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Pet 8296D no signs of life

Desperado

Veteran Member
Joined
Nov 25, 2017
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6,776
Hi guys,
I want to open this thread to try to repair this old 8296D that I had in the basement for quite some time..
I tried to turn it on but nothing happens, not even the power supply fan is spinning. So I opened the transformer cover and saw that there is a component or maybe two that have blown ... maybe a RIFA?

No desperation at this time!!
 

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Certainly looks like a RIFA. Remember the RIFA's are normally RF suppression capacitors across the AC input and the machine should work with them removed so it might have taken something else out.

But when they blow, they can briefly short and take something else out in the supply

Yep, I can see its a RIFA
 
I think the failed RIFA is C4 and the blue thing is a twin indictor L1.

The inductor can be tested by checking continuity between the pins through each coil (ie two pairs of pins with a low resistance between them)

Clean up the board and check the tracks are intact, test the inductor then get back to us.
 
It looks like C1 may have got 'singed' in the ensuing fire storm as well.

I suspect the burning on the PCB is due to the flame that erupted from the RIFA.

Dave
 
Where do you find them all ?
I was thinking the same...

They never show up around here and when they do it's immensly overpriced rust buckets.

I count 4 RIFAs from the pictures alone and most likely the IEC power inlet filter ("Funk-Entstörfilter") also tends to explode, please don't leave it plugged in unattended (even with the switch in the OFF position) and don't ask me how I know!
 
Yep, I have had three of those input filters eject there guts., the barrel ones in the 8032-SK's and some disk drives.
 
The Rifa capacitors are interesting in that they deteriorate when the equipment is not in use, quite unlike most types, except perhaps Tants which can short in storage.

The clear plastic shell of the Rifa initially develops fine cracks, and lets moisture enter and is absorbed by the metallized paper. This causes progressive volume expansion (swelling) of the capacitor, which then widens the surface cracks, letting in more moisture, so it is like an exponential process.

Heating of the capacitors with the equipment in use from their surroundings might possibly reverse some of the moisture influx. So regular use of the equipment might delay the failure.

Typically what happens is; if the apparatus has been in storage for a year or two and then the equipment is powered and if there is enough internal moisture, the paper becomes conductive and the capacitor evolves heat, carbonizes the paper and the Rifa spectacularly burns up in a mini smoldering fireball. The capacitors are not "fire-proof" they are "fire retardant" compared to other types but still nowhere near as good as older larger types in sealed metal cannisters. It is like the naming semantics of Pest Exterminators, since they can't exterminate a species, they are better called "Pest Control" in case somebody gets disappointed when the pests they paid to eliminate, return later.

What I do now is if I'm powering old gear I check if there are aged Rifas in the PSU, and check their bodies for cracks and physical volume expansion and replace them if they are like this before powering the unit. It can be a lot of work to clean up the mess they make when they burn up, especially if the access is difficult. And they can take out other parts. Though I got caught out myself recently in a rush when I powered a backup 2465B scope that had been in storage for a year and still had the original Rifas and I had a workshop full of smoke and a mess on the pcb to attend to.
 
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You certainly have a good eye for some rare machines :)

Where do you find them all ?
I look for them with patience on the sites where used items are sold .... every now and then I find someone at decent prices but obviously they are always broken ...
 
I was thinking the same...

They never show up around here and when they do it's immensly overpriced rust buckets.

I count 4 RIFAs from the pictures alone and most likely the IEC power inlet filter ("Funk-Entstörfilter") also tends to explode, please don't leave it plugged in unattended (even with the switch in the OFF position) and don't ask me how I know!
Why???
 
The Rifa capacitors are interesting in that they deteriorate when the equipment is not in use, quite unlike most types, except perhaps Tants which can short in storage.

The clear plastic shell of the Rifa initially develops fine cracks, and lets moisture enter and is absorbed by the metallized paper. This causes progressive volume expansion (swelling) of the capacitor, which then widens the surface cracks, letting in more moisture, so it is like an exponential process.

Heating of the capacitors with the equipment in use from their surroundings might possibly reverse some of the moisture influx. So regular use of the equipment might delay the failure.

Typically what happens is; if the apparatus has been in storage for a year or two and then the equipment is powered and if there is enough internal moisture, the paper becomes conductive and the capacitor evolves heat, carbonizes the paper and the Rifa spectacularly burns up in a mini smoldering fireball. The capacitors are not "fire-proof" they are "fire retardant" compared to other types but still nowhere near as good as older larger types in sealed metal cannisters. It is like the naming semantics of Pest Exterminators, since they can't exterminate a species, they are better called "Pest Control" in case somebody gets disappointed when the pests they paid to eliminate, return later.

What I do now is if I'm powering old gear I check if there are aged Rifas in the PSU, and check their bodies for cracks and physical volume expansion and replace them if they are like this before powering the unit. It can be a lot of work to clean up the mess they make when they burn up, especially if the access is difficult. And they can take out other parts. Though I got caught out myself recently in a rush when I powered a backup 2465B scope that had been in storage for a year and still had the original Rifas and I had a workshop full of smoke and a mess on the pcb to attend to.
Can i use PET without RIFAs or it is dangerous?
Thanks!
 
>>> Why???

Because the in-built RIFA capacitor(s) are located within the IEC connector itself and are BEFORE the PET's ON/OFF switch. As a result, they are permanently fed with voltage (and therefore under stress) even with the PET switched OFF. If they then fail, they let the magic smoke out and could cause a fire in your property. Always best to switch off AND unplug vintage computer equipment when not in use.

>>> Can i use PET without RIFAs or it is dangerous?

You can remove all of the RIFA capacitors. They are provided for noise suppression purposes etc. The government may not like every person in Italy buying a vintage computer and removing all of the RIFA capacitors though! They are provided to prevent electromagnetic interference from being conducted both up and down the mains power lead. The metal case of the PET provides a convenient means of preventing interference also.

They provide a means of passing the electromagnetic emissions laws in the various countries where Commodore equipment was sold.

Dave
 
1666098378813.png

If you look at the input part of the PSU, the mains comes in through the two black boxes (labelled L1) then there are four capacitors C1, C2, C3 and C4. The mains AC voltage is of quite a low frequency and wont pass through the capacitors, which if you look at C2 for example, if the mains did pass through it, it would be a dead short to earth. The RIFA's do allow high frequencies through them, so any interference high frequency noise, produced by the PET is shorted to earth and not allowed to propagate.

So, if you remove C1, C2, C3 and C4 all you do is remove this filtering and the mains still passes through R1 & R2 to the rectifier GR1 where is is turned into DC.
 
Hi guys, i removed the two Rifà, i cleaned the board and i changed fuse! Now when i turn i have nothing at screen but i see flashing leds!

 
So, how are you going to proceed?

You have three (3) sub-systems to diagnose:

1. The monitor.
2. The PET logic board itself.
3. The disk sub-system.

So what are you going to tackle, in what order and why?

Time for you to put the knowledge that you have gained to good use...

Dave
 
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