Need Help with Finding an Amdek V220A Flyback (FMC-1245NL) Replacement

[awewsomegamer]

Hello Hugo, great to hear that there is one more thing to try before measurement.

I figured to go and inspect these spark gaps further to get you some more information, so I got some pictures of the board back lit by centering it in a ring light I have on my work desk. See the first attached screenshot of the area.

It seems SG3 is the most dirty, all others look fairly clean. I made sure to move the cables attaching to the board out of the way.

Before taking the monitor to the repair shop, I did try cleaning both VR503 and VR501 to no avail. Further, I tried replacing VR503 with a new 2 MOhm trimmer resistor, but this also did not work.

I did take VR503 out of circuit and hooked it to a signal generator, and it did seem to behave similarly to the monitor. After squeezing it, the behavior seemed to disappear.



In this photo, the scope's probe was on 3 and the scope's signal generator on 2.

Thank you again for taking a look at this.

 

[Hugo Holden]

I have no idea at all why you scoped or tested the Focus potentiometer VR503 or did anything to it. I even remarked about that, irregularities in the focus voltage will not cause the fluctuations you are seeing. That potentiometer and the focus voltage on G4 were never in the diagnostic equation. Best to leave that preset potentiometer alone. At least you know that VR501 is probably ok.

The CRT electrode voltages , with variations that could cause the fluctuations you are seeing relate to the cathode voltage, the G1 voltage and G2 voltage only.They are the ones to scope & investigate.

I think you should run a drill through all the spark gap holes, pick a drill that is a little larger than the width of the gap, and do it with the drill bit in a hand chuck, in your hand, so nothing violent happens. Then re-try the VDU and see if the fluctuations have gone away.

 

[andy]

Have I missed the part where we figured out why there is no picture? I would start with that since it should be a pretty obvious problem. A few voltage checks should reveal something.

 

[awewsomegamer]

Hello,

The CRT has been reassembled, SG3 has been cleaned with a pin (I did not have a bit small enough to go into the hole), and everything should be ready for a test.

I will scope K, G1, and G2 by placing the probe before resistors R119, R523, R524 respectively.

Before I do that, I would like to pick up a test pattern generator, do you all have any recommendations for which to get?

 

[andy]

I'd check the voltage on the G2 with a meter before you put a scope probe near it, not that you will learn anything useful from scoping the G2 since it's a DC voltage.

 

[Hugo Holden]

I'd check the voltage on the G2 with a meter before you put a scope probe near it, not that you will learn anything useful from scoping the G2 since it's a DC voltage.

In this case he is looking for fluctuations in what should be a stable DC voltages, which is why the scope is handy, the response of the meter might be a tad slow to spot it. Also fluctuations in the relative G1 to cathode voltage are being searched for, as these three voltages, if fluctuating , could cause the effect he is seeing on the screen.

I think he should get a suitable sized drill to clear out the phenolic material between the sparks gaps because that is a location where any conduction can cause fluctautions of the voltages, if the surface of the phenolic material has carbonised.

 

[andy]

I was alluding to the fact that there could be voltages on the end of his CRT that are rather dangerous to a typical oscilloscope front end. Also, does he even have a picture yet?

 

[Hugo Holden]

I was alluding to the fact that there could be voltages on the end of his CRT that are rather dangerous to a typical oscilloscope front end. Also, does he even have a picture yet?

I suggested the x10 probe ( see post #20) as most can handle 400V easily. The k, G1 & G2 voltages are normally lower than this in a monochrome VDU of this size. But, when working on tube gear, mostly it is better to use the x100, probe. You certainly need that for the HOT collector voltage in a color set that is around 1.2kV peak in many cases.

x100 probe costs have dropped dramatically in the last decade, you can get a P4100 for about $40.

I recently wrote an article about scope probes and how they work and I reverse engineered the famous Tek P6137 400MHz probe. Tek never published its design.

And on the topic of x100 probes, most Technicians are completely unaware that there are two entirely different design philosophies with these probes, resultng in one type having a substantially lower input capacitance. I explained it in the article:

https://worldphaco.com/uploads/Tek%202465B%20Repairs.pdf

 

[awewsomegamer]

Hello,

I have tested the CRT just now. The video source was an RF modulator, that outputs color bars when no input is provided to it, connected directly to my VCR.

I measured the following voltages with my scope and meter: K, G1, G2 (across R523). For no particular reason, I also measured B, but only with the meter.

Voltage	Scope (10x)	Meter
B	N/A	12 VDC
K	Vrms=0.740V Pos=+0V	0.629 VDC
G1	Vrms=33V, 12.1kHz square wave, regular "blanking" periods Pos=+47V	-30 VDC
G2	(through R523) Vrms=~150V, stable	437 VDC

The square wave on G1 persists even without a video source.

Here is the video of the K and G1 measurements:

 

[Hugo Holden]

What to you see on the screen currently ?

The most abnormal recording is the cathode voltage. It does not look like any form of regular video drive voltage. We know from the circuit I traced out, the cathode is being driven from the output of the video amplifier.

What is the video signal you are currently feeding it ? saying where it comes from is not that helpful in confirming that the video signal is normal or not.

Do you have a known good stable 75 Ohm baseband video source (nothing related to an RF modulator) that you can 1) verify is ok with the scope and 2) check it on a known good VDU that it is ok and 3) then feed it to the Amdek, so we can see the result at the CRT's cathode on the scope ?

("The video source was an RF modulator"..... I'm sure that you know that you cannot feed this VDU from an RF modulator)

 

[awewsomegamer]

What to you see on the screen currently ?

The most abnormal recording is the cathode voltage. It does not look like any form of regular video drive voltage. We know from the circuit I traced out, the cathode is being driven from the output of the video amplifier.

What is the video signal you are currently feeding it ? saying where it comes from is not that helpful in confirming that the video signal is normal or not.

Do you have a known good stable 75 Ohm baseband video source (nothing related to an RF modulator) that you can 1) verify is ok with the scope and 2) check it on a known good VDU that it is ok and 3) then feed it to the Amdek, so we can see the result at the CRT's cathode on the scope ?

("The video source was an RF modulator"..... I'm sure that you know that you cannot feed this VDU from an RF modulator)

Hello Hugo,

The cathode's voltage was likely polluted by the scope as the scope is quite cheap.

The Amdek does not have an RF input. I have an RF modulator that produces color bars if it has no input.

I plugged the modulator into my VCR, and the analog output of my VCR into the monitor.

Attached is a picture of the video signal that was fed to the monitor from this stack.

 

[Hugo Holden]

That image still does not 100% guarantee that a video signal that generates that image is making it into the Amdek's video amplifier input. There was nothing like that at the CRT's cathode.

Can you take two scope recordings to double check, one at the video amplifier input which is the base of the transistor Q106 (yellow wire) on the Neck board, firstly we will see if that video signal is making it to the input of the video ampifier, and double check the cathode. The video amp might not be working.

Also to double check your scope can actually resolve a video signal, scope the video signal coming out of your VCR (the cable coming from it) and adjust the scope timebase to see a line or two of video. A scope recording of NTSC color bars should look like the attached over a couple of lines of video, the stair steps in the grey scale being cluttered over by the color carrier. With the cable from the VCR unplugged, and not terminated in the usual 75 Ohm resistor (that is in the Amdek) the peak to peak voltage in the cable should be about 2V. When it gets plugged in and terminated it would drop to 1Vpp.

 

[awewsomegamer]

That image still does not 100% guarantee that a video signal that generates that image is making it into the Amdek's video amplifier input. There was nothing like that at the CRT's cathode.

Can you take two scope recordings to double check, one at the video amplifier input which is the base of the transistor Q106 (yellow wire) on the Neck board, firstly we will see if that video signal is making it to the input of the video ampifier, and double check the cathode. The video amp might not be working.

Also to double check your scope can actually resolve a video signal, scope the video signal coming out of your VCR (the cable coming from it) and adjust the scope timebase to see a line or two of video. A scope recording of NTSC color bars should look like the attached over a couple of lines of video, the stair steps in the grey scale being cluttered over by the color carrier. With the cable from the VCR unplugged, and not terminated in the usual 75 Ohm resistor (that is in the Amdek) the peak to peak voltage in the cable should be about 2V. When it gets plugged in and terminated it would drop to 1Vpp.

I set my probe back 1X for these measurements, at 10X there is quite a bit more noise from the scope.

This is a zoomed out picture of the voltage at the base of Q106:


This is a slightly more zoomed in shot at the start of the above waveform:

This is the full view of the input video signal:


Zoomed in onto the start of the above signal:


I also probed the video input in the Amdek, and the voltage dropped to ~600mV.

 

[Hugo Holden]

What are you seeing on the screen, you did not answer that question.

And although you recorded the base voltage of Q106 (which is very abnormal, we will get to that later and the cause of that), where is the recording of the CRT's cathode voltage with the scope, can you chek that again and post it.

Plese always use the x10 probe even if you see more noise.

Also check the voltage on the blue wire on the neck board with the meter, report that, and double check R123, the 8.2k resistor on the neck board to ensure its not open. The previously low CRT measured cathode voltage and what could be a drop in input resistance of Q106's Base circuit (differentiating the video signal) could be because the B+ voltage on the blue wire has gone awol or the 8.2k resistor is open as two of about 4 possibilities.

 

[awewsomegamer]

What are you seeing on the screen, you did not answer that question.

And although you recorded the base voltage of Q106 (which is very abnormal, we will get to that later and the cause of that), where is the recording of the CRT's cathode voltage with the scope, can you chek that again and post it.

Plese always use the x10 probe even if you see more noise.

Also check the voltage on the blue wire on the neck board with the meter, report that, and double check R123, the 8.2k resistor on the neck board to ensure its not open. The previously low CRT measured cathode voltage and what could be a drop in input resistance of Q106's Base circuit (differentiating the video signal) could be because the B+ voltage on the blue wire has gone awol or the 8.2k resistor is open as two of about 4 possibilities.

My apologies, there is still no picture on screen.

I am pretty sure that the K voltage is the same as Q106's base, so I most likely did not see in the signal in the first measurement.

 

[awewsomegamer]

My apologies, there is still no picture on screen.

I am pretty sure that the K voltage is the same as Q106's base, so I most likely did not see in the signal in the first measurement.

I was in the middle of editing this message when it hit the time limit, so:

My apologies for not answering the question, there is still no picture, and I have mistaken the cathode voltage you wanted.

In the original measurement, the wire labelled "K" was measured instead of the cathode.

Voltage	Scope (10x)	Meter
B+	~80 V	81.7 VDC, appears stable
Cathode (pin 2 of socket)	Vpp=15.9 V Intermittent blanking periods of 11.6uS and and a ~2.77 MHz sine wave (measured using cursors)	70.3 VDC, appears stable

The resistance of R123 measured to 1.2 kOhms.

This is the scope's shot of the cathode's voltage:


A more zoomed in shot of the above signal:

 

[Hugo Holden]

Can you slow the scope's timebase down, so the sweep covers > than 63 uS (one line of video) tso we can see if the cathode volatge looks right or not.

16v pp on the cathode is somewhat low, normally it should be twice that.

If the CRTcathode is 70V, then the video signal dropping it 16V below that would unlikely put the CRT into beam current, so I would expect the CRT would still be blacked out.

To get any CRT beam currrent and illuminate the screen, the relative grid to cathode voltage must generally be less than 30V (grid more negative than cathode) and for full beam brightness 0V, they are equal.

So if the cathode is at +70V, and the signal peaks taking it down to about +54V, then to see anything on the face of the CRT, the grid of the CRT would have to be in the order of +30V or more unblanked, not about zero volts which is I think what you found before in the unblanked state.

This suggests the cathode voltage is likely too high and it should be more in the order of 30 to 35 volts and also that the video drive level should be higher too.

This could possibly mean that one or other of the two video output transistors is defective, unless the B+ voltage is running way higher than it was originally designed or the CRT's grid voltage is running more negative than it was designed.

See what the most positive voltage you can get is, on the CRT's g1(white wire), by adjusting the brightness control and the series sub-brightness control and report that. Also, turn up the Contrast control to get the highest peak to peak video voltage on the cathode of the CRT and report that, and with those things done see if you can see the CRT face illuminate.

We may have to trace out the circuit that powers the video amplifier on the main board.

 

[awewsomegamer]

Can you slow the scope's timebase down, so the sweep covers > than 63 uS (one line of video) tso we can see if the cathode volatge looks right or not.

16v pp on the cathode is somewhat low, normally it should be twice that.

If the CRTcathode is 70V, then the video signal dropping it 16V below that would unlikely put the CRT into beam current, so I would expect the CRT would still be blacked out.

To get any CRT beam currrent and illuminate the screen, the relative grid to cathode voltage must generally be less than 30V (grid more negative than cathode) and for full beam brightness 0V, they are equal.

So if the cathode is at +70V, and the signal peaks taking it down to about +54V, then to see anything on the face of the CRT, the grid of the CRT would have to be in the order of +30V or more unblanked, not about zero volts which is I think what you found before in the unblanked state.

This suggests the cathode voltage is likely too high and it should be more in the order of 30 to 35 volts and also that the video drive level should be higher too.

This could possibly mean that one or other of the two video output transistors is defective, unless the B+ voltage is running way higher than it was originally designed or the CRT's grid voltage is running more negative than it was designed.

See what the most positive voltage you can get is, on the CRT's g1(white wire), by adjusting the brightness control and the series sub-brightness control and report that. Also, turn up the Contrast control to get the highest peak to peak video voltage on the cathode of the CRT and report that, and with those things done see if you can see the CRT face illuminate.

We may have to trace out the circuit that powers the video amplifier on the main board.

!!! THERE IS A PICTURE (sort of, see end for more info) !!!

All measurements were done with 10x probes.

Here are the pictures of the cathode voltage with slower timebases:

100uS:

50uS:


I have also measured all other voltages on the socket, excluding S1 (not connected) and S4 (ground).

S3: 9.5 VDC, stable
S5:


I found some strange behavior on S6, the focus voltage, it looks like it is a result the behavior of VR503. It was unstable and around 6VDC. This is its behavior, despite maintaining good contact with the pin:

In the video the scope setup is 20uS and 5V per division. The cursor is set at 7.5V.

I am not sure how accurate these measurements are. Particularly, at higher frequencies in the MHz range.
This scope isn't too good, so I have ordered a better scope from Siglent (SDS408X).
When it arrives I will hopefully get more accurate measurements.

Adjusting VR502, user brightness control, and VR501 to maximize G1

VR501 changes the bias of G1 slightly.
From when it was working, I know that the maximum brightness was when VR502 was rotated all the way counter-clockwwise (when looking at the back of the board, counter-clockwise is to the right).
VR502, like VR501, biases G1, but this time the change in voltage is much bigger.

VR502 Setting	VR501 Setting	G1 Maximum (V)	G1 Minimum (V)	G1 Bias (V)	VR502's Left Terminal (V)
Maximum	Factory	-25	-63	-44	-13.5
Maximum	Maximum (fully counter-clockwise)	-24	-62.5	-43.25	-19
Maximum	Minimum (fully clockwise)	-16.5	-50.5	-33.5	0
Minimum	Factory	-5	-44	-24.5	-13.5

Attempting to adjust contrast control to maximize S2 (cathode voltage)

The contrast control changes the amplitude of the cathode's signal. With the contrast control all the way counter-clockwise the signal becomes flat, and all the way clockwise the signal emerges the most.

With the contrast at its maximum, Vpp=~16.5V on the sine wave portion of the signal, the maximum voltage of the cathode is ~80.5V, and the minimum is ~64.5V.

The changes did not produce a picture.

!!! THERE IS A PICTURE (sort of) !!!

After repeating the test for changing the value of VR502, VR501, and contrast, I turned the monitor off and went to turn off the extension cable when I noticed an EXTREMELY bright point in the middle of the screen, it then faded out, and it doesn't seem the point burned in the phosphor.

Every time the monitor is turned off now, an extremely bright point is drawn a little while after the switch off. The brightness of the point does not depend on the setting of VR501, VR502, or contrast resistors.

Further, observing the monitor now, there seems to be noise that is extremely faint across the entirety of the screen. This is most likely because of the V-HOLD, V-SIZE, and H-HOLD pots being set wrong.

I think today's frequent use of VR501 and VR502 has potentially cleaned them a bit, and having it on for a longer periods of time, more regularly for testing, has changed something with a passive component, maybe a resistor on the neckboard.

* The one other thing I did that could relate to this was solder R123 back into circuit as I took it out for testing, maybe the heat from the soldering iron "fixed" it or another resistor nearby?

The point's fading in/out looks like a capacitor discharging.

One more test I could do is to see if changing the focus trimmer has any effect on the size of the point.

Here is a recording of what this looks like:
View attachment AmdekDot.mp4

 

[Hugo Holden]

Surely you can see what that dot on the center of the screen means ?

Both the horizontal and vertical deflection systems have failed to operate.

While one could invoke the notion of a power supply failure to both the H and V deflection systems, this idea does not wash, because in the event of the H deflection system failing, there is no EHT and no beam current, so you would see nothing on the CRT face.

So the fact that the beam illuminated the CRT with the phosphor damaging high intensity dot, means there is a working H deflection system & EHT system, but there is no H deflection current in the H yoke coils. Also none in the V deflection coils either, it is as though you have disconnected the CRT's deflection yoke, and forgot to reconnect it/plug it back in.

 

[awewsomegamer]

Surely you can see what that dot on the center of the screen means ?

Both the horizontal and vertical deflection systems have failed to operate.

While one could invoke the notion of a power supply failure to both the H and V deflection systems, this idea does not wash, because in the event of the H deflection system failing, there is no EHT and no beam current, so you would see nothing on the CRT face.

So the fact that the beam illuminated the CRT with the phosphor damaging high intensity dot, means there is a working H deflection system & EHT system, but there is no H deflection current in the H yoke coils. Also none in the V deflection coils either, it is as though you have disconnected the CRT's deflection yoke, and forgot to reconnect it/plug it back in.

Yes, there is no horizontal or vertical deflection, because this dot appears only after the main power switch is turned off.

The connections for the deflection coils need to be soldered into the board, which I did do, so it is very unlikely for them to be disconnected.