Tektronix TM500 or TM5000 Series Boat Anchors

[Russ-KD4JO]

Hi All - I'm new to the group!!

I was tired of lugging test equipment back and forth from work (and the risk I took having it ripped off or breaking it).
So I am finally getting around to outfitting a pseudo test lab in one corner of the Ham Shack.
Really liked the idea of the old Tek TM series mainframe based plug-ins - especially for audio analysis.
Scoured the web and eBay and found a AA501A/2 distortion analyzer and an SG-502 oscillator.
The AA501A is a fine piece of equipment!! After I cleaned the SG-502's pots with DeOxIt all was OK.  The old Allen Bradley Mod Pots come apart because of their modular nature - so they are easy to clean - they also get dirty easily!! The SG502 is good for most day to day stuff audio tests (about .06% THD) but will not match the 501A's distortion measurement capabilities (.0008% I think) - still looking for an SG-505 (they are scarce and expensive).

So now for the fun..  The TM5000 mainframe I bought had a whisper fan that sounded like a cement mixer - luckily I had an exact replacement.  So the mainframe SEEMED to work - except when I looked at distortion products I saw a very spikey 60 hz, Then I noticed that the MF couldn't handle three plug-ins at the same time without overloading.  Turns out that for this PS, Tek rectifies the incoming AC and them produces a 20 kHz squarewave that they PWM, rectify and filter to get the plus and minus 26 and plus 8 volts "raw" dc for the modules. One of the two 1100 uf / 200VDC can caps that do the initial filtering from the AC line had gone west -  Was able to find 2 - 1200 uf / 250 VDC "snap-in" caps on eBay for $20 - and replaced both (each polarity). That's Fixed!!

The other problem is with an FG-504 I bought ("Powers on - no other tests done - from eBay - BEWARE")
Yup- it put out "functions" square/triangle/sine about -20 dBm - no more. as an added feature - I got a bias of 10 volts DC that the waveform was riding on. Now I have to make myself an extender and deal with that issue - alot more complicated than the PS!!
Caveat Emptor!!!!!

Anyway - long story short - unless the guy (or girl) that sells you this stuff has used it on his bench recently - there can be all sorts of  hidden cans of worms to deal with - but that's the FUN of it - isn't it!!

Russ KD4JO

[miguelvp]

On the FG504 if you could post a source for the connectors you are going to use to make the extender please share.
My mainframe is a TM503 so I can work on parts of my broken 504 from one side but it would be nice to extend it so I can work on the whole unit since this way I'm not having much progress.

Edit: And welcome to the forum!

[miguelvp]

And in case you don't have them:

http://w140.com/tekwiki/wiki/FG504
http://bama.edebris.com/manuals/tek/fg504/

[Russ-KD4JO]

Miguel:

I plan to just get female edge connectors and I may just have a PCB made my Express-PCB for the male side.  Probably just make up a "cable" - not a traditional extender - just for the plus and minus 26 and 8 volts.

Russ KD4JO

[commongrounder]

Check the fuses on the FG-504. That can cause the severe DC offset.  I got parts to make the TM extender cable from Jamma Boards.  I have the complete AA501 dual SG505 test set, and use it every day in my audio business. Oldies, but goodies!

[Russ-KD4JO]

Thanks for the suggestions...
Pretty sure I checked the fuses - but that was before I discovered the bad filter cap in the TM5006!!
Square one again - maybe the FG-504 isn't as messed up as I thought!
Even if you buy "tested" equipment from a reputable vendor - they probably haven't fully tested it under load - as Tek specifies.  If they had - it would have kept cycling...
Now I have to replace an old can type cap with a new "snap in" type - may have to make a new interface base since the pins do not line up.
Russ KD4JO

[miguelvp]

My FG-504 released it's magic smoke because of a faulty transistor on the TM503 mainframe. I believe it was for the +15V rail but I haven't look at it lately and it's been sitting there on my todo list.

[commongrounder]

I mentioned the fuses, because I had one of the fuses blow on my FG-504, but there wasn't any fault.  I think it was probably current inrush fatigue.  I mean, these plugins are only 35 years old!  It is a pleasure to look inside these and see the highest quality components available at the time in use.  I have been able to purchase most of my Tek plugins from sellers that clearly understand what they are and how they operate.  I'm not above buying a "fixer-upper", but I need to be sure there aren't any missing circuit boards or hidden damage.

[MarkL]

On these FG-504s, several times I've had the holders for the output power transistors separate from their mounts, which are screwed to the chassis for heat-sinking.  It looks like they were glued in place and the glue cracked.

It wasn't at all visible this happened until I accidentally touched one.  Ouch!

I used thermal epoxy to glue them back on.

While you're in there servicing other stuff, you might want to take a quick look.

[dom0]

SG-505: I've seen the price people are ready to pay for it and seriously, you can build your own sine generator with same / better specs(*) for less money than that, probably even if you put your work hours on the imaginary bill.


(*): Ultra-low distortion oscillators are limited by two factors, mainly:
- THD caused by active elements in the oscillator itself (i.e. op amps), not really an issue today anymore with the LM4562/LME series and some extremely good discrete designs
- THD caused by the gain/level control. Mainly caused by residual nonlinearities of the controlling JFET (solution: select it!) and ripple in the control voltage (solution: filter more, use more inputs to the rectifier, for example, in a state variable oscillator, rectify the quadrature signal, too. And it's inverted counterparts etc.)

[cncjerry]

Don't bother with the ridiculously priced extenders.  Either pull the top cover and reach in or buy a smaller one slot module and pull the supply.  I do all my debugging with the top pulled off the TM500/5000.

Also, some modules require a high power slot.

[commongrounder]

dom0:
It's true that a custom built low distortion audio oscillator is easier than ever to build, but I have to say there is some value to having the modules work together so well.  You can make backplane connections in the TM modules that eliminate patch cables.  The TS-4353/U audio test set I have, which incorporates the AA501 analyzer, SG505 mod WQ and SG505 Mod WR oscillators, connect to allow easily selected IMD measurements.  The time it would take to get all the user interfaces together would be considerable with a custom build.

cncjerry:
I would never buy one of those $250.00 extender cables when making one is pretty easy, and the wire gauge I used on the ribbon cable can handle the high current modules.  I do prefer to service the module outside the TM unit.  Most of them have access panels on both sides and being able to just flip it over to trace something out is very convenient.

[SoundTech-LG]

Check the fuses on the FG-504. That can cause the severe DC offset.  I got parts to make the TM extender cable from Jamma Boards.  I have the complete AA501 dual SG505 test set, and use it every day in my audio business. Oldies, but goodies!

Yep, make your own cable from a ribbon cable. Cheap, and makes working on the modules far easier, although the modules are not real easy anyway...

When I worked at Atari on the original Pong 7400 logic boards, the edge connectors were these very same connectors. Jobs was on the same shift secluded somewhere in a back room, but I remember him sleuthing around amongst us common idiots on graveyard shift.

[sdg]

You might want to get in touch with Mark Sims (holrum@hotmail.com) from the volt-nuts forum, he has some of his own making for sale, around US$20 apiece plus shipping (prices for the TM500 extenders, the TM5000 version is a bit more)

[commongrounder]

That is a great price for an extender cable fully assembled.  Jammaboard's kit is US $18.00 unassembled, plus shipping.

[soren]

The cheapest connectors I was able to find:

http://www.ebay.com/itm/121716443100
http://www.ebay.com/itm/111733867890