Mitchell Gant
Active member
I have a Hantarex MTC900 that I think was given to me as faulty some time ago that I had a go at fixing.
After some basic visual inspection I hooked it up to a CRT and applied 115V AC. Monitor appeared dead.
Measuring the B+ that should be 126V DC showed 168V on my meter, oops that's a bit high! Maybe regulator board is faulty. After carefully checking I could find no problem.
After a bit of probing around I found what could be a problem around transistor TR10 and pulse transformer TH2. Although feed resistor R97 was supplying a voltage, collector of TR10 was firmly stuck at 0V. Metering the TH2 transformer primary showed open circuit. Hmm that would kill the Horizontal Output Transistor drive then!
Close inspection showed that the fine copper wires of the primary of TH2 was broken/cut from the pin post that it should be soldered to. There are two enamel wires making up the primary, wired in parallel I assume.
Hmm, wonder how that got damaged? Doesn't look melted/fused, looks cut cleanly off. Maybe rough and tumble of years of poor storage? Malicious booby trap fault?
A bit of enamel wire managed to connect it back up to post, tedious and fragile work under a magnifying glass.
Plugging back in to a CRT an applying power showed a B+ voltage of about 105V now, and I heard the crackle of HV before seeing a white raster on the screen.
I attempted to adjust the B+ to 126V as stated in the schematic, but after 20 seconds or so there was a fizz, a puff of smoke and a drop in B+ down towards 80V so I thought I better switch off the power
Capacitor C51 on the neck board had expired, I assume from drying out over the years of being unused, so next up a a re-cap of the board now that it has a chance at living again.
After some basic visual inspection I hooked it up to a CRT and applied 115V AC. Monitor appeared dead.
Measuring the B+ that should be 126V DC showed 168V on my meter, oops that's a bit high! Maybe regulator board is faulty. After carefully checking I could find no problem.
After a bit of probing around I found what could be a problem around transistor TR10 and pulse transformer TH2. Although feed resistor R97 was supplying a voltage, collector of TR10 was firmly stuck at 0V. Metering the TH2 transformer primary showed open circuit. Hmm that would kill the Horizontal Output Transistor drive then!
Close inspection showed that the fine copper wires of the primary of TH2 was broken/cut from the pin post that it should be soldered to. There are two enamel wires making up the primary, wired in parallel I assume.
Hmm, wonder how that got damaged? Doesn't look melted/fused, looks cut cleanly off. Maybe rough and tumble of years of poor storage? Malicious booby trap fault?
A bit of enamel wire managed to connect it back up to post, tedious and fragile work under a magnifying glass.
Plugging back in to a CRT an applying power showed a B+ voltage of about 105V now, and I heard the crackle of HV before seeing a white raster on the screen.
I attempted to adjust the B+ to 126V as stated in the schematic, but after 20 seconds or so there was a fizz, a puff of smoke and a drop in B+ down towards 80V so I thought I better switch off the power
Capacitor C51 on the neck board had expired, I assume from drying out over the years of being unused, so next up a a re-cap of the board now that it has a chance at living again.
