Commodore PET 4016

[ScottishColin]

On further examination, both of the TIP29s have been replaced before.

The Q1 TIP29 was shorted between Base and Collector. However, once removed, it's not shorted on the TIP29 but is on the motherboard so I assume my next port of call is the Q2T2222 IC.

Q2 is not shorted.

Colin.

[TonyDuell]

The Q2T2222 is shown on sheet 3 of the schematic. As the diagram indicates it's 4 individual transistors and can be tested as such. The 2N2222 transistor is a pretty common part and could be replaced by many types of small signal NPN transistors if you have trouble getting them

[ScottishColin]

The Q2T2222s are on the slow boat from Poland.

Luckily they only affect the cassette port (so I will only be typing any programs in as I can't save them).

A cursory check of the IEEE port failed but I'll proper clean up the motherboard connectors and try again later.

Back to the memory ICs I think.

Colin.

[julie_m]

2N2222 is an example of what one of the magazines used to call a "TUN" -- Transistor, Universal, NPN.

BC547 should be a suitable replacement, or BC337 if it's expected to carry more than a couple of hundred mA.

[ScottishColin]

In other related news, a replacement original character ROM has arrived from Polyplay in Germany - 9 euros plus shipping so I'm happy with that.

Also I've bought a 4016 label from Corei64. This may turn out to be the most expensive part for this PET but they are the only sellers of a 4016 label I could find and they have been very helpful.

Two suppliers I would reccomend.

Colin.

[ortek_service]

Well ii seems both of the TIP29's appear to have survived (No short-circuits at least, but probably worth doing a proper transistor-test on these whilst out of circuit to measure the hFE etc (as b-c & b-e junctions diode-checks can sometimes read OK, even with no-gain as I once found)).
I assume your DMM can do this? / I recall you recently got one of those universal testers.
So ones you ordered will be for future spares, as it does seem like these do occasionally get damaged (maybe they should have fitted a fuse on the motor supply outputs).

And probably might as well remove the Q2T2222 and fully-test the transistors in that as well, whilst waiting for the replacements / could have a go at putting some discrete transistors in for now (maybe pushing leads into a spare IC socket, and fitting an IC-socket to the board, to put that into / eventually the replacement 'IC'.


With the memory tests, were you still waiting for Julie's test code to have some additions to diagnose the upper 16K more-fully? So didn't want to remove IC's until that confirmed the same as other testers? (Although, if socketing these, could always refit the old ones to see what happens with future versions).
And there's still the mysery as to why PETTESTER seems to crash (Only when Jumper is set to test upper 16K?), so can maybe see if fitting all-good DRAM's resolves that with it working OK (Like it did on other PET's - with all good DRAM's at least).

[ScottishColin]

In my cheap Multi-function tester, the extracted TIP29 tests as a diode which doesn't seem quite right.

With the meter in diode test mode, I get 0 readings wit hthe exception of:

(Red) Base -> (Black) Connector - +0.67v
(Red) Base -> (Black) Emitter - +2.6v

Colin.

[ortek_service]

Well you could double-check with your DMM, on diode-test, with red lead to the pin on the far-left (base) (looking at the front, with pins downwards) and black lead on middle (collector), then right (emitter).
But, unless you've got a bad-contact on that tester / you're not using the right contacts on the socket as it can only do certain combinations / it isn't working too-well. then it does look-like at least one of your TIP29's is faulty and you'll be needing the ones you've bought.

So a previous owner / storage conditions would seem to have managed to kill both the output transistor and the driver for it (Although maybe on different channels - won't know without testing all of the transistors).

[ScottishColin]

I have a short between pins 8 and 10 of the Q2T2222 so I'm assuming that has failed

Colin.

[ScottishColin]

That's correct. I'd still like to help the development of the software with this PET if that's possible.

I'm in no hurry and we go away Mon-Fri next week anyway so I won't be able to do a lot.

Colin.

Originally Posted by ortek_service

With the memory tests, were you still waiting for Julie's test code to have some additions to diagnose the upper 16K more-fully? So didn't want to remove IC's until that confirmed the same as other testers? (Although, if socketing these, could always refit the old ones to see what happens with future versions).

[ScottishColin]

In more better news after a clean of the IEEE port, my PET2SD works fine so there's (surprisingly) no MC3446s to replace (a common theme in previous PETs).

Colin.

[SiriusHardware]

With the meter in diode test mode, I get 0 readings....

Going back to past experience I'm going to assume that by '0' you mean overload / infinite rather than 'zero'. So that will be fine for some of the junctions / directions but...

(Red) Base -> (Black) Collector - +0.67v
(Red) Base -> (Black) Emitter - +2.6v

Base to collector is about right but for an 'ordinary' NPN transistor the base-emitter voltage drop looks too high. You can verify this by doing the same 'diode test' tests on one of the new TIP29s when they arrive.

I have a short between pins 8 and 10 of the Q2T2222 so I'm assuming that has failed

It does sound that way, provided there isn't some other kind of short (like a solder splash) between those pins or the PCB tracks serving them. How many actual ohms (if any) is the short between pin 8 and pin 10?

[TonyDuell]

According to the data sheet I've just looked at (OnSemi) the TIP29 is not a simple transistor. It's a darlington with a couple of base-emitter resistors and a collector-emitter diode.

I'd therefore expect the b-e drop to be about 1.2-1.4V

My guess is that one of the junctions is open and you're seeing the effect of the resistors. So it's faulty, but don't be surprised when the new ones don't test as you might expect.

[SiriusHardware]

Interesting one that, this is the first time I've ever come across a data sheet for a Darlington, in this case this one...

https://www.onsemi.com/pdf/datasheet/tip29b-d.pdf

...which doesn't even mention the word 'Darlington' and has the transistor drawn as a single transistor rather than the two-transistors-within-a-circle that I would expect.

There's a diagram showing a transistor symbol (again conventional, not Darlington) with a couple of resistors and a diode attached but that would appear to be a test circuit rather than the internal diagram of the device, since the accompanying text talks about varying the resistors.

However, in the electrical characteristics part it does say that the turn on voltage (when applied to base-emitter) needs to be 1.3V or more and that would indeed be typical of a Darlington.

Seems like overkill, since the transistor in the quad-pack device is already connected in a 'Darlington pair' configuration with the TIP29 so if the TIP29 really is a Darlington in and of itself, it's more like a Darlington 'triplet'.

[TonyDuell]

That's not the data sheet I found on 'alldatasheet.'. Mine explicilty shows a darlington schematic.

It's odd, the turn-on voltage would appear to be 2 diode drops, implying a darlington circuit, but Hfe is very low for such a device.

Colin, how does the 'other' TIP29 in the PET test?

[ortek_service]

I too wondered whether it was a darlington, as I had previously encountered some TIP14x? ones that were but not come across the TIP29 before.
Although, when I'd done a quick-google to check whether it was PNP or NPN (as hadn't got schematic to hand), to tell Colin which DMM probe to put on the base, I hadn't seen Darlington mentioned in the description.

Some of those automatic universal testers may not recognise Darlington's depending on their software / hardware (although I'd hope most do by now) / get confused by any built-in parallel diodes etc.
But the transistor hFE measurement built into many DMM's should be OK with Darlington's, and a diode-check of the junctions should also work / show if you've got twice the standard 0.65Vbe, that a Darlington will have.

Quite a few power transistors do have a rather low hFE (and even-lower DC-gain at high collector currents - Sometimes only around 10! Hence usually moved to PowerFET's these days), so a darlington version may not have that high DC-gain compared to a standard 2N2222 etc. small-signal transistor.

[SiriusHardware]

One of the defining features of Darlingtons is that they (usually) have a very high gain due to the fact that the second transistor is effectively amplifying the already amplified output from the first.

[ScottishColin]

OK. Q2T2222 (UE16) removed and a socket put in place while I await a replacement UE16.

4 x BC337 inserted as per page 3 of the schematics and a replacement TIP29 installed.

Both cassette ports are now working fine.

Thanks.

Colin.

[julie_m]

Guess who's back, and look what she's got!



I'm about ready to call this a release, now.

testrom_10.zip

The bit faults are now split into high and low chunks (it turned out to be easier than I was thinking) and there is now a refresh test; which just picks up on any zeros in the alternate block that have changed to ones during the last "picture pause" between tests, and indicates them with an "F" in the write failure bits for high or low memory. The sequence of page tests has also been altered. After testing the stack and zero page, there is a round of testing pages &02, &04, &08 and so on up to &80, against page &00. This ought to show up how much memory is actually fitted: on a 16K machine, page &40 will clash with page &00 and on an 8K machine, page &20 will clash with &00. If there were such a thing as a 4K PET, then page &10 would clash with &00. After testing as far as page &40, the program then goes on with the same process as before, testing page &02 against &01, &03 against &02 and so on.

[ScottishColin]

Hi Julie - thanks very much for this.

I don't have time to run a full test right now but here's a couple of screenshots:

1) showing success on the initial tests
2) showing a failure on page 40

I'll post more screenshots when I have time to go through a longer test.

Thanks again.

Colin.