daver2
10k Member
Use Google to search for "Zimmers Commodore PET schematics" and you will find most of the schematics for the Commodore PET series of computers.
Dave
Dave
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Neglected PET needing some love
- Thread starter Divarin
- Start date
Divarin
Veteran Member
Thanks! Looks like the schematics needed for this PET are in: http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/8032/index.html
In the 12" CRT display schematic it shows A/C In at 50/60 hz but not the voltage
Here are the images I took of the gun:
daver2
10k Member
The AC in on this diagram comes from the low voltage secondary side of the transformer identified in this schematic:
Dave
Dave
Hugo Holden
Veteran Member
can you take a photo of the gun directly side on. And also have a look to see if any of the electrodes in the array were connected together.
Divarin
Veteran Member
Sure here are a couple photos of the gun from the side (I won't bother making thumbnail size photos of these two):
Image1, Image2
I couldn't find a way to remove the wires from the gun without damaging the connector and I'm not sure I want to do that yet but I marked the back side of the circuit board where each wire connects and checked for continuity between any of them but didn't find any so it doesn't look like any are connected together.
daver2: Okay so that looks like 21 volts AC.
daver2
10k Member
You are aware that the pin numbers for the existing CRT are shown on the VDU schematic you linked to?
Dave
Dave
Hugo Holden
Veteran Member
I have drawn a diagram, but just to double check that it is correct, can you take a photo of the exact opposite side of the gun as shown in your image1, because sometimes there are variations, where the structure labelled as the left hand Green X is connected to a base connection and not A2.
OR can you confirm that the three electrode structures marked with the Green X's are all connected together and that the only wire running from this area to the CRT's base is the one connected to the Focus cylinder ? If that is not the case and the structure labelled as the left hand green X is connected to a base pin, I will need to re-draw the diagram to correct it.
The A2 (final anode connection) of the gun connects via spring tabs to the CRT's internal conductive coating to the EHT cap on the tube bulb.
Generally, if you look at the schematics that show the pin connections to the gun, the first dotted line near the cathode is the control grid, g1, the next is the first anode A1 (in many sets this is connected to a fixed DC voltage, but it can be used as an additional brightness control on a potentiometer as in the 12" PET VDU and it affects the gun sensitivity a little too) and the third dotted line is the Focus electrode, connected to the focus adjustment pot. But they sometimes label them as g1, g2, g3 etc, rather than g1, A1, F which is better naming. Also, it is often not always shown on the gun's diagram on the schematic, that the Focus electrode (cylinder or "lens") is surrounded by two electrodes, one on each side, that are both connected to the final anode (A2 or EHT). And in some guns there can be an extra anode and in that case the anodes are better labelled A1,A2,A3.
Also, in the heater area, you will see the fine heater wires enter the very small diameter Cathode cylinder. You will notice that they have a white coating on them. This insulates the heater from the cathode. That area where the heater wires enter is the area where you see the orange glow coming from when the heater is running. At the other end of the cathode cylinder that is hard to see, is a flat face coated with a white compound to help emit electrons.
If you have a look at the larger grid cylinder, you will see it has a largely blind ended like a cup, with just a tiny hole in the center to let the electrons from the cathode come through.
One you familiarize yourself with the basic arrangements, then it is much easier to work out the pin connections on the Amber CRT by looking through the glass.
There is a quick way to confirm that the Amber CRT in the Samsung VDU has a 12V heater (which is very likely as most are 12V). If you can get your hands on a 12V incandescent 50mA or 100mA lamp, and tack it across the heater connections and power the Samsung set, to confirm it looks about normal brightness, this is a very low risk test, as the heater supply in the set is more than able to supply this extra current and it is less risky than playing around connecting a power supply to the CRT's heater.
Also, if you look at the PET's VDU schematic, they did something that is very good. It turns out that the coupling capacitor for the Horizontal deflection yoke coils has a parabolic shaped voltage generated across it (as in all VDU's) they have amplified this voltage with a transistor and applied it, capacitively coupled to the focus electrode. This is called dynamic focus and allows for the fact the CRT beam focus is not perfect toward the screen edges because the beam travels a longer path length to get to the screen.
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Divarin
Veteran Member
Yeah the X's marked in your diagram are all connected together via A2.
Full Size Image
Full Size Image
Correct me if I'm wrong but are there (possibly) two ways to go here? 1) to try to connect the PET's VDU to the amber tube or 2) Pull out the PET's VDU and bring over the amber tube along with its driver board and try to connect the PET's video signal to it?
It might be a little while before I can try either. I'd like to get the case cleaned up and repainted before I try to fit a tube inside of it and I also haven't actually tested the PET itself yet. If I flip the switch and nothing comes up I won't know if it's because the PET isn't working or if the CRT situation is incorrect. I did happen upon this video (HDMI for the Commodore PET), since I already have an RGB2HDMI that I use with my Commodore 128 I'm thinking this would be a good way to go temporarily to see if the computer is working. Also it would allow me to output video to both the HDMI monitor and the CRT at the same time which would be helpful when I get to that stage. But I'll have to order some parts to make the cable.
Correct me if I'm wrong but are there (possibly) two ways to go here? 1) to try to connect the PET's VDU to the amber tube or 2) Pull out the PET's VDU and bring over the amber tube along with its driver board and try to connect the PET's video signal to it?
It might be a little while before I can try either. I'd like to get the case cleaned up and repainted before I try to fit a tube inside of it and I also haven't actually tested the PET itself yet. If I flip the switch and nothing comes up I won't know if it's because the PET isn't working or if the CRT situation is incorrect. I did happen upon this video (HDMI for the Commodore PET), since I already have an RGB2HDMI that I use with my Commodore 128 I'm thinking this would be a good way to go temporarily to see if the computer is working. Also it would allow me to output video to both the HDMI monitor and the CRT at the same time which would be helpful when I get to that stage. But I'll have to order some parts to make the cable.
daver2
10k Member
What test equipment do you have access to out of interest?
Dave
Dave
Divarin
Veteran Member
not much, an old CRT oscilloscope and a multimeter.
Divarin
Veteran Member
Had a little time to do some cleaning today, looks like the fuse and cap to the fuse socket are missing. Any idea what type fuse should be used? I may end up having to replace the whole fuse socket since the cap is missing.
daver2
10k Member
That is more than adequate. You should be able to check the majority of signals depending upon exactly what scope you have. Can you let us know what you have so we can find the manual?
The fuse is a fast blow (not a slow blow) but it depends upon what mains voltage you are running the PET on. 110V or 230V.
Dave
Hugo Holden
Veteran Member
The gun configuration is standard as as I indicated in my first diagram. Likely, the Amber CRT is the same. So it is only a matter of looking at it and verifying the pin connections (after the heater voltage was checked, that is likely 12V) If the heater current was different the resistor in series with the heater in the PET VDU can be altered to give 11V to 12V across the heater terminals while running. If, in the off chance it was a 6.3V heater in the Amber CRT, the resistor could simply be increased in value and power rating.
When you are repairing the pet, you can leave the VDU disconnected, initially at least, and check that the 60Hz (vertical) and 20kHz(horizontal) drive pulses are arriving at the VDU board with the scope, before you power the VDU.
I read that the Samsung set has a H frequency of 18.4 kHz (is that true ?) if it is is would still work with the PET signal adapter, but with the slightly higher H frequency it might or might not pull into horizontal hold (it probably would or could be made to) The picture width would be a little less than normal, but, that could be increased with the Width control inductor.
So you have two ways to go. I would fit the Amber CRT tube and keep the original PET VDU board, for originality, but that is just a preference because I'm comfortable substituting CRT's.
Ignore the image gun.jpg below I attached it by accident, haven't figured out how to remove it yet.
I drew in the electrodes around the Focus cylinder that are often omitted in schematics.
When you are repairing the pet, you can leave the VDU disconnected, initially at least, and check that the 60Hz (vertical) and 20kHz(horizontal) drive pulses are arriving at the VDU board with the scope, before you power the VDU.
I read that the Samsung set has a H frequency of 18.4 kHz (is that true ?) if it is is would still work with the PET signal adapter, but with the slightly higher H frequency it might or might not pull into horizontal hold (it probably would or could be made to) The picture width would be a little less than normal, but, that could be increased with the Width control inductor.
So you have two ways to go. I would fit the Amber CRT tube and keep the original PET VDU board, for originality, but that is just a preference because I'm comfortable substituting CRT's.
Ignore the image gun.jpg below I attached it by accident, haven't figured out how to remove it yet.
I drew in the electrodes around the Focus cylinder that are often omitted in schematics.
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Divarin
Veteran Member
oh I'm glad you said that i had read another post on this forum where someone said it was a slow blow fuse. I'm in the us so I'll be running at 110 volts.
about the scope, sure I'll post it tomorrow morning, I actually don't remember the model, it's something i picked up at habitat for humanity.
Divarin
Veteran Member
okay sounds good I should have some time this weekend to take some measurements on the amber CRT.
Divarin
Veteran Member
This is my scope, it's a Hickok Model 517
It's "dual trace" but actually I think channel B is broken.
It's "dual trace" but actually I think channel B is broken.
daver2
10k Member
Ignore what I stated in post #32.
I have just got back from holiday and checked my documentation at home and it is a 1.6A SLO-BLO for 110V... See F1 on http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/8032/8032051-2.gif.
Dave
Divarin
Veteran Member
oh okay good that's what I had in my amazon shopping cart. I think I need to just study all the docs on that site while I'm waiting for stuff to arrive: rgb2hdmi cable parts, rust converter, fuse & socket, etc...
br44
Experienced Member
It doesn't take much. I don't know if you watch The 8 bit Guy on youtube, but his most recent video was of his attempts to repair an old arcade cabinet. He had to remove and install the picture bulb, and he accidentally --tapped-- the neck against the cabinet. Pop whoosh, the CRT was destroyed.
Please please please be careful handling that old CRT! You do NOT want chromium poisoning! You should not be handling it with your bare hands, nor should you work on it anywhere near a living area. The dust could get all over the place, and get into your lungs.
Divarin
Veteran Member
Yeah I saw that video a few days ago, I was thinking: That looks familiar!