Meck XB-702 restoration

[bandersen]

That is the service info I have been using and it matches what I'm seeing. Same chassis, different cabinet.

Here are the three models using this chassis.

[bandersen]

Sencore TVA92 has arrived and is working

I injected composite video to the grid of the video amp with the tuner powered and it had the typical traveling distortion in the image,

Then I killed power to the tuner filament transformer and it cleaned up quite a bit. Even better than when I ran the tuner on batteries.

Next up, I'm going to unmount the transformer and attach long lead to the secondary. I'll power the primary from a separate wall plug. That should allow me to determine if the issue is AV on the tuner tube filaments or a radiated magnetic field.

[kvflyer]

Bob, one word comes to mind... Tenacity...

[vortalexfan]

Originally Posted by bandersen

That is the service info I have been using and it matches what I'm seeing. Same chassis, different cabinet.

Here are the three models using this chassis.

OK, I didn't realize they were all the same chassis, I kind of thought that was the case but wasn't sure.

[bandersen]

I dug up a real mumetal shield, attached extension leads to the filament transformer and experimented.

I can now 100% confirm the transformer does radiate enough to effect the image.

Rotating 90 degrees helps quite a bit but does not eliminate it. Unfortunately, there's not enough room to mount it this way.

Putting the shield over it helps even more.

Running the tuner off pure DC is about the same as using the shield. There is still some 60Hz getting into the horizontal sync but it's not very noticeable.

I'll see if I can mount the transformer under the chassis.

[bandersen]

Hmm. I didn't realize you could by a sheet of the stuff and make a custom shield.
There are smaller, cheaper scraps for sale on eBay.

https://www.digikey.com/en/products/...004-8/13160834

[jr_tech]

Originally Posted by bandersen

Hmm. I didn't realize you could by a sheet of the stuff and make a custom shield.

To restore maximum efficiency after cutting and forming, a special annealing process is necessary.
https://en.m.wikipedia.org/wiki/Mu-metal

jr

[bandersen]

Ah good to know. Meanwhile I cleaned up the chassis using CLR. Never tried it before and I must say I'm impressed!
Ony took a few minutes to clean it by using a soft toothbrush dipped in it. Followed that with a damp rag wipe down.

[bandersen]

I finished cleaning the chassis, reinstalled the filter choke and removed all the extra filter caps I had added.

I then twisted the filament transformers primary and secondary leads and remounted it. I reused one of the existing mounting holes and rotated as much as I could from its previous position. About 80 degrees.
I'll drill a second hole and mount more securely after some burn in time to make sure no problems crop up.

That reduced the distortion enough for me to call it done.



I'm tempted to replace that 50 ohm resistor in the filament string with a thermistor. That would give it a soft start and run the tubes at full strength.

[mr_rye89]

It looks pretty good

[bandersen]

Thanks. While in the process of putting reinstalling the chassis I notices that there is only a grille cutout on one side.

I could have saved myself a lot of speculating if I had noticed earlier!

[bandersen]

Not much to look at, but so glad it's finally working! I'll re-veneer the cabinet when the weather gets a bit warmer. I've never done a whole cabinet before and am looking forward to the challenge.

[old_tv_nut]

Nice!