Commodore 128 tune-up - Page 1
Forum section: electronics / repair

author: hernandp
author_rank: Newbie
timestamp: October 29, 2022, 01:43:03 pm
content: Hello EEVBlog community, this is my first post, I'm looking into some guidance on diagnosing and "tuning up" a Commodore 128 computer (NTSC) that I got from a local guy. The equipment boots, loads programs without any issue from disk. It has issues that I want to iron out for it to be fully functional, if its possible.I detected the following problems:No audio (buzzing / hum)Characters "vibrating" a little in composite-mode video outputColor RAM problem? Character color may change in editorI would like first to address the audio problem. To begin I want to show the motherboard naked.KeyDOS in U36 and JiffyDOS ROMs installed:A solder mess near CN3SID6581R3The 8502 and Z80 microprocessors.Red wire patch is present in all C128s, so its ok.So, here's the audio problem that I'm getting:The SID 6581 R3 chip may be fried after all. At least the audio circuitry, because POTX/POTY pins that read joystick input work.Taking into account the official C128 troubleshooting manual (https://retrofixer.it/download/C128_Diagnostic_Instruction_and_Troubleshooting_Manual_(314060-01-02).pdf)I did some measurements.SID pins 25 and 28 are connected with +5 and +12VDC rails, I got:Pin 28 ~4.9VDC (correct)Pin 25 ~10.2-10.4VDC (a bit low)Continuing with the troubleshooting steps, I got 10.49V / 11.4V for the C111 cap and CR10 diode ( <12V).Before testing the bridge rectifier I took the old C128 power supply and measured voltages, got proper 5VDC and 9VAC.So up to this point two things in mind I want to share and get opinion:The VICII video chip is fed by the 12VDC rail, so could the lower voltage there be the cause of tingling characters? (I need to meter voltages in the VIC chip itself first to confirm)It is possible for just 1.5V in difference to make the audio chip buzzing or not audible?I will continue posting with my new findings. I hope there are Commodore fans here in EEVblog forumThanks in advance.

author: Peabody
author_rank: Super Contributor
timestamp: October 29, 2022, 03:23:47 pm
content: I have no experience with the C128, but to me the buzz sounds like a power supply issue - maybe a bad electrolytic capacitor. Do you have software that normally makes the SID produce audio? If so, it would be interesting to see if the music is produced along with the buzz. If it is, then the problem probably isn't the SID.Or, do you know if it's ok to power up the board with the SID chip removed (assuming it's socketed)? If you still get the buzz with no SID chip installed, then maybe the SID is still good.But you might want to watch the Youtube videos on repairing C128 audio issues. And there may be a C128 forum somewhere, or a C64 forum where C128s are also discussed.Good luck in your quest.

author: hernandp
author_rank: Newbie
timestamp: October 29, 2022, 04:14:40 pm
content: It is ok to run system without the SID, and since this is socketed I pulled it off and turned on the system. No more buzzing. Probably SID audio circuitry is blown up.Anyway I will measure +12VDC again to see if my hypothesis of "noisy video" is due to lower 12V rail.

author: xmris
author_rank: Contributor
timestamp: November 04, 2022, 06:59:18 pm
content: as your C128 isn't dead and that's good , there are various testing PRGs like this :https://commodore.software/downloads/download/203-c128-testers/1158-c128-factory-test-v1-2some of them need special 'dongles' that you attach to ports - in order to check the ports, but I believe you can test the ICs without them, I ve used a similar as I had issues with the 9VAC of my C64c - luckily was an easy fix : bad power cordBTW I m not that C= repair expert , but if you can invest on a over-voltage power protection kit get it, it may save your C128 one day - unless you have a very good (non C=) PSU

author: hernandp
author_rank: Newbie
timestamp: November 05, 2022, 03:45:05 am
content: Thank you for the test program!The PSU is the 128 original -at least from the outside-, I measured the pins and still gives correct 9VAC and 5VDC power. But it's an old power supply for sure.After some time (~15 mins) of being powered up, the VIC II shows top and bottom border glitches. So another thing to investigate.Regards.

author: hernandp
author_rank: Newbie
timestamp: November 05, 2022, 04:54:08 am
content: A very interesting thing to note is that modern TVs try to “over correct” input frames causing unintended artifacts on old systems video output (especially composite). Image is much more stable in a LCD monitor:

author: Bud
author_rank: Super Contributor
timestamp: November 05, 2022, 05:13:19 am
content: Quote from: hernandp on November 05, 2022, 03:45:05 amLooks like you are overdriving the TV. Check the output composite signal with an oscilloscope, should be 0.7...1V peak-to-peak. If it is larger try inserting a 100 Ohm resistor in series.

author: hernandp
author_rank: Newbie
timestamp: November 05, 2022, 06:26:06 am
content: Quote from: Bud on November 05, 2022, 05:13:19 amJust as a sidenote, LCD monitor shows the same lack of chroma in top and bottom borders and the non uniform color across screen.

author: darkspr1te
author_rank: Frequent Contributor
timestamp: November 05, 2022, 07:22:38 am
content: I would look up Jan Beta on you tube, he did a excellent video on re-capping and re-doing the dodgy psu the 128 has that can kill the 128 if not addressed.also schematics are herehttp://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c128/index.htmland you can also buy a new 128 main board with supporting components and transfer all the magic chips if you so wish found herehttps://corei64.com/shop/index.php?route=product/product&product_id=287i also recommend you get a cartridge emulator as that can help testing rom issues by swapping the roms out in the address space and some test can be run with faulty ram from the same.good luckdarkspr1te

author: Bud
author_rank: Super Contributor
timestamp: November 06, 2022, 12:02:42 am
content: Quote from: hernandp on November 05, 2022, 06:26:06 amIf luma signal is too strong it will get into the chroma channel in the receiver (TV or display) and be falsely decoded as color. This is a fundamental problem with composite video. To remediate it, reject filters are used at the chroma carrier frequency, and I recall there is no such filter in Commodore. Regardless, an overly strong brightness channel would cause color interference.

author: hernandp
author_rank: Newbie
timestamp: November 06, 2022, 08:25:24 pm
content: What is really strange is that the VIC glitch wasnt there days ago until I tried to adjust the CT (trimmer-capacitor), I dont believe a chip be killed by this failing cap which is connected to 14.31818 crystal oscillator and the 8701 clock gen. Maybe the VIC was already on the course of it's demise.

author: darkspr1te
author_rank: Frequent Contributor
timestamp: November 13, 2022, 04:01:14 pm
content: Quote from: hernandp on November 06, 2022, 08:25:24 pmI Used to work at a c64 repair center for my summer time monies, we often found one main fault which we have normally repaired before then we open and do the repair work and find others, yes we were grounded all the time when handling the systems but still found some chips not related would just peg it. in some strange cases we found that two systems with same fault would just work by swapping the two chips between the systems. we did do 24hr burn ins and they still worked swap the chips back and they would black screen. I never worked there long enough to find a cause/true solution but it happens with MOS chips i was told by my teacher/boss.There was one series of boards sold in the EU/uk at the time which suffered board warpage from heat and that would 'crack' the internals of a well soldered chip. these were easy to spot as pressing a corner would allow boot but take the pressure away and it would crash. if i recall right it was the well made german boards that has this initial issue and was solved by putting sockets on all large chips and reducing the copper amount around the center of the board as that would soak up heat but expand sideways first causing the issue , we noticed the change after the first few boards hit the offical repair centers which is where would buy parts boards as in early days the centers would just send a new c64, some customer would send in bread bin and get the later c128 style case and really complain and demand the old case back, i dont know how that was handled though.darkspr1te

author: hernandp
author_rank: Newbie
timestamp: November 14, 2022, 09:57:22 am
content: Quote from: darkspr1te on November 13, 2022, 04:01:14 pmAccording to own words of Commodore engineers MOS (later CSG) was not known for high yield, and in the same semiconductor fab run the chips [the ones that worked] had a lot of variance between them. It is known that you can try e.g several VICs or SIDs of the same revision and get different video/audio qualities. This is not strange as CBM was always looking for radical cost cutting measures.I'm waiting for a replacement VIC, so I 'll be keeping posting updates here for anyone interested. Now I'm getting +3.9VDC instead of +5V on the SID socket, so another thing to diagnose. Power supply is perfectly working however.

author: hernandp
author_rank: Newbie
timestamp: November 25, 2022, 06:44:00 pm
content: Updates on the C128 tuneup quest. The VIC-II video chip on the machine was probably defective as I replaced it by another MOS 8564R5C and I got rid of the color bleedings and strange glitches.Comparison (the color bleeding on the borders is clear on the old chip, bottom border also was practically white). Characters experienced subtle chroma problems also.Defective VIC:New VIC:So far so good, I'll do some more deep tests to check.