View Full Version : Predicta Tandem 17DAP4 CRT for testing ?


vt105
01-06-2017, 06:51 PM
I have a Tandem chassis I am restoring but has a bad CRT.

Could I use a 17DAP4 for test purposes considering the heater voltage differences ? The 21EAP4 uses 2.34@ 0.60 17DAP4 2.68@ 0.45 what about HV?

I have a spare tandem yoke and video board (used on rear of CRT) that I would be using with the 17DAP4 and making a shorter cable to connect the test CRT to the chassis. Would this cause any problems being shorter vs the cumbersome 25 footer ?

As always thanks for any advice

bandersen
01-06-2017, 09:31 PM
The Tandem is a series filament set and using a 450 ma tube may cause problems. I suggest you get an 8YP4 bench CRT. They have a 600mA filament and are designed for subbing many types of Crts while servicing. No a shorter cable won't cause any problems but that's a lot of work. I'd try using the full length one

vt105
01-06-2017, 11:41 PM
Bob,

I value your expertise and enjoy reading and watching your restorations.

Just what kind of problem would that heater current difference present ? Could it damage the 17DAP4 ? Would the set not work ?

I do have an 8YP4 but thought a larger screen would be more realistic for adjustments.

So using an 8YP4 there has to be some crt socket pin changes correct ?

Thanks

bandersen
01-07-2017, 01:48 AM
Well I think what would happen is the other tubes would end up with somewhat less than 600 and the 17dap4 somewhat higher than 450. That's going to effect the operation of the tubes. Just how much I couldn't say. Not sure if you would need to make any changes for the 8YP4. I think mine might have come with an adapter. I don't recall making any wiring changes in a Predicta when I used it.

Eric H
01-07-2017, 02:17 AM
I may be wrong but I think if you stick a 450ma tube in a series string set it's going to get hit with a lot more voltage than the 600ma tubes, especially on cold startup.

vt105
01-07-2017, 01:55 PM
Thanks guys. I am going to try it out later. I see the 8yp4 has a 6.3 filament so
is there anything else I need to do ?

Electronic M
01-07-2017, 03:47 PM
I'd check data sheets and see is the pin out is the same and if not how to adapt it.

In series sets heater current is often more important than voltage. String sum voltage changes within 10% (10V) of entire string usually will be evenly split among tubes and not be harmful, but too low current in a sub can burn out that sub, and too high current may not allow the sub to light...As others have said.

Wow, I have that same Panasonic portable phono you have under your set...Yours is the second one I've seen.

vt105
01-07-2017, 08:55 PM
Tom C, you have a good eye ! I take it with me to shows and or record shops to check out the merchandise.

Meanwhile I removed the CRT board cover to double check filament on CRT and to my surprise it was good! Now I haven't messed with this set for awhile and I know I triple checked the crt. I checked the filament supply pins 5&6 on the 17 pin cable and it was open. (feeds crt and video amp tube on crt board.) Well after eliminating the other tubes I removed the Umbilical cord from the rear of the chassis and pin 6 on the cable had broken off creating an open so I ran a new wire outside of the cable and she came back to life. Picture tube appears weak (don't have a crt tester). So after it cooked for awhile it got a little brighter so I went for the gusto and sent a signal from the BT modulator and got picture and sound ! Not very good but its a start.

Vertical started to roll within minutes, so now I'll proceed with the usual cap and resistor change. I have a spare pc board from a holiday and I notice there are some differences from the tandem.
Will it work ? I figured I would ask the experts first.

Excuse the long reply and lousy photo and thanks for all the input.

bandersen
01-07-2017, 09:08 PM
The Holiday uses a 9L37 chassis and Tandem a 9L38. They use different flybacks and a number of minor component value differences and as you mentioned Tandem has the final video amp inside the CRT housing.

mrjukebox160
01-08-2017, 01:25 AM
I would keep that tube until after the rebuild then see how it looks after some hours of use. If it's still too dark you could get a rejuvenator from ebay and try to zap it.

Electronic M
01-08-2017, 11:36 AM
I would keep that tube until after the rebuild then see how it looks after some hours of use. If it's still too dark you could get a rejuvenator from ebay and try to zap it.
If you do get one of the newer gentler rejuvinators....Don't want to atomic blast that cathode away.

vt105
01-08-2017, 01:43 PM
Since there a lot of rejuvenators out there what do the experts say ?
I have read both pros and cons about "zapping" crt's as well as using brighteners

I have several old sets (including 4 predictas) and I cherish the ones that still have a working crt considering the lack of rebuilding services.

Electronic M
01-08-2017, 01:54 PM
There are loads of threads on that if you search.

Sencor CR-70s and some Beltrons are well regarded.

mrjukebox160
01-08-2017, 05:39 PM
I would only try it on a rare tube if it was too dark to use. To me, at that point, what have you got to loose.