Fixing my scope

Projects / Repairs  Original post date: 11/07/2015

One of my scope stopped working on a hot day. I heard a whine from the flyback and the screen was dark. I power cycled the scope and it went dead. I finally had some time to open it up the other day after recovering from HaD contest mental and equipment burnt out.

Getting my scope factory repaired or getting replacement power supply is quite a bit outside of my budget. Thankfully I did managed to find a scan copy of the service manual online.

Scope flyback HV circuit

The flyback transistor (VT1109) was blown. The part that was populated is an obsoleted BUL147 (probably a replacement of the obsoleted BUV28 on the schematic), so it is not going to easy to find one. I replaced it with a MJE13007 from the power stage of a blown $20 200W ATX supply. It is the closest match that I have on hand. The flyback circuit delivers about 12W of power, so that extra 2A of current rating isn't going to be making a difference. With the transistor replaced, the flyback starts to make noise again.

Flyback windings and pinout

My gut instinct tells me that the flyback might have been shorted (but I was wrong). So I actually took the flyback apart and rewinded it. The core was held together with spring clips and a small amount of epoxy. A bit of heating with my hot air tool later, the epoxy turns soft and I removed the core and unwinded the whole thing. I replace the magnetic wires. I wasn't going to pay $10 and wait 40-50 days for that, so I took apart a 5W AC transformer for the some magnetic wires from the primary side. It was around 7 mil dia. (vs the 5mil in the original) close enough for me as the flyback wasn't fully packed. Each layer was insulated with clear mylar(?) tape and the windings was kept away from the two edges. I wound the primary and the filament on opposite side on the first layer just as the original. I had around 85 turns per layer for the secondary and I leave less clearance than the original. The flyback manufacturer seem to be very conservative on their insulation.

After all that work, there was still not a proper waveform at test point at MV11. Eventually, I start to examine the rectifier circuits on the secondary side. Then I noticed C1132 has a 15K resistance across it! I don't exactly stock 3kV caps around. I found a 3.3nF 3kV cap used as Y cap on the EMI filter on the scrap PCB of my CRT monitor. I swapped C11321 to C1132 and put my 3.3nF cap at C1131 as you always want the higher capacitance after the inductor in a LC filter.

I put everything back together and this ghetto repair seem to work.

The post-mortem seem to suggest that the high voltage ceramic cap failed. That put a heavy load across the output of the circuit increasing the loss on the driver. Combined with the hot weather, the flyback transistor was destroyed due to overheating (after the power cycle).

I have changed the jumper resistor configuration so that the 24V variable speed fan supply is raised from a 22V upper limit to 26V. I placed an order for a bag of 10nF 3kV cap from Aliexpress just in case, but I don't trust them more than the other still surviving cap.

My camera, netbook and now my scope all seems to end up broken this year. At least I got the scope working again for $0.

Updates:
It has been close to 3 years now after this repair and my scope still works fine.
Netbook fixed with a new motherboard bought from Aliexpress.  Battery drained due to the long wait and lost its setting and beyond repair.
Old camera is broken and not worthwhile to be fixed.