• Please review our updated Terms and Rules here

Commodore Early 9" PET VDU Mods & Antics.

Hugo Holden

Veteran Member
Joined
Dec 23, 2015
Messages
4,643
Location
Australia
I bought a spare VDU for restoration for my PET and some NOS CRT's to test. I'm writing a detailed article on this VDU restoration and the LOPT. Including how to deal with the rust and re-paint the cabinet. Since I had it, it gave me a good opportunity to fully investigate the design and make some modifications to improve it.

The article is very large and not finished, because of the extensive work on the LOPT analysis (directed toward using substitute LOPT's if needed) so I thought I would post some of the other work more briefly. The idea was to make the modifications reversible with as little change to the pcb as possible.The things that require improving are:

1) Poor Horizontal scan linearity - reason, no magnetic linearity inductor- Solution add high speed high voltage germanium rectifier and alter S correction capacitor value to 8uF.
2) Poor focus- reason, inadequate focus voltage for G4 electrode, needs to be adjustable around 330V. Only GND & 85V available inside VDU - solution; add transformer/rectifier/focus control powered by 580V.
3) Inadequate brightness control range (both VDU's same problem)- reason, the manufacturer's "cheated" with the design of the video output stage because the voltage ratings of the chosen output transistors were not high enough for normal CRT cathode voltages around 70v and also they did not have a higher first anode voltage than 85V (ideally for the CRT ist anode its 120V if the cathode voltage was higher around 70V) so they were forced to regulate the cathode voltage down with a zener diode. This required a higher negative grid voltage to bring the CRT beam to cut off - solution; add turns around LOPT core to gain more negative bias for the CRT grid. Requires one 2.5mm hole added to the pcb for the wires to pass through.
4) Turn off spot problem- solution increase C22 to 220uF
5) Protect VDU from damage from abnormal H drive signals- solution; add 5W rated zener diode to H output stage.

So to do these things I made a sub-board which carries modifications 1,2 & 5, from 1/8" fiberglass with some silver plated brass eyelets, though it could readily be a standard thickness pcb (if somebody makes a gerber). It mounts above the LOPT, using an extension above one existing pcb mounting hole and requires only one 3mm hole added to the pcb for the other mounting post.

The H linearity, without the modification on the PET VDU, is pretty poor, as expected, stretched on the left side. The technical reasons for this linearity error and why this added diode improves it are explained in the article I'm doing. It is not widely known or published anywhere that this technique of combining a Germanium energy recovery rectifier with a Silicon HOT is a linearity solution. I discovered it myself back in the 1970's, it significantly improves the left sided scan linearity defect , see picture. It is not practical in the PET to improve the H scan linearity with the common method of using the magnetic linearity coil, because there is limited scan width & no width control.

Adrian, from Adrian's digital basement video, about the PET VDU, also noticed the CRT focus was soft. He added a pot in attempt to adjust it, but the voltage required to get the CRT through a focus voltage knee is not present on the pcb, you can only get it between 0 and 85V. These CRT's achieve corner focus at about 420V on G4 and center focus at about 220V. (this difference is due to the extra length the beam travels to get to the corners). However a good compromise is around 330V, but for any individual CRT, it really requires a control pot to allow for CRT variation and to get the perfect compromise between center & peripheral focus (some VDU's have focus modulators to correct this center-corner difference, but adding that for this VDU would be going too far, even for me). Initially I built some voltage multiplier circuits, however the superior solution was to step up and rectify the HOT's collector voltage to around 580V with an auto-transformer wound on a 1" pot core with very easy to handle 30 awg wire and a UF4007 rectifier. The potentiometer was selected to be suitable for the task, high quality Japanese 2M ohm 0.25W rated part.

Also, one thing that helps is to get the CRT out of the cabinet and make a stand for it, I used some 3/8" aluminium bar. This makes life much easier working on the VDU. Also adding single pin connectors helps with the yoke wires and those for the mod board so they can be connected & disconnected at will.

The screen shot shows the VDU working with all the mods in place using X's to make a test pattern. Generally after the mods the performance is pretty good and the individual pixels comprising the X's can be seen clearly at the periphery and at the CRT center.This is an original Amperex MW24-302GH CRT that came in the spare VDU. There is some mild barrel distortion of the raster more in the global upper R corner area, this yoke does not have any overall geometry correcting magnets on it as they were obviously keeping the cost down, hence the absence of a focus potentiometer.
 

Attachments

  • modss.jpg
    modss.jpg
    234.2 KB · Views: 24
  • modss2.jpg
    modss2.jpg
    758.4 KB · Views: 25
  • mods6.jpg
    mods6.jpg
    393.2 KB · Views: 25
Thanks for the kind remarks.

One challenge was that the paintwork on the spare VDU was in very poor condition with significant rust. (Unlike the VDU that came with my PET where there was rust only on the inside requiring repair & repaint there and the exterior was good).

Rust can be very deceptive. In the case of the type of paint used on the VDU, it is textured & porous and pits extend to the metal surface, so you have a pit with a rust crystal in its base.

Without having to remove the paint entirely and re-electroplate the metalwork, one way to deal with this is to apply Fertan organic rust converter. It is a light brown liquid. If applied to clean steel there is no color change, however if there is rust, even microscopic, the Fertan reacts turning it a dark blue-black inactive compound. So it is like a detective for rust. For example, when I rubbed it into the paintwork of the spare VDU's exterior, even where it appears there is no rust, examination shows the rust is there in microscopic spots in the paint pores. It is always surprising how there is more rust than there appears to be at a glance and Fertan unmasks it.

Of course there is no point in re-painting the housing unless the rust is deactivated first, or the problem simply progresses under the new paint. I have heard of PET computers and VDU's being "professionally repainted" but the ones I have seen, it was never explained how the rust problem was dealt with, if at all, or what paint was used. The problem is likely always worse on the inside of the VDU cabinet, because it only got a light dusting of paint, compared to the exterior surface. Also the edges of the ventilation slots in the rear back surface acquired little paint from new and these must be treated and care taken to make sure the cut edges get painted. One great thing about the small dupli-color spray cans is that it is easy to angle them to get the spray to all the surfaces, including the ones inside the cabinet.

One oddity is I found a "reject stamp" inside the spare VDU. I took a photo of it. It looks like when the stamp hit the surface it bounced and gave two slightly displaced applications. Maybe a lot of VDU's had this and they were supposed to write over the Reject word in felt pen after it was tested. Has anyone seen these reject stamps before?

Another thing about working on these VDU's, it pays to put some temporary rubber feet on the screws at their base, or the VDU might scratch surfaces it is placed on. Also its is worth putting some thick tape over the three tabs that mount the rear panel, so there is no risk of those scratching the CRT glass if the tube is removed. The tube needs to be angled and manipulated to get it out of the cabinet.
 

Attachments

  • PaintAj.jpg
    PaintAj.jpg
    370.8 KB · Views: 14
  • PaintBj.jpg
    PaintBj.jpg
    309.2 KB · Views: 13
  • PaintC.jpg
    PaintC.jpg
    309.8 KB · Views: 12
Last edited:
Nice work. I really ought to repaint and rust treat my blue bezel 2001-8. Good work. What is with the reject stamp?
 
Nice work. I really ought to repaint and rust treat my blue bezel 2001-8. Good work. What is with the reject stamp?


Apart from the poor original paint job inside the housing especially, I could not find anything else that would have made the VDU a reject.

Though one odd thing; the two pcb mounting posts were plastic, unlike the 6-32 metal threaded ones in my other VDU, and they had slipped 4-40 screws & nuts to mount the board.

Investigating this further I found that the spacing of the two rear mounting holes in the pcb itself, were in fact about 3mm closer together than the holes in the metal VDU cabinet.

So when I went to fit proper 6-32 stand offs, they were way out of alignment. I had to elongate the holes in the metal cabinet toward each other, each by 1.5mm, to get the pcb mounted. I also noticed that the spacing of the two nylon spacers in the metal housing for the front pcb holes is such that they are also further apart than the pcb holes, but they just tilt over a little to accommodate that geometry error.

Perhaps one other geometry issue, if you look at the punchings in the rear panel photo, the row of long ventilation slots, the alignment of those is somewhat erratic as though the panel was not properly secured in the punching machine, so perhaps that could be a reason, but there was no reject stamp on that actual panel.

Maybe the cabinet itself was a "paint and geometry reject" as it carried the stamp. I'm glad I took a photo of that stamp. At least it is not a reject anymore.

It reminds me of a story I once read to my children about a teddy bear called Super Ted. When he was made at the factory they found something wrong with him and "Threw him away like a piece of rubbish"
 
Last edited:
Back
Top