Tek FG-501 went VROOP!  What have I gotten myself INTO? :D - Page 1
Forum section: electronics / repair

author: mrz80
author_rank: Contributor
timestamp: December 01, 2018, 02:47:36 am
content: So here's the story. I loaned an engineer friend all my Tek gear a while back for a prototype project he was working on. The stuff was duly returned with a casualty: my FG-501 was dead. No noise, no smoke, just no function. My friend was good enough to buy me a replacement FG-501, which I promptly plugged in and tried out. FLASH! BANG! went the replacement, which IS, of course, how I determined that the TM503 chassis was ALSO bad.So I'm working on repairing the FLASH! BANG! module. Carbon comp resistors make awesome flares when you pass enough current thru 'em!

author: xrunner
author_rank: Super Contributor
timestamp: December 01, 2018, 02:58:27 am
content: Oh dear - very nasty. I have these units and I don't have to tell you how to proceed - one or more of the voltages coming from the chassis is ... not feeling well.Good Luck!

author: duak
author_rank: Super Contributor
timestamp: December 01, 2018, 06:53:32 pm
content: Ouch!What mainframe do you have? You probably know that there are a pair of power transistors for each module in the mainframe that are used by the voltage regulator circuit in each module. These are known to short out and cause this trouble.It's hard to say what will be damaged. With any luck, a tantalum cap shorted out and prevented damage to some of the harder to replace parts. The FG501 should only need +/- 15 V or so to run well enough to test by itself on the bench.I've got a TM504 mainframe with a few modules. It's real handy to have an extender cable to debug a module outside of the mainframe but not essential. I made up one from some debris I had laying around to help repair a couple of the modules. There was a posting here recently from someone who is selling kits to build one.Cheers,

author: mrz80
author_rank: Contributor
timestamp: December 01, 2018, 07:34:35 pm
content: Quote from: duak on December 01, 2018, 06:53:32 pmI have a pair of TM-503s. Fortunately the other chassis is good (as evidenced by the three non-failed modules in that chassis).QuoteOooh, yeah, I know about the transistors on the backplane, but I didn't know they were a known failure point. Looks like time for another Newark order. Thanks for the tip!QuoteI ordered replacements for all the zorched resistors (with some extras in case something else is sideways), along with a small heap of those 2n3904 transistors, just in case.QuoteYeah, I'm in the market for an extender. There was a guy selling some really nice ones on Amazon, but he's apparently dried up and blown away. I'll go looking for likely articles and pointers. That's definitely a "gotta have" for owners of this Tek gear, I'm finding out.Ooh hey, a known-working FG-501 just popped up on eBay. Maybe I'll scarf it up just to have on hand in case I can't get these dead ones on their feet any time soon. Besides, it'd be handy to have a couple working ones for two-tone tests and playing with RF mixer sections.

author: duak
author_rank: Super Contributor
timestamp: December 02, 2018, 02:47:51 am
content: Texaspyro sells extender kits, there's a bit of info at the end of this:https://www.eevblog.com/forum/projects/interest-check-extender-boards-for-tm500-series-modules/msg1972925/#msg1972925I gather Haakon is a Tek serviceman who has built a TM500 test module:http://www.hakanh.com/dl/homemadetm500.htm

author: texaspyro
author_rank: Super Contributor
timestamp: December 02, 2018, 03:24:33 am
content: Of the 100+ TMxxx mainframes that have crossed my path, I don't remember ever replacing a backplane transistor... btw, I seem to remember that they may have weird pinouts so if you replace one, check carefully.The number 1 .. 100 failures are shorted tantalums... oh, so very, very many shorted tantalums. Followed by bad op-amps (usually LM741's) in the module voltage regulator circuits.