CBM Pet 4032 DOA

[dave_m]

Yes, I agree, replacing the capacitors should fix this, and also the lingering dot in the middle of the screen on power off.

 

[mjnurney]

Okay 99% of the CRT is recapped now - I'm waiting for a couple to arrive in the post.

But ...it has made little to no difference at all , the screen still crackles a little at boot but the picture is unstable when the screen is cold , this reduces as the screen warms up. I cannot however adjust the screen to full size
as the tear / horizontal tear is too great. If i reduce the screen size i can almost hide the fault.

Now ....i have a hunch its power related.

AC PSU -
16.59v
18.35v

A/C to CRT 21.4v

Crt voltages at HTI7818p in 24.8v / out 13.50 / 13.60v

when the screen is tearing this drops to around 13.32v

Can anyone confirm what the voltages should be?

Video here http://youtu.be/W0bD4W4o-30

http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/8032/321448.gif

 

[dave_m]

Now ....i have a hunch its power related.

AC PSU -
16.59v
18.35v

A/C to CRT 21.4v

Crt voltages at HTI7818p in 24.8v / out 13.50 / 13.60v

AC to the CRT is correct. Mine reads 21VAC, but your DC reading of the CRT regulator output is way off. It should be 18VDC. You once reported it as 18V. I wonder what has happened? I would think either the full wave rectifier (D901) or the voltage regulator (IC901) is bad. Your voltage into the regulator seems too high. I would think it should be around 20 VDC.

 

[mjnurney]

Hi Dave

Output from the 7818 should be 20v ? I'm
Way below that ...

I have a L7818cv here would that replace it ? I cannot find any information on the Hitachi 7818p

 

[jmetal88]

Hi Dave

Output from the 7818 should be 20v ? I'm
Way below that ...

I have a L7818cv here would that replace it ?

Hey, just a little electronics part number information for you:

For the 7800 series voltage regulators, the last two digits are the voltage output. For example, 7805 outputs 5V, 7812 outputs 12V, and 7815 outputs 15V. So the 7818 should output 18V.

 

[mjnurney]

Excellent thanks I'll fit it tomorrow and see how if goes ... Not in flames I hope

What's the P and CV for ? Temp , construction ?

 

[dave_m]

Hi Dave

Output from the 7818 should be 20v ? I'm
Way below that ...

The output of the 7818 should be right at 18 VDC. The input to the regulator should be at least 20 VDC but the 24.8 VDC you reported seems too high.

Any brand of 7818 Voltage Regulator will do as long as it is in a TO-220 package. Also the LM340T-18 is an equivalent part.
-Dave

 

[mjnurney]

i better check again , the reg is DC isn't it but the PSU is AC , so what converts in on the video board ?

something isn't right...

 

[dave_m]

something isn't right...

It's OK. There is a full wave rectifier (D901) on the video board that should convert the 22 VAC to an unregulated 22 VDC (DC with ripple). Capacitor C901 smooths (filters) the 22V. This is what goes to the 7818 regulator.

 

[mjnurney]

excellent ....i better check that, maybe its killed the reg?

RB 151

 

[dave_m]

excellent ....i better check that, maybe its killed the reg?

RB151 rectifier

Possibly there is a problem with the full wave rectifier or the regulator. The rectifier is composed of four power diodes in a single package. Here is some good info on it: full wave rectifier

This schematic and board layout may be close to the video board you have. There are several versions I think.
video schematic and video board

 

[mjnurney]

Any idea where i can order one from ?

search are coming up with nothing ...

mike.

 

[KC9UDX]

I can think of a dozen local places I can buy them retail but that doesn't help you. Any of the mail-order electronics houses should have them. You don't need aa specific one, just one with a high enough current rating. If it's too high it doesn't matter.

 

[KC9UDX]

They are called "Bridge Rectifier" by the way.

 

[mjnurney]

what about amperage and voltage rating?

i can make one from 4 diodes. 1n4007 x4

However if the 7818 is rated to 35v why does it matter that I have 24v input? Unless the input is used elsewhere too?

 

[dave_m]

what about amperage and voltage rating?

i can make one from 4 diodes. 1n4007 x4

1N4007s are rated at 30A so quite a bit of overkill, but they would certainly work. You only need 2A and 25V reverse voltage.

However if the 7818 is rated to 35v why does it matter that I have 24v input?

It's not that it wouldn't work, the problem is that with 22 VAC as the input to a rectifier, one can not create 24 VDC out. There must be something wrong with the measurement or ???

 

[jmetal88]

1N4007s are rated at 30A so quite a bit of overkill, but they would certainly work. You only need 2A and 25V reverse voltage.



It's not that it wouldn't work, the problem is that with 22 VAC as the input to a rectifier, one can not create 24 VDC out. There must be something wrong with the measurement or ???

I would guess that the voltage is just higher because the transformer is not loaded to capacity. Many transformers I've measured have put out higher than their rated voltage when not under load (for example, a 24V transformer for an automatic vent damper I used in a school project recently actually measured 27V when the damper was not being powered).

 

[mjnurney]

All these measurements are taken under load , Pet running and screen powered.

i'll have to check the schematics...

 

[mjnurney]

here are the results with the 7818 changed , looks good so far. steady 17.98v

input is still 24v DC
PSU CRT output 22v AC

However i no longer have the flyback type crackle at power up , the screen is perfectly in shape and its stable too - no wobble or distortion.

mike.

 

[dave_m]

This looks good. You have done a great job. Did you have trouble replacing the 7818 due to the big heat sink? It looks like close quarters in there.
-Dave