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  #1  
Old 12-18-2016, 02:46 PM
spacediver spacediver is offline
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Results of my Sencore CR70 Field test, and some questions

UPDATE: video of some of these tests now included


Hi all,

I've been testing out my CR70, and it seems to be having some issues. I'm not sure whether they're fatal or not.

Here are the results of the CR70 Field Test.


Quote:
Connect the UNIVERSAL ADAPTER to the CR70. Set the CR70 controls to the following positions: F1 to 1, F2 to 2, K to 3, G1 to 4, G2 to 5, gun select to R/B&W, and CRT to Video. Connect a DC VOLTMETER to the UNIVERSAL ADAPTER with the POSITIVE lead to the G1 lead, and the GROUND lead to the K lead.
Quote:
Set the FUNCTION SWITCH to REMOVE G1 SHORT. Press the REJUV button. The external DVM should read greater than 350 Vdc.
Reads about 420V. PASS

Quote:
Set the FUNCTION SWITCH to REJUV. Press the REJUV button. The external DVM should read greater than 350Vdc.
Reads about 4.7V FAIL

Quote:
Set the FUNCTION SWITCH to AUTO RESTORE and the external DVM to DC CURRENT. Press the REJUV button. The CR70 should cycle on and off 3 times. When on, the external CURRENT METER should read between 80 and 120 mA DC.
reads 110 mA and cycles on and off 3 times. PASS

Quote:
Set the FUNCTION SWITCH to MANUAL 1 RESTORE. Press the REJUV button. The external CURRENT METER should read between 80 and 120 mAdc with no cycling. Set the FUNCTION SWITCH to MANUAL 2 RESTORE. Press the REJUV button. The EXTERNAL CURRENT METER should read greater than 150 mA DC.
110 mA and 160 mA for Manual 1 and 2 restore, respectively. PASS

Quote:
Set the FUNCTION SWITCH to CUTOFF. Set external DVM to DC VOLTS. As you SWITCH through the different BIAS VOLTAGES you should read them on the DVM. (-68, -52, -36, and –20).
When switch is set to -68V: DMM reads -57.4V (DMM value = 84.4% of Switch value)
Switch: -52, DMM: -44 (84.6%)
Switch: -36, DMM: -30.6 (85%)
Switch: -20, DMM: -17.3 (86.5%)

FAIL?

Quote:
Move the POSITIVE lead of the DVM to the G2 lead. When the CUTOFF control is adjusted, look for a voltage swing of 20 to 400 Vdc.
6.36V to 395 V

TENTATIVE PASS (That swing should be fine for testing the dynamic range of the CRT, but the swing starting at 6.36V instead of 20V is puzzling, and might indicate an underlying problem with unit?)


Quote:
Move the GROUND lead of the DVM to the F1 lead and the POSITIVE lead of the DVM to the F2 lead. You should be able to read the filament voltage on the DVM. The reading will depend on what the filament volts are set to on the CR70. NOTE: IF YOU CR70 IS A RUN 16 OR LOWER, YOU FILAMENT VOLTAGE WILL BE AN AC VOLTAGE.
TENTATIVE PASS (see below)

Now here I'm not sure what run number my CR70 is. I've tried looking everywhere, but cannot find any sticker or etching or print in the setup manual that gives this information. There is what looks to be a serial number etched into the top right corner of the inside of the top flap of the box, however. It is 361**76M (if those two asterixed numbers are useful, I can include them).

In the Operation Manual (which I downloaded online, as it didn't come with my unit), it says that the CR70 uses a regulated, DC power supply to provide the proper RMS filament voltage to the CRT.

I'm confused about why it would describe it as an RMS voltage when it is in the context of a DC power supply. Isn't RMS voltage a measure that only makes sense in the context of an AC voltage waveform? I'm guessing that the sencore emulates an AC waveform by modulating the DC voltage, but not sure.

Anyway, the test shows that the measurement needle isn't very reliable. It is sometimes off by a whole volt, and the needle itself sometimes needs to be re-zeroed with the screw, as it doesn't always return to zero when the unit is switched off. It is clear, however, that the needle indicator is in DC, and not AC volts.

When using this with my CRTs, would it be ok to use my DMM to set the filament voltage (connect the universal adaptor F1 and F2 leads to my DMM, set voltage, and then connect the F1 and F2 leads to CRT pins)?

So, the field test indicates to me the following :

Rejuvenation doesn't work at all (the G2 voltage is only 4 volts instead of 350). This might be ok for my purposes. I can still use manual and auto restore, and potentially get a secondary restoration unit that allows fine control over G2 voltage and timing of this voltage, if I need to revive a dead tube.

Bias voltages seem off, and needle isn't reliable. This will limit my ability to interpret cutoff. However, I only work with high end trinitron CRTs (mainly sony GDM FW900's), and they have a service cable that allows you to adjust voltages to calibrate the unit, in combination with a colorimeter. So I can get a sense of the dynamic range of the tube using those tools.

I'd appreciate any advice on what I can do to improve the situation here. I'm really new to electronics, and this is my first time using a CRT tester/rejuvenator.

Last edited by spacediver; 12-22-2016 at 01:24 PM.
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  #2  
Old 12-18-2016, 04:55 PM
Findm-Keepm's Avatar
Findm-Keepm Findm-Keepm is offline
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Set the FUNCTION SWITCH to REJUV. Press the REJUV button. The external DVM should read greater than 350Vdc.

Reads about 4.7V FAIL


Are you giving it some time after you set the function to rejuv? In that position, it charges a large capacitor - if you push immediately, the cap may not have charged fully.

Most important, the discharge curve for the cap once you push the rejuv button is quite steep - use a DVM pre-set to the highest range, as an autoranging meter takes time to "range up" to the highest range needed to read the voltage. By the time the autoranging gets there, the cap is discharged.

Me thinks your CR70 is fine, just gotta setup things correctly.
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Old 12-18-2016, 05:18 PM
spacediver spacediver is offline
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Thanks for the thoughtful reply Findm. I tried leaving the function on rejuv for a few minutes before pressing the button. And I set my DMM to the 600V range (it's not an autoranging DMM).

No change. It peaks at just over 4 volts, stays there for a few seconds, and then drops by about 0.01 volts every 2 seconds.

The Remove G1 Short function, however, peaks at about 430 V, and then immediately starts to drop about 5V every second. And it doesn't require much time between tests to recharge. If I quickly re-press the button, then it peaks at 425, but after only a second between presses, it peaks at the original 430.

I can take a video if that's helpful.

edit: I waited for an hour with the switch at rejuv, no real difference. I think it may have peaked at 7V very briefly.

Last edited by spacediver; 12-18-2016 at 07:02 PM.
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Old 12-18-2016, 10:09 PM
spacediver spacediver is offline
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Aside from the Rejuvenate function, what should I do about setting filament voltage?

Can I trust the needle? If not, will the DMM be a good substitute? I'm concerned about the DC and RMS issue, and also about whether the DMM will impose the same load as the CRT will.
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Old 12-19-2016, 10:23 AM
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The CR70 uses DC to power the CRT filament. DC and AC RMS are basically equivalent. So 6.3 VDC on a CRT filament is just fine. No, your DMM has a very high impedance and will not load the supply down like the filament will. You'll need to measure the voltage while it's connected to the CRT.

You meter might have a static charge built up on the plastic face. Try wiping it with an anti-static dryer sheet like "Bounce"
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Old 12-19-2016, 11:28 AM
spacediver spacediver is offline
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Thanks very much bandersen, really useful to know. I'll definitely be buying some hook clips so I can easily connect my DMM to the CRT pins.

I'll try the anti-static dryer sheet idea and will report back.

If I may, there's another area that I'm having problems understanding, and this concerns the way that trinitron CRTs are driven.

In the CR70 manual, it says that the when measuring emission, it applies no bias voltage to G1, thus simulating the conditions of sending a white level signal to the tube.

However, a few years ago, I had a discussion with a person who used to be a CRT service technician, and they said that trinitrons only have one G1, and so the tubes are driven at each of the three cathodes. If this is true, then removing the bias at G1 will not produce any beam current. One would have to change the voltage at the relevant cathode. And if this is true, this might explain why when I did my emissions testing, the needle didn't budge at all for any of the three guns.

Another wrinkle to this is in the schematics for the FW900. There are three G1s listed. When I tested my tube, hooking the G1 lead of the universal adaptor to pin 3 didn't seem to work, but it did work for pin 6. I was testing the red gun too, whose cathode is connected to pin 7.

So it could be that this person I was talking to was mistaken, but I'm not sure. I'll be testing a Sony CPD G520P tube next, whose pin setup is identical to the FW900, and I'll see if I can figure anything out from that.

But does anyone know whether it makes sense for a trinitron to only have a single G1?



Finally (last one!). Is it possible to measure the filament voltage of my CRT when it's under normal operation? I cannot for the life of me figure out what the correct filament voltage is (It could well be 6.3V, but I've heard of at least one other person measuring their trinitron filament voltage at 5V).

Here's what my CRT looks like when its connected normally. I'm not sure if I could connect a DMM to the pins while the white plastic circular thing is attached to the pins. Would I have to directly probe the appropriate points in the circuit board above?


Last edited by spacediver; 12-19-2016 at 11:46 AM.
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  #7  
Old 12-19-2016, 11:46 AM
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Some inline gun tubes (trinitrons are inline) have a single grid or a single cathode.

There is a difference between video (AC) drive and DC bias. You can drive a tube with AC video signal at grid and/or cathode (the CRT don't care) and things will work fine. DC bias (which is the test condition on a CRT tester) is what your CRT elements will be at if your video stage is not operating (and what the DC voltages on the tube will hover near when driven with video). DC bias is the relative DC voltage of the cathode and grid....assuming plate (which may be the focus electrode or similar when on the tester) much more positive than cathode.
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Old 12-19-2016, 12:11 PM
spacediver spacediver is offline
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Quote:
Originally Posted by Electronic M View Post
Some inline gun tubes (trinitrons are inline) have a single grid or a single cathode.

There is a difference between video (AC) drive and DC bias. You can drive a tube with AC video signal at grid and/or cathode (the CRT don't care) and things will work fine. DC bias (which is the test condition on a CRT tester) is what your CRT elements will be at if your video stage is not operating (and what the DC voltages on the tube will hover near when driven with video). DC bias is the relative DC voltage of the cathode and grid....assuming plate (which may be the focus electrode or similar when on the tester) much more positive than cathode.
Ah! So if I'm understanding correctly, during the emission testing, when the bias is set to 0, it doesn't matter if this is achieved by increasing the grid voltage until the potential between control grid and cathode is 0, or whether cathode voltage is decreased until the potential between control grid and cathode is 0. They both achieve the same condition. And this condition is what would occur when driving a gun to peak luminance.

I assume that when the bias between grid and cathode is 0, g2 is what is pulling the electrons, and creating beam current. How does the CRT tester know what voltage to apply to g2 though?
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  #9  
Old 12-21-2016, 07:34 PM
spacediver spacediver is offline
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Quote:
Originally Posted by bandersen View Post
You meter might have a static charge built up on the plastic face. Try wiping it with an anti-static dryer sheet like "Bounce"
I tried this, but it didn't work. I've just created a video where I test the remove G1 Short and rejuvenation functions, the bias settings, and adjusting filament voltage to compare the needle with the multimeter. I wipe down the plastic face with an anti-static dryer sheet in the video too.

Here it is.

https://www.youtube.com/watch?v=CSMIkESgn98

Am I doing something fundamentally wrong here? Or is my unit just bust?

edit: I realized soon after making this video that you meant to wipe the plastic face where the needle is. I can be a bit daft at times :p. Nevertheless, does anyone have any suggestions about the other issues with the unit?

Last edited by spacediver; 12-22-2016 at 03:14 PM.
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