IBM 5151 Monitor

[retrogear]

Ok the problem was there before the recap. That changes things a bit. Give me all the voltages on all the IC401 pins. Pin 1 at 8.6v is still not right.
I take back my comment it's not the yoke. Hugo's comment about one of the vertical coils bad with respect to the other is a possibility. I would be
using my scope to compare to the waveforms given. Using just a voltmeter is pretty limited in this circuit.

 

[stangman517]

I'm a bit confused because of a change in Pin 1 from 8.6v to now 5.2v. Checked 3 times. The first difference I can see because I forgot to solder back a few pins on those two caps.
Pins:
1 = 5.2
2 = 14.4
3 = 0.3
4 = 6.4
5 = 13.9
6 = 6.5
7 = 4.2
8 = 0.7
9 = 2.1
10 = 2.9
11 = 0.11
12 = 6.58

 

[retrogear]

According to the schematic of the TDA1170, pin 1 to pin 12 is two base/emitter junctions so 12 should be slightly lower than 1 but your voltages are the opposite.
Also pin 11 is only .1 when a .6 volt signal should be there. I think IC401 needs to be replaced.

 

[Hugo Holden]

According to the schematic of the TDA1170, pin 1 to pin 12 is two base/emitter junctions so 12 should be slightly lower than 1 but your voltages are the opposite.
Also pin 11 is only .1 when a .6 volt signal should be there. I think IC401 needs to be replaced.

I agree the IC is a possibility. However the most likely part internal to the IC to fail, is the output stage transistors. The one in question is the Szlikai pair that is responsible for sinking current in the lower leg, rather than the Darlington in the upper leg.

Yet, there is one measurement that suggests that the output stage is ok, and that the output amplifier is ok, that is the pin 4 voltage, being, on the average about 1/2 the supply voltage.

I think the OP should buy an IC as a spare anyway in case it is needed.

The problem could still be elsewhere.

One thing, if the OP measured the DC resistance of the yoke vertical coils with the meter. And somebody on the forum could do the same on another 5151 (I don't have one of these VDU's) then we could rule out one of the V coils being open circuit.

The problem would be very easy to sort out with a scope. On the balance of probabilities though, it is more likely the IC. But, it is not good when an IC gets needlessly replaced because other avenues were not exhausted first, but those avenues are blocked to a large extent without a scope. Though, in the Song Feel My Love, Bob Dylan was willing to crawl down the Avenue, and his son Jakob in the Wallflowers, was willing to drive home on "One Headlight", so it is amazing what you can sometimes do without test gear.

 

[retrogear]

I do own a working 5151 on my XT with MDA card but have never even opened it as far as I can remember. I've had it for decades and occasionally use it. I had it on this weekend.
The Sams schematic does show 3.89 ohms. If your reading is way off I could confirm on mine. The coils are in parallel but I wouldn't want to separate them to read individually.
My thought is if the yoke is bad, the only way to get a replacement would be with another parts machine so may as well try an IC to eliminate that possibility. We've pretty much
covered the alternatives. Back in my service days, I would ring the yoke with my scope and a small cap. It can be done on a vertical yoke as well as a flyback. If I recall correctly,
it was hard to get a good ring out of a horizontal yoke because of low ohms which this Sams confirms. I could never justify paying big money to get a ring tester from Sencore.
We would get a sales visit from their rep maybe once a year since they were based in the Dakotas and I'm in Minnesota. Oh the memories ...

 

[Hugo Holden]

I do own a working 5151 on my XT with MDA card but have never even opened it as far as I can remember. I've had it for decades and occasionally use it. I had it on this weekend.
The Sams schematic does show 3.89 ohms. If your reading is way off I could confirm on mine. The coils are in parallel but I wouldn't want to separate them to read individually.
My thought is if the yoke is bad, the only way to get a replacement would be with another parts machine so may as well try an IC to eliminate that possibility. We've pretty much
covered the alternatives. Back in my service days, I would ring the yoke with my scope and a small cap. It can be done on a vertical yoke as well as a flyback. If I recall correctly,
it was hard to get a good ring out of a horizontal yoke because of low ohms which this Sams confirms. I could never justify paying big money to get a ring tester from Sencore.
We would get a sales visit from their rep maybe once a year since they were based in the Dakotas and I'm in Minnesota. Oh the memories ...

That Sencore gear wasn't worth it because if the technician understood the Physics of the problem, a simple signal generator and scope would suffice.

Also, the reverse is true, low Ohms increases the Q value, so the ringing is enhanced.

You can calculate the inductance of a Yoke with a resonance test, especially if you add parallel capacitance to swamp out the lower self capacitance of the coils, normally a 0.1uF capacitor is sufficient. Then with the resonance peak, found with a signal generator and a scope, you can measure the inductance. Though these days inductance meters are pretty cheap.

The V yoke coils of course have a much higher inductance than the H coils, hence the ferrite cores. And they have a much higher DC resistance than the H coils too.

To the V output stage driving them, the V yoke coils represent a combination of a resistive and inductive load. (on the other hand the H coils represent a primarily inductive load at the higher operating frequency).

This is why the drive waveform to the V coils is a trapezoidal shape, to allow for both the inductive and resistive component of the V yoke coils as a load.

 

[stangman517]

My thought is if the yoke is bad, the only way to get a replacement would be with another parts machine

I'll give the IC a try. Soldering is no problem, but all the other testing you guys mentioned like the quote is out of my league. It would take some serious hand holding or experienced guys like you to do those tests. Those should probably be left with experienced CRT guys.

I found some on ebay and bought them; link below; hopefully these are the right guys. Please let me know.

3 PCS - TDA1170S "Original" STMicro 12P DIP/ZIP IC w/ Heatsink Tabs | eBay

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When I get them and put them in I'll get back of course. I've never seen an IC with the legs in different positions of each other!!! Nor I have ever seen a heat sink soldered on a PCB with an IC between it. It looks easy enough, but looks can be deceiving. Is there anything tricky about soldering these guys?

 

[retrogear]

That looks like the right one. The hard part will be desoldering the heatsink. You might want to desolder all the pins then try to pull out the heatsink lifting one side then the other while heating the ends then worry about desoldering it's pad. Or better yet clip the IC legs at the IC body leaving enough to pull out after the heatsink and body get removed. I remember changing a couple chips like that and it was a real PIA. Also when you put in the new one you could tack solder with just enough to make connections so it would be easier to remove if your new chip was a dud then after it's working properly solder it in better. I've done that strategy myself.

 

[stangman517]

The hard part will be desoldering the heatsink

Yep you're not kidding. I was a bit intimidated by the heatsink. I kept having to turn up the heat but with caution and fear. ;-) Around 880 degrees on the metal, but I was in and out. Use good ole wick and liquid flux (love the stuff). Then just wiggled it some while pulling slightly on it and the IC & heatsink came right out. Now I'll just patiently (no patience) wait for the IC.

 

[stangman517]

82K ohms ?

Something is wrong there. Both IBM's circuit diagram and SAM's circuit diagram indicate that VR403 is 50K ohms. That means that, measured in-circuit or not, 50K is the most you should ever read between any of VR403's three legs.

You will find procedures online on how to test a potentiometer.

I just realized something m7. I am wrong about saying these VR pots were two legs. I apologize! I did not see the middle leg behind the two outer legs in the back. So my mistake. Hope you accept my apology.

 

[retrogear]

Oh wow you did an excellent job getting that out !! The VR pots usually have the wiper tied to one end on the board.
The fact you could lower the resistance while turning is the important one.

 

[modem7]

Hope you accept my apology.

No apology required.

( Hopefully, the TDA1170 is going to be the fix. )

 

[stangman517]

I received the new TDA1170 ICs. Installed ok. A bit tricky getting the pins lined up in the holes. Sorry to say no change. I really appreciate your efforts from all of you! It's been educational for me.

 

[modem7]

One thing, if the OP measured the DC resistance of the yoke vertical coils with the meter. And somebody on the forum could do the same on another 5151 (I don't have one of these VDU's) then we could rule out one of the V coils being open circuit.

Two independent measurements are recorded in the top-right corner of the diagram at [here].

 

[retrogear]

the voltage on 12 is too high. are you sure R404 is ok at 470k and not open ? The C403,C404 R404 form the 4V RC ramp shown in the waveform at 12. Pin 12 should show the average DC of the ramp
at about 50% of 4V which they show as 2.5V but you got 6.6V which means the ramp is probably curved higher causing the linear compression at the bottom. You may have to replace C403 and C404
if R404 is ok. I remember those damn green caps like to go leaky with resistance.

 

[Hugo Holden]

I received the new TDA1170 ICs. Installed ok. A bit tricky getting the pins lined up in the holes. Sorry to say no change. I really appreciate your efforts from all of you! It's been educational for me.

I am not surprised by this because the other "avenues" I talked about were not exhausted.

I think there in only one cure.

You need to buy a cheap scope (I can suggest some) We need to have a look at the voltages, as they are changing with time, around the IC.

The problem is with meters, they only tell you averages, and it is far too limited to diagnose problems in circuits, like deflection circuits, where the shape of the waves are very important.

Unless you are prepared to do this, you might find yourself replacing capacitors, IC's and other parts and getting nowhere.

 

[stangman517]

Some confusion here for me. The first confusion is "First, check this suply voltage" part. I don't know where I'm supposed to put the probes to get a measurement.
Measuring pin 2 = .877v, measuring pin 5 = 13.92v. These were obtained without a PC connected to the monitor; don't if this changes anything or not. Obtaining a DC resistance "of the two deflection coils and R413 (all in parallel)" I do not understand how to obtain this value. I saw the V-DY and the H-DY connectors and how they go from the PCB to the yoke, but that's all I see. Honestly I can't find that R413 rascal. It must be where I'm not looking. ;-)

 

[retrogear]

pins 2 and 5 are supply voltages. you said earlier you had 14.4V on 2. they should both be about 14V and won't matter if connected to a pc.
The vertical yoke is two coils and a resistor all connected in parallel up at the yoke. there will only be two wires down to the plug. The resistor
is probably hiding up by the yoke coils. That's why I said I wouldn't separate mine to try and measure them. The manual shows 3.89 ohms
for them all together. What resistance do you read across the vertical yoke at the plug pins 1 and 2 ? I thought you had measured that but I must have read
somebody else's post. Hugo is right in that the next step will be to look at scope waveforms.

 

[stangman517]

The vertical yoke is two coils and a resistor all connected in parallel up at the yoke

This makes much more sense to the layperson.

If that resistor on the yoke is R403 no wonder I couldn't find it. I was looking on the pcb. Its resistance = 4.2ohms. I also saw the number 1 and 2 on the white piece of plastic. NOW it makes sense. ;-)
Pin 2 = 14.47v
Pin 5 = 13.92v

If waveforms are next and Hugo assists in me finding a decent, not too expensive scope what exactly will the waveforms diagnose? Does this mean cutting the glue on the yoke against the tube, adjust the yoke and then re-glue it with the power turn on? I do have electrical insulated rubber gloves but never used them!!! ;-)

 

[modem7]

If that resistor on the yoke is R403 no wonder I couldn't find it. I was looking on the pcb.

R413. Mine is pictured at [here]. Photo only just taken now.

Its resistance = 4.2ohms.

You are actually measuring R413 (180 ohms) in parallel with the two vertical deflection coils. Nothing else is being measured due to capacitor C408.

My same measurement is 4.1 ohms.