DM501 and FG502 repair. Sort of...

[sdg]

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... 

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... 

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 ?

[lapoltba]

Nice score!  I would go with whichever is cheaper, but I'm a tight wad.    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.

[sdg]

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.

[ElektroQuark]

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.

[SeanB]

Replace the lamp with another, and add a series resistor to drop the voltage by 10%. Will dim the lamp slightly, but life will be 1000 times longer. I have a pack of 36V lamps I bought for use on a 24V industrial panel, and they last forever on 25VAC. Bad thing is I now have some stuff with 36VAC supply ( yay for China in saving 1c on the cost of copper but make it impossible to get spares) so I now use them at 34VAC, where they do fail every few months.

[kayvee]

The old TM500 stuff is great.  I am scraping a few plug-ins together at present, although it's not that easy, without resorting to international shipping. 

Strangely enough the power supplies seems the hardest to source locally, everyone needs a few, and are hanging on to them.  Seemingly the exotic scope calibration ones are more readily available, at least around these parts.

[David Hess]

The old TM500 stuff is great.  I am scraping a few plug-ins together at present, although it's not that easy, without resorting to international shipping. 

Strangely enough the power supplies seems the hardest to source locally, everyone needs a few, and are hanging on to them.  Seemingly the exotic scope calibration ones are more readily available, at least around these parts.

I have really grown to like the floating and tracking outputs on the PS503 power supplies.  I can see why they remain in demand.  I had the same experience as sgd with the DM501 and DM502 multimeters; they are impressively accurate and low noise.

I make great use of my PG506 "exotic scope calibration" plug-in as pulse generator for transient response testing.  It replaces the PG502/PG503 which I do not have.  At some point I anticipate picking up the companion TG501 to replace my rather balky Nuvistor based Type 184 time mark generator.

Sometimes I even use the PG506 to calibrate oscilloscopes . . .

[lapoltba]

huh, alright.  I happen to have a TG501 and had no idea what I would ever use it for.  Now I know.

[David Hess]

huh, alright.  I happen to have a TG501 and had no idea what I would ever use it for.  Now I know.

The TG501 is for calibrating oscilloscope horizontal timebases.  It is more specialized then the PG506 which is for calibrating oscilloscope vertical amplifiers.

[sdg]

So, as suggested, I replaced the lamp and added another 200 ohm resistor in series with the one already in place. This brings the current down to 20 mA instead of 25, the difference in light intensity is barely noticeable.

And yes, one segment on the LSD of the DC503 is defective, but we're talking unobtainium here...

[David Hess]

So, as suggested, I replaced the lamp and added another 200 ohm resistor in series with the one already in place. This brings the current down to 20 mA instead of 25, the difference in light intensity is barely noticeable.

I need to do this to mine.  Tektronix often graded lamps for high reliability somehow and I wonder if that was the case here.

And yes, one segment on the LSD of the DC503 is defective, but we're talking unobtainium here...

You could ask over on the TekScopes@yahoogroups.com email list.  I think someone made a replacement display board for these.  Broken ones are cheap enough though to be used as parts mules.