Repairing an old CRT display

As a fan of retrogaming, I’m using an old CRT TV to play with my retro consoles (Famicom, Megadrive, etc.). My TV is a not so old Grundig ST 72-864 (board reference : CUC2033). It is a big 70cm display with exellent stereo sound and a nice picture rendering. It even provides a way to easily access geometry settings with the remote control. 

Unfortunately, the TV started to behave strangely and was switching to standby mode after a few minutes. I managed to repair it and I thought that this was worth sharing this story…

Symptoms / problem

All was working wonderfully when the TV started to “hiccup”, like a mini degauss after about 20 minutes of use. These “hiccups”, were more and more frequent until the TV started to swich to standby mode. Also, there was a “tic” sound at each “hiccup”.

I was able to switch it back on, but after a few seconds, same symptoms, same standby mode.

I opened the TV, and searched for any visual sign… And I found that the responsible for the “tic” sound was a spark on the high voltage transformer (more info on wikipedia). 

Below is the TV’s motherboard, the spark was visible in the area surrounded in red :

Parts replacement

Board clean-up

First things first, I started to cleanup the board with a “contact spray” from Facom and a tooth brush, which gave a pretty good result :

High voltage transformer replacement

 

Searching the web for the reference of the TV (ST72-864) and the board reference (CUC2033), I managed to find the service manual with all parts references. And guess what, the same brand new high voltage transformer could still be bought in 2022 ! I found it on http://www.donberg.ie (ref HR6517).

 

I desoldered the old one with a solder sucker, then soldered the new one :

 

Capacitors

As we all know, capacitors do not have an infinite life. So, while the main board was disconnected, I also deciced to replace all capacitors (about 50). I know that “if it’s not broken, don’t fix it”, but I did not want to risk any other failure or damage because of a leaking capacitor. CRT TV are becoming rare and expensive theses days !..

On / off switch

Ok that’s another “fix whereas it’s not broken”, but I was advised by a former CRT repairer that this very model had a known weakness with its power switch (if you read french, here are his notes).

This TV does not use a simple ON / OFF switch, but a fleeting action switch. The known issue with this switch is that it tends to have a small current leak after several years. This current leak makes the processor crashes and randomly put the TV in standby mode (yet another cause for this symptom).

So again, while the TV was opened, I deciced to change this power switch. I found a brand new one with its reference (5000636) on a french web site : https://www.indipc.fr

Below is the board with the original switch :

Here, the original switch (the black one) has been desoldered. The new one (white) is identical, except that it has a kind of metallic collar that must be removed :

The new switch in place : it looks like the old one

Calibration

With a new power supply for the CRT and all brand new capacitors, the old geometry settings was no more suitable. But I was not able to correct them via the service menu : the picture was like  “compressed” in the middle :

To correct the picture, I had to “play” with the two adjustable “things” (surrounded in red) bellow.

The one at the bottom right is the “+A” value. The technical manual says that this should be adjusted so the voltage on R60037 or R61313 reads 142V.  In my case, I had to set it so I read 139V.

The one on the top left is the tuning coil. The manual says that this should not be tuned because it is supposed to have a good factory value. This coil allows to adjust the width of the picture. On my TV, tuning it adjusted the width of the picture but not (or very little) on the corners. 

Together with “software” geometry adjustements throught the service menu (the code is 8500), I managed to get a pretty decent result :

Below are the geometry values with a standard 240P picture from my Playstation 1 in background (running Ridge Racer) :

That’s it : I hope this CRT will now last another 20 years 😉