WG4901 - mod for low impedance yoke

jrok

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Hey all,

Well I have this perfect Orion TV tube ( A48ACB32X ) complete with yoke that is also perfectly converged. Unfortunately I need to replace the tube from a WG4901 which had two very dead guns that even witchcraft could not bring back to life.

The WG4901 stock runs with a higher impedance yoke, this one measures 32ohm vertical. So the only way to get this going is modify the chassis to support the lower impedance. The replacement yoke measured at 10ohm vertical, 4 ohm horiz.

There's nothing technically that stop this from being possible. So it's just a question of how. BUT, and perhaps much more importantly, "how" without turning the chassis into a firey ball of G08 style goodness.

It'd be nice if there were some more apps notes and details on the HA11423 vertical stage drivers but there's enough to work with.

The overall change is to alter the feedback to the HA11433 vertical drive amp, size adjustment for the vertical oscillator and finally increase the current available to drive the Vertical yoke deflection.

In order:
Power resistor R313 was replaced with a 180ohm 25W, the 25W was chosen because it what I had to hand.

Current through the resistor increased from 0.0707A to 0.120A so it needs to upgraded to a 15W (at the least!) to deal with the increased current. Voltage drop across the resistor reduces from 33V stock to 25v.

I'll note that even with an unmodified chassis that resistor freaking COOKS. If my math is correct, with stock voltage/current then that thing is dealing with up to 6.5 Watts !

Q302 voltages measured are C=105v, B=48v, E=48v, stock gave C=93v B=44, E=44, so even with the mod it's still within the specs for the transistor.
Q303 C=48v B=0.56 E=0 ( stock was 44,0.56,0 ) so not a big difference from stock measurements.

I'll note in the manual Q302 shows Collector voltage being 79v, my own notes say around 90v (!) Anyone else like to comment on this ? Granted I've only had 2 other chassis like this ;)

Next, R310/R309 changed for the vertical amp, and R318 for the vertical size adjustment.

Final summary of component changed:

R310 = 56K ohm 1/4W Stock = 39K with 220k in parallel ( so 33.1k )
R309 = 18K ohm 1/4W Stock = 22K ( this can be left the same but I think the 18k improves the vertical linearity )
R318 = 270 ohm 1/4W Stock = 560 ohm
R313 = 180 ohm 15W Stock = 470 ohm 7W
( although a nice heat-sinked TO-220 cased non-inductive power resistor would really rock )

The monitor's been running burn in on for about 24 hours total on-time now without incident and nothing measured was really out of whack.
So, I'm pretty happy with the results.

Anyone free to jump in and say "you've forgotten about X".

- James

: edited for grammar & typos ;)
 
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I tried Jrok's mod today on a 19K4952. It works really well.

The yoke I used measured 2.6 ohms horizontal, 12.5 ohms vertical. With R309 at 18k, the vertical linearity was pretty bad -- crosshatch squares were squished at the top of the screen and stretched at the bottom. I put in a 100k potentiometer in place of R309 to test different values.

As I decreased the value of R309, the picture became very small vertically, but the linearity was clearly improving. Adjusting V-size restored the vertical height, and with R309 at 10k ohms, the linearity was perfect.

I changed the width capacitor, C365, from 0.27 uF to 0.68 uF to correct for an over-wide image. The new value is much better and should work fine.

Thank you, Jrok! This is significant, valuable work you've done.
 
I can honestly say I would have never tried this myself as I don't have the smarts for it. Nevertheless, this is great info to have available when trying to adapt something to work with what you have.
 
Somehow I missed this originally, but this is very nice work! I wanted to work on such a mod eventually as well, and the work still needs to be done for the g07, but I'm really glad to see you've successfully done this on the 4900. Thread subscribed!
 
One minor update to this, I did end up chassis mounting R313 using an isolated case.

The chassis at that spot has suffered some heat damage, before I'd even modified it (!) Also what remained of the traces were pretty screwed.

So I used a Caddock MP930-200-1% non-inductive power resistor, fixed a 3.96mm ( 0.156" ) pin header to the monitor chassis and used 3.96mm (0.156") molex KK housing with 22GA & trifurcon pins to run to the resistor mounted on the chassis. The nice thing about the Caddock resistors is the mounting tab is electrically isolated from the resistor, so you can mount directly onto a heatsink without making it live.

The thing now only gets warm to the touch, even the original resistor got to 'remove skin' hot ( well perhaps not that hot but pretty close ;)

As a long term test, this particular monitor has been out on loan for 9 months, getting about 8 hours of use a day, 5 days a week (!)

- James
 
You should print out the modifications and keep that piece of paper with that particular chassis for the future. At the very least, take a black Sharpie marker and mark the chassis "modified for low impedance vertical yoke".
 
Follow up on this old thread....

I just got back a machine that's been out on loan since July 2012 which was the first chassis I did this modification to. It's been in the break room of a friend's company for almost 3 1/2 years and was running for around 10 hours a day 5 days a week ( plus it would get powered up if folks wanted to play over the weekend ).

So the long term field report is that the monitor is still running like a champ with no problems in all that time.

- James

Edit: Just so it's clear the chassis is a Wells 19K4901
 
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This is pretty awesome stuff.

I actually posted in the monitors forum, that I have a Neotec 27-E with the crap Zenith tube. I won't get into the story of trying to rejuv it.... needless to say I started with one almost dead gun, had 2 guns at one time with over 10 gun current, but in the end, by the time I finished, all gun currents are 4.5 - 5. Balanced now anyway.

However, I did pick up a Zenith TV (thankfully with an RCA tube :) ) on the side of the road. I measured the yoke 8.5 ohms and 0.5 ohms on my Neotec tube, and 7.2 ohms and 0.8 ohms on the TV yoke. Will I need to make circuit mods to use the yoke on that TV tube on the Neotec, or are they close enough ?

I haven't seen good guidance on how close they need to be to sub a yoke with a tube swap.
 
Oh wow... I was thinking last night that this is probably a thing, and here you actually did it.

I wonder if I could do this to a U3000. And if I could adjust it to do a flat tube decently. I'm getting properly pissed at my 3414 and Pump only runs in VGA anyway. (Well, Prime has the HD thing going on, but that doesn't work with the 3414 anyway and I'd still need a newer computer.) I'd love to have a more common chassis that ISN'T known for frying goofy, very rare flybacks, while still having the ridiculously nice Samsung tube these come with.

There's more variables at play there though. Flat, obviously, and the deflection angle may be different. And I'd have to scrape the neck connector off the 3414 since it's a dual focus tube. And I don't know if the guns would be any different, or if the slightly bigger tube would be too much strain on the flyback or color drive.

Hm. My ghettoified 3414 is working okay for now, even if the picture brightness leaves a bit to be desired. I'm still saving this thread.
 
Wow...Awesome!!!!! Its all about swapping these nice clean TV tubes to make awesome looking monitors again!!! I have a Ms. Pacman cocktail with about the worst burn EVER and it still WORKS so yea it gonna get a swap one of these days...!!!!
 
That TV tube should work IMHO...Its the lower Horiz that might be the issue if anything..
This is pretty awesome stuff.

I actually posted in the monitors forum, that I have a Neotec 27-E with the crap Zenith tube. I won't get into the story of trying to rejuv it.... needless to say I started with one almost dead gun, had 2 guns at one time with over 10 gun current, but in the end, by the time I finished, all gun currents are 4.5 - 5. Balanced now anyway.

However, I did pick up a Zenith TV (thankfully with an RCA tube :) ) on the side of the road. I measured the yoke 8.5 ohms and 0.5 ohms on my Neotec tube, and 7.2 ohms and 0.8 ohms on the TV yoke. Will I need to make circuit mods to use the yoke on that TV tube on the Neotec, or are they close enough ?

I haven't seen good guidance on how close they need to be to sub a yoke with a tube swap.
 
And if that lower horizontal (I'm not even sure which one is the horizontal :) (the higher or lower ohm'd one)) would require something to be changed out on the monitor PCB... and if so, what ?

I had a tough enough time repairing the B+ section of my Sanyo 20EZ, not sure I want to be doing much guessing on monitor PCB board mods :)

I suppose I could change the yoke, but it would be nice to not have to reconverge it (though I need to learn how to do that). I need a medium res pattern generator (I should be on the waiting list for the TPG, but I don't have one yet).... I don't want to try to converge this in cab (Rush the Rock).
 
One minor update to this, I did end up chassis mounting R313 using an isolated case.

The chassis at that spot has suffered some heat damage, before I'd even modified it (!) Also what remained of the traces were pretty screwed.

So I used a Caddock MP930-200-1% non-inductive power resistor, fixed a 3.96mm ( 0.156" ) pin header to the monitor chassis and used 3.96mm (0.156") molex KK housing with 22GA & trifurcon pins to run to the resistor mounted on the chassis. The nice thing about the Caddock resistors is the mounting tab is electrically isolated from the resistor, so you can mount directly onto a heatsink without making it live.

The thing now only gets warm to the touch, even the original resistor got to 'remove skin' hot ( well perhaps not that hot but pretty close ;)

As a long term test, this particular monitor has been out on loan for 9 months, getting about 8 hours of use a day, 5 days a week (!)

- James

Wow, how did I miss this.. IMHO this one of the best mods I've seen for monitors!! James, could you post a pic of where you mounted the R313 to the chassis?

I'm excited to start this mod!
 
Wow, how did I miss this.. IMHO this one of the best mods I've seen for monitors!! James, could you post a pic of where you mounted the R313 to the chassis?

I'm excited to start this mod!

Here it is mounted against the side of the chassis edit: [ METAL FRAME ] with the heatsink. That type of resistor is fully insulated so can be mounted without needing a mylar spacer or isolation for the mounting screw.

- James
 

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Here it is mounted against the side of the chassis with the heatsink. That type of resistor is fully insulated so can be mounted without needing a mylar spacer or isolation for the mounting screw.

- James

Ok, glad I asked. At first I thought you somehow mounted it to 4900 chassis. It looks like you have a double mount heatsink, can I make due with a single mount?

And thermal paste between the resistor/heatsink and between the heatsink/chassis?
 
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Ok, glad I asked. At first I thought you somehow mounted it to 4900 chassis. It looks like you have a double mount heatsink, can I make due with a single mount?

And thermal paste between the resistor/heatsink and between the heatsink/chassis?

Yes ! It's mounted to the metal frame ( "chassis mounting" gives a whole different meaning here !)

You probably don't really need the additional heatsink, there's a lot of metal there acting as a nice radiator :) I just used some grease between the heatsink and the resistor.

I should add in the setup pictured I'm using a 3 pin molex KK 0.1" socket plugged into the legs of the resistor. You could just solder directly onto the then and cover with heatshrink for insulation.

- James
 
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I implemented JROK's low impedance mod on my 19K4903 this afternoon. It works and looks fantastic.

The yoke on the swapped tube measured 2.8 ohms horizontal and 13.5 ohms vertical. After a couple tweaks of the horizontal width coil and the potentiometers in back I had it dialed in. I did notice the mounted power resistor was running a bit hot so I added thermal grease between the resistor/heatsink and heatsink/frame. Now its warm and not hot to touch :D

Excellent mod jrok, I appreciate the research behind it!

IMG_1525.jpg IMG_1526.jpg
 
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