Electronics > Repair

DM501 and FG502 repair. Sort of...

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So, this was supposed to be a post about the repair of a DM501 plug-in. Got one off eBay, dirt cheap, supposedly displays only garbage, but only when the display decides to work... well...
Postman drops the package, unpacking, inspection: no obviously charred components, no bulged caps, no shorts on the power lines, the magic smoke appears to be wherever it's supposed to be. Hmmm... so far, so good. I just got a TM500 extender, so it's time for a live test : power on and...  :wtf:

It works perfectly ! And it's even perfectly calibrated, actually better than my DM501A on the ohms ranges !

Okay. Time for plan B : a FG502 sig gen, also off eBay, sold for parts, "doesn't power on". Visual inspection OK, no damaged parts, no bulged caps, no obvious shorts. Plugs-in and indeed, the power light doesn't turn on. A bit strange, since all the power rails check ok. Hmmm... what's on the output ?

Yep. A nice, perfectly stable sine wave. And the other waveforms are okay too.
Well you guessed it, a blown lightbulb...  :palm:

So much for a nice rainy afternoon spent repairing vintage hardware...

BTW, Mouser stocks the lamp (it's a CM7220). What's the general consensus: should I just change the lightbulb or go for a LED replacement ?

Nice score!  I would go with whichever is cheaper, but I'm a tight wad.  :-/O  I have a few of those plugins you have too.  I have two 7854 mainframes not working at the moment, but I would much prefer the 466 you have! 

One plugin you may want to keep an eye out for is the FG504.  I have two of them and they are pretty sweet.  The power supplies PS503 aren't bad either if they are working. 

Also, it looks like the pull tabs on the 501, 502 and 503 may not be original.  Any idea where you can get replacements?  I have a few that are FUBAR.

No, those are the original tabs that were used on early plugins. For instance the PS503 (aluminum faceplate) got a plain white tab, while the PS503A (grey faceplate) has a grey tab. Same for DC503/503A, etc. Probably someone at Tek realized at some point that the white ones were plain crap...
I haven't been able to find a reliable source for replacements. http://www.sphere.bc.ca/index.html might have some, but I never bothered to ask since S&H would probably amount to an arm and a leg...

I've been eyeing the FG504, as it would spare me the purchase of an RG501, but the current prices for a functional unit are out of my budget.

If you are restoring, the bulb.
If you are upgrading, the LED.

(I like the bulb most, do buy a couple spare just in case.).

David Hess:
I replaced the bulbs on my PS503s and they did not last long as long as they should have but I am planning on replacing them with bulbs again.

I am not surprised about the plug-ins working.  I have often had the same experience.

In your photograph, it looks like your DC503 has a problem with the right most digit.

I have the same plug-ins except for the DM501A.  I am still looking for the exact LEMO part number for the plug on the DM501 and DM502 multimeters which is used for the delta Vbe thermometer probe.  I have one of the probes and it works great with a replacement transistor on the tip.  For most of my meters though I rewired them with a temporary internal transistor as a temperature sensor so they read their internal temperature.


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