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Composite Black and White monitor dead

MikeS

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Yes, it is a DP switch. Not in the conventional 110V monitors (so not in the schematic) but it is in this one.
Odd, but that explains it

Ok. this might be a clue. I've been measuring in all in AC, thinking this part of the circuit is AC (Duh!). Oh course when I think about it, things like diodes have no place in an AC circuit do they? (or do they?). But then if it's all DC why should there be any AC voltage showing at all?
Normally an AC voltage on those filter caps would just be ripple of a few volts riding on top of the high DC voltage; however, if the cap were open and there were no filtering anywhere else then you would get a pulsating DC voltage (i.e. the 'ripple' would effectively be the entire applied voltage). But in no case should it be higher than or even close to the cap's working voltage of 175V.

The schematic doesn't specify AC or DC but I guess it assumes you just know. Having learnt a bit a about 5 and 12V DC logic circuits from problem solving and repairs, it looks like I still have a bit to learn about AC/DC conversion circuits. Where in the schematic does the current turn from AC to DC?
That's the 'tricky part' I mentioned in the beginning; in the 'low voltage' (relatively speaking) supply from the AC line anything past the diode(s) should be DC. However, the horizontal output stage is effectively actually a second switching mode power supply, shown in the lower section of the power supplies on the left of the schematic ('part of T101'), so you will see high-frequency AC there as well, rectified with a diode where DC is needed.

I'll do some DC measurements tonight. Promise me I won't fry my multimeter. ;)
Sorry, no promises!

Yes, I do have a scope now. I did drag it out and consider using it but, but given I'm a newbie at TV circuits I didn't want to accidently blow it up. If I did zap something I'd rather is be my $30 digital multimeter. However, I'll start using it.
Scopes are generally quite forgiving and an out-of-range trace will just be invisible off the screen, but if you're unsure measure with the meter first; as with the meter, set the voltage range to what you're expecting before you put the probe anywhere, especially in the horizontal output section.

I suspect that the horizontal oscillator is not actually running so you probably won't encounter any of its high voltages (unless you accidentally fix the problem ;-) ) but be careful anyway. When the monitor is working the dangerous voltages are not really the anode of the CRT which is what most people warn about because it is several thousand volts and can still be present after you turn it off, but the several hundred volts at much higher current around the HO transistor & HOT and the socket of the CRT.

Keep one hand in your pocket.
 
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Chuck(G)

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Tez, you've been measuring on the AC setting all this time? Oy.

facepalm.jpg
 

tezza

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Ok, let's try this again. This time in DC. Re-reading the previous posts I see you did specify DC for at least the big pot readings Chuck. Sorry about that.

2012-07-03-model1-monitor-voltages-DC.jpg


First the big Pot C113. 121.2V on the yellow lead (from the two diodes in figure 1 in a previous post) and 77.5 on the RED.

The transistor. 27v. Way lower than the 92.2v it should be. 0.1 on the base.

Around the tube. 21V, 1v and 0v as marked
The tube neck. One pin is 28v, another 2 are 0.7v and the rest are 0v

I checked out those large block like resistors. Some are slightly over their ratings but I wouldn't have thought they were significant.

22 ohm one was showing 22.4 ohm
150 ohm one was showing 147 ohm
2200 one was showing 2080 ohm
180 one was showing 190 ohm

Tez
 
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Chuck(G)

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Thanks Tez, that's starting to make some sort of sense! The resistors are fine--on those old "sand block" resistors +/- 20% is manufacturer's spec.

I'm kind of rushed today, but I'll try to find some time to give you a few more things to check. Oh, one question--when you take the measurements of the HOT, are you providing the monitor with a video input?
 

tezza

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No worries, there is no hurry.

No, I don't have anything plugged into the video input.

Cheers

Tez
 

tezza

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I don't think it's relevant to diagnosis but I'll mention it anyway. Those block resistors get very hot! Testimant to this is the layer of skin cells I left on one of them.

The monitor had only been on for a few minutes as I read a few voltages . Time to measure the resistors. I switched off and unplugged it from the wall. The resistors had a dust layer making hard to read the values printed on the surface. I wet my finger and ran it along the surface of the component to clear the dust.

Ouch! Big mistake. The next 20 mins was spent with my finger in a glass of cold water.

Live and learn. At least I haven't got zapped by high voltage yet. Mind you, there is still time... :)

Tez
 

Chuck(G)

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At least the resistors are in range, the voltages seem reasonable and something's drawing current.

We're getting there. You said you had a 'scope. Let's run some stuff down.

Apply your video signal to the monitor. Set up your scope for AC coupling, about 2V/div (vertical) and free-running horizontal (usually labeled "auto" or some such) and about 10 µsec per division horizontal. Apply a probe to the base of Q11 (near R472). Do you see a pulse train?

If so, look at the base of Q101. See a pulse train there? If not, look at the collector of Q12.

If you need help in using s scope, just ask.
 

tezza

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Thanks Chuck,

I shall see how I get on. When you say a pluse train, you mean what exactly?

Tez
 

tezza

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At least the resistors are in range, the voltages seem reasonable and something's drawing current.

We're getting there. You said you had a 'scope. Let's run some stuff down.

Apply your video signal to the monitor. Set up your scope for AC coupling, about 2V/div (vertical) and free-running horizontal (usually labeled "auto" or some such) and about 10 µsec per division horizontal. Apply a probe to the base of Q11 (near R472). Do you see a pulse train?

If so, look at the base of Q101. See a pulse train there? If not, look at the collector of Q12.

Ok, video signal applied to monitor from TRS-80 and scope setting as specified.

Pulse train at the base of Q11. Kind of a squarish wave. Emitter 3.9v Dc, Base 0.55v DC and Collector 0.27v DC. These voltages don't square up with the circuit diagram though where E=0.6v, B=0.3v and C=8.5V. I'm sure I've got the emitter and collector identified correctly. I quadruple checked.

There was a pulse train at the base of Q101.

The collector of Q12 was a little weird. There was a voltage of 9.5AC/5.1DC. Not the 1.2V it was suppose to be. There was no pulse train. Also, the scope registered the voltage on DC but not on AC even when it was switched to AC. It showed just a flat line on zero despite a multimeter registering 9.5V AC. Even when I turned the volts/division up to 5? Is this normal behaviour for the scope or is there actually no AC current but my multimeter just thinks there is?

The Base for transistor Q12 was 4.4 DC?

Do you want me to post up pics of the scope wavs?

Tez
 

Chuck(G)

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The pulse train at the base of Q101 might be interesting to see.

Right now, since the horizontal sweep signal is getting through to the horizontal output transistor, my suspicions are leaning toward either the transistor itself or the flyback transformer as the most likely candidates.

On your scope's AC setting, DC components of a signal are removed. So you could have a 3 volt AC signal riding on a 100 volt DC level and all you'd see is the 3 volt AC signal. On your multimeter, the principle is a little different--on AC volts, the input is passed through a rectifier and the displayed reading is displayed on the assumption that the input is a perfect sine wave with a root-mean-square value of 0.707 times the peak voltage. Unless you're measuring the AC mains, that assumption rarely holds, which is why the AC scale on a multimeter is of very limited use.
 

tezza

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I understand. Thanks for that explanation.

Here is the wave form on the base of Q101

2012-07-05-model1-monitor-q101-base-wave.jpg


Figure 6

Tez
 

Chuck(G)

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That looks nice, Tezz--just the way it should.

So, what do we have left in the chain? Well, we've got the transistor, Q101 (possible), the damper diode CR106 and two capacitors, C108 and C105. Since they're all hooked to the horizontal output transformer, we need to lift at least one end of each 2-terminal component to thest (capacitors should be tested for short; diodes pass current only in one direction--if they're shorted, they pass in both directions; if blown, they pass in neither direction).

Transistors can be tested using the diode setting on your ohmmeter (just like the diodes); they look like two diodes facing one another when tested emitter-to-base and base-to-collector.

After that, we may be looking at the transformer itself--not a good thing.
 

tezza

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Ok, progress I think.

A bit of a battle with confined spaces and years-old solder globs but after some persevering I managed to get a leg of the components free for testing.

CR106 - Fine. One way traffic only
C108 - Fine. No connection in either direction

Transistor Q101

Base to collection - Fine. One way traffic only. (575 Ohms)
Emitter to base. - Perhaps NOT FINE. Current can be passed in both directions (344 Ohms).
Collector to Emitter - One way at about 930 ohms. There should be no connection either way right?

I haven't tested C105 as it's very hard to get to, but am I right in thinking this is strong evidence that the transistor is the fault here?

Tez
 

Chuck(G)

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Given the cost of a small horizontal outputl transistor, it's a relatively inexpensive to check, I'd say. It's not unusual for one to go--they're under a lot of stress when operating.

You've got one of the most common ones: just look at the number of equivalents. I used to be able to buy these things in a bubble pack at Radio Shack--I don't know the parts situation in Kiwiland, but it shouldn't be hard to find something that works locally.
 

tezza

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Thank Chuck, both for stepping me through the diagnosis and the link. Hopefully I can source this part next week.

I'll update with progress once I fit it.

Tez
 

TRS-Ian

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Tez, is it fixed? No post since the 8th.

Anyways, I'm upgrading the b&w tube in a model 1 monitor to a green one this weekend so if you want I can do some comparison voltage tests if that helps.

Ian.
 
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