Problem Hanty MTC900 (not E version)

[Stickman]

Got an MTC 900 on the bench, it looks like it has a full recap down in recent times, fired up and it work beautifully for 1/2 hour then screen shut off when I came back to it, checked fuses on PSU seemed ok, but B+ at 97v and cannot adjust. Had another look today and as I have a couple of spare chassis I swapped the PSU. Fired up and as you see attached picture was great, B+120v. It ran on the bench for an hour and a half went to check on it and picture was dark and fuzzy, then went out, checked B+ and was back down at 90 ish volts. I rechecked the initial PSU board and it seems to work fine so would suggest something breaking down on the main chassis. Starting to check schems but as there does not seem to be a lot on the web on repairs on these thought I would see if anyone has any pointers or experience before I dive in.

I get HV when it starts and pretty sure it is running as my neon glows when near the LOPT (Patent Randy Fromm test 😁).

Appreciate any pointers.

 

[tb2000]

First thing i'd do is disconnect the psu and run it on a bulb to see if it's breaking down. Although if you swapped another in then it's probably not that. Did you swap the regulator over when you swapped the psu? If it was the driver transformer i'd expect the B+ to go high rather than low. Otherwise i'd be checking poly caps and things on the deflection board. Or possibly the lopt. If you didn't do the caps yourself i'd double check they're all in the right way around. Maybe swap the TDA horizontal deflection chip.

 

[Stickman]

First thing i'd do is disconnect the psu and run it on a bulb to see if it's breaking down. Although if you swapped another in then it's probably not that. Did you swap the regulator over when you swapped the psu? If it was the driver transformer i'd expect the B+ to go high rather than low. Otherwise i'd be checking poly caps and things on the deflection board. Or possibly the lopt. If you didn't do the caps yourself i'd double check they're all in the right way around. Maybe swap the TDA horizontal deflection chip.

Thanks Tony, yeah focusing on the deflection board, was just going to step through the logical stuff like testing power supply under light bulb load, I did not recap, someone elses work, will have a closer look, I have swapped out the TDA2593 which seems to be associated with the horizotal drive. I did notice if I tapped (sharp tap) I got a flick of the picture sonetimes come on teh screen, which made me think cold solder, have reflowed a lot of it on the deflection but no changes and did not see anything bad. I have a feeling this is going to be a fun one . First thing is track down whats pulling supply to 97v and not allowing any adjustment

 

[gunblade]

tda2593 socket new?

 

[tb2000]

Yeah the socket does get flaky on those chassis (I can't remember if they're only single wipe sockets), same as on the 900/E. Also, I assume you are measuring the B+ properly? I only say this as you can't use the surrounding grounded HOT heatsink or frame like you can on the 900/E, you need to put your negative probe on the vertical deflection chip heatsink to get a proper reading of the B+.

 

[Stickman]

tda2593 socket

Yeah the socket does get flaky on those chassis (I can't remember if they're only single wipe sockets), same as on the 900/E. Also, I assume you are measuring the B+ properly? I only say this as you can't use the surrounding grounded HOT heatsink or frame like you can on the 900/E, you need to put your negative probe on the vertical deflection chip heatsink to get a proper reading of the B+.OK did not measure like that, but originally when it was working it say at 120v, I

I did test it using chassis as ground and got 120v when working but will retest as you suggest, will look at the socket as well, its a crappy looking one (technical term!)

 

[tb2000]

What type deflection chip do you have? Same as the MTC900/E or the version with a TDA1470 with a U shaped heatsink attached to the top of it? I can't remember where you put the negative probe on those, it may be that the chassis is ground on that version.

 

[Stickman]

I have both types this has the U type one on it. Going to have a little poke around now as its lunch time

 

[tb2000]

Actually I think the U shaped heatsink on those screws to little posts coming through the chassis which clamps it down to the chip so I think you probably still use that heatsink for the negative probe.

 

[Stickman]

Did a quick change out of the socket, also found a lifted pad on B5 coil. Did the light bulb test and works fine. Retested and the B+ climbed from 80 ish to 122v, still no picture, then it dropped down, I could adjust up and still no picture, then it dropped to 96v and no adjustment. Feels like something is breaking down, I was wondering if the xray protection is kicking in, but then I can feel static so HV is running