High pictched noise coming from terminal I got from eBay - no screen

[alank2]

So I rolled the dice and ordered a dumb terminal from eBay and it doesn't work.  Power on = high pitched noise, no video or retrace line that I can see even after changing the brightness/contrast knobs from edge to edge.  Any ideas on what might be wrong?

[floobydust]

Do you see the CRT filaments light up, any high-voltage crackle?
I would guess the power supply is sick or overloaded, assuming it's the horizontal (flyback) transformer making the noise?

Wilder guess says HV rectifier shorted and loading things down.
Best to check some PSU voltages.

[alank2]

No high voltage crackle.  It is going back today so no chance to work on it.

[alank2]

A good friend of mine helped me troubleshoot it and we found a diode G1856 was shorted near the flyback/transistor connected to a large metal heat sink around the flyback.  Removing this diode fixed the power supply being shorted condition and allowed 5V on the CPU rail.  The CPU boots up and makes a noise on a piezo now.  I can hear beeping when I type or hold a key down.  We initially tried to put another diode in there to replace the shorted one.  At first power on, the brightness control was out of bounds to where you could not turn it down enough.  Changing a master brightness pot allowed it to be turned down to black.  The entire screen was filled/can be filled with amber when turned up and if you turn it too high you can see the raster line.  We scoped the video signal and could see a blinking from a cursor presumably, but you can't see it in the video.  I adjusted the v-size and can see border on the top and bottom, but I can't see any adjustment or adjust the h-size to get a border on the left or right, it goes past the display edge on the left/right.  One last thing I tried was to cut the diode we added and see if it made a different, but no difference.  Any ideas of what to try next?  I am unfamiliar with video signals though I could see the contrast increase the voltage of it, what voltage range should it be in?

[james_s]

You should start by checking the voltages on the G1, G1 and cathode pins on the CRT, I'm a bit fuzzy on the specific values but if G2 (also called "Screen") is too high, the picture will be washed out. It sounds like you have an almost-working monitor, you just need to figure out what issues remain. You might start by turning down the screen/sub bright control as someone may have cranked it up all the way trying to get a picture with the shorted diode.

[alank2]

I agree, I feel like it is so close to working.  What should the voltages be on G1, G2, cathode?

[alank2]

There is a shielded small RF type cable connecting to the pcb on the next of the display.  It has a black/white and I connected my differential probe to it and came up with:

I figure it is 60 Hz.  This was after holding down a bunch of A's and sending characters to the terminal over its serial port.  The voltage range was from 0.6V to 3.2V.  This is with the front panel control contrast all the way up.  Turning it down reduces the voltage.



There are five wires coming off of the pcb that are blue, yellow, red, brown, green.  blue/yellow are on a tiny small cable going to a 3pos connector on the neck PCB.  The third wire in this connector is separate (white) and goes to a pcb post.  The red/brown/green are heavier gauge wire and go into a 3 pos connector on the neck PCB.  I used my diff probe (pintek DP-25) on its highest 700V setting and the red wire set the overrange LED on.

There are also another set of red/blue/yellow/green heavier gauge wires coming off of the pcb that go to the side of the white ring near the windings.  Of the four, only the red one seemed to have a lot of voltage on it:



I'll post a picture of this in a minute.

[alank2]

pics

[alank2]

I tried adjusting the master contrast to its largest amount which barely pushed the output signal to around 4V.  I read that sometimes more is needed so I hooked up my bench power supply to the socket on the pcb neck to see if 5V, 6V, 7V would light the display more.  It started at 30mV and was up to 90mV with 7V and the display did not change.  Should the pcb on the neck amplify the output signal?  I could try to probe the pins on the socket that it plugs into while it is running to see if I can find that...

[james_s]

Ok slow down a bit, it sounds like you don't really understand how a CRT monitor works, learning a bit of the theory will go a long way toward a successful repair and not hurting yourself in the process. A decent general overview is here http://lateblt.tripod.com/bit71.txt The coil assembly around the neck of the tube is the deflection yoke, this is actually two separate coils, one horizontal and one for vertical deflection. Since you have a full raster the yoke is fine, no sense in messing with it. The neck board is that shielded board on the end of the neck, be very careful with that as pushing it to one side or the other very far will break the exhaust pip off the end of the tube where the air is evacuated during manufacturing. If that happens you'll here a sickening crack followed by a whoosh and then you have a paperweight. Anyway on that board is a video amplifier that drives the cathode, this is what switches the beam to paint the picture on the tube. That first scope trace looks like the video signal driving this, there is a video amplifier on that board which is a single transistor amplifier to convert the low level video signal to the higher voltages required to drive the electron gun and also inverts the signal since pulling the cathode negative turns on the beam. In many designs the contrast control sets the gain of the video amplifier and the brightness control adjusts the DC offset. G1 is often tied to ground on small monochrome monitors and G2 comes from a voltage divider pot and is typically somewhere around 50V-250V, the higher the G2 voltage the brighter the picture. he sub-brightness or screen control will be internal and not customer accessible, normally it's located near the flyback transformer which is what supplies the HV to the anode on the tube. Don't run it with the contrast and G2 too high for very long as it greatly accelerates wear on the cathode and nobody is making or rebuilding CRTs anymore so when they're gone they're gone.

What kind of terminal is this? You might be able to find a service manual for it which will give you an idea of the voltages needed. Otherwise a service manual for any monochrome video monitor ought to be close enough. You can find some here http://vt100.net/ and elsewhere is a manual for the popular Wyse 60, I know because I found it when I was working on mine.

T

[alank2]

Thanks james_s.  You are right I only have a basic understanding of how a CRT works.  I'll check out the link you posted - I've been searching for similar.  It seems peculiar to me that I can adjust the brightness and bring the entire screen up so it is redrawing properly - I just can't get the signal to turn on or off the gun to show it.

Do you think the board on the end of the CRT has the issue, or could it possibly be that another voltage that the amplifier is working it isn't at the right level?  I am a bit leery of doing much with that board!  It has a white goop holding it in the stocket, but could I pull the PCB directly rear ward and unsocket it?  Should I check any other voltages before trying to do much with this pcb?  I can get access to some of the socket pins between the socket and the pcb and could try to see if I can find the amplifier signal to see what voltages it is at?  How can I identify G1, G2, etc?

It is an Aspect telecommunications terminal - I've searched far and wide and found nothing, not even a user manual.  Model 3039, PN 9500-0274.  It is a cute little terminal.  The only thing I don't love is that it has an adjustable angle for the CRT - the case is made so you can swivel the CRT vertically!  I can see that this puts pressure on the rear pcb on the back of the tube though.  I am going to put it in one position and superglue the case so it can't move like that any more if I get it working.  It is also on a stand that swivels too and that is good enough.

What controls the width?  There is a white large thing next to the flyback with a ferrite adjustable core inside it, does this adjust the width?

[alank2]

The top pin on the socket is negative in respect to ground.  It was like -50V to -90V depending on the brightness.  I don't see any of the output signal in it.  Oddly now something has happened and I don't see an output on my signal line any more.  It still goes up/down with the contrast control, but no more pulses or the cursor or what might be on the screen.

[alank2]

Also, there is a 3 legged component that the output signal goes into on the neck pcb.  When I still had an output signal, it was passing the signal through, but I didn't see any amplification of it.  It seems to go to a diode and resistor after the 3 legged component, probably a transistor.

[james_s]

CRTs are totally cool tech, I miss working on them, it was always a rush to hear the crackle of HV coming up after a successful repair.

The 3 terminal part on the neck board is the driver transistor, it's what is translating the video signal to a level required to drive the cathode.

Check out the manual for the Electrohome G05 monitor, this is a vector monitor so the deflection circuits are completely different but the video amp or Z amplifier in vector terms is clearly described in what I consider to be a very well written manual.

http://www.arcade-museum.com/manuals-monitors/Atari%20Monitor%20TM-146%20Issue%201%20Electrohome%20G05-801%20Monochrome%20XY.pdf

The neck board should pull straight off, you may have to cut a blob of silicone or peel it but be careful because there's a fragile glass pip in the center of the pins and if you crack that it's game over. This monitor has a shield over the back of the neck board which makes probing difficult but sometimes it's not too hard to remove. Looking at the end of the CRT and starting with the first pin after the gap the numbers go around clockwise, this is the most common pinout for those small neck tubes.

1. G1
2. K
3. Heater 1
4. Heater 2
5. G1 (internally connected to pin 1)
6. G2
7. Focus


Pin 2 should have the video signal on it, don't worry about pins 3-4 because if the heater wasn't working you wouldn't have anything on the screen. Pin 7 is not important yet either. Pin 1 and 5 may be connected to ground, it is on many monitors. Pin 6 should have a DC voltage that varies with the control usually marked Screen or Sub-Brite, look around for that control and verify that it works. So here's the thing, a CRT has limited dynamic range, so if you crank up the brightness too far the gun will never go into cutoff and the whole tube will light up. This can be due to a circuit fault, or the screen control can be turned up too high. Since this control is not customer accessible it typically has a very wide range to account for variation between CRTs and if you turn it up too far you will end up with a solid glowing raster. Depending on where the diode that was shorted is it may be part of the circuit that produces the voltage that powers the video amp.

So at this point I'd say scope pin 2 on the CRT and verify that you have a video signal, let us know what range it is relative to ground, you should be able to ground your scope to that metal shield or to the ground strap over the bell of the CRT but do check just to make sure that it's tied to the earth ground first. Next check the voltage on pin 6, find the Screen or Sub-brite control and verify that the voltage on pin 6 varies as you adjust that, let us know what range it covers.

[alank2]

I got the neck board off ok.  Can I power it up with the neck board off even though the suction cup is on and the coil wires are connected?  I can disconnect those too if I need to.  There is a C3950 transistor connected to the signal in pin.  The negative voltage I was looking at earlier was on pin 5.  Only pin 3 seems to be grounded on this board.  I'm going to take the ground cover off and see what it yields.  My bigger problem is that the main pcb is no longer transmitting a signal on the its output wire though, I'm not sure what happened there as it was working, maybe I shorted it or something.

[alank2]

Pics of the neck board:

The signal is coming in the small white connector in the upper-left, right pin.  It does indeed go through a transistor (q702) which then goes through another transistor (q703) and on to pin 2 of the socket.

[alank2]

pin 5 seems to go with the brigthness

brightness adjust

-62V all the way bright
-90V all the way black


probing the input signal it is at 1.21V which goes into Q702.  The other two pins of Q702 are 0.54V and the center one goes into Q703.  Its center pin is 0V resulting in 0V to the socket terminal 2.

Pin 6 is 645V DC!

[alank2]

I pulled the main pcb again and am trying to figure out why the video signal output is dead.  I opened up a grounded box and found what looks like the video generation components.  There is an IC called Sherwood ITT3000-01 that looks like a main IC.  I traced the signal to a 74S09N and traced that to a pin going into the ITT3000-01.  No signal coming out now.  Has anyone heard of this IC?  Any datasheet for it?

[Armadillo]

No high voltage crackle.  It is going back today so no chance to work on it.

I thought you returned back to ebay?

[alank2]

I was going to until the seller said I could just throw it out.  He refunded me so it is a if I can fix it great, if not it ends up in the trash type of situation now.

[alank2]

Is it possible that the video IC will not generate any output, if it doesn't receive proper signals for a new line, new display, etc. ?  Or would a video IC typically generate these things?  There are two metal can oscillators and both are functioning.  I am temped to cut the connection to the 74S09 and see if that allows it to work again.

[alank2]

Another hint is that the pcb says Sherwood 7000ST on it.  Does anyone have any experience or documentation with this board?

[james_s]

So you had a video signal before and now it's gone?

What is showing on the screen now? Does it still light up solid orange? 645V on G2 is way too high, I'd expect a solid bright screen with no visible picture with such a high G2 voltage, it normally should be around 200V from memory. G2 goes through a resistor directly to that 3 pin connector, if you follow that it likely goes to a potentiometer on the main board.

The main IC is probably a custom chip, usually a mask programmed version of something like the 8085 or Z80, if it's bad there isn't much you can do but it sounded like that was working fine previously. Some of these use a CRTC chip which will not put out sensible video signals without being set up by the firmware running on the CPU chip.

[alank2]

Yes, signal present before and now it is gone.  Nothing I've tried will bring it back so that might be the end of it.  What I find odd is that there is plenty of stuff between that custom chip and the output signal that I would have thought would have been blown first.  A resistor, an open collector SN74S09N, etc.  The custom chip doesn't appear to be doing much now.  I saw some memory chips next to it and probed them and nothing is going on, so I'm not sure what happened.  It has a separate Z80 with about half of its pins going into the custom chip.

ETA - thanks for your help on this james_s!

[james_s]

Sounds like the Z80 is the CPU and the other chip is a CRTC. It's hard to say what's going on now, it sounds like it was mostly working prior to this. Did you check carefully for shipping damage such as cracked traces or loose connectors? Does it beep or anything when you power it up? What does the reset pin of the Z80 look like?