Pet video board 320064-02 bad

[daver2]

The OP was considering buying one of this little €20 oscilloscopes. I ordered one and it arrived yesterday. If the OP ordered one today, it would be with him by the end of the week.

Dave

 

[Desperado]

Maybe this job is too big for me ... I'm thinking of selling Pet ... I'll sell it as a repair, hoping to recover at least the money spent ... I don't see any other solutions ... You are very good and very kind but I don't have the skills to repair it

 

[Dwight Elvey]

I don't think there is anything wrong with the regulator. We determine a little more as to why the circuit is not doing as we'd expect. Each step we are making some progress. I hate to lose a patient when we only need to understand the symptoms a little better.
I'm guessing that the vertical is oscillating and pumping up the voltage. We can quite it down some to see if that is the case. Can you disconnect the vertical yoke coil. You can disconnect either wire. There is a blue wire and a red wire. The yoke is that heavy part wrapped around the tube. I don't know if they are soldered to the board or there by a connector.
With that pin grounded take your measurements on Q11, Q12 and Q13 again. I'm guessing that we are seeing something related to capacitor failures.
We need to find what is bringing the voltage up to 14v.
Dwight

 

[Hugo Holden]

I suspect some type of oscillation.

Dwight

I agree, assuming the center pin and tab of the regulator are properly connected, we know the regulator is good and we know that 78XX regulators are prone to oscillate if they don't have suitable capacitive bypassing on their input and output pins and if that was happening, that could also fool the meter into showing 14V and not 12.
(but there other explanations as noted)

Ilpaninaro:

I would think the first move here (without a scope) is to connect a 0.1uF 50V monolithic ceramic capacitor on the input pin to the ground (center) pin of the 7812 regulator VR1, and another from the output pin of that regulator to its ground pin. Keep the capacitor leads as short as possible and put these two capacitors on the solder side of the board, right on VR1's three pins.

(C1 and C3 may have worked when new but as they age their ESR goes up and they become even more useless, like most electrolytic caps at radio frequencies, they should have really had 0.1 or 0.22uF ceramic caps in parallel or some Tants, the 7812 manufacturers recommend low ESR capacitors near the pins or the 7812 will oscillate)

If the 7812 is oscillating that will stop that most likely and your meter will read 12V. Then with that issue solved move back to the Q11, Q12, Q13 area, but you did not measure the voltage on test point 3 (on C16 yet)

just to show you an example of the type of capacitors to try, they will have 104 printed on them and likely be yellow or blue in color:

https://www.ebay.com/itm/50-Mono-Ca...558559?hash=item3d8b637e9f:g:PqIAAOSwNSxVfVbG

 

[Hugo Holden]

Now i ve connected again central pin of Vr on board...
i ve measured Q11,12 AND 13 collectors and i ve these measures:

Q11=14v
Q12=0v
Q13=14v

Three measurements, but where is the 4th ? from post #255: "please provide the 4 measurements asked for"

(ignoring the 14V issue) These are exactly the voltages one would expect with the absence of the H drive signal on test point 3, which is why I have asked for you to measure it. If this point is stuck high, or low, (no drive signal) then Q11 will be off, its collector 14(12v), Q12 will be off, its collector 0V and Q13 off, its collector 14(12v) pulled up by the 470R resistor you have put in there.

Don't forget to remove, test and/or replace Q14. I think you monitor is very close to being fixed, so don't give up yet.

IF the HOR.DR.IN signal is missing, then its back to the computer board to fix that.

 

[Hugo Holden]

Ilpaninaro,

So there is no confusion I have attached an image of the capacitors tacked on to the 7812's solder connections. Other capacitors that work for this are 224 (0.22uF caps). In some data sheets 0.33uF is recommended.

Electrolytics C1 and C3 may need to be replaced later, but don't worry about that now. In any case its pays to have these extra ceramic capacitors for these regulators, and not rely on electrolytics to prevent oscillations in 78XX series regulators.

 

[Hugo Holden]

We have clearly all fallen into a parallel universe where the Laws of Physics are different!

We only think we have fallen into a parallel universe when odd ball things happen in circuits with unexpected RF oscillations and it messes with our minds. I used to comfort myself with the notion that the laws of Physics applied everywhere in the universe, except Black Holes are in our universe and its quite different inside them!

In the usual course of events, servicing a computer or monitor or anything else a scope is often the tool used and that would have revealed most likely that the power supply rail had oscillations on it and that was affecting probably not only the average voltage but the meter's interpretation of it too. So this is a potential trap (no pun intended) when using very limited test equipment.

As you can see from the design of the Pet monitor, they actually violated recommended practice by not having suitable capacitors immediately around the regulator. This problem of course, found along the way, probably had nothing to do with the original fault that took out the monitor and its Q13 & Q14, which would have been an abnormal H drive signal from the computer board.

 

[Desperado]

Thanks for your patience....
now i am seeing Q14 (mje13006).... this transistor have central pin cut off...only two laterl pins are soldered on board....
it's correct?
In these 2 lateral pins i measure 0V...
on C16 always 14V

Unfortunately i don't have a 13006 spare...can i use equivalent?

 

[Hugo Holden]

Thanks for your patience....
now i am seeing Q14 (mje13006).... this transistor have central pin cut off...only two laterl pins are soldered on board....
it's correct?
In these 2 lateral pins i measure 0V...
on C16 always 14V

Unfortunately i don't have a 13006 spare...can i use equivalent?

Yes there are the equivalent parts noted on the circuit diagram.

But I would think it would be better to get the original part. These days there are a lot of copy & fake transistors. However, the fakes seldom (if ever) have gold plated lead wires.

If it was my monitor and I was wanting a genuine replacement part I would go for these, they are not expensive, so buy three of them to have some spares in case of accidents. I'm pretty sure these are genuine Motorola parts, but obviously you will have to wait the time for the post to get them:

https://www.ebay.co.uk/itm/2-count-...607913&hash=item2edbf9bc48:g:7FAAAMXQZ7dTlye8


Often,the central pin is cut off, because it connects to the metal tab that screws the transistor down (the collector connection) and the connection to the transistors's collector is via the screw that secures it.

While you are waiting for these transistors to arrive, you can get on with the fault finding.

 

[Desperado]

Yes there are the equivalent parts noted on the circuit diagram.

But I would think it would be better to get the original part. These days there are a lot of copy & fake transistors. However, the fakes seldom (if ever) have gold plated lead wires.

If it was my monitor and I was wanting a genuine replacement part I would go for these, they are not expensive, so buy three of them to have some spares in case of accidents. I'm pretty sure these are genuine Motorola parts, but obviously you will have to wait the time for the post to get them:

https://www.ebay.co.uk/itm/2-count-...607913&hash=item2edbf9bc48:g:7FAAAMXQZ7dTlye8


Often,the central pin is cut off, because it connects to the metal tab that screws the transistor down (the collector connection) and the connection to the transistors's collector is via the screw that secures it.

While you are waiting for these transistors to arrive, you can get on with the fault finding.

Thanks, but it's normal have 0V to Q14?

 

[daver2]

Post #288. C16 should only have a signal between 0 and 5V on it. It couples the horizontal drive signal from the PET logic board. Anything above 5V (depending upon which side of C16 you are referring to) has either destroyed a logic IC on the pet main board, or there is a fault with Q11 (C-B short possibly)?

The information you require regarding replacement parts for Q14, and the signals to expect on the base and collector, can be found on the schematic at http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/video-3.gif. Have you downloaded and printed out this schematic?

If you think the project to repair the monitor is beyond your skill set, do you have a TV repair shop in your village, town or city? If so, they should be able to repair the monitor for you (assuming the logic board is working).

Did you purchase one of those portable oscilloscopes that MikeS pointed us at?

Dave

 

[Hugo Holden]

Thanks, but it's normal have 0V to Q14?

Yes, currently you should measure 0V on the base and emitter pins of Q14. The collector (its metal tab and cut off center pin) with have the power supply voltage on it, that passes from the +12V rail via inductor L1, through the horizontal output transformer primary to the collector of Q14. So, I'm sure if you check on the metal tab of Q14, it will measure about 14V that you have been measuring elsewhere.

Obviously, the emitter of Q14 will always measure zero volts, its connected to negative or ground or "common".
The Base connection of Q14, is driven by the transformer T1. Currently there is no power applied to T1 (remember you have disconnected Q13's collector wire and have that connected via the 470R resistor to the 12V rail. So there cannot be any base voltage on Q14). It is better to leave it like that for now until the horizontal drive voltage from the computer board can be made to be normal and Q11,Q12 & Q13 are checked & Q14 replaced, before re-connecting Q13's collector to the driver transformer T1.

 

[Desperado]

Yes, currently you should measure 0V on the base and emitter pins of Q14. The collector (its metal tab and cut off center pin) with have the power supply voltage on it, that passes from the +12V rail via inductor L1, through the horizontal output transformer primary to the collector of Q14. So, I'm sure if you check on the metal tab of Q14, it will measure about 14V that you have been measuring elsewhere.

Obviously, the emitter of Q14 will always measure zero volts, its connected to negative or ground or "common".
The Base connection of Q14, is driven by the transformer T1. Currently there is no power applied to T1 (remember you have disconnected Q13's collector wire and have that connected via the 470R resistor to the 12V rail. So there cannot be any base voltage on Q14). It is better to leave it like that for now until the horizontal drive voltage from the computer board can be made to be normal and Q11,Q12 & Q13 are checked & Q14 replaced, before re-connecting Q13's collector to the driver transformer T1.

Ok thanks...so i think that Q14 it's ok....
what's next test please?
No i don't have buy scope ma i ve logical probe....

 

[daver2]

Until Dwight 'wakes up' and continues with the question 'why have we got +14V instead of +12V' - and if you want to power the PET up, then can you use your logic probe again (but fairly quickly) on the following points:

C16:
Q11 collector:
Q12 collector:
Q13 collector:

And tell us if there are pulses at each point or not please.

This assumes we have the 470 Ohm resistor in the collector of Q13 to +12(14) Volts - instead of transformer T1?

Switch the PET off after the test, report the results and wait for Dwight...

Regards,

Dave

 

[Desperado]

Until Dwight 'wakes up' and continues with the question 'why have we got +14V instead of +12V' - and if you want to power the PET up, then can you use your logic probe again (but fairly quickly) on the following points:

C16:
Q11 collector:
Q12 collector:
Q13 collector:

And tell us if there are pulses at each point or not please.

This assumes we have the 470 Ohm resistor in the collector of Q13 to +12(14) Volts - instead of transformer T1?

Switch the PET off after the test, report the results and wait for Dwight...

Regards,

Dave

C16:High
Q11 collector:low
Q12 collector:low
Q13 collector:high

no pulses...
Yes Q13 collector it's connected with 470R to 12+

 

[daver2]

I assume you have the monitor connected to the PET main board?

If so, you are not getting any HORIZONTAL DRIVE signal from the PET main board again. That is the only way I can think of that C16 would be HIGH.

Also, if Q11 collector is LOW, this should turn ON transistor Q12 and its collector should be +12(14) Volts.

Let's wait for Dwight. What I would like to propose next (unless Dwight has a better idea) is to get some high-quality, close up photographs of the front and rear of the monitor PCB if that is possible. You will have to post the photographs on a file-sharing site, and link us to them. I would like to see if this PCB has been modified at all...

Dave

 

[Desperado]

I assume you have the monitor connected to the PET main board?

If so, you are not getting any HORIZONTAL DRIVE signal from the PET main board again. That is the only way I can think of that C16 would be HIGH.

Also, if Q11 collector is LOW, this should turn ON transistor Q12 and its collector should be +12(14) Volts.

Let's wait for Dwight. What I would like to propose next (unless Dwight has a better idea) is to get some high-quality, close up photographs of the front and rear of the monitor PCB if that is possible. You will have to post the photographs on a file-sharing site, and link us to them. I would like to see if this PCB has been modified at all...

Dave

Ok Dave picture it's been uploaded here:

https://postimg.cc/gallery/31tul49be/

 

[daver2]

Thanks.

Can you get me an 'overhead' shot of the VR1 voltage regulator and Q14 please on their heatsinks?

One thing I have noticed about the voltage regulator is that both the rivet and the bolt has fibre washers on them - so they can't be making good contact with the GND copper...

You stated that the middle pin was cut-off the VR1 voltage regulator at one point and one of Dwight's tests was to connect the middle pin (via a wire) to GND.

I just want to see a photograph of the top of these two transistors please.

Regards,

Dave

 

[Dwight Elvey]

Hugo seemed to be doing some good and I has busy fixing my Ford. It was throwing a fuel pressure high error. It could be a wiring issue ( I've already had to fix crumbling insulation on the wires from the EGR valve sensors ). I just installed a new sensor for the fuel pressure. I hate working on the old wiring. The car is not that old. It is like Ford put a time bomb in the wiring so that it will fail long before the engine fails. ( it is a 2005 car ). Anyway, it is intermittent. It will be fine for weeks and then I get the check engine light. I figure the EGR wiring was mostly because they ran the wiring harness right next to the PCV. These always seem to leak a little oil that had saturated the wiring harness and turned the wiring to goo. If I have to replace more wiring, there won't be much original wiring in the engine.

A while back I asked if you have a 5K to 10K resistor handy. Do you have one? We can use it to test some stuff. If you don't have one, please get one, it will be useful.
Also, I asked if you can remove a lead of the vertical yoke coil. Have you done that? I'm hoping we can stop the 14V by not having a way for it to be pumped up by the vertical coil. I want to see if this is causing the oscillation that is pumping up the voltage and it will be easier to test things with it quiet.
Also, while ordering the Q14 as Hugo has suggested can you also get the small capacitors that he'd recommended. We may still replace one or more of the electrolytics capacitors but for now, we may be able to look at other things first.
A scope will be easier but there are a lot of things we can do until then. We just have to be clever. I'll try to explain what we are doing as we do it.
Your measurements of Q11 trough Q14 look a little suspicious but I'm not all that sure there is an issue there yet. The logic probe is not the best thing to use with the rail voltage oscillating around +-14V some place. It could be hitting peaks over 18V for all we know. When you get the resistor I asked for we can run some experiments.
Dwight

 

[daver2]

Morning Dwight.

I will go back to my 8086 firmware and then disassembling some PDP-8 programs (the 4K disc monitor in particular).

dave