An oldie but a goodie(?)....Tektronix 7L12 SA repair

[RedShoeRider]

Good Evening, All:

Though I realize this is a shot in the dark, but perhaps someone else has run into this one!

I recently acquired a Tek 7L12 spectrum analyzer plugin for cheap in an unknown condition. Besides looking like it was run though a dirt machine, it's in ok condition. Gave it a good cleaning; it looks not-halfway-bad now! Inserting it into the 7903 mainframe I have, it rapidly (1sec on - 1sec off - 1 sec on) cycles. After a few seconds, I shut it off, not wanting to hurt the mainframe or the module.

Starting to dig though it, I found that if I removed the connector P872, located on the rear of the unit, it worked! Well, mostly. It powers up, communicates properly with the mainframe, changes modes, etc. It does not display a spectrum though, as I believe that that connector is the power that runs the IF section.

Being that there is a ton of work to go though to troubleshoot out that whole board, has anyone run into this particular problem before, and do you recall how it was fixed? I know it's an old module, well past it's "normal" lifespan, but if someone has seen this before and can point me in the right direction......terrific!

Thanks a lot
-Red

[tautech]

Not that I have had experience with that module or board, you might like to check for any tag tantalums that have shorted. Have seen it in scopes in the past.

[N2IXK]

Have never worked on a 7L12, but the single biggest problem that I have seen with 7000 series plugins are shorted tantalum caps on power rails. For whatever reason (probably cost), Tek allowed very little margin on some of these caps (16V caps on 15V rails, 6V caps on 5V rails, etc.).

Tantalums are a bit cheaper nowadays, so you should uprate your replacements a bit for reliability.

[RedShoeRider]

I am so glad that I asked here first!

Thank you both for the input on the matter. I was starting my hunt with the electrolytics and largely ignoring the tantalums, as I haven't really dealt much with them in the limited amount of repair work that I've done in the past. I've mostly stuck to stereo equipment repair; this is one of my first forays into test equipment repair. From the error mode, I largely suspected a power cap issue, but now....now I know where to start my hunt, and that makes life so much easier.

I'll let you know who it is when I find the culprit(s).

My thanks!
-Red

[mazurov]

Also, check the plugin compatibility to your mainframe. Some newer plugins draw too much power and 79xx (non-A) MF has weak PS. Both of my 7904 won't work with 7S12 plugin if any of the other 2 slots have a plugin in it.

[JoeyP]

I worked on quite a few of those back in the day. Can't speak to the specific issue you're having, but can tell you that in general all caps in that unit are suspect. That unit can accurately be described as "20 pounds of crap in a 10 pound bag". They run very hot in the mainframe, which makes them very very failure prone (especially electrolytics - not just tantalums). When they work, they work OK, but MTBF could probably be measured in minutes rather then hours. If you get it working, do yourself a favor and find some way of adding a cooling fan.

[RedShoeRider]

"20 pounds in a 10 pound bag".  Funny...I've read that very same sentiment other places. Someone else called the 7L12 a half-baked apple pie that wasn't really cooked until they came out the the L14 years later.

Sounds like I'm in for a recapping adventure (and watching out for the power supply on the mainframe itself!). At least that's not too awful to do, though the engineers must have had a degree in origami to get everything to fit in there like they did. I think I can figure out a way of cooling it if I leave the 3rd bay open, or come in from the bottom. I'm not worried about asthetics; it's all about it just doing its job. I realize this is going to be interesting to see if it even works, but for the 50 bucks I paid for it (and that I don't have a SA)....it's worth some time and headache.

Cheers!