VR201 Repair Opinions

[NeXT]

While working on my Rainbow, the screen started rolling vertically. It stopped after a while but the readout is now extremely faint tot he point only half of most characters are readable. (it's a monitor issue. I checked.)

This particular amber phosphor VR201 seems to of been with the unit since it was sold back in '83. When I got it at first all I was dealing was cataracts and a slightly dim screen. With on and off use the screen got more and more dim until tonight. I can easily repair the cataracts however will I get more time out of the monitor if I gave it a recap? It seems odd it got so dim so fast.

 

[Lou - N2MIY]

Next,

I would doubt that it has to do with capacitors. There are number of things to check, but I'd start by finding the old-school TV repair guy in the neighborhood with a CRT checker/rejuvinator. If the tube checks out, next place to look is in the HV supply. I have never seen a VR201 fail by going dim, but perhaps yours has high hours on it.

Of course, I would say you should fix it.

Lou

 

[Qbus]

Check the power supply in the monitor, you may have a week or dying filter capacitor and that’s giving you reduced voltage that will cause the vertical roll and reduced brightness. The HV and fly back never decrease output to the extent that when the fly back fails you get nothing at all. From what you describe its sounds like a power supply issue to me.

 

[NeXT]

I'll talk to the guy at the local electronics shop today. Off hand I have both a flyback tester (Eico 944) that's too old for this application and a tube rejuvenator for the wrong make and size of tube.

For future reference, it's a Philips M31-341LA CRT.

Check the power supply in the monitor, you may have a week or dying filter capacitor and that’s giving you reduced voltage that will cause the vertical roll and reduced brightness. The HV and fly back never decrease output to the extent that when the fly back fails you get nothing at all. From what you describe its sounds like a power supply issue to me.

That's what I was suspecting as well. Thing is that the monitor is powered over the video cable and I don't think that's a high voltage line so would it be better to suspect filter caps int he computer itself?

 

[Lou - N2MIY]

The VR201 gets +12V from the connected terminal or computer. Somehow I totally missed the vertical roll problem and my mind only focused on the dim screen. Forget the CRT checker - Ray is right, it must be a power supply problem.

Lou

 

[NeXT]

Power supply problem as in the Rainbow itself or a secondary PSU circuit in the monitor?

By the way. Cataract surgery was completed this afternoon.

 

[Lou - N2MIY]

I think the power supply would be in the monitor, but you could check it by scoping the 12V output from the computer to the monitor while everything was powered up.

The "mould" removal job looks fantastic! Great job there!

Lou

 

[Qbus]

This is all pure speculation being that I have not worked on that but would assume that the +12 volt supply to the monitor feeds the Horizontal stages only and all other voltages developed in the display are provided by additional windings on the fly back (Horz output transformer). The filter caps for that will be smaller than the usual AC power supply capacitors but will still be can electrolytic. Did not know there was no low voltage supply in the monitor itself. Still first rule of troubleshooting is check the power supply, try looking at the +12 volts input on the monitor itself.

 

[NeXT]

+12v seemed okay out of the PSU so I went ahead and recapped what I could. The only cap I couldn't replace was a large grey 25V 10UF bipolar cap because I didn't have one. In all I replaced some 18 caps.
The result was mixed.

Brightness control funky. This photo was taken with the brightness and G2 pot cranked down as low as it could go and the screen was stills so bright it was putting it out of focus and displaying a raster. You can turn up the brightness until the screen is a blindingly bright orange lamp. I'm not smelling anything and nothing's exploded so I don't think I put a cap in backwards.

Edited: I did put ONE cap in backwards it seems however reversing it did not rectify the issue.

 

[NeXT]

There was another reversed capacitor. 10uf 250V.

Most of the caps I replaced were axials and radials were cheaper so I ran into problems several times where the older caps eiher had incorrectly marked polarity on the board or the cap itself was indicating the positive side, not the negative like we expect.

After final adjustments the screen is still a little dim but for its age that's more than enough for me and probably more related to the age of the tube.

The capacitors that were replaced are as follows:

5 - 25v 100uf
4 - 50v 50uf
4 - 16v 1000uf
3 - 16v 470uf
1 - 30v 47uf
1 - 250v 10uf

 

[Lou - N2MIY]

Next,

Very Good! Bad caps again! I am getting the feeling that capacitors are the singlemost common cause of failures in electronics anymore.

The sharpness looks beautiful with the bad RTV cleaned out from the coverglass. Really nice and clearer than when the tube was new (I think that RTV always made things look hazy.)

Lou

 

[RSX11M+]

Next,

Very Good! Bad caps again! I am getting the feeling that capacitors are the single most common cause of failures in electronics anymore.

Yeah, this is true - period. MTBFs on electrolytics are responsible for the vast majority of failures. It got a lot worse when PCs converted to linear regulators on motherboards during the PIII phase and later. Capacitor manufacturers were cheating their temperature specs.

In the telco equipment industry, many specs mandate all power supplies be constructed without electrolytics at all. Tantalums were also a big no.

Of course reverse polarity installation doesn't help. I've seen reversed tantalums turn to ash without any other sign of problem. Electrolytics tend to burst in power supply filter situations, but in small signal instances where potentials are low enough they just go leaky or short. Open caps happen too, but I've not discerned any common causative factor.

Instances of non-polar cap failure (like in my BA23 supply a few years back) are fairly rare in my experience. In that case, being right across the AC line, I tend to think long term moisture infiltration is partially responsible, or perhaps just the drying and aging of the encapsulation material. (some sort of molded epoxy or polyester?)


Reminds me - what are the temp specs on those new caps? Specific manufacturers and sources might be nice to share too. Nothing is available locally around here any more - everything mail order.

Your persistence is admirable - congrats on the victory. Very clean.

 

[NeXT]

I didn't look at the temp ratings of the caps when I purchased them but I believe they're all generic ratings. The monitor doesn't really get all that warm when in use.

 

[willseward]

Hmm... I replaced the caps in my VR201 recently as well. I also noticed that C6 (10uF 250V) and C211 (47uF 30V) were backwards on the board.

I flipped them around to no avail. I seem to be having the same problem you were experiencing. Maybe next weekend, I'll open it back up and double check my work.

 

[thunter0512]

There was another reversed capacitor. 10uf 250V.

Most of the caps I replaced were axials and radials were cheaper so I ran into problems several times where the older caps eiher had incorrectly marked polarity on the board or the cap itself was indicating the positive side, not the negative like we expect.

After final adjustments the screen is still a little dim but for its age that's more than enough for me and probably more related to the age of the tube.

The capacitors that were replaced are as follows:

5 - 25v 100uf
4 - 50v 50uf
4 - 16v 1000uf
3 - 16v 470uf
1 - 30v 47uf
1 - 250v 10uf

What was the key fix to bring the brightness/contrast back up?

Thanks
Tom

 

[NeXT]

A full recap went miles towards fixing the dim screen in this instance.

 

[thunter0512]

A full recap went miles towards fixing the dim screen in this instance.

You seemed to struggle with the polarity of a few electrolytic caps ending up with a very bright picture, and then you wrote about some "final adjustments" and showed a perfect image.
What did you adjust other than brightness and contrast?
I am asking because after doing cataract surgery on my VR201's CRT, I have a beautiful and clear but dim image.
First I checked the anode voltage and it is a stable 12kV as expected.
I also checked the ESR of the electrolytics in-circuit and they all seemed fine, so I am reluctant to just replace the electrolytics without understanding the root cause of the dim image.

 

[NeXT]

Hmmm. I think I tweaked the sub-brightness while the screen was full of text until I had raster lines barely visible when I was up at 90% brightness. The tube itself had a lot of hours so it was never going to be like-new brightness.

 

[thunter0512]

Hmmm. I think I tweaked the sub-brightness while the screen was full of text until I had raster lines barely visible when I was up at 90% brightness. The tube itself had a lot of hours so it was never going to be like-new brightness.

When you say sub-brightness do you mean the externally accessible brightness, or is there another internally accessible pot for that?

The capacitors that were replaced are as follows:

5 - 25v 100uf
4 - 50v 50uf
4 - 16v 1000uf
3 - 16v 470uf
1 - 30v 47uf
1 - 250v 10uf

The second line of your list has a typo (50uF), it should be:
4 - 50V 22uF

I will try to change the electrolytic caps in mine and report back the outcome.

 

[thunter0512]

I read chapter 9 of the Rainbow 100 Technical Manual and it describes the contrast and video amplifier circuit including R119 the preamplifier preset which is hidden away behind the 15 pin D-Sub connector on the PCB. You have to remove the small rear-panel covering the keyboard and 15 pin D-Sub connectors and the contrast and brightness pots to be able to access R119 for adjustment. It only required a small adjustment counter-clock wise to get display to full contrast as expected.
No capacitors had to be sacrificed to "un-dim" the VR201 display.