Atari SC1435 Monitor Troubleshooting and Repair

I was recently contacted by forum member Mikro, who had read my 1084S monitor repair thread, and asked if I would be willing to take a look at his Atari SC1435 monitor.

The Atari SC1435, which is virtually identical to the Philips CM8833 MkII, was working fine until one day the picture collapsed and was never to be seen again. The power light was still lit however.




I decided I'd like to get more experience with these things, so I arranged to get the monitor from Mikro, and began to troubleshoot it.

Initially I disassembled the unit, and resoldered key joints such as flyback, HOT, switching transformer, yoke. Then reassembled I began to take some test measurements.

Using the high voltage probe I built last year I was seeing only around 13kV on the picture tube - pretty much half what it should have been (should have been around 25kV).

Checking the power supply B+, I was seeing just 68V (this should have been around 125V) :



Many of the other power rails were approx half what they should have been, but the power supply section all checked out otherwise.

I disconnected the B+ supply before the flyback, connected a load (lightbulb) and re-tested - supply voltages were normal :



Thus it was quite possible that the flyback transformer was at fault, dragging the power supply down, but I had no way to properly test it.

I'd read about ring testers before - a ring tester applies a pulse to the flyback transformer primary circuit, and counts the number of damped oscillations or 'rings' produced :

"`Ring' testing gets its name from the fact that when a fast pulse is applied to the primary winding of the LOPT, the total inductance and capacitance in the circuit will produce an electrical `ring' - a decaying AC voltage which can have a duration of a dozen or more cycles before it reaches a low value. It's the electrical equivalent of tapping an empty glass; in each case, an energy impulse generates damped oscillations."

The ring testers that I had seen had 8 LED's - the idea being that if it lit up all 8 LED's then the transformer was likely ok, but if it only lit up a couple then the transformer likely had internal shorts in the windings.

Dick Smith used to sell a ring tester kit which was designed by Bob Parker:



Luckily, Bob Parker has kept all documentation for this ring tester kit on his website here, including the assembly manual, PCB artwork, and mirrored artwork for toner transfer methods.

So, I decided I'd build one using the documentation that was available, and etching my own board. The finished article (I'll put it in a case at some stage!) :



I used the toner transfer method but was a bit lazy cleaning the board beforehand, and the transfer was a bit ugly. A couple of tracks had small breaks in them after etching - Thus i went over the whole pcb with a bit of solder and repaired the breaks :



Main thing is it works!

Testing the flyback in circuit across the primary coil on the flyback :



Only 3 LED's were lit on the tester, which is not a good sign :



I also hooked up the scope - the ringing waveform :




Testing another transformer - all LED's lit up as they should:



And the scope trace :



This is pretty much what the flyback trace should have looked like - a gentle attenuation with plenty of "rings".

Incidentally, this is the pulse that is injected :




Thus the flyback was classified as faulty, and removed. I was able to order another one from WES which I should receive later today

The old one :



I'll update this thread once I've installed the replacement.

 

Boy I love these threads. So in depth. Thanks for sharing, as always.

 

So is this a service you offer aXLe?

I will be at a later date (probably months from now considering my backlog lol) looking at repairing a couple of IBM monitors I have. Nothing drastic for now, just a cap replacement for starters. If it ends up being anything more (which I suspect one might be) I might hit you up for advice etc. I'm comfortable with replacing caps and other small components (after taking all precautions of course) but not particularly keen on dealing with the flyback.

Good work with the monitor. It's good to see that there are still people out there willing to keep old CRT's going.

 

Thanks for the kind words (and from elvis too) - I really haven't had a lot of experience yet with monitor repairs but I'm certainly willing to have a go at it and am learning a lot on the way. I'm happy to help with yours if you run into trouble

Quick update on this one - new flyback arrived :



This one does light up all 8 LED's on the ring tester (the old one only lights up 2 out of circuit) :





I've installed it, however it hasn't resolved the issue - in fact if anything it's worse!

B+ (125V supply) is getting totally dragged down now (or PSU is shutting down) and I'm wondering if the new flyback means that there is some additional stress on some of the components including the horizontal output transistor and the snubber cap.

The transistor and capacitor both test ok, however they aren't being subjected to the same voltages when testing on the bench with a meter. I have some parts on order and will update the thread properly once I've had a chance to have a bit more of a look at it.

 

When the weather cools down and I can get back in the shed, I've got a few monitors that need fixing up. If it's bigger than a cap/transistor replacement, I'm lost.

If you're willing, I'd love to call on you to take a look. I'd be happy to pay you for your time, regardless of fix or not (these are often 30+ year old arcade monitors, or side-of-the-road TV pickups, so I don't expect magic, but I also value that people's time and expertise is worth paying for, because I know mine is too).

 

Sounds good elvis

On another note, I've been corresponding with Bob Parker and have permission to produce a version of his original ring tester.

I thought I'd like to do an smt version using the PCB as the front panel and with the economics of PCB manufacture I figured that I may as well get permission to sell a few since I will only need one!

Won't be a big seller of course, but handy to have for those dabbling with vintage gear. Anatek do a version (blue ring tester) but its through-hole and to be honest I like the original design. I will likely add a sensitivity switch (normal/high) but otherwise it be unchanged (except smt of course)

 

Amazing read and investigation, can't wait for an update. Great work, aXLe!

 

Nice work, hows my board going btw?

All way over my head

 

I think these are the same as the Commodore 1084S.

The bung flyback is a common issue in those, and everywhere I've read about repairs they say if the flyback is gone, to replace the HOT while you are at it regardless.

EDIT: you probably already know this but the new flybacks ship with the screen pot will be turned right down and will need turning up to get a picture.

 

Quick update. Play had been called off due to bad weather (too bloody hot in the garage!!) - just getting back into this again now that it is starting to cool down a little.

I've had a bit of a look at the current issue and am pretty sure that the unit is going into HV shutdown (there is an SCR that "crowbars" the PSU, shutting it down if various voltages go a little too high).

I'm starting to investigate the cause - if I run the board on the bench without the HV anode attached, the PSU sits there happy (measure HV output was only 10kV though) and all PSU output voltages appear to be in spec.

If I put the board back in the chassis and connect the HV anode the PSU will almost immediately go into shutdown, but before it does I am seeing 20kV on the tube which is promising!

I've scoped the drive to the HOT and I am seeing 15kHz pulses - I'm pretty sure that the PSU and horizontal drive is all good (including horizontal output transistor) so I'm investigating components around the SCR to see if perhaps the new flyback has a higher output on a secondary coil that could trigger the SCR, or whether there is a bad component in that area (a number of zener diodes are used plus a few caps that I need to check out).

Hopefully now that I'm back working on this I'll be able to get to the bottom of it!

 

This is a great thread, I only wish I knew enough about CRTs to repair them, beyond obviously blown caps etc.
I think this is going to be a lost art one day...

 

Does the power supply have over-current protection? That might also cause this if the new flyback draws more current. But I can't find a schematic for the power supply.

 

Probably a lot quicker than we realise, too.

 

I made some progress with this monitor last night after delving a little into the High voltage shutdown circuit.

In the image below, the flyback is highlighted in pink - you will note that there is a small coil in there between terminals 1 and 2 which is used to generate the heater voltage for the picture tube (heats the filaments in the tube - giving the "neck glow") via the A11 and A12 lines, and is also used as feedback for X-ray protection :





So the output of the coil is a 15Khz pulse that goes off to the filaments, but it also gets rectified by diode 6546 and filtered by the capacitor 2547, where the output is supposed to be a stable 23.4VDC according to the notes on the schematic. There are 2 zener type diodes (6547, 6548) that total up to 23.6V - if the voltage at the output of the rectifier is greater than this, then there will be excess voltage fed to the "X-RAY Protection" input on the PSU section.

In the diagram below we can see that the X-ray Protection voltage feeds into the gate of an SCR (highlighted in the red box) - if this SCR turns on, it shorts the 128V B+ rail on the PSU to ground - killing the input voltage to the base of transistor 7417 which controls the feedback optocoupler (bottom left corner) - shutting down the supply.



We can see that there are also a couple of other over/undervoltage feeds running to the gate of the same SCR.

Firstly the orange path which originates from the +28V supply output - again a couple of zener type diodes (6454, 6452) which add up to 30V - if the +28V supply goes above 30V, the excess voltage will trigger the SCR and shut down the supply.

Next the green path, which appears to be an under-voltage shutdown - if the base of transistor 7470 (a PNP transistor) drops to around 5.5V (emitter is at 6V2) then it will turn on and will put 6V2 on the gate of the SCR, again shutting down the supply.

I focused my initial investigation around the X-Ray protection circuit as this is operated from the flyback, so I decided that I'd bypass it by removing one of the diodes (6547 marked in blue below) from the circuit - breaking the path to the gate of the SCR :



I found that the PSU came up fine, and I was able to adjust the B+ to 128V and was getting approx 24.8kV on the anode cap of the tube!



(Voltage probe is 1000:1 - thus 24.8V on the meter is 24,800V(!) )


This was stable and I was able to measure all of the other various PSU outputs in spec (I haven't measured the 5V3 output - hope to check tonight).

Now the voltage at the cathode of the rectifier (diode 6546) was 30V - quite a bit above the 23.4V that the protection circuit was designed for. As this voltage is generated by the flyback, I'm wondering if the new flyback is out of spec. I could possibly substitute the 18V and 5V6 zeners in the protection circuit for 2 x 15V ones and that would then operate, however I'm not sure about the long term effect on the filaments in the picture tube - maybe ok??

The x-ray protection is taken off this secondary coil as the voltages are a lot easier to deal with - the assumption is that if the voltage on the secondary has gone beyond the 23.4V then the HV output must be in the danger category, however in this case the HV output is fine at 24kV.

I may need to consult with someone who knows about this stuff (and possibly the manufacturer of the flyback transformer) to see whether this higher output will be an issue. If not I'll replace the protection diodes with higher voltage ones (or maybe they will send me another flyback to try).

Unfortunately whilst I'm now able to get a stable PSU and high voltage, I'm still not getting a raster! So there is more work to be done - I'll be looking at the vertical deflection circuit next to work out what is going on there (I have tried adjusting the screen voltage on the flyback - no dice).

Some progress anyway and I'm learning along the way so all good

 

I extended one of the ground cables so that I could run the board outside of the chassis with all the CRT connections in place - much better access to everything for testing :



I was able to properly adjust the screen and focus voltage adjustment pots on the flyback transformer such that I was seeing a horizontal line across the screen - this confirms that the horizontal section is working and that I now have a vertical deflection issue :



The heater filaments in the tube glowing ("neck glow") :



The electron beam is essentially controlled by two coils on the rear of the picture tube - the horizontal deflection coil and the vertical deflection coil :



Normally the electron beam sweeps the face of the picture tube - starting at the top it will sweep to the end of one line (under control of the horizontal deflection circuit), then very quickly fly back to the beginning of the line while at the same time the vertical deflection circuit will move it down a line (moves in a "Z"). It then continues sweeping across and down the tube line by line until it reaches the end of the bottom line, after which the beam will go back to the top (during the vertical blanking interval) and repeat over and over for your viewing pleasure . With no input to the monitor, if you turn up the brightness this is what you are seeing - the raster as the beam sweeps the screen.

If the vertical circuit is faulty, you get the beam sweeping from side to side only, with no vertical control - giving the bright horizontal line in the middle of the screen (you don't want to run it too long like that as it will "burn in" (the beam burns out the phosphor coating inside the front of the picture tube leaving a permanent ghost image).

I've checked the horizontal and vertical coils (if they burn out then its probably fatal as the parts are likely not available any longer) with the ring tester and they are fine. The vertical deflection on this monitor is controlled by a Philips TDA3653 vertical deflection IC :



I now need to probe around this IC for the various voltages and signals and see whats going on - its possible the chip itself is dead, or it could be as simple as a bad capacitor/resistor or diode.

 

Ok - probed the supply pin to this chip (pin 9) and nothing - zero volts.

Tracing back I found that I had 28V on the input side of resistor 3563 (top right of the last picture above - also highlighted in pink below.

This resistor is a 4R7 0.33W resistor - measuring it in circuit showed it as an open circuit so I removed to double check it. Yep - it was dead.

I replaced it and tested again - still no vertical deflection and both the vertical IC and the resistor were getting quite hot.

Measuring the voltage either side of the resistor I found I had 27V on one side, and 22V on the other - a 5V drop across it.



Using Ohms law, the current through it is therefore V/R = 5/4.7 = just over 1 amp. Note that the 28V rail has sagged to 27V under this load.

Power dissipation would be = I * V = 1A x 5V = 5W!

The original part was a 0.3W resistor - you can see why it measured open circuit - it cooked.

So where is this current flowing? The only place it can go (other than through the capacitor 2563 - I checked this cap and its fine) is through the chip.

Looking at the datasheet for the TDA3653, the total supply current (with no load) should typically be 50mA with a max of 85mA.



To be sure I unplugged the deflection coils and the current draw was the same.

Thus, I'm sure that this chip is dead so I've now got to hunt down another!

I feel I'm getting somewhere at least.

 

Love your work man Wish I has them electronic abilities.

 

Well the chip is out, and I have one on the way - it's coming from the UK though so will take a week or so.

Removing large SIL packages like this is super fast with a 32mm blade tip You can heat all the leads at once, and pull it out. Then a quick clean of the pads with some braid, and the boards all ready to accept the new part.



While I'm waiting on the new part to arrive, during the week I'll have a look at the degauss function on the monitor. When I first received the unit I noticed that the it wasn't degaussing when powered up (that thump you hear on some CRT's when they first power on) - or at least it didn't sound like it was working - I'll be checking out the posistor, which may possibly be faulty.

I must say that this monitor has been an excellent one for learning about monitors! It had a fair number of issues, but I reckon I'm pretty close now to having it going again

 

This is a great thread.

Do you have any more information on that high voltage probe you're using? I want to make one.

What method did you use to check the cap? It's possible for a capacitor to measure open circuit with a multimeter but still have high leakage at rated voltage.

You can test this by removing the cap and placing it in series with a resistor with 28V across both (then just measure current draw of power supply or voltage over resistor).

 

Check my signature for links to my retro projects including the probe I can supply my KiCAD project if interested.

In regard to the cap, I removed it from circuit and checked capacitance and ESR - tested ok.

I'm still waiting on the replacement vertical IC - should arrive this week I hope.