HP334A Distortion Meter - foam problem.

[Diabolical Artificer]

There's been a few threads concerning the problem of degrading foam eating into component leads on over on the UKVRR forum. I have a HP334A distortion meter that wasn't nulling out the fundamental, degrading 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 forms 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  multistrand cable on to it.

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.

In pic two you can hopefully see a rust like substance on the gold photo resistors. It's probably more like an acidic residue off the foam. I cleaned it odd with meths and my thumbnail, careful not to touch the board with my fingers.

Andy.

[ppppenguin]

Old foam rubber frequently rots and sometimes goes very horrible indeed. Well done for finding the problem and fixing it before it did really serious damage.

Be aware that the black and pink antistatic foams are conductive and might give problems if in contact with high impedance circuitry. I measured the conductivity of a few samples of black foam and they vary widely, over several orders of magnitude.

Look out for foam inserts in some CD cases, usually those with 2 or more discs. Battery compartments in radios sometimes have a foam insert. Also quite a few items have been packed in foam rubber over the years so it's worth checking your stores to see if anything is suffering from the dreaded rot.

[Mark Hennessy]

Yuk - I've had similar problems with anti-static foam, where it's rotted the legs of ICs stored within. As Jeffrey says, well done for spotting it, and for alerting us all

[Refugee]

I have had problems with the anti static stuff.
Jumping from the frying pan into the fire sounds about right.