Author Topic: Help fixing a CRT terminal  (Read 3595 times)

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Offline Renate

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Re: Help fixing a CRT terminal
« Reply #75 on: May 09, 2021, 11:25:49 pm »
When you're hunting for bear and you don't see any, you don't use a magnifying glass to see if you missed any.

Keep the sweep on V rate. We're not interested in anything happening in H.
Keep the gain on 5V/div for a single trace.

Double check that you actually have +36 (+38? I can't read it) supply.

Q303 is not getting its bias. Check voltage on C305.
 

Offline rea5245

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Re: Help fixing a CRT terminal
« Reply #76 on: May 09, 2021, 11:26:36 pm »
Renate, thanks to you, too, for your help.

What's collector of Q303 doing?
It should have a nice big (20V?) sawtooth on it.

Go to emitter Q306, it should have basically the same thing.
Then the minus end of Q309, basically the same thing but centered on zero volts.

In this display, yellow is Q303's collector, cyan is Q306's emitter, and magenta is C309's minus end. I guess the ~0V on Q303's collector is a real problem. That means there's no vertical deflection signal, and we're just picking up noise from the horizontal?

- Bob


 

Offline Renate

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Re: Help fixing a CRT terminal
« Reply #77 on: May 09, 2021, 11:35:02 pm »
You've got a flat line.
Your patient is dead.
You don't zoom in and say, "Hey, we've got a pulse! The heart is going 10,000 beats per minute."
"He's dead, Jim."

Seriously, leave the sweep at 2 mS/div

You've got a massive lack of interesting voltages there.
Check +36 supply and C305.
 

Offline rea5245

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Re: Help fixing a CRT terminal
« Reply #78 on: May 09, 2021, 11:43:00 pm »
Seriously, leave the sweep at 2 mS/div

You've got a massive lack of interesting voltages there.

I was just hitting the Auto button on the scope, and it chose the sweep and voltage ranges. I did check, before posting the images here, whether there was anything in the 60Hz range and there isn't.

So yeah, I guess it is, as you say, flatlined. I need to hunt down what's going on with the 36V supply (which is coming is from  another board). Thanks for your help, and I'll be back either with good news, or more pleas for help.

Thanks,
   Bob
 

Offline Renate

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Re: Help fixing a CRT terminal
« Reply #79 on: May 09, 2021, 11:58:31 pm »
"Otto is not your friend, Luke. Use the force!"
 

Offline rea5245

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Re: Help fixing a CRT terminal
« Reply #80 on: May 10, 2021, 03:13:32 pm »
It turns out the 36V supply is fine.

I've measured voltages around the transistors using a voltmeter, not a scope. Here's what I found:
[attach=1]
 

Offline xavier60

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Re: Help fixing a CRT terminal
« Reply #81 on: May 10, 2021, 04:07:39 pm »
Try removing Q303. The effect to check for is the output voltage going high. If it does go high, don't leave it powered for long.
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Offline rea5245

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Re: Help fixing a CRT terminal
« Reply #82 on: May 10, 2021, 04:12:44 pm »
Try removing Q303. The effect to check for is the output voltage going high. If it does go high, don't leave it powered for long.

The output voltage where? What point should I measure at?

Instead of desoldering Q303, could I just jumper the base to ground?
 

Offline xavier60

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Re: Help fixing a CRT terminal
« Reply #83 on: May 10, 2021, 04:15:44 pm »
The output of the vertical stage, the + end of C309.
There is no voltage measured on the Base of Q303, so shorting it to ground should do nothing, try anyway.
« Last Edit: May 10, 2021, 04:18:26 pm by xavier60 »
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Offline rea5245

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Re: Help fixing a CRT terminal
« Reply #84 on: May 10, 2021, 05:19:42 pm »
With Q303 removed, the plus end of C309 reads 29V.
 

Offline xavier60

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Re: Help fixing a CRT terminal
« Reply #85 on: May 10, 2021, 05:25:27 pm »
With Q303 removed, the plus end of C309 reads 29V.
If you replace Q303 with something common like a BC548, the vertical stage should work in a fashion. 
Q305 might run at a higher temperature, so don't leave it powered for long.
Hioki AS100D vom, HP 54645A dso, Fluke 87V dmm, AN8008 dmm, Agilent U8002A psu,  FY6600 function gen, New! Brymen BM857S-(With Battery)
 
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Offline rea5245

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Re: Help fixing a CRT terminal
« Reply #86 on: May 10, 2021, 05:41:53 pm »
Q303 is an MPSA13. Are you thinking that it's defective?

The BC548 does not appear to be a Darlington. MPSA13s are still being made (https://www.digikey.com/en/products/detail/nte-electronics-inc/MPSA13/11649904). Should I get one of those?
 

Offline xavier60

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Re: Help fixing a CRT terminal
« Reply #87 on: May 10, 2021, 05:48:32 pm »
Q303 is an MPSA13. Are you thinking that it's defective?

The BC548 does not appear to be a Darlington. MPSA13s are still being made (https://www.digikey.com/en/products/detail/nte-electronics-inc/MPSA13/11649904). Should I get one of those?
Yes, get the original. Seeing it work with a BC548 will help to confirm that Q303 is really faulty.
Hioki AS100D vom, HP 54645A dso, Fluke 87V dmm, AN8008 dmm, Agilent U8002A psu,  FY6600 function gen, New! Brymen BM857S-(With Battery)
 

Offline rea5245

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Re: Help fixing a CRT terminal
« Reply #88 on: May 10, 2021, 05:57:37 pm »
Sitting in my parts box are some BC547s and MPSA42s. Looking at the data sheets, I'm thinking the MPSA42 might be the better one to try. Do you agree?
 

Offline xavier60

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Re: Help fixing a CRT terminal
« Reply #89 on: May 10, 2021, 06:06:21 pm »
Sitting in my parts box are some BC547s and MPSA42s. Looking at the data sheets, I'm thinking the MPSA42 might be the better one to try. Do you agree?
The BC547 has much higher current gain so is likely to work better. Or you could make a Darlington out of both with the Bc547 driving the MPSA42 with a 47K between B-E of the MPSA42.
What ever you do, I would like to see the waveform at the Collector of Q303. I suspect that it spikes to a high voltage during retrace.
The MPSA13 is rated for 30V.
Hioki AS100D vom, HP 54645A dso, Fluke 87V dmm, AN8008 dmm, Agilent U8002A psu,  FY6600 function gen, New! Brymen BM857S-(With Battery)
 

Offline rea5245

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Re: Help fixing a CRT terminal
« Reply #90 on: May 10, 2021, 06:40:57 pm »
OMG, it works! Thank you!!

Here's Q303's collector:


And here's the Heathkit H9:
 
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Offline xavier60

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Re: Help fixing a CRT terminal
« Reply #91 on: May 10, 2021, 06:48:05 pm »
Good to see. Is that with a made up Darlington?
The raster appears to be rhomboid shaped. The vertical yoke core might need to be rotated.
Hioki AS100D vom, HP 54645A dso, Fluke 87V dmm, AN8008 dmm, Agilent U8002A psu,  FY6600 function gen, New! Brymen BM857S-(With Battery)
 

Offline rea5245

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Re: Help fixing a CRT terminal
« Reply #92 on: May 10, 2021, 07:12:37 pm »
That's with just the BC547.

Rotating the yoke helped. I need to center the image horizontally and vertically - it's getting smooshed at the bottom.

 

Offline xavier60

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Re: Help fixing a CRT terminal
« Reply #93 on: May 10, 2021, 07:36:32 pm »
 I wouldn't expect it to work properly without Q303 being a Darlington.
Hioki AS100D vom, HP 54645A dso, Fluke 87V dmm, AN8008 dmm, Agilent U8002A psu,  FY6600 function gen, New! Brymen BM857S-(With Battery)
 

Offline james_s

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Re: Help fixing a CRT terminal
« Reply #94 on: May 13, 2021, 11:07:17 pm »
You're about 98% of the way there, install the correct replacement part and then get everything dialed in. Looks like the tube is in good shape and so is the HV transformer and the yoke is at least in reasonable condition. Pretty much everything else is easily replaced.
 


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