Tektronix Circa-1988 492 Spectrum Analyzer Power supply Failure

[Dan Gibbs]

Hello All

Our shop has one of these Tek 492 SA's here at work that was long dead when I started this job two years ago. I'm trying to get it working again for our workshop and I have poured over this post here and there over the last few years when I had some free time. I followed the recommended parts replacements in the post by Gazucha last updated in Oct 2013 found here:
https://www.eevblog.com/forum/projects/tektronix-49xx-spectrum-analyzer-series-power-supply-failure/msg1269906/#msg1269906

Our unit had a blown Q2061, diode CR2065, and mains-fuse along with some aged-looking capacitors. I ordered replacement 2SC4237 transistors from a trusted vendor and mounted them securely and isolated to the heatsink for the originals. I replaced all electrolytics except for the two large 750uF/200V units. I replaced the two small 47u/6v tantalum C5084 and C5085 caps. I have a bag of NOS 6A100 6-amp 1000V diodes and used one to replace the shorted CR2065 diode.

(07/28/17)
I reassembled and powered the full 492 SA on after double-checking everything I could think of. It ran for 3 or 4 minutes and seemed fine before blowing the mains fuse again. It blew the exact same parts as when I found it - Q2061, CR2065, and the mains fuse - no more and no less.

(07/31/17)
I finally ventured forth to power the supply on it's own and temporarily fusing the collector and the base of the replacement transistor for testing purposes - I hate the idea of replacing transistors like fuses if they keep blowing before I can get to anything to test for the remaining problem.

I wired in new Q2061 (2SC4237) and CR2065 (6A100) using the flying-wire method using #16-AWG solid pvc-jacketed wire salvaged from a new light-ballast replacement. I fused the collector with an AGC-8 fast-blow fuse and the base with an AGC-4 fast-blow fuse. I also fused the CR2065 diode with an AGC-5 fast-blow fuse. I replaced the 492's main AGC-4 fast-blow fuse. I powered on the power-supply module by itself disconnected from the main chassis. It is unloaded of course but has been running fine as long as I want to try. It tells me a little but not much that it runs by itself when unloaded.

(08/02/17)
I reattached the power supply to the main chassis to test operation with the three test fuses visible. I placed a piece of 2"-wide elecetrical tape folded over the Q2061 to protect against any accidental shorts. I powered up the unit and all started up and ran. I ran it for approx 3-5 minutes when I noticed that the 2" electrical tape started smoking and I then clicked off the power switch on my old B&K isolation transformer (personal home test-gear inventory). The Q2061 had almost went into thermal runaway, regardless of the small heatsink that I had attached to the 2SC4237. None of the test fuses blew and the transistor had not shorted or opened up - it's check on my old Fluke 79 diode check showed it was still good/viable. Since I didn't yet have an oscilloscope in our new shop, I had to wait and bring in my personal home-inventory old Tek 2245 oscope the next day for further in-depth testing.

08/04/17
I brought in my dusty and cigarette-smoke coated Tek 2245 scope today - 40 minutes with a microfiber cloth and a bit of Windex and looks 9 of 10 condition. Unfortunately only one of the two factory probes is 100%, so one lead it is for now, and hence the single-trace pictures. I again fired up the 492 PS by itself unloaded and it runs as long as I want it to, even with the heat-stressed 2SC4237 still on flying leads.

Once I re-familiarized myself with using my oscope (terrible I know), I finally started to get some insight into how terrible the signals hitting the Q2061 and Q2071 drive-transistors are.

- The oscillator output at TP6053 is spot on at 66kHz/15uS as-per the schematic, nice/crisp shoulders. Osc-adj pot fine and adjustable.
- The U6063B pins 12 and 13 outputs are nice/crisp and square also.
- TP6065 is a guess for me, that it's alright also as there is no current demand.
- U6069 outputs 1 and 13 look good to me, albeit, a bit of leading-edge ringing on each output - the waveforms look pretty-darn square otherwise.
- At the base of Q6077 after R6074-10k, something has went awry. What should normally be a low-transition in the squarewave is now a quick-exponential, almost full-voltage occurrence of approx 5.5Vac which likely equates to an almost dc-drive of Q6077.
- At the base of Q6078 after R6076-10k is a 9Vac 98% squarewave - some ringing on the leading edge, but just like the output of U6069 pin-13, a voltage-gain analogue.
- It seems that Q2061 is getting nearly twice the square-wave voltage drive of Q2071.

Thanks,
Dan G

[Dan Gibbs]

If anyone is interested I can provide approximately 12 good-great resolution pictures.

Edit - uploaded the remaining pictures today with a crop of the annotated schematic copy that I'm using - I had to crop the schematic to keep the file size under 1MB and still keep it zoomable/readable.

[Dan Gibbs]

Adding 11 more Pictures as I can of the steps throughout the SMPS.... hopefully numbers 01 through 11.....

[KE5FX]

Wonder if you've got a short in the transformer?  Hard to say what would be a good way to test for that, I've never run into that particular problem on these.

[Dan Gibbs]

Woo-hoo! I finally figured out how to post multiple pictures at once.

KE5FX asked: "Wonder if you've got a short in the transformer?  Hard to say what would be a good way to test for that, I've never run into that particular problem on these."

I'm not sure yet. A transformer short was alluded to in Gazucha's thread too, but he was able to fix his without any tranny trouble. At some point I'll probably pull T6081 but I'd need DC-resistance info from a known good one to get any idea if this tranny is good or bad. It doesn't smell or look bad at all.

Dan