53131A counter PSU - Missing bits

[tombi]

Hello,

Some time ago I got this broken 53131A counter with damaged PSU. Unfortunately someone had started repairing it and there were a few parts missing. It's one of the older SMP-43DL supplies and is different to all the other ones I've seen in teardowns. I started a thread on this before (https://www.eevblog.com/forum/repair/bad-53131a-psu/msg938315/#msg938315).

Basically the main switching transistor (Q2) has left a big burn on the board and is missing. In addition a zener diode (ZD1) and a diode (D3) is missing. While poking around I figured out Q1 is dead (a dead short).

Anyway it's been sitting on my shelf and I finally got round to having another go at it.

I found someone I could ask about the missing transistor and it is a BUV48A (big bipolar switcher). There was a diode missing but the one next to it is a RGP210 2A fast recovery diode so I got one I think is equivalent. I can't go back and ask them about the zener (pushing my luck getting what I did).

The problem is I have no idea what the zener voltage would be. I attempted a reverse engineering of the circuit but I'm not sure I got it right and I'm not sure how to calculate the voltage required. See schematic attached (sorry not pretty).

The unit doesn't have a chip controlling the switching but it seems to be based on the resonant frequency of the transformer. The feedback from the secondary side is via a small transformer rather than an opto.

Here is a photo of the top side of the board
https://photos.google.com/share/AF1QipO1Wpvf2u4Qomk_mDREy0FHIsAULwXoIAGFzgsV5UUkRQDTFD8izltJfya6k9vRig/photo/AF1QipPJphKsn41wYCKTseenSxcu_mH0qchjz-KM8WD0?key=di1oUGNYWDQ3X04tbDFDZ2lKZ3p3bExReXdSWkJn

And the bottom
https://photos.google.com/share/AF1QipO1Wpvf2u4Qomk_mDREy0FHIsAULwXoIAGFzgsV5UUkRQDTFD8izltJfya6k9vRig/photo/AF1QipMT4nNpVgD9_n9UwO9lu4Mns-8mSScElCIkkBqA?key=di1oUGNYWDQ3X04tbDFDZ2lKZ3p3bExReXdSWkJn

Here are the specs for the switcher BUV48A (Q2)
http://www.st.com/content/ccc/resource/technical/document/datasheet/3d/6d/7c/e8/c5/73/42/58/CD00001493.pdf/files/CD00001493.pdf/jcr:content/translations/en.CD00001493.pdf

Here are the specs for the other transistor 2SD1247 (Q1)
https://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwjL5fCY66_PAhUFF5QKHRToDQAQFggbMAA&url=http%3A%2F%2Fpdf.datasheetcatalog.com%2Fdatasheet%2Fsanyo%2Fds_pdf_e%2F2SB927.pdf&usg=AFQjCNEvRpl_JCnCrPxeJyGyG7p64grOTw&sig2=lgberD_2YUUZl7JPj4e_4g

Thanks,

Tom

[tombi]

Wait - wrong version of schematic. This has a couple more values.

[tombi]

Ok I think I have an answer.

Somebody on the HP Yahoo group helped me out.

ZD1 is used to disable the supply in case the input voltage is too low. Q2 needs about 1mA to be biased into operation so using a 180V zener will prevent this running if the input is too low.

Tom

[Paul Moir]

I was scratching my head.  Thanks for letting us know! 

[tombi]

So all the bits came in the post.

I haven't figured out how to clamp the main transistor onto the heatsink (and it gets hot).

Otherwise it looks good!

[CJay]

You need a spring clip like the ones here:

http://www.fischerelektronik.de/en/latest-news/press-releases/releases/new-retaining-springs-for-transistors-for-fitting-to-heatsinks-etc/

I'm sure I'm preaching to the choir but make sure you desolder your transistor and then clamp it to the heatsink before you resolder it so you're not mechanically stressing the PCB tracks and solder joints.

[tombi]

Thanks for the good advice. I did what you suggested. Found a slightly too small spring clip from a local electronics store that I could adapt. Temperature seems much better. Counter is running really well and either both it and my signal generator are off by the same amount or it is very close to being in adjustment.

Mental note - don't use your finger to test the temp as the heatsink is at 300V! - Don't worry I didn't do this.

Tom