This will be a fairly short log:
3 x MTC900e
1 x mtc900e / USA
All 'do nothing'
Checked B+ reading at 160v, and non adjustable (should be 126 or 115v if usa spec)
Checked HV: nothing
All turned out to be open windings at TH2 horizontal drive transformer, flybacks/lopts were all fine.
Key points leanred
(1)My first assumtion was B+ was too high causing x-ray protection to kick in but as others have previously posted it was the total lack of load causing the rise, not helped I suspect by the 220 ohm bodge resistor at R17
(2)Having ruled out (1) I found the power input (pins1/2 TDA2593 - IC2) were at about 7v not 11v so spent some time looking at the section of the power supply where the voltage is dropped via a resitor (R96) + zener diode (ZD16) from the B+ but presumably this voltage is normal when not being boosted via flyback and 7812 reg (IC4)
(3)Repair TH2:
Invest in a magnifying glass and a needle (with thread so you don't drop needle) and reattatch tranformer windings on TH2 using suitably sized wire or legs cut off old resistors to extend the post(s) down to where they can reach the wire. I made loops out of the resistor legs and gingerly wrapped the tranformer winding round/across the loop using a needle. You can then pool solder into the loop.
The 75ohm winding with the thin wire is the one that usually breaks, you should remove the laquer/insulation off the wire really in order for it to make good contact but if you go mad you'll break the wire.
One side should read about 1ohm, the other ~75ohms. you can get away with doing this with TH2 still mounted on the board but it is fiddly.
3 x MTC900e
1 x mtc900e / USA
All 'do nothing'
Checked B+ reading at 160v, and non adjustable (should be 126 or 115v if usa spec)
Checked HV: nothing
All turned out to be open windings at TH2 horizontal drive transformer, flybacks/lopts were all fine.
Key points leanred
(1)My first assumtion was B+ was too high causing x-ray protection to kick in but as others have previously posted it was the total lack of load causing the rise, not helped I suspect by the 220 ohm bodge resistor at R17
(2)Having ruled out (1) I found the power input (pins1/2 TDA2593 - IC2) were at about 7v not 11v so spent some time looking at the section of the power supply where the voltage is dropped via a resitor (R96) + zener diode (ZD16) from the B+ but presumably this voltage is normal when not being boosted via flyback and 7812 reg (IC4)
(3)Repair TH2:
Invest in a magnifying glass and a needle (with thread so you don't drop needle) and reattatch tranformer windings on TH2 using suitably sized wire or legs cut off old resistors to extend the post(s) down to where they can reach the wire. I made loops out of the resistor legs and gingerly wrapped the tranformer winding round/across the loop using a needle. You can then pool solder into the loop.
The 75ohm winding with the thin wire is the one that usually breaks, you should remove the laquer/insulation off the wire really in order for it to make good contact but if you go mad you'll break the wire.
One side should read about 1ohm, the other ~75ohms. you can get away with doing this with TH2 still mounted on the board but it is fiddly.
