Email Address:
Password:

Lost your password?

This is the legacy website; please use the new website.

Serviceman's Log

Occasionally, when I am allowed out by myself, I wander off to the auctions for the day. I always enjoy the experience but I have learnt that what you might gain is at the expense of a day's lost wages and if you do buy something, it's often a pig in a poke with no guarantees.

I’ve got to admit that I buy a lot of stuff at auctions. But you can never have enough stuff and so there I was recently at another auction, recklessly bidding for all sorts of ex-government/ex-university electronic gizmos I didn’t really need. And that was when I spied her. She was gorgeous and just what I had always fancied.

Now before you all get too excited, "she" was in fact a Gould Classic 6000 4-Channel 200MHz True Trace Digital Storage Oscilloscope with an LCD readout. It looked in good order apart from two gouges on the edges of the front escutcheon where it may have been dropped and I was advised it didn’t work.

That didn’t worry me too much as it would reflect in the price which it indeed did and I was a very happy bidder when I walked out with my prize for only $100! Subsequently, I found out that these units were made in Ilford, Essex in the UK about 12 years ago (my old stomping ground) and at the time sold for about $5000 plus.

When I got it back to the workshop, I found that the 2A mains fuse had blown and a replacement also did the same immediately. Oh well, she was never going to be that easy!

Opening the case revealed what looked like a full-blown computer and a large complicated power supply unit which I removed. Obviously, I didn’t have a circuit diagram but I figured that this must be similar to an ordinary computer power supply.

Click for larger image

I carefully measured all the components in the primary circuit but couldn’t initially find much wrong. There were no obvious short circuits and the only components I could possibly find a slight problem with were TR1 & TR5 (MJE13009), C10 & C11 (100mF 63V), R4 & R5 (22kW 1W) and the MOV1 & MOV2 varistors across the two main electrolytic capacitors (C1 & C2, 680mF 200V). These VDRs were marked 20N241KJVRXHT and are not easy to source. I also noted a 3300mF 10V electrolytic capacitor in the secondary with a domed top, so this too would require replacement.

I tried tracing out the circuit diagram of this part of the power supply but it required removing a lot of components in order to trace the tracks of the double-sided PC board. Anyway, I replaced all the above suspect parts with the best equivalents I could find but found that the main electros (C1 & C2) were now getting hot and the fuse kept blowing.

After further investigation, I removed T3’s FL black lead that was stretched tightly across transformer T4. And when I did so, T4 fell out, severing its remaining leads to the PC board.

I now realised what had happened. The oscilloscope had indeed been dropped and transformer T4 (Part No. JK9501-01) had broken one of its primary winding legs.

Share this Article: 

Privacy Policy  |  Advertise  |  Contact Us

Copyright © 1996-2018 Silicon Chip Publications Pty Ltd All Rights Reserved