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Cbm 8032 capacitor

Lololobolosse

Experienced Member
Joined
Oct 29, 2021
Messages
88
I just got a cbm 8032 and the big green capacitor only produce 0.1v does it is dead ?
Wath schould i replace it with ?
Wath is the reference of the capacitor ?

Sorry if my english is bad
 
I recently replaced mine with a KEMET ALS30 because the green can didn't charge past 8 V. Maybe I just didn't let it sit long enough, but as I had to order capacitors anyway I just went for it.

What I was looking for was a 22000 uF 15 V (minimum) cap, ideally with screw terminals.


Hope it was the right one, at least the PET is working fine for now
 
The capacitor doesn’t ‘produce’ Volts, it smooths it.

It could be short circuit, or there could be a fault with the transformer, bridge rectifier or wiring.

Can you post a photograph of your transformer - especially the numbers on each terminal and the wire colours, and whether they are thick wires or thin wires please. Failing that, can you describe what you see.

No problem with your English... If you put your location in your profile, we will know where you are from. Your English will probably be better than our ???!

Dave
 
Just an aside, if this machine has the common PET arrangement where the unregulated +9V goes from the bridge to power connector J8, out to the +VE of the large offboard capacitor and then back onto the board via another pin on J8 and onwards to the inputs of the 5V regulators, I have been involved with a couple of recent cases where the J8 plug / socket power connection between the output of the bridge and the +VE of the offboard capacitor has gone high impedance - so there is voltage as expected on the output of the bridge but nothing on the capacitor +VE. This sounds like it could be a similar case.

In these cases the test measurements can sometimes be impossibly contradictory, if (with power off) you measure for continuity between the bridge +VE and the offboard capacitor +VE you may see good continuity as expected, but power on and measure for voltage instead and it is seen to completely disappear across the J8 connection. The fault can be proven by running a temporary wire between the bridge +VE output and the offboard capacitor +VE, bypassing the J8 connection.
 
The original capacitor in my PET was not electrically leaky, and its ESR was still acceptably low, but it had lost uF capacity. Because of its very high capacity it was beyond the range of my capacitance meter to get an exact measurement. However in the PET the voltage across its terminals was dropping between diode conduction charging peaks, to the extent that ripple appeared in the 5V regulator output. It may have come right with re-forming, but I always replace Electrolytic caps that have any detectable defect with new parts. The Vishay part has the same screw terminals and diameter as the original, but the body is a little shorter, even though it is a 22,000uF 40v rated part. It just goes to show how the volume of electrolytic capacitors of some uF value and voltage rating have shrunk in size over the years.
 
On the machines where J8 terminal 4 has failed high resistance / open circuit, what seems to happen is that the connection initially develops a small amount of resistance, but this small resistance is enough to cause heating of the connection, which damages / burns the connection, which increases the resistance which makes it dissipate even more heat which makes it burn even more, causing more damage. Eventually the damage is such that the connection goes high resistance or open circuit, so at that point it no longer heats up, but on the other hand there is a large voltage drop across J8 terminal 4 so you have the normal unregulated voltage on the +VE output on the bridge and little or no voltage on the +VE of the offboard capacitor and therefore no input to the +5V regulators.

We have a possible theory about how this problem starts. The value of the offboard capacitor is very, very large (23,000uF)? so the inrush current into that capacitor through J8 terminal 4 at switch-on must be enormous.

In the two cases I have come across, on the first there was no visible / physical sign of the problem and it was only possible to detect it by following the voltage until it vanished across J8 terminal 4. In this case it was possible to restore operation by cleaning the terminal 4 connections on the J8 plug in header and PCB pin row connector.

In the second case there was visible browning of the plastic housing of the J8 plug-in connector showing that the terminal 4 connection had been running very hot.

In both cases resistance measurements through from the bridge +VE to the capacitor +VE were misleading as they appeared to show the connection to be OK, (very good continuity) but when power was applied there was a complete loss of voltage across the J8 terminal 4 connection. Since these were obviously contradictory results, the problem was then proven by running a bypass link from the +VE output of the bridge to the offboard capacitor +VE.
 
maybe the capacitor is good but when i power it on the crt start glowing but no sign of life of the computer and the transformer is making a loud noise and in the cbm there is the ram extension with the processor moved on can this be the problem ?
 
I am afraid I don’t understand what all these extra bits are. Can you post a photograph showing what the cards are?

Also, you haven’t posted a photograph of the transformer terminals (etc.) that I asked for back in post #3.

Dave
 
Lololobolosse, If your transformer is making a loud, angry noise (a buzz?) when you turn it on, don't turn it on any more for the time being.

Do you have a meter, and do you know how to use it to measure voltage, measure resistance, and measure current?
 
I know how to use a meter but im actually on vacation all the pictures i have are one of the transformer and a picture of the upgrade i found on the internet64k_board_cs_tu-min.jpgIMG_20220601_140527-min.jpg
 
Well, according to the text your large PCB is a '64K' memory expansion. If the transformer is taking off when you power it up I think you need to look at the main PCB closer to the power supply end first and yes, you may need to check that the green capacitor has not gone short-circuit as that is one possible cause for the transformer to behave that way.

Come back to us when you are back from your vacation, in the meantime enjoy it.
 
What an interesting looking board. I see it has what looks like a delay line UF10.

Why would a memory expansion board have a 6502 processor on it ?
 
No idea, but it has the words 'PET 64K Memory' in the upper right corner of the image. Perhaps if the diagram can be found on zimmers that would shed some light.
 
Here's a bit more about it. It seems that the 6502 on the board replaces the 6502 on the motherboard via a ribbon cable that plugs into the 6502 motherboard socket.


A bit more searching says that the actual board part number is 8032099 but I can't find schematics.

Hope this helps.

Colin.
That would explain it, it simply takes the place of the main board CPU and it is a way for it to plug onto the main board.
 
Thanks for digging that out, Colin. I am currently thinking that the initial checks need to be in the power supply input area on this machine but it is useful to know that this add-on can probably be removed to rule it out, if necessary.
 
I notice the picture shows the board has tantalum capacitors. These are notorious for going short circuit, sometimes even explosively! This may be the cause of your problems. I would advise removing them and replacing them with electrolytic capacitors. The tantalum capacitors are the red or orange globular shaped devices. You may have them on the main board too, so check there also.
 
Sometimes also mustard yellow, sometimes green, and long ago, with liquorice-allsort colour codes on them as well.
 
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