Repairing the output stage of the Intellibox

After 23 years of use, I managed to burn out my Intellibox!

I think it happened when I was measuring the voltages of my turntable and I must have touched the track center rail contact with one of the probes. The 3A quick blow fuse on the power supply line to the Intellibox blew. I replaced that and then the Intellibox (IB) complained about a short. I disconnected things and tried a few times and then it said "Attention Extern. voltage".

I disconnected everything from the IB and all it would display was the same error.

This is how I went about repairing it. Though unsuccessful, the method of doing the work may be useful to others.

I recalled online discussions from years back that one had to drill out the plugs on the underside in order to open the IB case. I did so (but used a drill bit somewhat oversized) and opened it up. Nothing  could be seen to be destroyed, but the obvious candidates are the two power output transistors. These were BD535 and BD536.

I could not find replacements online and I asked in the IBX online forum for suggestions and also emailed Uhlenbrock support. As usual, J.J. McVeigh was extremely helpful and said that he had successfully replaced the transistors with NTE152 and NTE153, and that their 4 Amp rating would not be a problem since the IB output is limited to 3A. I ordered some online and started removing the old transistors. Uhlenbrock replied that they replace these with BD911 and BD912 (Also not readily available in the USA.)

This comprises cutting their legs off and unscrewing them from the heat sink:

and pulling the pins out the underside while melting the solder from the top:

To get the solder out of the holes I used some flux paste and some copper solder wick

I then cleaned up the flux with some alcohol.

One of the transistors tested to be bad, so I was confident I was replacing the correct items.

The new components arrived:

and it was pretty easy to solder install them. After careful cleaning, I used the existing insulation pads, screwed the new transistors in place and then soldered the legs in. The effect of the heat sink meant that the middle legs called for a 700°F soldering iron.

I reassembled all the parts and tried it out, and sadly I still get the same error. Something else in addition to the power outputs appears to be damaged. I may need to ship the IB back to Germany for repair at Uhlenbrock.