Hp 334a Distortion Analyzer help

[Mikeh3]

I'm trying to repair an Hp 334a analyzer. I found a manual online but the manual does not match the unit I have.  My serial number is 1140a05815.
When I turn the unit on, the meter moves to the right (full scale) and then returns to zero on the left side of the meter, this seems to indicate the meter driver is working correctly.
With function switch in "voltmeter" position and a signal applied to the input the meter never indicates anything. Changing meter range switch does not help. Plus & minus 25 v supplies are good. Signal tracing indicates problem is in board A2(impedance converter). The manual online indicates part# 00333-66512 for A2.
My A2 board has part#00333-66508 rev B option 1. Does anyone have a schematic of my board? I've Googled the board # without luck. Any help appreciated.

 

[PRR]

> indicates part# 00333-66512 for A2.
> My A2 board has part# 00333-66508

You may have to figure it out.

Is this a major change or a minor change? What IS the difference? Do you have the same number of transistors? Do you have a large ~~500uFd electrolytic near the output? Is one end of that cap near negative 11 V DC? Is there AC/audio on one side but not the other?

 

[moamps]

You may wish to look for schematics and SM manual here:
http://bama.edebris.com/manuals/hp/334a/
A2 board is  66502, I believe it is identical or very similar to yours.

 

[Mikeh3]

Thank you both for your replies.
You're correct, I will have to spend a little more time without the proper schematic but to have a working unit it will be worth it.
I will check out the other A2 board 66502.
Thanks again for your input.

 

[Mikeh3]

Good news!
Since I didn't have the correct schematic I just started checking the capacitors.
I found a 130 mfd @  16 vdc cap at TP2  that read 3.5 ohms. Scrounging a 100 mfd @ 25 vdc cap and installing for a test, I found the unit works pretty well. Since the unit is 51 years  old I think I will just replace all the  electrolytic caps on this A2 board. I was surprised to see the voltmeter was still in calibration.
I will recalibrate the unit some time in the future.  I'm looking forward to using this instrument on some tube amps I'm planning to build this summer.
Thanks again for the replies.

 

[PRR]

The meter calibration "should" not go out enough to matter. Good resistors gently used don't drift. Good Alnico meters in theory lose force, but apparently not much in a lifetime. Typically what happens is something just goes BAD, no reading at all. If it reads, and nobody has messed with trimmers, it is likely within 5%, which is plenty good. (Your IM number will be a *ratio* of levels, so absolute accuracy is not essential if the ratios are stable.)

 

[Mikeh3]

Sounds like I just need to replace the defective cap with a good quality cap and just use it.
Now to find a good low distortion oscillator.
Thanks PRR for your reply.

 

[Mikeh3]

Update.
I thought I had my HP 334a fixed.  After trying to check some distortion levels on a couple of signal generators, I found the distortion measurements  would not go below 8.5 %  no matter what source I applied to the input.
Using the troubleshooting procedure in the manual it lead me to the rejection amplifier (A3) board.
Since I had already found a shorted cap on the A2 board I thought I would just start checking for shorted caps on the A3 board.  Sure enough I found C9 with 5.1 ohms, installing another cap close to its value solved the problem until I can get the correct values.
Seems strange to find two shorted caps in two different sections.

I plan to let the unit run for a couple of days to see if any other problems pop up. For now the reading are believable.
If you have a problem with one of these units, it may be best and quicker to just may make a quick check of the electrolytics with an  ohm meter first.

 

[PRR]

Those caps were not born yesterday.

I don't know if there is a pattern, but I do know I have found multiple bad e-caps in older H-P gear.

It's like a 40 year old car. Unless you got it from my neighbor, you *know* it will need tires, brakes, maybe steering repairs. (My neighbor does that stuff for fun and profit, won't let a car go until it is safe, but doesn't know H-P stuff.)

 

[JohnRoberts]

Update.
I thought I had my HP 334a fixed.  After trying to check some distortion levels on a couple of signal generators, I found the distortion measurements  would not go below 8.5 %  no matter what source I applied to the input.
Using the troubleshooting procedure in the manual it lead me to the rejection amplifier (A3) board.
Since I had already found a shorted cap on the A2 board I thought I would just start checking for shorted caps on the A3 board.  Sure enough I found C9 with 5.1 ohms, installing another cap close to its value solved the problem until I can get the correct values.
Seems strange to find two shorted caps in two different sections.

I plan to let the unit run for a couple of days to see if any other problems pop up. For now the reading are believable.
If you have a problem with one of these units, it may be best and quicker to just may make a quick check of the electrolytics with an  ohm meter first.

In fact it is not strange at all... Those capacitors may have come from the same production run, so failing close to the same time is more expected than unexpected.

My general advice to people recapping consoles because they have too much time and money on their hands, is to measure path performance and if that identifies an out-of-tolerance cap... replace all of the same value caps in all of the boards. Like cockroaches there is rarely just one.  8)

JR