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Saga of my Model III...

A

Al Hartman

Veteran Member
Joined
Aug 13, 2013
Messages
912
Location
Keansburg, New Jersey, United States
Last year, I bought a Model III from M.A.R.C.H. at the estate sale last summer (a local collector had died, and M.A.R.C.H. was helping the widow sell off his collection.)

It was a two disk drive 48k unit. One disk drive had a broken latch. The unit powered up to garbage, and there was no floppy access. The floppy controller to logic board cable was bad.

I got it home, removed all the RAM, cleaned the corrosion of the legs and reinserted them and the unit worked OK as a cassette unit. My friend came over and we soldered in headers and used a 40 pin cable to replace the floppy controller cable, but the floppies still didn't work because the power supply was bad.

I bought a replacement supply in January, but didn't get to put it in until yesterday.

It now boots to floppy though the drive 0: is bad. Hopefully, the bad drive can be fixed. We took the good door from the 0: drive which didn’t read disks, and moved it to the 1: drive which worked but had a bad door latch.

My friend Tom is going to fix the broken latch and see if the drive will work after a cleaning and speed adjustment. I was out of alcohol, and didn’t have a florescent lamp for the speed test.

The model III also has a bad keyboard, so if anyone runs across a Model III keyboard that works, let me know.

So, incremental progress was made. And I know what I need to get it running fully.

If anyone wants to buy the bad supply and fix it, drop me a line with an offer. I'm trying to pull together cash to buy a new keyboard.
 
Hi Al, sounds like you are making good progress.

Just some info, most of the time I see disk drives with broken doors, it's due to the early design doors which used the two small white pins, they are a friction fit into the door, and a minute crack prevents them from being retained in place so they fall out and the door buggers up. Recognising this, TPI/Tandon improved the design with doors using a long brass bar, and you rarely see these give any trouble. I sell these doors (removed from dead disk drives) but the shipping to the USA would outweigh the cost of the door assy ($5).

With regards to drive speed, I almost never see the drive speed far off its original setting, even after 30 years. Don't play with the speed control, but make sure the head is clean. the felt pad is in place, and that the carriage assy moves back and forth smoothly with slight resistance. If in your testing you find you need a felt pad or logic board assy I sell them also...

With regards to your keyboard, basically if 15 or fewer keys don't work then I repair the keyboard rather than look for a replacement. The individual key switches can be unsoldered, disassembled (4 lug type), and replaced if necessary (both 2 and 4 lug variety). Yes you guessed it I sell the key switches but this is not meant to sound like a sales pitch.... just stating that these things can be fixed frugally and properly if needed.

Cheers,

Ian.
 
My friend Tom is going to replace the bad hinge with a real brass hinge, so that is taken care of.

He has been repairing Tandon drives since the early 1980's, so he'll get it working again.

Next time he comes, we'll test the keyboard a bit better and see which keys aren't working.
 
I would certainly replace that yellow one marked "0,1uF @ X" on top. That is one of the Rifa brand paper dielectric caps which commonly burst into flames these days. My Model 4 supply had three of them (with different values), but I only see one of them in your picture.

You need to use the right kind of capacitor to replace those. They are what are called "safety rated" capacitors, and they are designed to fail in a specific way to help keep you breathing. They are used for EMI (electromagnetic interference) filtering, and are directly connected to the AC mains in operation.

These are the capacitors that I used in my models II, 4 and 12:

ECQ-U2A103ML CAP FILM 10000PF 275VAC RADIAL
ECQ-UAAF104K CAP FILM 0.1UF 275VAC RADIAL
ECQ-UAAF224K CAP FILM 0.22UF 275VAC RADIAL

The ECQ-UAAF104K capacitor is likely to be a suitable replacement for the cap that I see in your picture.

Regarding the transformer, I'd say that if the supply works, leave it alone. But I replace those yellow Rifa PME271 series capacitors on sight, because they have a bad habit of catching on fire after aging for a few decades.
 
These are hearty old PSUs which I have seen looking pretty tarnished/dusty/burned but still run.

On those PSUs the only real problematic capacitor is the one Mark mentions and you should replace that one with a Yellow X2 rated cap.

Ian.
 
This PSU is not working. I'm going to try to change this capacitor to see if it will help. I'll also change the ones in the working supplies in the Model III. Next time I open it up, I'll take pictures of the supplies and see if I need to order other caps.
 
Changing that particular capacitor is unlikely to help bring the supply back to life. Its only purpose is to make the power supply emit less radio interference, and the usual failure mode results in a very obviously burnt capacitor. Removing it entirely from a working power supply would probably still result in a working power supply that may or may not have trouble passing FCC certification.

Ian and I, and others who have been burnt by that capacitor (pun intended!) replace that style of capacitor as a safety concern, but it's probably not your culprit in the bad supply. Replacing any Rifa PME271 series caps in your working systems is a good idea though, to help keep their smoke inside. They're easy to recognize by having either a translucent yellow shell, or a burnt crater of minor devastation. There's a pretty good chance that the three part numbers I provided above are suitable replacements for the Rifa PME271 series caps in your other machines, but feel free to ask if there's any doubt once you see what's in there.

Unfortunately, debugging switchmode power supplies can be challenging, even for experienced electronics dudes or dudettes. If it was my pile of computers, I'd probably set the dead PSU aside as either a source or parts or something to fix later, and put off fixing it as long as I had other supplies to make my systems work. If the transformer that looks burnt to you is actually burnt out, then you will not find a replacement anywhere other than in another power supply of the same or nearly identical design. The transformers and coils in computer-sized switchmode power supplies are custom-designed nearly 100% of the time, and are almost never off-the-shelf parts that an electronics supplier would stock. If the power supply was done in by one of its capacitors or small discrete components, then it may be fixable without having to gut a donor supply.

In any case, good luck making progress!

I'm working on a Kaypro II right now in which a previous owner relocated a brightness potentiometer which is in series with the video signal between the main board and the monitor board. He extended the wires with speaker cable, spliced in with wire nuts. Yes, WIRE NUTS. Big orange ones. In the video signal path. :headslap: I'll probably make a blog post with pictures late this evening.
 
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