course. I would like to know if your HP 334A also has this drawback.
The second one is even more unexpected. The sliding top cover of my HP 334A is often failing to proper make electrical contact with the chassis, causing rather intense hum (mains, in this case) to appear during low distortion measurements if you touch the conductive the aluminum strip on the front of it. Pressing it agains the chassis may fix the problem for a while, but it is sort of loose so it is often loosing contact again and again. I guess I would have to use some kind of gasket or conductive tape to fix that, but those are hard to find in my country and I wonder if anybody else has ever had such an unexpected problem with a HP cabinet of this vintage.
Any returns will be most appreciated.
Flavio
 

Offline alanep

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Re: Help with HP 334a
« Reply #3 on: October 23, 2020, 05:33:44 am »
Hello Flavio,
I recently acquired a HP334A with a 5/73 mains transformer date code. It had a couple of leaky/shorted Sprague electrolytics (A2C27 - 50uF 3V, A5C2 - 500uF 3V) & also the PCB push on pins were transposed on the A5 board pins 16,17. This put 25V on one of the LDR lamps instead of a maximum of 10V! This has no doubt reduced the lamp life but the HP334A now works well with a residual of about 0.012% distortion (1KHz).

Answering your questions, my unit's Sensitivity & Vernier Sensitivity brass control shaft is isolated/floated from the chassis via nylon washers near the switch thread/mounting nut (see photo). This forms part of the +/- 400VDC signal ground to chassis dielectric rating - see the HP334A handbook DC isolation specification. The electrically floating brass switch shaft can carry noise & RF to the switch wafers and couples it straight into the low level circuitry of the impedance converter & the rejection amplifier. I'm about 4km away from a 100KW AM broadcast transmitter so moving my hand in the vicinity of the Sensitivity controls induces all sorts of problems. My solution was to AC clamp the switch shaft to signal ground via a 100nF 630V capacitor. The capacitor also isolates the shaft from a +/- 400VDC floating signal ground whilst still meeting the spec. I installed the 100nF 630V cap between a new solder lug mounted on the wafer switch assembly & the signal ground (see photo). Mind you, I won't be testing the dielectric rating anytime soon by raising the signal ground +/- 400DC above the chassis (death wish) as the HP334A user manual (written in the 60's) doesn't really explain the hazards.

As to the lid, my unit has two screws (see photo) that when tightened, really "pull" the bare metal side of the lid down hard onto the diecast aluminium chassis i.e. a good electrical connection is formed & no hum problems.

I made one other change to the analyser. Whilst it has a 400Hz high pass filter for hum/earth loop reduction, it lacks a low pass filter to remove out of band noise especially with the analyser's 3MHz BW & a 100KW AM transmitter nearby. My solution is to install a switchable passive 80KHz 2nd order low pass filter which still measures distortion up to the 4th harmonic at 20KHz. This only operates in the Set Level & Distortion modes (like the 400Hz filter). Switching the 80KHz filter in & out helps identify any extraneous noise that might be present. If you're interested, I can provide the details.

Alan
 

Offline alanep

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Re: Help with HP 334a
« Reply #4 on: October 23, 2020, 06:06:14 am »
Apologies DtSG, I just realized that I've gone a bit off your original topic re the A3 DC adjustment in answering Flavio. I'll start a separate topic/thread if there's any interest in the 80KHz filter.

Alan
 


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