VIC-20 repair log #3 – Power tests

We’ve recently acquired a pair of non-functioning Commodore VIC-20 and are looking at fixing them, but quickly realized we’d need an AC-AC transformer.

Power Transformer

The original VIC-20 AC power transformer pop online for auction from time to time, but prices are ridiculously high. Replacing the original brick simply requires an appropriate AC transformer; which is much less complicated than replacing a C64 power brick, or a Colecovision power brick. Readings online suggest to have a transformer of at least 3A in the 9-12VAC range. Sadly, the 9VAC section of the C64 power brick is only 1A, much too low to properly supply the AC-only VIC-20. The spare parts bin does have a few 9VAC and 12VAC wall warts, but the strongest one was 2A, which is also too weak.

There are a multitude of 9-to-12 VAC transformers available online, some more practical than others. Some pricier than others. Some much bigger than others. One that seemed practical and economical was meant for alarm systems, rated 9VAC 3.34A; hopefully more than what’s required. Upon reception we quickly tested its output to read a satisfying 9.95 VAC without loads.

Alarm system 9VAC 3.34A transformer

The transformer outputs a nice 9.95VAC without load.

Power Connector

To connect to the VIC, the proper connector is needed. In these VIC model, the connector also include an imbedded switch – likely the specific power receptacle was also used on some battery-operated apparatus and that insertion-switch was used to disconnect batteries and connect AC power.

Nevertheless the correct size is required for pin connection, but also to activate the switch and permit power to reach the PCB. Not every 8-shape connector would fit the bill.

There are somewhat similar connectors on apparatus here and there, including IKEA (no affiliations with wereallgeeks) but none seemed to have the proper physical sizes and pin sizes to work correctly.

Various connectors

In the end the spare parts bin served – a few old radio power plugs fit perfectly. Apparently some razor plugs also work well, but the hunt ended with our radio plugs.

Recycling radio plug for VIC-20

Splicing this cable and screwing it to the power brick’s power output is all that’s needed to proceed with testing.

Safety first

Before proceeding… some of those chips are well over 40 years of age now. The CPU, VIC (Video Interface Chip) and VIAs (Versatile Interface Adapters) as well as all the ROMs are socketed on both boards, so we pulled them out.

The board with the mention “bad VIC” had the VIC inserted with a pin pulled out.

VIC-chip with a bent pin.

The pin seems to have been pin 18, which is one of the paddle potentiometer pins so this should not have prevented normal operations.

We labelled the VIC-20 with the “dead VIC” mention as #1, and the other “graphic mess” as #2. The chips also got markings to differentiate them from one another.

Labelled VIC boards and chips

With all the chips pulled, we could test the main power circuit. The 9VAC reads clean when the switch is off, but it reduces to just above 5VAC with the added load of the motherboard as soon as it is powered on. This seems on the low side. There was similar readings on both VIC-20s

AC Brick reads just above 5VAC with load

That said, testing at multiple locations proves the 5V DC rail to be appropriate, on both units, as well as the user port 9VAC output.

5VDC at MOS 6502 CPU

5VDC at VIC-20’s user port

9VAC at VIC-20’s user port

VIC20#1 has 4.89VDC and VIC20#2 has 5.05VDC. With the power block and power conversion circuit giving these readings, we feel safe to continue with testing.

Next step: VIC-20 repair log #4 – clock signals

2024/04/172024/05/07 by wereallgeeks Categories: 100% Geek, Computers and technology, Geekyness, Projects, VIC-20Tags: 100% Geek, commodore, commodore computer, Diagnostic, DIY, electronics, fixing, for parts or repair, Geek, Repair log, retrocomputer, Retrocomputing, VC-20, VC20, VIC-1010, VIC-20, VIC20 2 Comments

VIC-20 repair log #2 – Initial inspection

We’ve recently acquired a pair of non-functioning Commodore VIC-20 and are looking at fixing them.

Overall condition

The boards are both somewhat clean, and visual inspection does not reveal anything weird. No missing chips, no lifted pad. No bodge wires. Except that one of the boards has had its game port replaced in the past, but not with a correct right-angled board-mounted one, rather with a set of wires.

VIC20#1 sideplate

VIC20#1 DB9 port

While we applaud the resilience of whoever fixed this, we will likely replace it with a more appropriate part.

Square 2-pin power supply?

When bidding on this specific set of VIC20s, we thought we’d be powering them with our trusty modern Keelog (no affiliation with wereallgeeks) replacement Commodore 64 Power Brick. But something we didn’t notice in the auction that was crystal clear when we got them, is that it won’t be possible.

Both VIC-20 boards are of the original, 2-pin power connector, with only 9VAC power input, power conversion being done onboard. These are the early types VIC-20s, often gold or silver labels, and perhaps maybe even a PET keyboard. The PCB was designed in 1981. Latter, reduced cost versions VIC-20 used the same connector and power brick as the Commodore 64.

VIC20#1 sideplate

The power brick on these early VIC-20 were 9VAC to 10VAC at 3A or 30VA. With a square connector. They sometimes pop on auction sites, but pricing is ridiculously high for just a power transformer. Readings online on atariage or other forums demonstrate the connector itself isn’t exactly easy to locate either.

Bypass the AC and power them with DC?

The same forum threads shows there are people who simply power those VICs with DC power – sometimes using the onboard power conversion trough the original connector, using the unmodified power circuit; and sometimes bypassing it with alterations to the circuitry; either with the original connector or an added one.

But starting from an unknown board, it may be a better idea to begin with proper AC power and diagnose the whole thing. Alterations may come at a latter date.

So, the hunt for a power solution begins.

Nest step: VIC-20 repair log #3 – Power tests

2024/03/272024/04/23 by wereallgeeks Categories: 100% Geek, Computers and technology, Geekyness, Projects, Retrocomputing, VIC-20Tags: 100% Geek, 9VAC, broken, commodore, commodore computer, computer, computers, Diagnostic, DIY, electronics, fixing, for parts or repair, Geek, PET keyboard, power transformer, repair, Repair log, reparation, restauration, retrocomputer, Retrocomputing, VC-20, VC20, VIC chip, VIC-1010, VIC-20, VIC20 2 Comments

VIC-20 repair log #1

We’ve recently acquired a pair of non-functioning Commodore VIC-20 and will be looking at bringing them to life, posting about progress in here.

There is a lack of love for the VIC-20 on the internet; one reason might be because they are scarcer to find compared to the more popular Commodore 64, but the VIC-20 paved the way to the world of friendly computing at home.

These have been obtained from the your favorite auction site with mention that they are for parts or repair, with one not even showing a black screen. No case, no keyboard, no power supply, just 2 populated VIC-20 motherboards.

two damaged VIC-20 motherboards. Are we up to fix them both?

Notice the bad VIC and graphic mess labels on them. One of the ROM even has an X label on it. What are the blue dots? Lots of the chips are socketed, which should make for easier early diagnostics…

Will they work? will they not? we’ll see in upcoming log entries. Stay tuned.

Next step: VIC-20 repair log #2 = Initial inspection

2024/03/062024/04/23 by wereallgeeks Categories: 100% Geek, Computers and technology, Geekyness, Projects, Retrocomputing, VIC-20Tags: broken, commodore, commodore computer, Commodore VIC 20, Commodore VIC-20, computers, Diagnostic, DIY, electronics, fixing, for parts or repair, Geek, Repair log, retrocomputer, retrocomputers, Retrocomputing, VC-20, VC20, VIC chip, VIC-1010, VIC-20, VIC20 3 Comments