Denial

Before I go off on an exploration of my other two-prong VIC-20's for a potential answer, I was wondering if anyone has had the following issue:

I have two two-prong power supply VIC-20's I'm working on. They both boot just fine. When I run the diagnostics on both of them, I get a BAD result for the cassette port test on each of the VIC's. Yet, newer VIC's (5-pin power connector) pass the diagnostics without any issues. No, I haven't actually connected a datasette up to them yet. I just find it odd that these older VIC's can't pass the diagnostics. I'm starting to question the cassette diagnostic harness.

The diagram for the harness from Ray Carlsen shows two 27r (2.2KOhm) resistors are needed, while the diagram from Zimmers shows one 1KOhm and a 2.2KOhm resistor. And the harness I'm using has a 1.2KOhm (4 bands - BROWN, RED, RED, GOLD) resistor, and a 11kKOhm (5 bands - BROWN, BROWN, BLACK, RED, RED) resistor. Strange, at least to me.

Has anyone else seen this issue?

Thank you!

Joe

(mod: post moved from Collecting and History section to Hardware and Tech section)

The reason for the two resistors is to provide a voltage divider that reduces the voltage of the CASSETTE MOTOR line to TTL/NMOS compatible levels. Some mainboard revisions actually deliver a (badly regulated) "+5 V" there, others put around +6 V there, which exceeds the allowed voltages on TTL/NMOS inputs. In the case at hand, an NMOS input on one of the VIAs pins.

First, you describe the resistor values on your tape harness as being clearly different from either the Carlsen or Zimmers variant. Both happen to reduce the 5 .. 6 V from the activated MOTOR line to voltages in the range of 2.5 .. 4.2 V, all of which are within allowed high levels. In all cases, the 0 V from a de-activated MOTOR line also leads to 0 V on the voltage divider for the VIA input pin, i.e. low level.

If the voltage divider in your tape harness has the 1K2 resistor at the bottom, the voltage of the activated MOTOR line will be reduced to 0.5 .. 0.6 V, which is still low level. While electrically harmless (so to speak), the voltage divider would not work as intended, as the VIA input pin then sees a low level all the time.

The other way round gives 4.5 .. 5.4 V for the VIA input pin and there, the upper range limit is barely acceptable and may lead to unexpected behaviour (latch-up or the like).

Thank you for the reply. I think I'm going to try Ray Carlsen's design while using a breadboard to host the harness wiring connections, including the resistors. I have a few cassette port connectors that I can put to use as well. With this configuration, I can swap out resistors quickly if I need to find what works.

Joe

I found a solution to the problem with the cassette diagnostic harness. The harness that would give me false BAD test results has the following design:


The harness above works fine on CR VIC-20's, but not with the original (two prong power) design VIC-20's.

I found Ray Carlsen's hand drawn design for the cassette harness in a document that showed all of the harness devices. I cut the following cassette section out of the design:

Using Ray's design, I built a harness using parts I had in stock for projects, and some 27ohm resistors I received today via Amazon. Here's a photo of the completed harness (the solder I was trying to use is junk. I need to get some better solder):


With the new harness attached to the cassette port, the older version of the VIC-20 finally passes all of the diagnostic tests.

Why is this important to me? Well, when I list a Commodore computer for sale on ebay, I always include a photo of the diagnostic test results in the listing. Having a photo with a false BAD result is not something I wanted to include. I could have shown that it does work by posting photos of the loading and saving from cassette, but that's not my desired style. And now that I can build a replacement two prong power supply for less than $10, I can provide a more complete system (less video cable).

If anyone would like to acquire a pre-built design of the harness, let me know and I'll build some and list them on ebay for cost plus shipping (typically $5 or $6). No international shipping - sorry!

If you have any questions, please ask!

Thanks, yet again, to Ray Carlsen for the invaluable information!

Joe

UPDATE:

I spoke a little too soon. It turns out that some of the VIC-20's work with the cassette harness that does not have the 27ohm resistors, while others need the harness with the 27ohm resistors.

I still have one VIC-20 two prong system that can't pass the test regardless of which harness I use. The cassette works fine with it, but the system just fails the test. I'm going to start replacing the diode and transistors one at a time to see if the problem goes away. But first, maybe I should swap VIA chip positions to see if anything happens there. Both VIA's passed testing in my chip tester. But real world application can give different results.

Then I have a CR VIC-20 with the DIN power connector that fails the cassette test when using the harness that does not have the 27ohm resistors, but passes when using the harness that does have the 27ohm resistors.

So it's a mixed bag of which VIC-20's like which cassette harness. But at least it's nice to have both on hand.

Joe

I need to acquire some replacement parts for a VIC-20 (two prong power) board.

Of the items that I need to acquire and replace is the transistor at Q4, the transistor at Q1, and the Zener Diode at CR1. I'm trying to resolve a false "BAD" result that is appearing during diagnostics, even though the cassette functions work perfectly. Measurements show the voltages widely varying between 4 and 5 volts on the transistor at Q1 and cassette edge connector 3 during the diagnostic. It's my understanding that I should see 9 volts at these measurement points (please let me know if I'm wrong here).

Looking in the VIC20 technical manual, the parts list shows the listed. But it shows two entries for the transistor at Q4. One for a NPN 2N4401, and the other for a NPN 2SC1815. The marking on the transistor itself shows C1815, as does it's neighbor at Q3. Which transistor do I need?


I need to order the Zener Diode and the NPN 2SD880 Transistor as well.

Any help is appreciated!

Thank you!

Joe

I just did another test measurement on a different VIC-20 (CR version). The voltage on the cassette edge connector 3 stays close to zero before the cassette port test is run, and then it goes to 5 volts during the test. On the system with the false BAD test result, the voltage hovers around 4 to 5 volts when the test is not running. So assuming that the results on the CR model (which passes the test), then the components on the two-prong power unit are allowing voltage through when it shouldn't?

Very odd!

Joe

FINAL UPDATE:

I replaced the Zener Diode, and it made no difference. So I replaced the two transistors at Q3 and Q4 at the same time (just to not do one, test, then do the other in case the first failed). And now the diagnostics pass the Cassette Test with a "OK".

Good thing too! I haven't received one of the other transistors as of yet.

Joe

Joe,

Is your CP Diag Plug now performing such that you can consistently, and reliably, verify CP performance on all VIC-20 MB designs, or is it still a "mixed bag" of sorts?

OGM

I haven't done a thorough test as of yet. But I do intend to try both CP tester styles all on of my VIC's and post results.

Merry Christmas!

Joe