Amstrad CTM-640 CRT

[SpidersWeb]

Last time I was here I not only fixed a CRT with the forums help, but I also learned a lot from you guys. So I'm hoping to repeat the process (or at least the second part!).

I've attached the PDF of the entire schematic.

This time around it's a power issue. The first problem was that the monitor's 5V DC (which powers the computer) was showing 0V DC. I traced this back and I've found that between D511 and Q502 there is the 9.3V advertised on the scehmatic. But for this part of the circuit to work, the LM78MG needs some action on pin 1, and it's getting 0V (including Q504,Q505, and Q506)

Following this back I end up to pin 3 on the flyback - also showing 0V.

Now my understanding of the flyback setup in this circuit, please let me know if I'm wrong or right, is that:

- high voltage is applied to pin 6 for input
- it's flow is snapped on and off  by Q405 (pin 11)
- this coil then induces current on the secondary side
- that current flow on the secondary then induces current back on the "feedback" side - so between pins 2 and 8

Have I at least got that part right?

Also, this is an unearthed setup, I've noticed my multimeter shows 112Vac (I tested this because when my meter probe touched, the "Danger, Will Robinson" light immediately came on) between the exposed connectors that hang out the front (GND) and mains earth. I'm being very cautious with this, but I'm not sure if it's normal or indicative of a fault.

Any information or advice welcomed. I can very easily access the bottom of the board while the unit is on - each component is also labelled on the bottom - so taking measurements is a very easy process.

[SpidersWeb]

Trying to get my head around this. Half the time I try and follow things it feels like a chicken or the egg situation - so clearly I'm missing something. I have been using the oscilliscope on the back of the board to look around and generally beyond T501 - all I'm finding is dead flat DC, often 0V.

The 7800 IC isn't triggering Q405 - which from what I can tell means no pulses to drive anything else in the flyback - it's just there with flat DC (I measure the advertised 100V DC).
Looked up the datasheet, and pin 12 of the 7800 is Vcc, circuit matches this with an "11.7" voltage marking - choice... but this measures 0V.
Follow that line and I get to the base of the secondary winding (and I see no action on this part of the flyback). Follow the line the other way, and I get to T402 which comes off a deflection coil.

Any comments welcome, just trying to get my head around how this thing powers itself up.
I'm also curious if this type of shenanigans is normal for later model CRT's? And is it shenanigans or am I just thick?

Cheers!

[PA0PBZ]

I remember a Philips monochrome television from the 70's that was powering itself from the flyback, relying on a few electrons going the right way on power up.
All fine until it didn't work anymore 

Can you measure the anode of D509 and pin 15 of the LA7800?

[SpidersWeb]

Thanks for the reply, sorry it took me a while (work/wife etc).

My oscilloscope isn't super accurate, but D509 has an oscilliating input greater than 100V AC, and thanks to a cap on the output, I measured this at a stable 119V (multimeter to confirm value). I followed this around the circuit and through the flyback to Q405 which also read 119V DC.

I've checked that Q405 has no input, and it's transformer is not being oscillated / sent a signal.  The pin to control this on the 7800 IC is 0V flat.

Pin 15 has DC power, I didn't measure this with the multimeter, but looking at the scope it appears to be around 12V DC and dead flat. The Vcc pin for that chip also shows 0, but this gathers power from the secondary side of the flyback, so it's not surprising.

Wondering if perhaps part of the chip was powered via Pin 15, I also checked out the other inputs to the horizontal sync parts of the chip - the sync input pin is getting a clean H/V Sync signal - the same measured as coming exactly out of the computer itself (I powered it with a seperate power supply). Pins 1 and 2 show 1.5V and 2.05V which isn't what you'd expect to see - but I wonder if that's because the chip isn't powering up.

I followed to the connections to pin 1 and 2, and watched the voltages drop from 100V to 77V as I got passed the HOLD variable resistors, doing resistance checks, I can't see any failures with connections.

[PA0PBZ]

I think that the horizontal oscillator is powered by pin 15, and once that runs and activates Q404 and Q405 the flyback is powering the rest of the circuit.
Can you see anything on pin 3 of the LA7800 with the scope? If so, what about Q404?

I followed to the connections to pin 1 and 2, and watched the voltages drop from 100V to 77V as I got passed the HOLD variable resistors, doing resistance checks, I can't see any failures with connections.

Pin 1 and 2 are powered from the 12.5V zener on pin 15, so you should not see anything exceeding 12.5V on the right side of R405.

[SpidersWeb]

I think that the horizontal oscillator is powered by pin 15, and once that runs and activates Q404 and Q405 the flyback is powering the rest of the circuit.
Can you see anything on pin 3 of the LA7800 with the scope? If so, what about Q404?

Pin 3 is completely flat 0V, Q404 isn't getting a signal to cause it to switch.

Pin 1 and 2 are powered from the 12.5V zener on pin 15, so you should not see anything exceeding 12.5V on the right side of R405.

I think Pin 2 is a reference input for the Horizontal Hold. Following down (immediatley after R405) the signal goes through the two Hold VR's - where they finally drop to ~77V - this leads to pin 2 which shows the 2.05V (the lowest I could get it was 1.95) which is a bit over the 1.5V on the diagram, this also joins up to Pin 1 via a resistor, so I did expect to see some kind of voltage reading there even if it is an output. I dunno, just thinking out loud.

Edit: cancel that, I just read the diagram again, and there is no way this is possible. It must be ~7.7V. I'll measure it again.

I'm now starting to wonder if perhaps it's this IC that is bad.

Edit: also, pin 2 is supposed to be at 6.5V, not 1.5V, I was referencing a printed version that was harder to see 

[PA0PBZ]

I'm now starting to wonder if perhaps it's this IC that is bad.

Yes, me too. I still think it should output the horizontal signal when pin15 has power. Unfortunately all I could find is a 3-page datasheet without any theory so I could be wrong of course.

[SpidersWeb]

Sorry for slow reply, I ordered a replacement (can you believe you can buy 7800's new and get overnight delivery??) and was waiting for time to install it.

Anyway, desoldered the chip. It's amazing how easy it is to work with boards that were designed to be maintained. Popped in a socket, and threw in the new 7800.
I don't think it was the fault however, because during this process I also found Mr Leaksalot. Because of position and lighting, and the fact the top of the can was not bulging, I didn't notice it earlier.



I thought these were supposed to pop out the top? This is the second time I've been snagged by a cap that blew down.

So new 7800, new C405, powered it up and the CPC464 powered up - meaning we had 5V! But only a few random and very dull dots on the display. Adjusting the sub brightness on the flyback I got the distinctive blue and yellow but a mess, further adjustments of hold settings etc and tada



Thanks PA0PBZ for your posts, and also thanks to anyone else who read this thread and checked out the circuit diagram, the fact people didn't call me nuts was a good sign that I hadn't completely misunderstood things.

[PA0PBZ]

Glad you found it, so it was not the 7800 itself (did you try the old one?) but we were close. Is that C405?
I've seen caps fail like that before, I think when the contents get bad slowly it doesn't blow the top but just presses the seal on the bottom out.
The green jumper wire should have been a clue.

[SpidersWeb]

Yeah the green wire should've been an indicator, I'm just so used to seeing corrosion like that I didn't even pick up on it - too busy looking at top vents I guess :/

Yes it's C405. 47uF.

I'm not sure which part was at fault. Pin 15 was showing a steady DC signal - but on the other hand it's more probable that the cap which has clearly blown out was the cause. Since I socketed the new IC, you'd think it'd be an easy swap out, however the idiots lovely people who sent me the 7800 IC sent it with no protection at all - so it arrived with the pins completely bent and folded over - 30 minutes wasted cleaning that up and I finally got it in to the socket. I'm avoiding reinsertion as the pins are already a bit weaker (and I didn't think to do one-at-a-time when soldering).

[PA0PBZ]

I'm avoiding reinsertion as the pins are already a bit weaker (and I didn't think to do one-at-a-time when soldering).

Understandable, so case closed and enjoy your new-old monitor