CRT fault (CBM PET)

mjnurney

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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

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

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mjnurney

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found the fault , low power on a regulator but over power on a rectifier.

L7818
RB151 rectifier AC-DC I'm having some trouble finding this
 

roy_bates

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bridge rectifier

got a picture? and pin spacing maybe we can find a drop in replacement.input voltage and amps would be nice mate:)


ok,lol...this any good to you?

http://www.ebay.co.uk/itm/RB151-Dio...981?pt=LH_DefaultDomain_3&hash=item461a0866f5


if so...these will do,lol

http://www.ebay.co.uk/itm/5-X-MULTI...511?pt=LH_DefaultDomain_3&hash=item5b025bd847



EDIT:hmm,if i was you i would try to replace the regulator as well mate.

one of these should do:

http://www.ebay.co.uk/itm/L78xxCV-S...t=LH_DefaultDomain_3&var=&hash=item35d3e860a9

and change the thermal paste:)(18 volt regulator)
 
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mjnurney

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not quite Roy

Full wave rectifier

i can't find one anywhere :-( sad face
 

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mjnurney

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ok bud.

Its a 22v AC to 22v DC full wave rectifier

- - - Updated - - -

tricky to find isn't it?

I've tried Ebay, mouser, RS, Digikey farnell,and amazon.

i don't know if something else would fit?
 
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roy_bates

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you see?

attachment.php
 

mjnurney

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yes 4 Rectifiers ? the RB151 is just 4 in one package.

this could be the only option bud , thanks
 

roy_bates

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a full wave rectifier is a bridge rectifier mike:)

just do it with 4 1N4001 diodes:)
 
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Dr_Diesel

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From what I can tell this is just a bog standard full bridge recitifier IC, with a full bridge just like in the pic above. RB151 is rated at 35 VAC max input. If you put in 22VAC, output voltage will be higher than 22VDC, around 22*1.4 - voltage drop of diodes, but if it's going into the 7818, that shouldn't be a problem.

All I'm saying is that most standard bridge rectifier IC's should work as long as the input voltage is OK, but also keep an eye on max cuurent it can supply...you didn't say how much the circuit is drawing, but since the 7818 is rated at max 1A, it shouldn't be drawing more that that.

Hope you get it going!
 
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mjnurney

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thanks guys , i'll look in to this tomorrow

- - - Updated - - -

The 7818 is in circuit to knock the voltage down to 18v

however at at the moment I have 13v out of it but nearly 25v going in to it
 

roy_bates

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thats why i said replace the regulator and thermal paste as well mate:)

check edit in post#3 bud:)
theres a link there as well...
 
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mjnurney

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I've got a couple of 7818s and paste so not a problem ...don't have diodes tho :)
 

mjnurney

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Problem solved it think..

changed the 7818 and done a bit of board cleaning and now I have a rock solid 18v going to the crt.
the picture is rock solid and stable and as a bonus no sparking flyback type noise at power on.

so I haven't touched the rectifier

mike.
 

roy_bates

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good news then mike:)

im glad you didnt have to mess with the rectifier,although its useful to know how to get around it if you need to.
 

mjnurney

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Thanks for the info Roy , it's good to know these things. I suppose these are obsolete parts now?

anyway all good. Solid picture, full screen and no wobble, it obviously didn't like 13v when it needed 18v

mike.
 

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roy_bates

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dont know if there obsolete,as its just a bridge rectifier.

i just couldn't find that particular one.they dont go very often even though i have seen a few partially fail.it would have to literally have a short the other end to kill one.

ill have a look at a later date and see if it was just a case of having different markings near the legs.for input and output.
 
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