HP334A Distortion Meter - Potential problem with degrading foam.

[Dr Wobble]

There's been a few threads concerning the problem of degrading foam eating into component leads on a UK vintage radio forum I'm a member of. I found a potential problem for owners of the HP334A distortion meter. Mine wasn't nulling out the fundamental. I found that degraded foam was the cause of one fault.

Whilst trying to trace the fault I opened a black box that contains one lamp, called a diode in the manual and a four photo resistors on a PCB as well as another lamp and single photo resistor. This is part of the automatic rejection filter.  In the lid of the box are two sunken sections which had expanded foam in them. This had degraded and eaten through the lead of one lamp. Luckily it had a tiny stub of the lead, which I was able to clean and solder a single copper wire from a multistrand cable on to, and thus getting it working again.

This has fixed the lamp but the meter has other issue's and will need more work. However if any one has one of these meters, it would be worth checking on the foam, and removing it. I used that black anti static foam that IC's etc come on.

Andy.

Apologies for the quality of the pics.

[Electro Detective]

Many thanks for this post      

Mine has been parked unused for a while, and this is a perfect excuse to 'take it apart'     

Is it just those 2 side screws to undo, so the black box tips up out of the way and clears the board like in the photo?

Just wondering if tossing the old foam and replacing with new stuff, if it might affect any calibration (which still appears ok)


Again, thanks for posting and pics, and hope you sort out yours asap 

It seems my life dabbling in electronics is doomed to suffer ongoing plagues of concealed yellow glue and perished foam..
with the occassional obese capacitors ALWAYS in difficult to reach locations, doing their unsanitary business all over the board   

[SeanB]

Antistatic foam is conductive, not a good thing there. Probably best to grab a brand new 3M green or yellow foam pot scourer and slice off some slices to fit there. You just have to check every year or so and replace as needed. The green seems to be the most stable, I have seen them on a shelf still fine after a decade of non use just in the open.

[Messtechniker]

You just have to check every year or so and replace as needed. The green seems to be the most stable, I have seen them on a shelf still fine after a decade of non use just in the open.

It might be a good idea to stick a label on giving the "Best before date" 
just as a reminder in due course.
All my equipment except battery operated clocks - where I simply don't care,
with those CR 2032 batteries carries such label whereby a service life of 8
years is assumed in most cases. This certainly helps to avoid
unexpected surprises.

[Electro Detective]

What's the exact function of the foam anyway? 

As a temperature stabiliser/equaliser or to keep the components intact,

or both, or neither ? 

[SeanB]

Thermal insulation and absorbing vibration, and here also as a light absorber, as there are light sources and phototransistors.

[Electro Detective]

Thermal insulation and absorbing vibration, and here also as a light absorber, as there are light sources and phototransistors.

Thanks, looks like a fun day ahead going through foam bits stash, to find something similar and stable

---------------------------------

EDIT: did not get that far, upon inspection the original foam bits are still 100% factory condition, see post and pics below

[Electro Detective]

A lucky bench day for me (it had to come one day, lol) 

The original foam thingies are intact and look as though they don't need to be changed... for another 40 years!   
Perhaps they put different foam in later board revisions? or just pot luck on degradation?

These ones look and feel like double sided foam tape,
with one sticky side stuck to the black plastic box, and the other side facing the electronics with the peel away tape left on, hopefully that makes sense   

i.e. I think the most simple solution for those afflicted with a bad batch of foam, is SOFT-ish double sided foam tape and leave ON the peel off layer which will form around the components. I don't think there's a lot of heat there by the looks of it.
The photos may tell the story better 

Since you're in that far  , I would suggest to work back and forth ALL the boards' connection plugs as well, especially if your unit is having issues, before you go sniffing about for phantom problems, because the black tarnish and contact resistance due to age isn't doing you any favours, as you can see on my boards in the pics.
Only takes a few minutes and a good way to spot any loose ones too, and give them a gentle squeeze (with the right tool and PATIENCE) to give better tension.
Please do that with the unit's POWER OFF !!!  or...   

After re-assembly, note in the pictures that this old school beast can measure down to 0.025% distortion with a 20 volt (metered with Fluke 189) audio transformer balanced 1 khz sine wave coming out of a signal generator also the same vintage (early 1970s),
a rough quickie test using a pair of untwisted red and black generic alligator clip leads.
Not bad performance for the 50 year old audio transformer inside the AWA oscillator and may be it's quoted spec, I'll have to check the manual.

With a good spring clean, switch lube and tarnish removal on the HP334A, I reckon with a pure sine wave I can get it down even further to __ Ludicrous Levels __   
with an    ~ ANALOGUE OSCILLOSCOPE ~ plugged into the 334A's output to monitor the waveform as I tweak it to below sanity levels
..that may send even the most hardassed AUDIOPHOOLS running for cover !!!    

(but who can be bothered, hey it works good enough as it is for me, and then some!)