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Old 12-18-2016, 01:46 PM
spacediver spacediver is offline
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Join Date: Jul 2014
Location: Toronto
Posts: 14
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 12:24 PM.
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