Saturday, 11 February 2012

Patching geometry controls (and tube swapping!)

After messing around with my analogue chassis Blaupunkt IS 70-33 VTN last week, I started to realise how fast and easy it is to adjust the sizing and geometry of these TVs compared with their digital counterparts. With digital, you have to enter the service menu, navigate to the required section or menu, select the value you want to edit, save the value, exit the service menu, reset, etc. Of course, every brand is different but, basically, these types of TVs give you great options for setting up a perfect picture for a single resolution. However, when using MAME, we want to change the resolution quite often (unless you only every play the same game, of course!) That means frequent trips to the service menu which gets very time consuming.

With analogue TVs (the ones I've tried at least) there are a series of trim pots that adjust the picture size and shape. On my Blaupunkt you have to take the plastic casing off to access them. They're also on the mainboard at the very rear. To adjust them, watch the picture and also avoid getting electrocuted is pretty hard to do! So, I got thinking: "what if I ran a patch cable from the mainboard to the front of the TV so that I could adjust the picture and see the resultant changes?" Well, today I did just that! Instead of attacking the Blaupunkt straight away, I decided to mess with my analogue Grundig first. Since GroovyMAME can be setup to provide near perfect resolutions for each game with only height adjustments needed on the monitor, I decided to start with this control.

The great thing about this Grundig is that there are a series of daughter boards that can easily be removed for servicing. All the controls for size and geometry are on the one card.

First, I desoldered the vertical size trimpot using some solder braid:

Nice clean holes
Here's the potentiometer in question:

100 ohm variable resistor
Then, I soldered the 100cm patch leads to the legs of the pot:

Nothing beats colour coded heatshrink!
After that, I soldered the other ends to the daughter board:

Where's my V-size pot gone?!
Stuck the board back in the TV alongside the others:

Various daughter boards including RGB, audio, tuner, etc.
Moment of truth time... I turned on the TV and hoped for the best. Initially, the picture was all messed up because the other pots had moved position when I cleaned the daughter board. However, the externalised vertical size pot worked just fine! And, it was such a joy to be around the front of the screen while adjusting the height!
The vertical amplitude pot is on the floor in front of the screen
After that success, I made patch cables for the other 5 controls (vertical linearity, vertical position, horizontal size, pincushion and trapezium). I made this shorter (250 mm) to avoid a big tangle. I just wanted to get them out of the electrocution zone since they were really tricky to get at while the TV was running.

The other trim pots removed
Leads attached
After patching the leads, I was able to work out exaclty what each control did (the list I gave before was unconfirmed before this process). Unfortunately the vertical position pot was extremely brittle and the turning part literally crumbled after a few turns. Oh well. Gonna need a new one. I guess that's what 20 years of cooking inside a TV does to you. Also, I couldn't find a horizontal position pot. Not sure if this will be a problem or not...

The long term plan is to get hold of some proper panel mounted pots and drill holes into the front panel of the TV, underneath the front of the screen. That way once GroovyMAME has done it's best to get everything sized and centred I can do the last few tweaks with hardware. I reckon it's going to be a killer system!

Now... this is where the day got wacky. After the success of my trim pot experiment I couldn't help fiddling with some other stuff. Long story short, I ended up performing a full tube swap! I took the awesome Phillips tube out of a faulty Loewe CT1170 and attached it to this Grundig chassis. Amazingly, it worked first try! Considering that I'd never even discharged a monitor before today I was pretty pleased with myself. The best part is that this old school Grundig chassis hooked up to the high-end Loewe tube looks AWESOME.

This whole process is worthy of a blog entry in itself but time is getting away from me... crazy day of television hacking. Gotta get some sleep then clean up the huge mess I made in the morning.

Grundig analogue chassis with the tube from a Loewe digital! Oh yeah!
Minecraft at 240p anyone?
Home made discharge tool
Super vivid R-Type Leo
Wow, what an adventure. I can sense a new obsession coming on... tube swapping! Enough already!


  1. Where exactly did you clip the discharge tool to when you dicharged the cap?


  2. I didn't discharge any capacitors but I did discharge the tube. I just attached the alligator clip onto the metal monitor mounting frame and then touched the anode with the tip of the screw driver. There were no sounds. When I discharged again (after taking the anode cap off) I did see a small blue spark. No loud cracking or arcing like some people have reported though.