Commodore SR4148R restoration finished

Hello from Commodore :)

Last time, I have installed a temporary alkaline battery pack into the calculator, to try whether it works or not. After that, I looked up the internet, to collect information about the AC adapter. I have found that, the original could provide 6V 400mA DC, through a normal, mono 3.5mm jack plug, where the pinhead of the plug is the positive polarity.

I ordered a configurable AC adapter, with 1A output, and option to set voltage from 3V to 12V. Also got a 2000 mAh NiMH battery pack, which is a low self-discharge version, instead of the original 500 mAh NiCd pack. Since the original pack was designed to 50 mA charge for 16 hours, there is no charge termination circuitry implemented inside. Which is fine for a NiCd battery. Fortunately, there is not much difference between the charge profiles of NiMH and NiCd, however a bigger capacity battery would miss the C/10h constant charge (fe.: 800mAh/10h=80mA), never charging up the batteries. However a low self-discharge version of the battery will be satisfied by the lower current.

Swapping the packs, and turning the calculator on, everything was fine. Plugging in the adapter I could measure 60 mA charge flowing into the pack, just as I expected.

It seams, that the device is expecting the battery inside heavily as a mediator/regulator, since not having the battery inside, just plugging in the adapter didn't do anything usable, just random characters on the screen (probably too low voltage for operating, didn't debugged any further).

Quickly made some new pictures, and TADA! Ready to use... erhh... charge. :)

Commodore calculator SR4148R repair

Last week I managed to put my hands on a 1974 Commodore SR4148R calculator. It was said to be non working. Never the less, I was thinking that this one seems to be in very good visual condition, just a little bit dirty, and in worst case, it could be used as a spare part donor, whenever I manage to get an other one. There was unfortunately no leather case, nor AC adapter packed with it, and the operating manual was also missing.

Getting home, I quickly opened the case, and saw that the original 500 mAh, 3 cell NiCd battery pack was in very bad condition. Electrolyte was flown out of the batteries, and made a big mess all over, inside the casing.

My first move was to take it apart, into as small pieces as I could, and wash the casing, and display. I soldered out the battery pack, and unscrewed the jack plug to be able to remove the display from it's place, and unfold all of the electronics, which consist of two PCBs, where the button back panel is a double sided PCB, and the logic board is a single sided, through hole mounted one. Awesome! The display has a series of small lenses in front of the very small LED display, melted together.

As I took it apart, when unscrewing the back panel of the keyboard, I had the feeling getting stronger and stronger, that the keys would be something very unusual these days. When lifting off finally the panel, I saw the following: every key on the keyboard is a separate small piece of plastic, with rubber/carbon circuit touch contactors, and a small spring for every single button. It also seems, that the buttons are not just simply painted ones, but there must be some protecting layer above the paint, preventing the signs to wear off. All of these features add a very pleasant, and very distinct touch feeling of the buttons, even though they are small.

I have the feeling, that this calculator was designed for eternity. The buttons practically were never meant to wear off, the PCB is small, simple, through-hole type, and very easy to fix, in case something goes wrong. Since I didn't have any jack plugging adapters right at hand, I decided to quickly wire up a small temporary replacement battery pack, from AAA alkaline batteries, just to try whether the thing as operational at all. Turned on, nothing happened. Measured the electric flow, and it was 800mA when turned on, but no display. There must be some shorts, or disconnects.

As I suspected, I have found some disconnects, since the electrolyte during the years managed to make complete PCB track segments to disappear. I have created some jump routs out of wire, and TA-DA! It works! It's consumption is around 70 mA, when simply turned on. Put together everything, and started to joy-key on some expressions.

Testing it I started to like it more and more. There are three blue keys on the top, marked EE, EE *down*, EE *up*. I have never seen such thing before. Ok, the EE is quite self explaining the exponential enter button, however with the other two, one can modify the punctuation on the fly, converting even a calculation result into other exponential result. This also makes counting 10s, 1000s, millions, very easy! I have also discovered a bug! 2^31 is OK, However, 2^32 is 4294967297 according to the calc, instead of 4294967296. However, when I divide the result of 2^32 by 2, I will get the correct 2147483648.

Even more strange, when I type in 4294967297, and divide by 2, I will get the mathematically correct 2147483648.5 as the result. 2^33 is also problematic, it is also bigger by one then the correct answer. Strange. I also very much like the small, thin LED numbers, and the strange delay we are not used to these days, when hitting the equals sign. :) After all, it is a very attractive, good to handle small scientific calculator, which was made for eternity! :)

Update: testing further the x^y functionality, it seems that it is a rounding problem inside the calculation, since 2^3-8 yields 1.6E-10. This also means, that the internal floating point representation is more precise than 10 decimals. Since single precision floating point numbers (32bits) can represent only 2^23 precision, it is too small for that purpose. Double precision (64bits) however would be a waste. My guess is it uses some arbitrary floating point representation, like 1bit sign, 8bits exponent, and at least 35bits fraction, which gives at least 44bits all together. Strange.

How I rescued a Commodore 128

I have got a Commodore 128 for free, when buying some Polski Fiat 126 parts some times before. The C=128 was waiting in his ashes, to rise, since it couldn't be turned on, and got some other issues as well.

When I pulled it out last weekend from the storage, to take a closer look, I instantly realized that the main problem was the power switch. Took one of the spare part C=64 as well, and soldered out the switch and installed it on the C=128. Checked, connections are OK, power is OK. However, there was a small discontinuity on the ground connection for the power LED. Soldering done, and shines as before.

Checking luminance on VIC

Since I didn't have an analogue TV at hand, I used my laptop with AverMedia Hybrid USB TV, to capture the display of the machine. First, I connected it to the RF antenna slot, to try to tune in the C=128 boot up screen. I only got screen pictures for some prompt times, mainly static pictures buffered by the TV card. And it was all black & white. I tried to fine tune, without chance, also tried to fine tune the RF modulator on the Commodore. The best I could get with PAL settings, is a black & white picture scrolling upwards, with great flickers on every second row. Also connected the video output to the composite in of the TV card, with the result of somewhat nice picture... in black & white.

C=128 main board

I quickly opened up the troubleshooting guide (which is for NTSC versions anyways), and followed the steps to narrow down the problem. Fortunately I have an old analogue oscilloscope, with 20 MHz measurement frequency. I quickly checked the different clock lines on the motherboard, and measured their frequencies with a rough estimation, since there is no such things on this 'scope as freq counter. The pin 16 of the VIC chip gives the luminance encoding, and pin 17 the chroma. Both were OK, however, the chroma seemed somewhat low on amplitude.

Donor C=64

So either the chip, or the M1 (the RF modulator) was faulty. Anyways, it seemed unreasonable, that the VIC chip give good signal with bad amplitude, so I turned to the RF. The modulator also contains an amplifier for the luminance and chroma output for the video connector. I measured with 'scope, and got a pretty distorted, low level output on it. So, the problematic part should be the RF modulator's chroma amplifier.

C=128 & C=64 RF modulator

I quickly soldered out the modulator from the C=64 in under 2.5 hours (since practically it is the same as the C=128 has). The procedure was pretty frustrating, not having the right tools to do it. What I missed for this action, is some kind of an IC pin heater head for my soldering-iron, and a solder sucker pump. Anyways, in 4 hours, the "new" RF modulator was in place in the C=128. Check with the oscilloscope: higher amplitudes on VIC chip, less distorted and higher level chroma on the video connection.

When I connected to the TV card again, with composite, from time to time, colors appeared and disappeared, wasn't usable. On the forums I quickly searched through, somebody mentioned that the frequency of the PAL oscillator crystal is very important, and should be very accurate. I didn't have a chance to measure frequency as accurate as 8 digits on an analogue oscilloscope, unless the 'scope can do XY plot option, and have an accurate freq generator. I lacked the later one, so I went blind. Exchanged the xtal and the variable capacitor from the C=64, and fortunately enough, it did the trick!  :)


Now I have an (other, also) fully functional Commodore 128, in nice visual condition. Later, I will try to test it also in 80 column mode, when I have a chance.

Also, I have a newcomer, an even older, Commodore CBM 720 to be restored. But this will be an other story.