Author Topic: CRO - Intensity stuck at maximum  (Read 788 times)

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Offline Areku

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CRO - Intensity stuck at maximum
« on: November 26, 2018, 06:04:33 am »
Hello all!

I recently bought an used Hitachi V-212 CRT oscilloscope, generally in pretty good condition - Knobs work smoothly after a bit of contact cleaner, both channels seem pretty accurate, even the caps look to be in reasonabe shape.

However, I’ve noticed the beam intensity seems to be stuck at the maximum level. Neither the front intensity knob, nor the internal CRT bias pot have any effect on it, and I can see the return traces on the upper and lower edges any waveforms displayed, indicating blanking is not taking place either.

According to most of what I’ve read, including Tek’s Troubleshooting Your Oscilloscope, this all seems to point at a failing DC restorer on the HV section - Still, this is my first time working with this particular scope, and I’d like to hear your opinions on it, in case I’m missing something.

Essentially, I intend to replace diodes D1021 to D1024, on page 69 of the service manual. Is this the correct course of action? Should I worry about the condition of the neighboring capacitors, as well?

For reference, the service manual can be found here: http://bama.edebris.com/download/hitachi/v212/service/v212-service.pdf


Thanks in advance!
 

Online tautech

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Re: CRO - Intensity stuck at maximum
« Reply #1 on: November 26, 2018, 06:27:29 am »
Welcome to the forum.

P 69/96
Can you measure the EHT is correct @ ~ -1900V ?
Check the brightness circuit carefully for open or drifted components.
Also check the CRT bias circuit components.
Check voltages on P 61/96

Also any M ohm resistors, check for drift or open......ones like R1024 10M

I guess you've checked all the low voltages already......everything but the EHT ?
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Offline tggzzz

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Re: CRO - Intensity stuck at maximum
« Reply #2 on: November 26, 2018, 09:59:34 am »
Check the low voltage blanking waveform before it is translated up to -2kV by the DC restorer. The LV waveform measured with a voltmeter is likely (but check your service manual) to be in the range 50-150V, and should vary when controls are changed. In this case "controls" are the brightness, relative settings of A&B timebases, holdoff period.

If the LV waveform's average voltage does change as you would expect, then the problem is in the HV circuits.

The first time I had to diagnose such a problem, the CRT was a good impersonation of a torch. It turned out the grid had become disconnected inisde the CRT; game over for that CRT!
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline Areku

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Re: CRO - Intensity stuck at maximum
« Reply #3 on: November 27, 2018, 03:51:29 am »
Thanks for the quick responses.

I had checked the low voltage side of the blanking circuit before - It varied along with the brightness adjustment, and was easy enough to set to the 15V indicated on step 3 of the calibration procedure.


I hadn’t measured the high voltage rails - I don’t own an HV probe, and currently don’t have the parts to make a resistive divider to measure it with my DMM. Still, I assume it must at least be close to the correct value, considering the CRT is otherwise producing a very sharp image.


A few minutes ago, I opened the HV cage to check the value of R1024, and... Well. I’ll let the picture speak for itself.




It’s a bodge if I’ve ever seen one. Seems like someone’s been in there before to replace D1012-1013, and not having the correct part (The Y10GA has been discontinued for a while, it seems), used a series of IN4007 diodes instead. I am not sure how much of a problem this is causing, especially considering the forward voltage of of that setup will be roughly 5v, instead of the Y10GA’s 22v. R1017 seems to have been messed with as well, with a wire soldered directly to the side of the component.

More than that - I’m afraid my picture is a bit out of focus, but if you look a bit to the left, D1023 and D1024 both have a break in one of their leads, with 1023 having been completely disconnected, while 1024 was essentially conducting via a spark gap. That can’t be too good.


Now, I could probably just use a bit of solder and a jumper to fix the leads, but I’m a bit worried about the stability of this old fix at 1.9kV. Should I try to unsolder and replace all the involved parts?
 

Online tautech

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Re: CRO - Intensity stuck at maximum
« Reply #4 on: November 27, 2018, 04:07:24 am »
Should I try to unsolder and replace all the involved parts?
Yep, if you plan to do this this properly and keep the scope.

BTW, might be a good idea to increase the wattage of R1024....for good measure I'd put a 2W resistor there.

For Y10GA I'd guess it's a 10 KV diode and you can find many suitable replacements on Google with a '10KV axial HV diode' search.
« Last Edit: November 27, 2018, 04:17:50 am by tautech »
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Offline Areku

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Re: CRO - Intensity stuck at maximum
« Reply #5 on: November 28, 2018, 07:37:24 pm »
Well, I removed the main board and flipped it upside down, and...



Seems like whoever was in here before was worse at soldering than me, and that’s saying something. Thankfully the transistors themselves are fine, but I’ll need to do a lot of cleanup.

In regards to R1024, my local parts supplier didn’t have 2W 10M resistors, but I’ll bump it up from 1/4 to 1/2W nonetheless. I had to buy R1017 in new old stock, but it seems to be the correct part.


About Y10GA, I am thinking of replacing it with a 2CL72 - Most of the specs seem to match, and the original part was only rated to 6KV instead of 10, but I’m a bit worried about the voltage drop of 36V as opposed to the previous 22V. Would it cause problems with the tube bias?

The part I’m considering, for reference: https://belchip.by/sitedocs/29493.pdf


EDIT: Correction, the 2CL72 is not available in the area, I’d need to go for a 2CL74. Alternatively, I have access to CL01-12 as well.
« Last Edit: November 28, 2018, 07:42:40 pm by Areku »
 

Online tautech

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Re: CRO - Intensity stuck at maximum
« Reply #6 on: November 28, 2018, 08:49:13 pm »
The Vf at 51V does seem excessive and if you only need a 6KV diode I'd look for other lower Vf options.
There is no EHT (-1.9KV) adjust other than changing the value of R1016.

IIRC the EHT range was ~+50V and as there's no adjust the risk is lower than optimum brightness.

Good progress.  :)
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Offline Areku

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Re: CRO - Intensity stuck at maximum
« Reply #7 on: December 12, 2018, 01:26:31 am »
I’m back for a quick followup on this.

I ended up replacing the HV diode with an ESJA08 - the voltage drop of 28 as opposed to 22V made the trace a bit dimmer, but still well within adjustment range. The hardest part ended up being the 22M resistor, I could only find it in a very large package, and it took some creativity to make it fit under the metal shield.

As it is, the scope works very nicely, and the intensity is adjustable throughout its entire range. As a bonus, reflowing the solder joints got rid of the buzzing noise from the HV circuit, and the scope only needed minor calibration after being put back together. Everything worked out in the end!

Thank you all very much for your help in this. Your suggestions really put me on the right track!
 


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