HP 403B rebuild

[joebog1]

So I have started working on the 203B. BADLY corroded switches!! Circuit boards look to be OK. Meter movement ( beautiful thing !!! ) has white traces of water? on it. Ill post as I go so those interested can follow along. I seem to leave some people behind at times. I write atrociously and speak worse, so if I lose somebody and you want to have me explain, just ask.

The pics I think are self explanatory, but maybe a little explanation.

Switch shaft is that!, the scratch mark along its length is my testing the "fungus" for hardness.

Clicker plate is the spring plate and ball bearing assembly for switch ident. Its completely lost in furry corrosion. Switch spacers are also badly corroded.

Meter mounting tabs are bright white and also covered om some furry corrosion. Its much finer texture than the switch parts. I suspect they were Cadmium plated rather than Zinc plated.

[Diabolical Artificer]

Is this some sort of oxidisation due to its plating failing or reacting over time? Is there a way to reverse this with a chemical, EG like using vinegar with battery corrosion?

Andy.

[joebog1]

I have no idea Andy !!

I have been working on gear for more than half a century, but have never seen this type of corrosion before.
Was a stinker here today with humidity over 90%.
I unsoldered the meter and front panel and removed same. More pics and perhaps not so much wisdom tomorrow.

Its very sad to see such superb equipment "ROT" in such a way. I unfroze the switches on the attenuator, but they did not have anywhere near as much corrosion present.
I have been soaking the switches in solvent for several days, but they refuse to even budge so far.
Perhaps they were from " a leaky boat" ?

sorry mods

Joe

[Radio Wrangler]

It seems to have only come from some types of metal and not from others. Some sort of metallic salt formation in hairs rather like dendritic growth. Try dipping a cotton bud or two in different materials and touching a small area. Vinegar is a good start. You can get citric acid as kettle descaler

David

[MrBungle]

Seen this in old heathkit stuff from the 50s. Sure it’s something to do with alloying and migration.

[Julesomega]

The 403B didn't by any chance come with a sticker saying something like "CFM Portsmouth"? I'm just peering into a defunct meter here with exactly the same sort of corrosion to steel parts, must have taken a dip in the briny before being written off in Portsmouth

[joebog1]

No Julian, no stickers or identification of any sort on any of the units. The distortion analyser service manual has "cal lab" hand written by a doctor on top right corner.

Meter movement has Sept 81 sticker on it for calibration date so I know when it was made. Apart from that its not yielding much info at all.

More pics:
Batts?
There are no batteries, but there is this bodge. Two asian capacitors sitting in what I think was the origional battery holder.
on-off switch
shows how bad the corriosion is around the clicker plate
range switch
The range switch is inside a screening can and is not as badly affected and only has mild corrosion.
grunge
A "salty" looking substance that had/has me thinking these units had been swimming. The aluminium screen is anodized and doesnt show any attack, so maybe the anodizing is hard enough to protect it.

Joe

[Diabolical Artificer]

Try soaking the shaft/bearing etc with diesel, better than most penetrating oils and if you can without setting fire to the wafer's and wiring, give it a bit of heat with the diesel on there.

You get a similar bd of a situation with modern pushbike pedals, the aliminium alloy shafts sieze onto the steel thread of the pedals. It's almost as if the ali has grown some crystaline structure ( like yours) that bonds with the steel. It's one of those jobs where knuckles are split and the air is blue. Of coarse this heavy smacking the hell out of a recalcitrant bit of metal with a big hammer approach isn't ok here. A bell is ringing at the back of the noggin, something to do with dissimilar metals.

Your corrosion couldn't be tin whiskers could it? Just a thought.

Andy.

[joebog1]

So I have cleaned up the switches somewhat. Range switch now as smooth a silk.
ON/Off/Batt check switch seized solid !!! PCB,s somebody mentioned were nickel plated, I disagree!! These are gold plated!! so no corrosion..
Pics today are of the switches. Its very difficult to get in and actually polish/rub the switches any better than I have already done. I don't want to remove all the wiring as I will never achieve a job as nice as the original.

I still don't understand the addition of a couple of electro caps, and the removal of the batteries. From the circuit it would seem that the caps add a little lower impedance to the output of the supply. Nowhere the output impedance of a set of charged NiCd's though.

Joe

[Radio Wrangler]

I didn't mean all HP PCBs were nickel plated, just that HP had a period when nickel on copper was the usual process. Gradually gold on nickel on copper replaced this.

I just wanted to warn you that if your boards were nickel surfaced you needed to use an aggressive flux and be sure to clean it off if you wanted to make reliable soldered joints. This was the reason for the shift to the gold process.

Boards were plated up to the intended copper thickness and through holes plated up at the same time, with etch resist on the boards. Then on went a layer of nickel because gold directly on copper isn't good. On goes a thin plate of gold, off comes the resist and the board goes in the etcher. The gold protects where conductor is wanted. There is usually a little under-etching, but the boards are tough, and beautiful. And very nice to work with.

They were a vast improvement on the nickel plate only ones.

I joined HP when nickel was the norm and enjoyed the transition to cu-ni-gold.

From a point of resistivity, the gold is trivially thin, the nickel lets current get down to the copper layer that does the heavy work. At RF the gold is trivial and over a significant range of frequencies, skin effect stops the current getting down into the copper and the resistivity of nickel makes for a slightly lossy board. Needs considering on precision RF stuff only.

David

[dave cox]

Cadmium and Zinc salts will produce different colours in a flame !

I doubt any Cd plating is damaged, Zinc will corrode readily in a salt solution.
Maybe the switches were sprayed with something unsuitable ?

dc

[joebog1]

So a few weeks of spraying every morning, and a tweak with some bull nose pliers
( gently) and voila!!!! I have freed up the switches on all units, i.e. 203B and 403B.

Now to reassemble them and ( holds breath ) apply some electrickery and check power transformers/power supplies. Will keep you posted.

Joe

[Diabolical Artificer]

Well done Joe, slowly slowly catchee monkey.

Andy.