Composite Black and White monitor dead

[Chuck(G)]

Let's try some very simple measurements. See the big "can" electrolytic cap C113 at the left side, right by the flyback transformer? Note that there are connections to the lugs on the can case itself. Call that point "ground". Set your meter to the 200VDC range and measure the voltages between "ground" and each of the two inner lugs. One should measure about 133V; the other, 110V. (careful--set your probes up, then power on--the voltage isn't that high, but the current is enough to hurt).

If you don't get anything, we've found the problem.

 

[tezza]

Thanks Chuck,

I'll try that tonight. I have been using the ground wire on that can as a convient ground point. I haven't tried the lugs themselves though.

 

[Chuck(G)]

Same-oh, same-oh, the lugs are connected (or should be) to the can.

 

[TRS-Ian]

Some time ago I took some photos of the inside of the US m1 monitor and an Aus one (same as your one), and the differences I noticed was that instead of one power switch behind the power button, there is 2, I'm guessing one for each of the 2 transformers? Check they are both functioning properly when the power button is pressed in.

BTW I hate that tacked on 240V->110V transformer, its mounting screws mean that the monitor never sits properly on top of the expansion interface.

Since I have on hand both a dead Aus m1 monitor and a good US monitor, I'm going to see if the add-on transformer and 2nd power switch lends itself to being transferred to the US monitor.

Also as I stated in the email, if you want to check your tube it can be removed from the M1 monitor chassis and connected to a model 3 or 4 video board and tested on that computer. Just don't get zapped. It might be as simple as a dead tube.

Ian.

 

[tezza]

Also as I stated in the email, if you want to check your tube it can be removed from the M1 monitor chassis and connected to a model 3 or 4 video board and tested on that computer. Just don't get zapped. It might be as simple as a dead tube.

Ian.

Thanks Ian,

I don't want to drag the tube out (and mess with my model 4) unless I have to so I'll (carefully) check these voltages and see if I (or anyone else) can link the failure to a specific component first.

Tez

 

[TRS-Ian]

LOL i forget that not everyone has 1/2 a dozen open model 3s, 4s and 1s on work benches ready to test everything.

 

[modem7]

Tony Duell from the Classic Computer mailing list has some familiarity with this monitor and it seems the step-down transformer is also an isolating transformer so there is no hot chassis.

If a short has developed between the primary and secondary windings, then a hot chassis is created. Rare, but it does happen. In such a situation, had the chassis been earthed via the mains power cord, a fuse (in monitor or house fuse box) would have blown or a circuit breaker tripped, or a winding blow open. Irrespective of what blows, the chassis is still earthed and thus the chassis does not pose a threat.

By the way, assuming a good transformer, if you feel a tingle when you touch the chassis, that's because:
1. Of stray capacitance between primary and secondary windings of the transformer, together with
2. Chassis not earthed, together with
3. You are earthed or partially earthed (e.g. touching ground clip of cro probe, touching earthed parts of other test equipment).

Explained in the article at http://www.minuszerodegrees.net/misc/Does a tingle mean that your appliance is unsafe.pdf

 

[modem7]

I have been using the ground wire on that can as a convient ground point.

In your scenario, perhaps the use of 'reference point' is better than 'ground point'.

 

[tezza]

Ok, here is an update.

First, the first few components up to C113 seem different than the SAMS schematic (This is in the SAMs document in an earlier link but I have just the schematic itself linked here).

Measuring across the two AC lines after they leave the isolating transformer shows 115V. I traced the two lines through until they connected to other components. Measuring voltage from the two lines when they link up to the first piece of circuitry to ground, gives the voltages shown on the following picture. One line gives 129V, the other 137V. I've shown where they connect.

The yellow wire shown (arrow points to lug it's connected to) goes to C113, - Voltage is 263V? SAMs (and Chuck) says this should be 133V. It seems to be double?? The other lug on C113 measures 172V. I've checked these voltages several times.

Figure 1

There is another thing I noticed tonight. One of the legs of a small component attached to the bottom of the tube has snapped. I'm not sure how important this component is...or even what it is...it looks like a clear LED. Anyway, here's a pic.

Figure 2

Any advice on what this is and how to replace it most welcome.

So..it seems like some unusual voltages are floating around this circuit but on the other hand, it's not the same as the schematic either so maybe they are normal? The mutimeter seems to be working ok as it does accurately measure 240V and 115V respectively prior and after the step down transformer across the two lines.

The ground or reference point I'm using to measure the AC voltages (apart from across the two wires prior and after the step-down transformer) is a lug off C113...which is directly wired to the chassis.

Tez

 

[tezza]

By the way, assuming a good transformer, if you feel a tingle when you touch the chassis, that's because:
1. Of stray capacitance between primary and secondary windings of the transformer, together with
2. Chassis not earthed, together with
3. You are earthed or partially earthed (e.g. touching ground clip of cro probe, touching earthed parts of other test equipment).

Explained in the article at http://www.minuszerodegrees.net/misc/Does a tingle mean that your appliance is unsafe.pdf

Thanks. No tickle at all if I touch the chassis while power is applied but the step-down transformer has developed an annoying buzz since I started working on this thing. I wonder what that is?

Tez

 

[tezza]

The following image shows THE BACK of the connection board shown in figure 1

Figure 3

Tez

 

[modem7]

The ground or reference point I'm using to measure the AC voltages (apart from across the two wires prior and after the step-down transformer) is a lug off C113...which is directly wired to the chassis.

So that particular lug must simply be one that is intended to ground the metal case of C113. The two other lugs are for the actual capacitor.

The yellow wire shown (arrow points to lug it's connected to) goes to C113, - Voltage is 263V? SAMs (and Chuck) says this should be 133V. It seems to be double?? The other lug on C113 measures 172V. I've checked these voltages several times.

263V on one lug minus the 172V on the other lug would mean that C113 has 91V over it (263-172). You could verify that placing one probe on one lug and the other probe on the other lug.

So, why isn't one lug at 0V (chassis potential) and the other at 91V ? Parts of the power supply circuitry that one may expect to be sitting at chassis potential may be sitting at a potential above that of the chassis. It comes down to the design. Remember, so far, the power supply has differed to the document you have.

There is another thing I noticed tonight. One of the legs of a small component attached to the bottom of the tube has snapped. I'm not sure how important this component is...or even what it is...it looks like a clear LED.

It is a spark gap (for protection). Ignore it for now.

 

[Chuck(G)]

Too many cooks here, so I'll bow out of this, lest Tez go bonkers. I'll only add that the transformer setup appears to be a full-wave center-tap setup (note the diodes in his penultimate photo of the tie strip).

 

[tezza]

Too many cooks here, so I'll bow out of this, lest Tez go bonkers. I'll only add that the transformer setup appears to be a full-wave center-tap setup (note the diodes in his penultimate photo of the tie strip).

Chuck, that analogy usually applies to some sort of creative task where something is being made whereas I would argue that the more diagnostic eyes and minds (especially yours!) the better. Given that the schematic doesn't seem to be much use, and I've found out from Ian Mavrick that there seem to be at least two versions of this 240V Model 1 monitor (oh, great!) I'm well on the way to bonkerism already.

However, I appreciate it's a project that's difficult to step-by-step diagnosis with someone because I can only devote snatches of time to it not to mention vast time zones between me and everyone else. However any comments or clues from anyone reading is always most welcome.

When I get back to it (probably Wed evening), I'll try to follow the circuit through and see if voltages seems to disappear anywhere before they get to the tube.

I'm assuming that those readings I took could well be ok so far, depending on what circuitry is employed?

What intrigues me about this varient is the lack of fuses...despite a huge notice inside the case saying any replacement fuses must be the right type!

Tez

 

[Chuck(G)]

Okay Tez, Can you read the engraving on the electrolytic can? Something doesn't seem right here to my eye.

Okay, so you've got power supply voltage at the filter cap at least. Let's measure between the collector and emitter of the horizontal output transistor. You should get somewhere around 100V betwixt the two.

 

[tezza]

Okay Tez, Can you read the engraving on the electrolytic can? Something doesn't seem right here to my eye.

Okay, so you've got power supply voltage at the filter cap at least. Let's measure between the collector and emitter of the horizontal output transistor. You should get somewhere around 100V betwixt the two.

Will do. I'll report back either tonight or (more likely) Wednesday. Thanks!

Tez

 

[tezza]

Ok, some more readings as shown on the image. The horizontal output transistor values are at the bottom of the screen. As you can see, only 46 volts on the collector instead of the 92.2 the circuit diagram tells me to expect. Some low values on the tube too, yes?

It's interesting that that's half the voltage on the collector of the transistor, yet the big can (CR101) carries twice the voltage the schematic says it should on the input lug.

I also measured the voltage on the neck pins. Only two had any voltage as shown.

Figure 4

Chuck, you asked about that big can. Here are the details:

U72187-65
175WV 200MDF
175WV 250MDF


MAX OPER. TEMP. 85 C
CAN. NEGATIVE
SAMPO


7826

Could a faulty Horizontal output transistor be the problem here?

Tez

 

[Chuck(G)]

Let me think about this, but here's something to do in the meantime. See all of those big power resistors (block-ish rectangular tan-colored things) scattered around the chassis? Measure each one with your ohmmeter (power off, of course). In no case should the resistance ever be substantially larger than what's marked on them.

 

[MikeS]

Another cook heard from ;-)

Some things don't seem to make sense; could you have mixed up some wires? If those white wires really go to the switch then closing the switch would short out the 220V unless it's a DP switch.

And are you sure about the voltages (DC) between the case of the big can electrolytic and the other (2?) terminals on it? Anything greater than 175V is definitely a serious problem.

Yes, the horizontal output transistor and/or the flyback transformer are among the most common failure points, but let's do some more checking first

The 'spark gap' is actually an NE-2H neon bulb (DS101 in the schematic); similar to a spark gap but instead of shorting above a certain voltage it 'clamps' the voltage at around 70-80V. It should probably be replaced, but I don't think it has anything to do with the current problem.

Just simple voltage measurements can be misleading in circuits like these; I think you have a 'scope now (or was that Phil's?) so why not see whether those voltages on the deflection yoke (for instance) are DC or whether there's actually a signal there?

 

[tezza]

Another cook heard from ;-)

Some things don't seem to make sense; could you have mixed up some wires? If those white wires really go to the switch then closing the switch would short out the 220V unless it's a DP switch.

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

And are you sure about the voltages (DC) between the case of the big can electrolytic and the other (2?) terminals on it? Anything greater than 175V is definitely a serious problem.

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?

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?

I'll do some DC measurements tonight. Promise me I won't fry my multimeter.

Yes, the horizontal output transistor and/or the flyback transformer are among the most common failure points, but let's do some more checking first

.....

Just simple voltage measurements can be misleading in circuits like these; I think you have a 'scope now (or was that Phil's?) so why not see whether those voltages on the deflection yoke (for instance) are DC or whether there's actually a signal there?

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.

Tez