Replication of B128 – CBMII series

[vmarcus]

Hey, I would like to know, has anyone ever replicated any of the old Commodore B128 computers (the ones utilizing the 6509 processors)?
Yes, I found multiple replications for the Commodore Pet.
Even here, I saw the Micro-PET (http://www.6502.org/users/andre/petindex/boards.html)
I suppose, I could also use another alternative https://www.glensstuff.com/pet2001/pet2001.htm , follow closely the overall design and modify the architecture as to implement the quirky bank switching of the mos 6509 processor.
My goal, was to achieve software compatibility for the B128.

Yeah, originally, I was lucky enough to acquire at B128-80 (CBM 710). http://www.zimmers.net/cbmpics/cb4s.html
As stated from the seller from eBay, "all it needs, are replacement rom chips." Turns out, computer booted up, without doing a memory check. When I did a simple test to print text, it displayed gibberish. And the built-in display eventual died after being on for a few minutes. Eventually, I discovered a number of ICs suffered the fate of corrosion from numerous pins due to age.

Having discovered, that a number of local Computer repair shops, would not touch a vintage 8-bit computer, I took the journey of learning on how to de-solder and re-solder components. Unfortunately, alone the way, I’ve learned on what not to do. Eventually, learned the proper way to solder. However, the in mist of the activity, apparently, I reached the point of, no recover , since some where down the line, I got short between +5v line and a ground. (While looking through magnifying glass, I couldn't find any obvious signs of any solder beads, touching any adjacent points.)

When, since it is really extremely rare to come across spare mother board for the CBMII series, I am wondering if anyone ever attempted to reproducing PCB board, based upon existing schematics? http://www.zimmers.net/anonftp/pub/cbm/ ... index.html

Though, I’m really not sure, if there was an attempt by Ruud Baltissen, to design a new CBM-II using modern ICs like CPLDs and may be even FPGAs. But I can’t seem to find anywhere if he was successful or not.
I sure would hate, being a newbie, to attempt this effort from scratch.

Thoughts??

[GARTHWILSON]

vmarcus wrote:

I got short between +5v line and a ground.

Did you find the shorts? I've been thinking I ought to post a number of circuits I've built, one being a short finder. We had a batch of very dense audio (not digital) boards with ground planes and a reference-voltage plane, and in some cases, there were shorts between the two planes. How do you find it, if anywhere you connect an ohmmeter, you get zero ohms? It was very frustrating, and at first I just considered them "beyond economic repair," and replaced the board. Then I had an idea to put a very low duty cycle high current (a couple of amps) through, and use the 'scope on a scale of 5mV/div and probe around and see where the voltage drop was the lowest, meaning that's where the short is. It worked wonders. Actually, it turned out the shorts were in a particular batch of capacitors we got, one voltage, size, value, manufacturer; so the seldom times I get one of these for repair now, I just replace all of those even if they haven't failed yet (although one always has, and that's why the repair is needed). Enough writing. I'll just scan it right now, from my 3"x5" loose-leaf binder. Here it is:

The unlabeled op amp is half an LM358.

[BigDumbDinosaur]

vmarcus wrote:

Yeah, originally, I was lucky enough to acquire at B128-80..."all it needs, are replacement rom chips."

The B128 was the Rodney Dangerfield of the Commodore family, and for good reason. While BASIC 4.0 was easy enough to program, trying to write and run assembly language routines on the machine was an exercise in extreme exasperation, thanks to the brain-dead design of the 6509. The thing quickly acquired a bad reputation, so it was no surprise Commodore unloaded their inventory on Protecto Enterprises. Protecto let them go at fire sale prices. I almost bought one because I liked its looks, but quickly came to my senses after reading the 6509’s data sheet. Nobody wanted the thing, it seems, and around the time Protecto was trying to get rid of their inventory, Commodore released the C-128, which was more capable (other than not having an IEEE-488 port).

Anyhow, enough with the history. The B128, as was typical of its contemporaries in the CBM product line, was plagued with crappy PCBs—the copper would peel off if you looked at one wrong—DRAM problems, and dismal assembly quality control. The corrosion issue with the chip pins was typical—apparently, Commodore didn't thoroughly deflux their PCBs after stuffing and wave-soldering. I recall from my experiences with wave-soldering in those days that the fluxes used were more active that the flux found in rosin-core solder. Apparently, the flux remained active after soldering and had to be removed to avoid long-term corrosion.

The original ROMs, if memory serves me correctly, were mask-programmed and seldom went bad. ROMs were usually in sockets, so it is possible to removed them, put them in an EPROM programmer and read them to see if the content is still there. If it is, you could burn new EPROMs (27C series or similar) to eliminate flaky ROM as a cause of a DOA problem.

Other than a PLA, the B128 didn't use programmable logic. The rest of the glue logic can be repaired with industry-standard parts. Most of the logic was 74- or 74LS series, the latter which is still readily available. However, if you don’t have a schematic, pinpointing a bad IC is going to be a real exercise in patience. See here for more B128 info.

Given that your description of your machine suggests a major short, I’d begin by focusing on the electrolytic capacitors. Electrolytics of that vintage didn't age well. The short-circuit detector Garth described may be able to help you find the electrolytic(s) that may be causing the most trouble. Or, if you are ambitious and know how to desolder capacitors without wrecking the PCB, you should consider replacing all of them (desoldering braid is your friend in clearing through-holes). Radial-lead electrolytics are cheap and readily available.

Regarding corrosion on the IC pins, a cotton swab dampened with isopropanol can be used to clean up any flux residue left from wave-soldering. Following that, a careful inspection under magnification should reveal any questionable joints. If you find any, resolder them using 63/37 rosin-core solder. Just be careful with the heat so you don't damage the PCB or the chip. Above all, don’t use RHoS solder—its melting temperature is much too high.

Good luck with this. Even after remediation, the B128 may still refuse to work. I was able to resurrect a dead C-128 and a dead C-128D, both of which had bad electrolytics. The 128D also had a bad transistor in its switching power supply, which was an interesting bit of troubleshooting to find. In both cases, it was slow and tedious work, but luck was on my side and both machines ran after repairs. You'll have to decide if your B128 is worth the effort.

[BigEd]

It's not what you're asking for, vmarcus but I notice Jim Brain built Fake6509 which is, I gather, a workalike board to replace the 6509 - see
MOS 6509 look-alikes?
which links to the Nu6509 repo.

For backgrounders on the CBM-II line, see
https://www.floodgap.com/retrobits/ckb/secret/b128.html
http://www.6502.org/users/sjgray/comput ... index.html

[vmarcus]

BigEd wrote:

It's not what you're asking for, vmarcus but I notice Jim Brain built Fake6509 which is, I gather, a workalike board to replace the 6509 - see
MOS 6509 look-alikes?
which links to the Nu6509 repo.

For backgrounders on the CBM-II line, see
https://www.floodgap.com/retrobits/ckb/secret/b128.html
http://www.6502.org/users/sjgray/comput ... index.html

Hey,
thank for your response.
However, this is just a opposite of what I was looking for.
Basically, I have a fully functional 6509 cpu, but the issue is with main mother board.
If I'm not mistaken, the Fake6509 is replacement of the extremely rare to 6509 cpu for a mother board missing that cpu.

[vmarcus]

Hey, I like to give thanks to all who responded.

Well, like me reintroduce myself. Basically, I am career programmer for the past 29 years, primarily in Cobol and IBM mainframe assembler. So, I’m pretty much been a purely software guy, now attempting to dive into the world of digital electronics. Though, my introductory experience was a C64, of which I then ventured with C128.
Been decades ago, when I first learned to comprehend the differences in little endian vs big endian, between 6502 and z/architecture.

Yeah, I recall back when I was teenager, in the mid-eighties, of those ads from Protecto Enterprises for B128, been wondering what was the deal. http://www.6502.org/users/sjgray/comput ... tecto2.jpg

BigDumbDinosaur wrote:

The B128 was the Rodney Dangerfield of the Commodore family, and for good reason.

Though, after seeing Michal Pleban's demo, where he reversed engineered one of those 8088 adapter boards and had written all the necessary "glue" (in software) in bridging the interrupt calls for keyboard and screen I/O, I was pretty much impress, he was able to run MS Dos on this machine. Seemed an incredible miracle in my book. https://www.youtube.com/watch?v=u4O8dIvz3WI

I suppose, that sparked my curiosity.

[Dr Jefyll]

Nice to have you with us, vmarcus

C64 to IBM mainframe sounds like an interesting journey...

-- Jeff

[BigEd]

I think I'd go with replacing all the capacitors, as BDD suggests - it's a relatively mechanical operation, and it stands some chance of fixing the problem.

[rpiguy2]

vmarcus wrote:

BigEd wrote:

It's not what you're asking for, vmarcus but I notice Jim Brain built Fake6509 which is, I gather, a workalike board to replace the 6509 - see
MOS 6509 look-alikes?
which links to the Nu6509 repo.

For backgrounders on the CBM-II line, see
https://www.floodgap.com/retrobits/ckb/secret/b128.html
http://www.6502.org/users/sjgray/comput ... index.html

Hey,
thank for your response.
However, this is just a opposite of what I was looking for.
Basically, I have a fully functional 6509 cpu, but the issue is with main mother board.
If I'm not mistaken, the Fake6509 is replacement of the extremely rare to 6509 cpu for a mother board missing that cpu.

Did you try sending it to Edward Shockley? He is pretty much the last B-Series repairman in the US and will repair for free for price of shipping (if no ICs are required).

https://www.insectria.org/b128/forsale.html

[vmarcus]

Well,
as a status update, alone way of my journey, I also got a hold of B128 – model CBM 610. This one had a with a 220-v power supply.
Being here in the US, I decided to replace the power supply on my own.

Lucky, I was able learned from others. Basically, I gutted out the original power supply circuit board and replace it with a MeanWell model PT-65B. Then I had to supply the 60Hhz timer feed, as required for the Video. Finally, taking the easier way out, I got pair of adjustable 555 timer module. Using a oscilloscope, I turned the variable resistors to desire frequency. Using one of the two +5v leads (at 5.5a) from power supply, I supplied the power as it, the timer module. Assuming, the module would pull in power as needed, would out burning out one of the modules. So, to remedy the situation, I limited the current with a 0.5 amp fuse, using the other module.

Well, while I was at it, since I had available +12v, I decided plug in the SBC Arm board that I had, into the mix.

So, now there are 2 video outputs from the same box, for this demonstration. I figure, I'll need fans at this point, but I'll need to first resolve issue some keys on not working on the original Keyboard. Yeah, some reason, the available ram in bank 3, is significantly reduced. Ugh, more hardware issues.

Ah, nothing like comparing an actual machine to the Vice (Commodore) emulator. A

[commodorejohn]

I really need to get around to troubleshooting my B128 sometime...power comes on, but nothing happens. Of course, I have no peripherals for it anyway...

[vmarcus]

Hey,
it appears I must be one of the luckiest guys, on the planet.

As with my original CBM 710 (B128) mother board, I just knew I toasted this thing. So, luck would have it, I just happen to come across a CBM 710 mother board, that someone was selling on eBay. Apparently, there was issue with the PLA chip, but luckily, I swapped in the one from my original board. Even thing worked out. Surprisingly, this one worked out better than the origin. Looks like this one had some nice work done. They, replaced the original rom chips with a daughter board with one containing three 64k-bit HN4827G4G EEProm chips. They also, soldered in IC Sockets for banks 3 & 4. For a 40-year-old board, this one appears almost like pristine condition, compared to the original board. Originally, upon several attempts at cleaning the contacts for the keyboard port, I thought my keyboard some shotty keys. But when I plugged in the keyboard this time, all keys seem to work.
Next, I would like to follow up with the following:

• 1. Fix/recondition monitor, which seems to dim out, go totally black within 5 minutes.
  • a. Still not something, I’ll attempt without some serious instructions.
  • b. Saw a YouTube video, where a guy used a screw driver with a dangling alligator clip to discharge the CRT Yoke. I’m still like hanging, dude was the other end of the wire, attached to a ground? Serious, concerns about discharging residue high voltage charges.
  • c. While looking at the CRT Schematics http://www.zimmers.net/anonftp/pub/cbm/ ... matics.pdf, my only clue is to follow only the +12v supply line, replacing capacitors along the way.

• 2. Construct an external (composite) monitor interface.
  • a. Looking at CN10, not sure if this is technically inline 6 pin molex connector?
  • b. But basically, I’m assuming I just to replicate the comp video line, like that from the CBM 610 model.

• 3. Replace power supply with an ATX supply.
  • a. I already have an ATX PSU that would fit in this area. And I have ATX power break out board.
  • b. My challenge, is replicating the 60Hz line, and addressing the difference in current (i.e. +5v - 15a to 4a)