Electronics > Repair

Philips PM5131 function generator repair

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Got lucky and scored three Philips PM5131 function generators for 20€ each.
They were sold as working but "defective".
I called up the seller and he said that the signal was unstable and noisy.
Hmm do i smell crusty switches? Perhaps scratchy pots?
The seller also said that he couldn't be bothered to fix them, which sounded weird to me but to each their own i guess.  :-//
He was selling three of these function generators for 20€ each and at first i thought about only buying one but the deal was too good to let go so i got all three.
I just brought them from the parcel machine and am waiting for them to warm up before i look any deeper into them.
They're actually so cold that all pots except one are very hard to move because the viscous grease is all frozen.
Anyways, while they're warming up i started taking a look at the outside and it looks like they've been banging around for a while.
There's dents on the sheetmetal case and even the BNC sockets are bent.
Found a cal sticker and looks like these came from Germany, last cal date shows 10th month of 2012, so it's been a while.

I don't think there's much to go wrong inside these. I've looked through the manual and it says that the outputs are "short-circuit-proof" so i don't think the will be much to replace inside other than old electrolytics perhaps. I hope there's no RIFA menace inside.
Also took a look at the schematic and read about the working principle of the device. It's completely analog and everything is controlled through the switches and potentiometers so any bad contact will manifest itself in the output, which i'm pretty sure is what's going on with these.

What do you guys think? Will this be an easy fix?
I'm super keen to get these fixed.  :-+

I'd say they would be an easy fix, just a little care and attention. Nice analogue [Edit:sweep] function generators without digital artefacts [and decent output drive and attenuators].

The circuitry is pretty much discrete and the Instruction manual includes full service information and schematics... https://elektrotanya.com/philips_pm5131_sm.pdf/download.html

Definitely worth the money, Philips made some nice stuff in that era.

One of the generators has some missing caps for the switches and the seller said that ironically that's the best one out of the bunch.
And now that i have them i can see why. The other two have very wobbly frequency adjustment knobs, while the one with the missing switch caps is much tighter.
But all three share the same wobbly DC offset potentiometers.
So it looks like the frequency adjustment potentiometers might be worn out or might have taken a hit even.
The frequency potentiometers feel like they're wirewound so i hope they're not completely busted otherwise finding replacements might be tricky.

Looking at the manual, there doesn't look anything special about the pots, they're just panel mount single gang carbon ones (I didn't notice a wirewound at first glance), connected to the PCB by flying leads (same with the BNCs I think). It might even just be loose mounting nuts. At least you have no worries about PCB footprints!

Worst case, you'll probably end up with a good pair and a parts mule.

Edit: Ah yes, the frequency one might be wirewound for linearity but those aren't hard to find either - it's only when you get to multi-gang ones that things get tricky.

The cases on these slide off the chassis much like the Tektronix 22xx scopes,
If severe dents prevent removal, the rivets can be drilled to allow removal, dents repaired and aluminum rivets replaced.


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