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  #31  
Old 03-14-2017, 03:08 PM
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dtvmcdonald dtvmcdonald is offline
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Quote:
Originally Posted by old_tv_nut View Post
Another thought is to float a VOM across the cathode and G1 so you can see any differential drift on a sensitive scale. (Just be careful to have it on an insulated surface and don't try to touch it while in operation!)
I've tried all your suggestions. But for this last one ...there is a problem.
That is, there are big blanking pulses on both cathode and grid. This means that the voltages measured on a voltmeter are not necessarily representative of
what they are during the active scan time.

I will retry this evening with the two blanking pulses disabled. And I will
look at the two pulses with a scope and see if they drift.
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  #32  
Old 03-14-2017, 05:37 PM
Ralph S Ralph S is offline
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Add me to the list of working block camera owners!

I've had my CRV59 camera working for at least 15 years now and it seems to be about the same today as it was when rebuilt that long ago. I agree about it possibly needing the rear mosaic lamp. I've never had the picture fade out on me as you describe as the bias lamp(s) on this camera do work. (It's been a quite a while since I've opened the camera up so I've forgotten whether or not there's just one or two inside!)

I revamped the scans circuits to standard def sweep rates so I could use a regular broadcast monitor for viewing. No problems there. The camera power supply is solid state.
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  #33  
Old 03-14-2017, 06:23 PM
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dtvmcdonald dtvmcdonald is offline
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Ralph, is yours a different camera from the ones I mentioned a couple of posts ago?
How long does yours take to stabilize from turn-on?

My lamp does work.

But old_tv_nut's suggestion did the trick, with my new angle.

Removing the blanking pulses generated a "correct" grid bias of -20 to -22 volts.
The picture faded when I adjusted the control, turned right on, to -26 volts.
When the picture faded with the control all the way up the
picture faded at .... -26 volts! I had not realized how much the blanking pulses
caused a measurement problem.

I will investigate the divider chain drift further, with all pulses sent to it
disabled. If I can't figure out what's the problem and fix it,
replacing a couple of resistors with Zeners should do the trick. That's unless
the blanking pulses cause problems by drifting.

Off to dinner.

Last edited by dtvmcdonald; 03-14-2017 at 06:33 PM.
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  #34  
Old 03-15-2017, 11:28 AM
Ralph S Ralph S is offline
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Same type of camera as yours

I checked on the camera and I can't find the original ID tag that was riveted on, but I did find what I think is RCA Camden's inspection tag; see pix attached. Interesting that they didn't call it a camera, rather a "Conversion Unit!"

The schematic I followed in my work is exactly the same as the excerpt you showed earlier, so we're both talking about the same model of camera.

This camera is definitely not one of those you listed earlier.
Attached Images
File Type: jpg Iconoscope RCA Manufacturing Tag.jpg (37.5 KB, 11 views)
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  #35  
Old 03-15-2017, 11:47 AM
Ralph S Ralph S is offline
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Fan added

One more thing: I added a square fan to the top via a replacement slide (instead of the tube access slide.) I did this after noticing how much heat there was inside the case when the original slide was in place. Considering how many tubes there are in such a small container, it might be helpful for longevity to do the same with yours even if it doesn't reduce the "blackouts" you've mentioned.
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  #36  
Old 03-15-2017, 05:17 PM
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dtvmcdonald dtvmcdonald is offline
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The problem is discovered and apparently fixed. I already had bought the
necessary parts.

The key, and it was absolutely necessary, was to disconnect C134 and C107,
the caps that send the vertical and horizontal blanking pulses to the grid-cathode circuit.
This done, all the voltages measured with my 200MOhm
probe made sense.

C109 and especially C108, the oil HV filter caps, were leaking badly: C108
was at best (cold) 25 MOhm, worse when warm. I replaced them with
0.47 uF 1600V film caps. The voltage on the iconoscope went too high
so I added a 1 MOhm 1 watt resistor in series with R125 (330K). The new
caps are larger than the originals, so I mounted them on a terminal strip
under the chassis. (They are polypropylene high reliability ones, and are very large.)

Its now making better images, the problem being mainly horizontal streaking
after bright or dark areas. Its quite stable.

I will now check C116, C117, and C118, the other oil caps. Only C116
is critical: if leaky it would kill V104. Well, no, I looked at the schematic
and it would just raise the cathode current. But I checked and its OK.

And I'm now the owner of two good iconoscopes.
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