Sencore CR70 Help
 

I did not get a full instruction manual with it, just the quick set-up guide. Problem is that when I am set to read emissions, the cutoff set knob seems to have a dramatic effect on the reading of the emissions. I am guessing that this is not ok, unless the cutoff set knob is supposed to be in a certain position before checking emissions. I don't know for sure without a full manual.



Thanks,

Scott

OK, I did find a manual online.

When I test a 16EP4 that tests fair on my trusty CR31 Super Mack, I can't get the needle to stop in the correct cutoff range with the CR70. The needle either jumps higher or lower and is virtually impossible to make stop in the correct cutoff range.
I then switch to emissions and the meter reads way down in the bad range until I turn the cutoff set knob nearly fully clockwise- then the meter jumps right into the good range. Should this be happening? I mean, should the cutoff set knob be changing the emissions reading?

No, not typically. It sounds like it may be gassy and when the gas ionizes, the emissions jump up. Can you see a blue or purple glow in the next while the emissions are in the good range ?

No Bob, no glow. Does the cutoff set knob need to be in a certain position to test emissions?

Yes, the knob should be positioned so that the meter needle is inside the black box when the function switch is set to cutoff.

OK. So because I can't position my needle in the little black box for the cutoff test, (Needle jumps above or below), then I can't test for emissions?. Maybe if I had a 10 turn pot for the cutoff set I could get it. It is just crazy sensitive once I get close to that black box.

I don't know what to think of this 16EP4. My SuperMack says it is OK on emissions once I run the tube for about 5 or 10 minutes. The CR70, which is a new purchase, seems to tell me the CRT is N/G.

Thanks for the help Bob.

Sounds like you may need to clean the cutoff pot.

Do you have any other CRTs you can test with the CR70 ?

I also recommend you run through the calibration instructions to make sure it's working properly.


Set the CR70 controls to the following, F1 to 1, F2 to 2, K to 3, G1 to 4, G2 to 5 ,gun select to R/B&W ,CRT select to Video, function to Remove G1 short.

Hook a DC Voltmeter ground lead to G1 lead of universal adapter and the positive lead of the DVM to the K lead of universal adapter.
When the rejuv button is pressed the DVM should read greater then 350 volts DC.


Set the function switch to the rejuv position and re-press the rejuv button and check again for greater than 350 volts DC.


Set function to Auto Restore and set external DVM to DC current. Press the rejuv button and the CR70 should cycle three times. When on the current should read between 80 and 120 mA.

Set function switch to manual 1 restore.Press the rejuv button and the external DVM should read between 80 and 120 mA with no cycling.

Set function switch to manual 2 restore. Press the rejuv button and the external DVM should read greater than 150 mA

Set function switch to cutoff. Set external DVM to DC volts. You should be able to read the BIAS voltages.

Move the positive lead of the DVM to the G2 lead of the universal adapter. When the cutoff control is adjusted look for a voltage swing of 20 to 400 Volts DC.

Move the DVM ground lead to the F1 lead of the universal adapter and the positive lead of the DVM to the F2 lead . You should be able to read the filament voltage on the DVM. It will be dependent upon where the filament volts settings are set to.

Note: if your CR70 has two power transformers on the bottom panel the filament voltages will be AC volts.

16EP4 is a metal cone tube, I bet it's gassy. Easy way to tell with the CR-70 is start with the cutoff knob all the way counterclockwise, flip the switch to read emission, then slowly advance the cutoff knob till it does something. If the needle slowly rises and you can control it like normal, the tube is probably good (able to display a picture anyway). If however at some point the needle pegs full scale while advancing the cutoff knob, you have a gassy tube. This is called out specifically in some B&K manuals, but not in the Sencore one. B&K call it 'gas regulator' action, whereby the gas inside breaks down at some voltage allowing full conduction at that point. If that's what you're seeing, I'm sorry but the tube is shot.

Do you have any other CRTs you can test with the CR70 ?

I just tested 2 - 10BP4's and on one the cutoff set would not budge the needle. The other I was able to set the needle in range with great difficulty. The needle wanted to jump higher or lower. Both of these test good on my Super Mack. Pulling a set with a known good tube is a little difficult at this time as my back is acting up and I don't want to move my 8TS30

I also recommend you run through the calibration instructions to make sure it's working properly.

Thank you for this information Bob.


Set the CR70 controls to the following, F1 to 1, F2 to 2, K to 3, G1 to 4, G2 to 5 ,gun select to R/B&W ,CRT select to Video, function to Remove G1 short.

Done

Hook a DC Voltmeter ground lead to G1 lead of universal adapter and the positive lead of the DVM to the K lead of universal adapter.
When the rejuv button is pressed the DVM should read greater then 350 volts DC.

It reads -430v

Set the function switch to the rejuv position and re-press the rejuv button and check again for greater than 350 volts DC.

It reads -425v

Set function to Auto Restore and set external DVM to DC current. Press the rejuv button and the CR70 should cycle three times. When on the current should read between 80 and 120 mA.

Reads 120 ma

Set function switch to manual 1 restore.Press the rejuv button and the external DVM should read between 80 and 120 mA with no cycling.

Works reading 120 ma

Set function switch to manual 2 restore. Press the rejuv button and the external DVM should read greater than 150 mA

Measures 180 ma

Set function switch to cutoff. Set external DVM to DC volts. You should be able to read the BIAS voltages.

They measured a few volts low for each.

Move the positive lead of the DVM to the G2 lead of the universal adapter. When the cutoff control is adjusted look for a voltage swing of 20 to 400 Volts DC.

Reads 62v low to 501v high. This is off.

Move the DVM ground lead to the F1 lead of the universal adapter and the positive lead of the DVM to the F2 lead . You should be able to read the filament voltage on the DVM. It will be dependent upon where the filament volts settings are set to.

Reads slightly higher than the CR70 meter reads. I guess this is to compensate for the voltage sag? maybe...

Note: if your CR70 has two power transformers on the bottom panel the filament voltages will be AC volts.

One transformer. Output is DCV.

I appreciate your help Bob. I just don't know if this is working correctly or not. Is my Super Mack telling me the wrong result or my CR70.....

Is there a CR70 service manual in electronic format for download? I only was able to find the user manual.

BTW, this universal adapter is ridiculous to use and I really need to make a proper adapter for the older CRT's.

Many thanks,
Scott

I appreciate your response below. I need to determine if my CR70 is working properly before I can trust the results from it. I will print and keep your advice. Thank you.

On edit: I did try this with the 16EP4 and the needle pegs when you turn up the cutoff set. It is nearly impossible to set it in the range as the needle jumps.
On the emissions setting, the cutoff set knob takes the needle up rather fast and not particularly smooth, but it stops in the middle of the good scale and goes no higher.

If the cutoff set will not budge, try dropping the cutoff bias range switch in the lower right. If you are seeing the same jumpy behavior with other CRTs, I'd suspect your CR70 isn't working properly.

Maybe try hooking up a voltmeter between G1 and ground while you change the cutoff control and see if it's changing smoothly or jumping around.

You got it Bob.
It is not jumpy along the way, but rather at about 7/8 fully clockwise the voltage from K to G2 jumps from a volt or so up to 430 volts. It is like turning a switch on. I guess it could be the pot broken, but I need to find a schematic. I was able to set the cutoff a couple of times, but others, and now the needle just jumps up like the voltage measurement is indicating.

I have heard that these are notorious for having bad solder connections...and as with most Sencore late model test equipment, it looks like a real PIA to service because of it's mechanical assembly.

Anyone have a schematic copy?

Looks like cut-off pot is bad. Now I just need to find a suitable replacement. It has a plastic shaft. I assume that is for safety? Original value is supposed to be..??


http://img.photobucket.com/albums/v2...d/IMG_4479.jpg

If you don't know the value, just measure it with your ohm meter, from one leg to
the burn spot, then the other leg to the other side of the burn spot, then add the values.
It might also be written on the case of the part....

So if you add it up, and get 48K ohms, well then its a 50K.....