Meck XB-702 restoration

[Kevin Kuehn]

Originally Posted by bandersen

I like that I can reflow it with a heat gun if I mess up. I'm about half done with the paper caps. Then, I'll move on to the half dozen axial electrolytics. I think those are the most challenging.

That looks really neat. Any concern about the hot glue or gun tip melting into the insulation of a typical axial lead film cap, or are we talking much lower temps? Also for us hot glue dummies, could you provide a link to your brown glue source? Understand if it's proprietary info.

[bandersen]

Much lower temps. A glue gun won't melt the capacitor or pvc wire insulation. I got these off eBay years ago. Many, many places carry them. Amazon, etsy, etc.

Just be careful you get the right size - mini vs full.

https://www.etsy.com/listing/2589412...lue-sticks-for


Heck there's even hotmelt.com
https://www.hotmelt.com/products/inf...nt=25974897099

[bandersen]

Inevitably I was going to find some out of spec resistors. The two 47K horizontal amp plate resistors are over 100K. Luckily, I have a limited stash of NOS carbon comps. I found two that are about 50.8K. That's only about 10% high and they match each other so I'll use them.

[bandersen]

The final caps to rebuild are the axial electrolytics. I carefully un-knurl the negative end of the cardboard tube and push the cap out.

The can is thin, soft aluminum - very easy to cut through. I then wash out the insides and drill a small hole in either end.

Then a little hot glue to close the can back up. Finally, I knurl the cardboard tube back over and use a little glue.

[bandersen]

Here's the rebuilt cap. I do have trouble with paint flaking off the resealed end. Perhaps it could be touched up with artist acrylic paint.

[init4fun]

Maybe the careful application of a small bead of your brown hot glue would dress it up a bit ?

[bandersen]

Perhaps. I just left them as is for now. I finished up the sweep and audio amp side. While doing so, I replace 5 resistors.

The end result? Loss of height and bad horizontal linearity!

Now there's still a lot more work on the other side so I'm not too concerned yet. At least it looks nice

I did some quick spot checking and am finding more than half of the resistors in the IF and sync circuit are way off. 50%-100+% high. That's going to take a while to do

[bandersen]

White dot = bad resistor

[bandersen]

I restocked by axial film capacitor stock using JustRadios. Was surprised to see the white HV caps are SHV rather than ASC. A new player in town?
I'm just happy they are available somewhere from someone.

[bandersen]

I picked up a bunch of 1.5" diameter cardboard tubes for new capacitor covers. I can cut them down and paint as needed for this and future projects.
The one think left to figure out is a good method for sealing one end. The originals were knurled over and had a cardboard disc glued in.

[Tom9589]

Just a thought. The guys who reload their shotgun shells have a press which rounds the end after they have reloaded the gunpowder and buckshot. The trick would be finding one which has the same diameter as your cardboard tubes. Off hand, I don't know of any shotgun shells which are 1.5" in diameter, only smaller.

[bandersen]

Finally, all caps have been restuff and a couple dozen resistors replaced.


I then adjusted the HV supply to get 5kV. That improved the brightness at the expense of the image shrinking. The deflection plates need to be driven harder as the HV increases.

Swapping the 12SN7s around helped somewhat but the height control is maxed out. This is definitely a case where tube can test great but not work very well in a set. I'll dig around for some more tubes to try.

The contrast had improved as a result of replacing all those bad resistors in the IF.


The horizontal linearity needs some help too. There is a trimmer cap under the chassis that has a major impact on the width and linearity. I'll have to adjust it with a test pattern generator.
I think the funky interference pattern is due to the HV box top not being fully secured.

[bandersen]

Here it is after a bit of tweaking. I outlined the mask in green sharpy and can almost fill the space.
Linearity is pretty good, but there is some hum making its way into the horizontal deflection.



It has a really simple IF design so the poor bandwidth wasn't a surprise.


There has also been a loud hum since the start of this project. Some of that may be due to this super simple audio coupling design. Just a cap from the volume control to the grid and a 10M resistor to ground. Just bring my hand near that circuit increases the hum.

Wrapping the cap in grounded foil helps a little.
I'm going to recheck the tube to make sure there is no heater cathode leakage.

[old_tv_nut]

Originally Posted by bandersen

...
There has also been a loud hum since the start of this project. Some of that may be due to this super simple audio coupling design. Just a cap from the volume control to the grid and a 10M resistor to ground. Just bring my hand near that circuit increases the hum.

Wrapping the cap in grounded foil helps a little.
I'm going to recheck the tube to make sure there is no heater cathode leakage.

Did you try reversing the cap? Hum pickup can be different depending on which end is the outer foil.

[bandersen]

It makes no difference. That would only shield the cap anyways. The hum gets worse if I put my finger near the 10M resistor for example. They used coax for getting the signal to the volume control, but no shielding afterwards. Seems a little odd.

Grounding the 6AT6 grid completely eliminates the hum.