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Commodore B128 (C610)

H

Holmes

Experienced Member
Joined
Dec 13, 2010
Messages
313
Location
Marietta, GA
About 1-2 months ago, I came into possession of a very nice Commodore B128 / C610. It has a 220/240V power supply, and I'm in the US, so I bought a transformer unit from ebay for about $30.

When I got it, I tested it out, and it seemed to work fine. I wrote some simple BASIC looping print programs, and ran then, without issue. I then connected my hand-made PetSD+, which is a SD-based IEEE disk emulator. I built it 1-2 years ago, and have been using it regularly on my PET.

I loaded up some images onto the SD, and tested them out. They appeared to work fine. I was testing Superscript, and suddenly there was a puff of smoke and the computer went out. I disconnected the power immediately, and opened the computer up. On this computer, there is some sort of metal-cylinder which the power wires from the IEC connector go to, and connect to the power-supply from there. I researched online and found this website, which actually documents almost exactly what happened to my B128:

http://mikenaberezny.com/hardware/cbm-ii/cbm-610-rfi-filter/

I removed the filter as he did, and tried the computer again. The fuse on the transformer had also blown when the filter went out, so I replaced that as well.


The computer came back up! The problem was it kept dropping down into the machine-language monitor. Sometimes it would boot to BASIC, then weird characters would come up, sometimes the keyboard didn't work. Sometimes it would come up in the monitor, sometimes BASIC, then beeps, then drop into the monitor, and then freeze.

I checked online, and on one posting board someone said if the computer (they were talking about a Commodore 128 ) drops into the monitor, then the BASIC ROM is failing, and he had fixed his computer by replacing that. Maybe a power spike fried the ROMs.

In my B128, there appear to be 3 ROM sockets on the motherboard, but there appears to be a small daughter-board with 3 ROMS on it (two for BASIC, and one for the KERNAL), and this daughter-board is connected to the motherboard with a plug to one of the ROM sockets, with 2 additional wires soldered directly to the motherboard. The motherboard ROM sockets below are empty. Turns out, this daughter board is an adapter board, converting the motherboard from a 2364 EPROM to a standard 2764 EPROM.

I tried reseating the ROMs in this daughter-board, and also rocked the daughter-board in its socket. It didn't do anything.

I then pulled the 2764 EPROMs, and tested them on my EPROM reader. I compared them to the downloaded images from ZIMMERS, and they appeared to match perfectly. I burned the downloaded images onto three new EPROMs, and tested them. I figured BASIC ROMS issues may be causing the drop into the monitor, and KERNAL issues may be causing the locking up.


It appeared to fix my problem. My B128 would boot fine, and I could run programs. I loaded Superscript again, and typed a small document. No trouble. I was paging through some of the Superscript menu items, and the computer locked up. I turned off and turned on again, and it went into BASIC, but then quickly dropped into the machine-language monitor again.

I left the computer for about a week, and then returned to it. I tested it yesterday, and I just did BASIC, with some small programs. It seemed to work just fine. I ran it for about 15-20 minutes with BASIC.

I disconnected the power-supply and checked the voltages. I get about 5.13V on the +5V line, 12.07V on the +12V line, and -11.99V on the -12V line. I also checked the 50/60Hz line from the power-supply (pin 1?) on my oscilloscope, and there's a steady square-wave of 59.9Hz.


So right now I'm a bit afraid to check the IEEE interface again. I just wanted to check in with you guys and see if there was anything else I might consider. I was worried about if my power-supply was the problem and was inconsistent when warming up. Or if I had fried some memory chips when the filter blew and that might be causing my issues. Or anything else anyone might suggest.

Any help would be appreciated!
 
5.13VDC is too much. I recommend a Power saver to prevent the chips that use the 5VDC from getting fried, or a new power supply that can be easily repairable.
 
retro-pc_user said:
5.13VDC is too much. I recommend a Power saver to prevent the chips that use the 5VDC from getting fried, or a new power supply that can be easily repairable.
Click to expand...

No, 5.13V is perfectly fine for TTL-based logic. 4.75 to 5.25 VDC is the allowed power supply range on the 74LS family (other families allow a broader range).

I'd check electrolytic capacitors in the power supply for high ESR and liquid leakage and replace them if necessary.

I would also replace the AC filter that has been removed with a similar unit.

Frank IZ8DWF
 
retro-pc_user said:
5.13VDC is too much. I recommend a Power saver to prevent the chips that use the 5VDC from getting fried, or a new power supply that can be easily repairable.
Click to expand...

Wow, that was quick! Thanks for the suggestions.

Really? I did not think 5.13V was too high.

I actually do want to replace the power-supply, as I don't want to continue using the transformer. It may be a bit of a pain, though, as I have heard the B128 needs the 50/60Hz line from the power-supply in order to work. I may have a solution to this, though. I have been experimenting with a Digispark ATTINY85 board. I programmed it with my Arduino IDE, and just wrote a simple program with a TONE command in it (to generate a signal with a certain frequency). I set it to 60Hz, and tested the output on my scope, and it looks pretty good!

If there are no other suggestions from others, then I will probably go this route. Thanks!

Anyone else have any thoughts on this?
 
iz8dwf said:
No, 5.13V is perfectly fine for TTL-based logic. 4.75 to 5.25 VDC is the allowed power supply range on the 74LS family (other families allow a broader range).

I'd check electrolytic capacitors in the power supply for high ESR and liquid leakage and replace them if necessary.

I would also replace the AC filter that has been removed with a similar unit.

Frank IZ8DWF
Click to expand...

Thanks! Question: how would this affect the output on the power-supply? Would it cause the voltage changes? Spikes? I did not test my PS under load, so could it be causing voltage issues when actually plugged in to the motherboard?

Either way, if people think the PS is the issue, I think I'd rather replace the whole thing than try to fix it, as it's not an ideal solution for me anyway, as it's 220/240V.
 
Holmes said:
Thanks! Question: how would this affect the output on the power-supply? Would it cause the voltage changes? Spikes? I did not test my PS under load, so could it be causing voltage issues when actually plugged in to the motherboard?

Either way, if people think the PS is the issue, I think I'd rather replace the whole thing than try to fix it, as it's not an ideal solution for me anyway, as it's 220/240V.
Click to expand...

Electrolytics in the switching PS are the first source of malfunction.
Regarding the AC filter, I can't tell how and if it affects the supply itself, but you're using a step-up transformer and now the switching noise reaches the transformer and we don't know how and if it affects the waveforms and all else. Probably nothing bad happens, but when troubleshooting an intermittent fault, we must remove all unknowns.

Frank
 
5.13V is really good, if it's clean. I wonder what it would take to convert the power supply to 125V. I have the 125V variant, I wonder if mine is changeable to 250V.
 
KC9UDX said:
5.13V is really good, if it's clean. I wonder what it would take to convert the power supply to 125V. I have the 125V variant, I wonder if mine is changeable to 250V.
Click to expand...

Assuming we are talking about switchmode PS:

The AC input goes into some filters, usually fuse on one side of AC, 2 x Y1 capacitors from each side of AC to earth, then common-mode choke (looks like a small transformer most of the times, but it's two parallel windings on the same core, one for each side of AC), then X2 capacitor across the AC sides, then a temperature-variable resistor (resembles a ceramic disc capacitor), then diode bridge either monolitic, or 4 discrete diodes, then big electrolytic smoothing capacitor, that can be one single or two in series if your PS is rated at 220-240V AC or in parallel if rated at 110-120V AC. Some parts might be missing for cost reduction.
If it's one single smoothing capacitor, you can't usually convert the input voltage, unless it's a modern unit that runs from 98 to 250V AC by design.

If there're two capacitors in series/parallel look at the center of the series, where one capacitor "minus" connects to the other "plus", there might be wire jumpers to change the series to parallel and vice-versa. Be sure to understand the circuit before doing something stupid. There won't be a second try if you make the wrong mod.
Messing with AC-connected circuit isn't a joke, so I repeat again: make sure you know what you do.
Quite a few times, two capacitors are present just to share the voltage or have more capacitance, and the power supply can't be converted to another input voltage.

Frank
 
I would also consider the replacement of the AC filter; but as the page you pointed out describes, what's that?, a combo between two caps and two inductors?:

0.1uF+2x2500pF XY
+2x100uH 250V~2A
F11.171/65 W-Germany
Click to expand...
 
Thanks for all the comments.

I don't really feel I have the knowledge or skill to try modifying the existing PS, so I am leaning to replacing it with a modern equivalent.


I've been reading the schematics online, and on the schematics for the CBM II low-profile (which I guess my B128 is), on the last page, there's a breakdown of the power consumptions of all the components:
http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/b/8256043-14of14.gif

At the bottom, it says the 5V line total is 5530mA, but then subtracts 20% to get to 4424mA? What is this reduction for? Is it standard practice?

The schematics for the PS show its specs are 5V (5.5A), 12V (0.7A), -12V (0.3A). It's buried in the schematic:
http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/b/4256004-01-right.gif

So, I have been considering this:
https://www.digikey.com/product-detail/en/PT-65B/1866-3929-ND/7705918

It's rated at 5V @ 5.5A, 12V @ 2.5A, and -12V @ 0.5A. It seems to meet the specifications to me. I will probably need to put in a high-wattage resistor on the +12VDC line, as it has a minimum current of 0.2A. I calculated that a 56-ohm, 5W resistor would do the trick here.

Other than this, it seems like it will fit this application. I would appreciate if others would glance over this and make sure I'm not missing anything.
 
Holmes said:
T
It's rated at 5V @ 5.5A, 12V @ 2.5A, and -12V @ 0.5A. It seems to meet the specifications to me. I will probably need to put in a high-wattage resistor on the +12VDC line, as it has a minimum current of 0.2A. I calculated that a 56-ohm, 5W resistor would do the trick here.
Click to expand...

Why on earth would you want to add a resistor on the 12V line? If you suspect it wouldn't regulate, then consider it will probably regulate on the highest amperage supply, which is the 5V line and the others are either linear regulated on filtered DC out of calculated transformer windings (the -12V probably is of this kind since it's spec'ed at so low current) or not regulated at all and they "move" as the regulated supply "moves" to meet current demand.
Modern SMPS designs can however run and regulate with no loads at all or have internal resistor bleeders to meet the minimum current needed for proper regulation.

Frank
 
iz8dwf said:
Why on earth would you want to add a resistor on the 12V line? If you suspect it wouldn't regulate, then consider it will probably regulate on the highest amperage supply, which is the 5V line and the others are either linear regulated on filtered DC out of calculated transformer windings (the -12V probably is of this kind since it's spec'ed at so low current) or not regulated at all and they "move" as the regulated supply "moves" to meet current demand.
Modern SMPS designs can however run and regulate with no loads at all or have internal resistor bleeders to meet the minimum current needed for proper regulation.

Frank
Click to expand...

I was thinking I needed to do that as the datasheet for the PS shows the +12VDC line provides 0.2A - 3.2A. I have read online that some power supplies sometimes have a minimum current for certain outputs, and I was planning on putting a small resistor to ground in order to draw the baseline 0.2A. I was planning on testing the core PS just by itself first, but wanted to be prepared to do this if I needed it.

Does that make sense? I'm very much a novice when it comes to power supplies, so I am happy to get the opinions of those who understand them.
 
Holmes said:
I was thinking I needed to do that as the datasheet for the PS shows the +12VDC line provides 0.2A - 3.2A. I have read online that some power supplies sometimes have a minimum current for certain outputs, and I was planning on putting a small resistor to ground in order to draw the baseline 0.2A. I was planning on testing the core PS just by itself first, but wanted to be prepared to do this if I needed it.

Does that make sense? I'm very much a novice when it comes to power supplies, so I am happy to get the opinions of those who understand them.
Click to expand...
Ok, I don't know that particular one, but I can guess it needs a minimum current on at least one output, probably not on every output.
I would test it with only the final load (the CBM-II) and see if it's well behaved.
I'm sure the original supply can however fully restored and it's surely matched to its intended load. It would probably need only cleaning and recapping (and checking solder joints).

Frank
 
iz8dwf said:
Ok, I don't know that particular one, but I can guess it needs a minimum current on at least one output, probably not on every output.
I would test it with only the final load (the CBM-II) and see if it's well behaved.
I'm sure the original supply can however fully restored and it's surely matched to its intended load. It would probably need only cleaning and recapping (and checking solder joints).

Frank
Click to expand...

OK, thanks. The +5VDC line it has a minimum of 0.4A, but there shouldn't be any issue meeting that. The -12VDC line has no minimum.

The main reason I want to replace the PS with a new one, is that the original is for 220/240V and I'm in North America. I don't want to have to use an external transformer, if I can avoid it. I do plan to keep the original PS for the future just for completeness, though.
 
I'd repair the power supply and see if there's someone in Europe with a 110V supply. But if the supply you're buying fits and works this may all be moot as it would be a good replacement in any case.
 
KC9UDX said:
I'd repair the power supply and see if there's someone in Europe with a 110V supply. But if the supply you're buying fits and works this may all be moot as it would be a good replacement in any case.
Click to expand...

I think I'm pretty much going to replace the PS with the MeanWell one.

I just checked the dimensions of the factory B128 power supply, and it's 9.5" x 3.5" x 2.5", so there's plenty of room for the new power supply. I may also fit a fan in there.
 
Turn the fan on with a temperature control if you can. These things do need ventilation but are nice when they're dead silent.
 
KC9UDX said:
Turn the fan on with a temperature control if you can. These things do need ventilation but are nice when they're dead silent.
Click to expand...

Good idea! I love computers that are silent as well.

If I did put a fan in, I'd probably try a Noctua 40mm or 60mm. Noctuas in general are really quiet, and even if you hear then, it's a pleasant hum, not a raspy buzz like with many other fans.
 
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