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Mono composite screen issues

Dave: Yes, the ground was on the chassis for the measurements.

Chuck: OK, I'll flip it over and post an image today. I might even have a crack at sketching out a circuit diagram myself. Re: the resistor..I guess I could always lift an end and take a measurement?

Mike: I'll have a look at what that resustor connects to when I get back up to the shack later today (in a couple of hours).

I appreciate the help guys. If you think of anything else I should measure once I get up there, let me know.

Tez
 
Chuck(G) said:
It could even be the resistor itself. It's not completely unknown for a power resistor to change value.
Click to expand...
Excellent point!

@ Tez: Removing the resistor will almost certainly allow the voltage to rise; assuming the resistor is OK, the question is what is drawing more current than it should. This could be as simple as the traditional shorted or leaking cap; since everything is more or less working it's not likely to be anything serious like a bad IC etc. and it looks like some non-essential part is just creating a partial short across the V+.
 
MikeS said:
This could be as simple as the traditional shorted or leaking cap; since everything is more or less working it's not likely to be anything serious like a bad IC etc. and it looks like some non-essential part is just creating a partial short across the V+.
Click to expand...

Mike,
That makes sense that it is the filter cap especially as the power transistor that Tez measured has so much AC on it (assuming it is part of the regulator which we do not yet know).

I didn't know that troubleshooting in the blind could be so interesting!
 
dave_m said:
Mike,
That makes sense that it is the filter cap especially as the power transistor that Tez measured has so much AC on it (assuming it is part of the regulator which we do not yet know).

I didn't know that troubleshooting in the blind could be so interesting!
Click to expand...
Indeed...
At this point I probably would not connect a high-current 12V supply, e.g. his car battery, without a good fast-acting fuse in the circuit; mind you, that is one technique for making shorted tantalums obviously visible ;-)
 
MikeS said:
Indeed...
At this point I probably would not connect a high-current 12V supply, e.g. his car battery, without a good fast-acting fuse in the circuit; mind you, that is one technique for making shorted tantalums obviously visible ;-)
Click to expand...

I've already had one set of tantalum fireworks up here in the shack Mike, (when fixing a shorted apple drive). It adds some excitment to the diagnostic process but I don't really want another episode :).

Ok. below is pic of the bottom of the board. This is the good one which is a lot cleaner. Below that you can see the non-working one which has (or rather had) a lot more gunk on it. I removed some of it with contact cleaner after I snapped the photo so it doesn't look quite so bad now.

I pulled out one leg of the large resistor on both machines (No easy task, I can tell you!), and took measurements. With the meter set at 200 ohms resolution, I got a reading of 23.4 on the bad and 24.1 on the good. This is approximately the 22k it should be, yes? The bottom leg of the resistor was attached to ground, Mike.

Unless you can think of anything definitive I can measure, I think trying to figure out how all these bits connect together is the next step. I'll have a go at drawing something...or there are a couple of original electrolytic caps which MAY be associated with the circuit. Maybe I'll just replace those first, given its easy to do.

Tez

Fig 1. Working monitor (underside)

2010-10-24-power-section-DS-monitor-bottom.jpg


Fig 2. Faulty monitor (underside)

2010-10-24-power-section-DS-monitor-bottom-faulty.jpg
 
Ok, replaced five electrolytic caps in the near vacinity. No change from that. Also, checked out a single diode just down from the 7W resistor for a short. It was ok.

The ceramic caps among the diodes at the top look in bad shape. Any chance they might be the problem?

Looks like I might need to sketch out a diagram after all. I've never done anything like this so it will be a challenge. Cup of coffee first, then I'll give it a crack.

Tez
 
tezza said:
Ok, replaced five electrolytic caps in the near vacinity. No change from that. Also, checked out a single diode just down from the 7W resistor for a short. It was ok.

The ceramic caps among the diodes at the top look in bad shape. Any chance they might be the problem?

Looks like I might need to sketch out a diagram after all. I've never done anything like this so it will be a challenge. Cup of coffee first, then I'll give it a crack.

Tez
Click to expand...

Tez,
You are doing fantastic. It may be time for you to give up your cushy software job and get a job as chief hardware engineer for a New Zealand electronics company.

You are an impressive bloke.
 
Do check the diodes and the caps (not strictly necessary to have them at all) across them. One shorted diode or cap could create the problem you're encountering.
 
Hmm...I checked across all tracks and couldn't find and resistance readings that differed from the good monitor. I didn't unsolder the components but I figure that even if one is shorted, that would drop resistance wholesale across connecting tracks.

After several attempts which ended up looking like spagettii I gave up trying to draw a schematic. I did however sketch out what components were connected to what as viewed from the bottom of the board. Please don't laugh at my feeble photoshop attempts (or at least if you do, don't do it in public! :)). My attempt at circles sucked, so I've represented the two transistors just with a label and three angled lines.

Does this look like the whole of the power circuit? I've just noticed an error...Where the diagram says "to a C9016 regulator" it really means a C1096 regulator. The regulator is screwed to a plate screwed to the outside of the main transformer. The same plate I've been using for my ground.

Tez

2010-10-24-power-section-DS-monitor-bottom-components.jpg
 
Ok Guys, I found something. I wonder now if we are barking up the wrong tree?

When I was doing my resistance testing I did notice that that variable resistor was showing different values between the monitors. If figured that might affect the power levels.

I just adjusted that resistor on the faulty machine with the power on. Guess what, adjusting it bought the power levels up to match the good monitor. They are now BOTH showing about 10.5VAC on the input lines when warm.

However the symptom is still there. It may be unrelated to the power then?

I'll keep looking.....

Tez
 
Ok, end of the day. I've replaced all the capacitors in the monitor except 5 very high voltage level ones that I don't have spares for. The symptom persists unfortunately.

It's getting close to the stage where I now consign it permenantly to the parts bin (or the recyclers). Without a circuit diagram to problem could be just too difficult to navigate.

I'll get replacements for those other 5 caps (might as well) and see if they make a difference. I took another photo of the symptoms, this time with the TRS-80 Model 1 welcome message on the screen. You can see that not only are there lines, but the image is also blurry and pulls a little to the right.

@ Dave - "You are an impressive bloke."

Just have to point out that typo Dave. What you mean is "obsessive bloke" (Just ask Annette, my wife!) :)

Tez

2010-10-24-ds-monitor-symptom2.jpg
 
To my eye, it looks like the CRT is being overdriven. Most likely, there's some adjustment pot somewhere that can relieve this or at least improve it. Without any idea of what you're working with, however, it's pretty difficult to say what that might be.
 
Chuck(G) said:
To my eye, it looks like the CRT is being overdriven. Most likely, there's some adjustment pot somewhere that can relieve this or at least improve it. Without any idea of what you're working with, however, it's pretty difficult to say what that might be.
Click to expand...
It does, doesn't it... maybe a problem in the video amp, driving too hard and pulling down the +V at the same time?
 
As with mike, I've seen it from a blown video drive transistor too, and also with a blown driver to that transistor (that doesn't pull charge out of the base of the transistor, meaning it's slow to turn off)
 
Hi Nige and others.

Flying blind due to no circuit diagram, but is there a quick transistor measurment which would indicate if one has a problem...either the transistor itself or something being fed into it?

Tez
 
MikeS said:
It does, doesn't it... maybe a problem in the video amp, driving too hard and pulling down the +V at the same time?
Click to expand...

Yes, although I've got the AC voltage back to comparable levels by adjusting a variable resistor in the power circuit on the faulty machine, I've only managed to do this by taking the resistor right to it's limit. The good machine has the resistor about 3/4 of the way there, but not right to its stop position.

Tez
 
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