Tatung Computer Forum

Hi all,

I've got a Tatung Einstein TC-01 and a matching TM-01 CRT display in for repair - the computer was a black screen fault which I've repaired (bad system RAM), and the display has vertical deflection issues - see pictures below.
Someone else has tried to repair the display in the past, having replaced a few electrolytic capacitors in the height circuit (10uF 35V @ C301, 6.8uF 35V @ C304, 1000uF 35V @ C307, 100uF 50V @ C308, 4.7uF 35V @ C403) but obviously didn't get anywhere. The capacitors they fitted weren't of very good quality, so I've replaced them and checked that the polarities are correct, but there was no change in symptoms.

I've checked for bad solder joints, burnt or damaged traces, burnt or damaged components, bulged/leaked electrolytic capacitors, etc, but there doesn't seem to be anything obviously wrong.
All of the actives on the neckboard seem to test OK in-circuit, I've yet to check the other boards.

Has anyone seen and/or repaired these symptoms before, please?
Can anyone please offer any advice as to the most likely potential causes of this fault, or how best to diagnose it?

I've put a schematic together from the service manual: Many thanks,

Adam

I’ve got one of these with a vertical collapse but it’s been on the back burner for a good while now. I did similar steps to you but I never did get to the bottom of it, I was a bit concerned there was corrosion in the yoke but I need to take apart a worker to compare. I really should get it out and have another look at it!

I’d start by having a poke around the TDA3651 and check that it’s getting the +25v for a starter.

arcadesy wrote: ↑Tue Oct 10, 2023 9:17 am I’ve got one of these with a vertical collapse but it’s been on the back burner for a good while now. I did similar steps to you but I never did get to the bottom of it, I was a bit concerned there was corrosion in the yoke but I need to take apart a worker to compare. I really should get it out and have another look at it!

I’d start by having a poke around the TDA3651 and check that it’s getting the +25v for a starter.

Thanks Arcadesy, I appreciate it

So, I've tested the various power supplies as follows, and nothing seems to stand out to me as being immediately wrong:
- 12V rail is 12.05V at I401.
- 117V rail is 118.2V at +117V jumper.
- 18V rail is 18.75V at +18V jumper.
- 25V rail is 25.73V at I301.
- 121V rail is 118.2V at R423 (the other side of which is 7.98V).

I've swapped out all of the electrolytic capacitors on the mainboard (but not the daughterboard) for high quality replacement parts using the correct values and voltage ratings as specified in the schematic and parts list, being careful to ensure the correct orientation.

I've swapped D301 and D809 (both 1N4003s) for 1N4007s, being careful to ensure the correct orientation, but no change.
I've replaced Q401 with a new-old-stock ZTX651 as these seem to be quite unreliable, but no change.
I've replaced the 12V regulator with a 2A-rated 78S12, but no change.
I've replaced Q402 with a new-old-stock BU508A in case it was leaky / baked out, but no change.

D403 (BY159), D404 (BY159), and D812 (BY399) could potentially be problematic.
D403 in particular must get quite baked out as it's underneath the flyback shielding. It has 117.8V on its cathode, and 209.5V on its anode.

I have some parts on order and will continue to investigate.

Many thanks,

Adam

Hi all,

I'm making some slow but sure progress!

I've managed to fix the vertical deflection issue - several potentiometers were dirty/bad, primarily the height potentiometer, and there were a lot of bad solder joints on the mainboard, particularly on the larger power resistors.

So, I now get a recognisable image with the full height of the display.
However, I have two issues remaining:
#1: the raster is very intense and noticeable, even with the HV on the flyback transformer turned to its minimum.
#2: the monitor doesn't display any colour.

I've done some testing with a known-good display, and issue #1 is definitely a problem with the mainboard, while issue #2 is definitely a problem with the daughterboard.

The daughterboard has been recapped, and I've visually checked it carefully against the known-good daughterboard that I have.
The colour control potentiometer has been cleaned and tests OK.
I can't find service information for the daughterboard, but it seems to be based on the reference design for the TDA3505.
The RGB potentiometers all seem to be OK, but I'll double-check them.

Does anyone have any advice on both of these issues, please?

Many thanks,

Adam

TheMightyMadman wrote: ↑Tue Dec 05, 2023 11:07 pm I've managed to fix the vertical deflection issue - several potentiometers were dirty/bad, primarily the height potentiometer, and there were a lot of bad solder joints on the mainboard, particularly on the larger power resistors.

So, I now get a recognisable image with the full height of the display.
However, I have two issues remaining:
#1: the raster is very intense and noticeable, even with the HV on the flyback transformer turned to its minimum.
#2: the monitor doesn't display any colour.

In addition to this, I've attached a schematic that I've put together for the TM01 (from the service manual), the schematic for the TDA3505 reference design (from the datasheet), and oscilliscope traces from some of the major TDA3505 I/O that I've taken on the bad daughterboard.

Well done Adam in getting your monitor very nearly perfect.
For issue #1, is the problem the frame retrace not blanking? See
http://easkcom.blogspot.com/2012/05/why ... occur.html
for descriptions of the problem. Can then look for possible causes.
Regards,
John

1980s_john wrote: ↑Sat Jan 20, 2024 9:44 am Well done Adam in getting your monitor very nearly perfect.
For issue #1, is the problem the frame retrace not blanking? See
http://easkcom.blogspot.com/2012/05/why ... occur.html
for descriptions of the problem. Can then look for possible causes.
Regards,
John

Thanks John I'm sorry for the delay in getting back to you, the TM01 has been on the back-burner for a little while.
The link you've provided is very useful, thank you.

I've managed to make some progress since I last posted - the colour issue with the daughterboard was due to R155 (U gain potentiometer) being open-circuit, replacing it with a new 1kR potentiometer brought colour back on TTL.

I'm still getting the bright vertical lines, the screen is extremely bright, and the screen voltage seems very high (I get retrace lines even with the HT voltage on the flyback transformer at its minimum, and there's very little increase before the image blows out). Adjusting the potentiometers on the mainboard doesn't make any difference. Could the flyback transformer itself be the issue? Is there anything else in the schematic that could be an obvious cause of this problem?

Many thanks,

Adam

TheMightyMadman wrote: ↑Fri Mar 08, 2024 10:48 pm

1980s_john wrote: ↑Sat Jan 20, 2024 9:44 am Well done Adam in getting your monitor very nearly perfect.
For issue #1, is the problem the frame retrace not blanking? See
http://easkcom.blogspot.com/2012/05/why ... occur.html
for descriptions of the problem. Can then look for possible causes.
Regards,
John

Thanks John I'm sorry for the delay in getting back to you, the TM01 has been on the back-burner for a little while.
The link you've provided is very useful, thank you.

I've managed to make some progress since I last posted - the colour issue with the daughterboard was due to R155 (U gain potentiometer) being open-circuit, replacing it with a new 1kR potentiometer brought colour back on TTL.

I'm still getting the bright vertical lines, the screen is extremely bright, and the screen voltage seems very high (I get retrace lines even with the HT voltage on the flyback transformer at its minimum, and there's very little increase before the image blows out). Adjusting the potentiometers on the mainboard doesn't make any difference. Could the flyback transformer itself be the issue? Is there anything else in the schematic that could be an obvious cause of this problem?

Many thanks,

Adam

Hi all,

I can answer my own question here!

I decided to try and replace the flyback. The fitted part seems to be an Orega 37013 marked "01 SD C0" which I couldn't find any hits for. The TM-01 service manual says the recommended part number is a Tatung 85-3878-6, and according to Donberg Electronics the replacement part for this is a HR 6028 - the order link for this is below.
https://www.donberg.ie/descript/h/hr_6028.htm

I removed the old part and fitted the new one, which included removing the CRT neck socket from the neckboard to replace the screen cable (there is a white cover which pops off, and the cable is soldered inside), and the display seems to work just fine now!

Thank you all for your assistance.

Well done, glad you fixed it!

Regards,
John