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-   -   Non-working Commodore PET 3016 (https://www.vintage-radio.net/forum/showthread.php?t=174829)

ScottishColin 3rd Jan 2021 8:28 pm

Re: Non-working Commodore PET 3016
 
1 Attachment(s)
This is what the motherboard looks like now - happy to clean it further if you think it needs it?

Thanks.

Timbucus 3rd Jan 2021 9:02 pm

Re: Non-working Commodore PET 3016
 
It does not look too bad - I assume the close up shows the missing solder mask only and not bare board on the Ground line? Check anywhere there seems to be any furry marks - especially between pins or between the leads of capacitors/resistors and give it a light brush - be careful not to use nylon if possible - make sure you have a ground strap and do not do it for too long if you have to.

duncanlowe 3rd Jan 2021 9:09 pm

Re: Non-working Commodore PET 3016
 
There's some evidence of green corrosion on the right of the last photo, and a bit of grey fur in other places.

I also note that there ar lots of tantalum bead capacitors all over the board. In other devices they are a 'change on sight' component. Generally though they will be on the power supply rails, and they seem to be at the right voltages so may be something to worry about later.

Timbucus 3rd Jan 2021 9:17 pm

Re: Non-working Commodore PET 3016
 
My understanding is that tantalum's are usually fine as long as they have plenty of headroom on the voltage for transients they will experience in use - I.e. 10V tants on a 9V line will likely go bad. I think they are 25V ones so hopefully they have a bit.

I agree with Duncan U11, UG2 and EF1 def needs the green dealing with and the leg on UH2 seems rusted as well and if it should not be connected the the rail below it may be shorting to that.

SiriusHardware 3rd Jan 2021 10:36 pm

Re: Non-working Commodore PET 3016
 
Back to work tomorrow so unfortunately an early night for me otherwise I won't be able to get up at the right time in the morning, but just to observe that VR6 does seem to be OK now that you are looking for minus 5V on the -bottom- pin as indicated on the diagram attached in #9. ;) So we can now say that the supply voltages are all OK, which is a good start. Good work, Colin.

With no display you are going to start to feel the need for more than just a meter fairly soon, do you remember I asked you if your meter could measure frequency? You said your old one could not, but can the new one?

The monitors on these are quite primitive, they don't have their own onboard free-running line oscillator and instead rely on receiving a steady stream of sync pulses from the main computer PCB in order to activate that part of the monitor circuit.

If you don't have a picture on the monitor there are two main possibilities,

-The mainboard is not generating the sync pulses which the monitor needs, therefore the monitor is not being enabled.

-The mainboard is generating sync pulses, but the monitor has a fault.

To determine which of the above cases is true, the ideal tool at this point would be a scope or, failing that, something which can measure frequency because when the mainboard is working it generates pulse waveforms which have steady frequencies on the horizontal and vertical sync signal outputs to the monitor and if they are there, the frequency of those signals can be measured. We need to know whether those sync waveforms are there or not.

Conventionally, you can detect the sync waveforms with a scope, with a frequency meter, or with a 'logic probe' as suggested by Mark earlier. I'm open to suggestions for any related checks or any other checks which can be done with just a meter to narrow things down further.

One thing which has been said to be a problem on these is bad IC sockets - if you have the confidence to do so, with the power off, try removing each socketed IC one at a time, inspecting them for bent pins and then reinserting them, taking care not to bend or fold any of the pins over as you do so. Try powering the machine on again after reseating each IC. Take plenty of photos before you start and only remove and reinsert one IC at a time so you can't ever get two of them mixed up.

Edit: Just looked at the images and it does seem to have suffered from damp / moisture. If it has caused actual problems, ie, corroded away tracks or VIAs this could be be quite a challenge.

ScottishColin 4th Jan 2021 12:33 pm

Re: Non-working Commodore PET 3016
 
2 Attachment(s)
Hi - thanks for your patience through this. This is the meter I bought:

https://www.amazon.co.uk/gp/product/...?ie=UTF8&psc=1

I attach a photo showing the frequency specs. Let me know if it won't do the job and I'll ask around to borrow a scope.

I'll get the motherboard out today and give it a better clean as per the messages above. I have a chip puller so I'll pull them out as suggested and examine them.

Thanks again.

SiriusHardware 4th Jan 2021 1:10 pm

Re: Non-working Commodore PET 3016
 
I think you've hit lucky (or chose well) as regards your choice of meter as it does indeed have a 'Hz' (frequency) section on the dial. It claims to be able to read up to 10MHz, if that's true it could prove quite useful for this project.

ScottishColin 4th Jan 2021 6:01 pm

Re: Non-working Commodore PET 3016
 
I don't suppose this will help - leg missing on chip from UD6....

Wasn't me; honest.

Colin.

ScottishColin 4th Jan 2021 6:01 pm

Re: Non-working Commodore PET 3016
 
2 Attachment(s)
Photos.

Timbucus 4th Jan 2021 6:14 pm

Re: Non-working Commodore PET 3016
 
I think that is A7 on one of the ROM's so no it wont help - you can usually solder a pin from another scrap tip if there is a stub - you may be lucky.

SiriusHardware 4th Jan 2021 6:36 pm

Re: Non-working Commodore PET 3016
 
1 Attachment(s)
Well, it's good that you found that. If this had been any ordinary logic IC we would have said just replace it. Unfortunately that IC is one of the PROMs, it contains factory programmed data which is specific to this model or model range so it is a chip which you can't easily 'just replace'.

Can you see if the other half of that snapped IC lead is still inside the corresponding socket hole? If it is you are going to have to get it out or remove and replace the whole socket. If you do have to replace the whole socket you could replace it with a turned-pin type, plug the chip into it and spot-solder the broken pin to the 'well' on the top of the socket contact.

If the socket hole is vacant what you can do, if you feel up to it, is to first find an unused new component like a resistor or capacitor which has leads which are a nice smooth fit in the hole in the socket, and then solder the end of one lead to the inside of the remainder of the snapped pin. Once you've got it in the right place and at the right angle, trim the lead off to the same length as the other leads on the device. The attached crude sketch illustrates what I mean.

You can also bend the original upper half of the pin inwards a bit and solder the 'new' lead to the outside of it instead. That would be easier to do but it will make the repair more obvious. You may not mind about that as long as it works.

Edit: Top tip from Tim, I've never thought about using another whole leg from a duff chip (rather than a bit of component lead) as a replacement leg. Will have to remember that one.

ScottishColin 4th Jan 2021 7:20 pm

Re: Non-working Commodore PET 3016
 
2 Attachment(s)
Hi - I clipped a leg of a potentiometer I had lying around and soldered that on. It's all back in the socket again, but no joy on the screen.

Attached are photos of the soldering for any criticism, plus a phot of the top of the chip showing the number.

SiriusHardware 4th Jan 2021 7:34 pm

Re: Non-working Commodore PET 3016
 
I'm honestly surprised you managed to get a potentiometer leg to go into an IC socket but clearly you did, so... continue on around the rest of the socketed ICs if you have not already, you may find one or two more like that, or you may find one which has been removed in the past and put back in with a pin folded over. (That is very easy to do, still happens to me sometimes even now).

In the meantime we'll muse over what to look at next. I think probably the simplest go / no go indication for the mainboard is to see whether it is producing sync pulses for the monitor and work forwards or backwards from there. I'll look into where to look for that: Back shortly.

Timbucus 4th Jan 2021 7:45 pm

Re: Non-working Commodore PET 3016
 
Looks really good - now we need Sirius magic numbers to measure with your frequency equipped meter - he may correct my view below

(Aside: I was looking at that very one the other day but settled on a set of their test leads for my old meter which is all I really needed... now maybe I will reconsider).

I expect we will be working in this area of the board

http://www.zimmers.net/anonftp/pub/c...N/320349-6.gif

- I think the circuit in the top left is the master clock but, its 16Mhz frequency will be beyond your meter so we may need to look at a later part of the chain - it seems to be generating a lot of phases.

You could usefully check the 1Mhz (Pin 7) and 8Mhz (Pin 3) outputs on the chip just below it G5 74191. As you are using a multimeter you should be safe enough in that area to measure it in reference to the ground pin on that chip (pin 8) https://www.elektropage.com/default.asp?tid=597

ScottishColin 4th Jan 2021 7:48 pm

Re: Non-working Commodore PET 3016
 
I have checked all of the chips and none have bent or missing pins.

Thanks again all for you patience and help - I really appreciate it.

fyi I have not done anything about the monitor electronics - no cleaning or anything.

You'll need to be patient/specific with me regarding testing frequencies - I've not done that before.

Thanks.

Timbucus 4th Jan 2021 8:04 pm

Re: Non-working Commodore PET 3016
 
Its a pleasure the one bit that is good on this forum is people have patience and enjoy helping.

As I have never used a meter to test frequency before I am going to guess you set it to Hz and then put the black lead on Pin 8 (rest it on the base of the pin so it is firm) and then touch the pin with the red probe to measure 7 for 1,000,000 Hz (1Mhz) and pin 3 for 8Mhz - Be very careful not to slip to short leads - that will likely be fatal to one or more of the chips.

Timbucus 4th Jan 2021 8:07 pm

Re: Non-working Commodore PET 3016
 
As an aside - in case the monitor electronics are causing a problem (that is an area due to the HT - thousands of volts- that you need to be very careful around) you can always unplug the monitor connections on the corner - this board should still run fine while we are just looking at signals to see if the board is operating.

SiriusHardware 4th Jan 2021 8:10 pm

Re: Non-working Commodore PET 3016
 
I think the relevant diagram for the timing circuit section is this one:-

http://www.zimmers.net/anonftp/pub/c...N/320349-6.gif

Among other things, this part of the circuit generates the 'Horizontal Drive' and 'Vertical Drive' signals. You can find these signals on pins 3 and 5 of connector J7 which looks to be the mainboard's output connection to the monitor.

If those connections are difficult to get a physical connection to you can, alternatively, find the 'Vertical Drive' signal on pin 11 of IC 'UG10' and the 'Horizontal Drive' signal on pin 2 of IC 'UH7'. Try looking at each of those (with the power on of course, and with the meter in 'Hz' mode) and see if you have steady frequencies on both of them. 'Vertical Drive' I would expect to be in the region of 50-70Hz, and 'Horizontal Drive' somewhere in the tens of Khz. If you do find steady frequencies on those points it would also be interesting to know what the 'Duty Cycle' (%) figure is for each of those signals as well - that's another unusual, but potentially useful feature your new meter has.

If you don't find anything which looks like a steady frequency on those two points try looking at pin 3 of IC UG5, where you should see a frequency of 8.00MHz, and pin 7 of IC UG5 where you should see a frequency of 1.00MHz. (The main clock frequency, at 16MHz, is a bit too high for your meter to be able to measure).

All measurements to be taken with the black meter lead on a known 0V / GND point.

Edit: Crossed with Tim, who said much the same in more concise terms.

SiriusHardware 4th Jan 2021 8:17 pm

Re: Non-working Commodore PET 3016
 
Quote:

Originally Posted by Timbucus (Post 1327035)
the one bit that is good on this forum is people have patience and enjoy helping.

Is that the only good bit? ;)

Timbucus 4th Jan 2021 8:19 pm

Re: Non-working Commodore PET 3016
 
Quote:

Originally Posted by SiriusHardware (Post 1327045)
Quote:

Originally Posted by Timbucus (Post 1327035)
the one bit that is good on this forum is people have patience and enjoy helping.

Is that the only good bit? ;)

The other bit is they have a very dry humour


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