Predicta Debutante - voltage rail being dragged down
 

Hello all, it's been a little while since I've posted about my adventures but now that school has been canceled till the end of the year, I've had basically unlimited time to work on projects! :D The most recent one being this nice Predicta Debutante which I picked up about a month ago shortly before things started getting majorly shut down due to the coronavirus situation.

I first saw it on Facebook Marketplace, and after a brief road trip, I discovered the guy had it sitting in a booth in an antique store for $50 more than the listed price on FBM. After a brief phone call, I confirmed the FBM price was correct and happily walked away with my very own Predicta! :music: The store did end up having a bunch of old radios in other booths and even an Admiral BW tube portable set that also made its way into my car, but mostly everything already worked or needed very minor servicing, so I don't plan on making threads for them.

To put things short, this set has been through hell and back. The seller claimed he turned it on and it did nothing, which I confirmed with a quick power up test - the tubes all lit up except for the horizontal output tube, and then a particularly blackened resistor in the audio circuit connected to B+ started smoking and sparking. :tears:

I first turned my attention to the dead horizontal output tube, which I was able to resurrect; there were some bad solder joints in the tube pins and after reflowing it came right up. The 6U8 sound IF tube had completely gone to air and was replaced. EVERY SINGLE electrolytic was visibly leaking electrolyte, and most sections were also electrically leaky if not outright shorted (this is what was causing the resistor to smoke as B+ was shorted to ground), so I replaced them all, tacking some in as I did not yet want to pull the PCB. I decided to test the CRT before I tried powering it up, and discovered the filament was OPEN. No current draw nor glowage from the heater... :sigh: Thankfully, I had a fair-testing replacement from another Debutante I had parted out about a year ago, so I swapped it in and cleaned the head unit while I had it apart. I have a suspicion someone tried to test the original CRT at 6.3V, as it is a Philco OEM 2.68V tube, and blew the filament.

FINALLY, I applied power with fingers crossed, and was greeted with a very dim horizontal line and a high voltage rectifier tube glowing a pretty purple! Okay, so it's gassy...popped a NOS RCA 1B3 in, and I was greeted with a reasonably bright line. Very good news, as I had not even touched the PCB yet. :thmbsp:

At this point I was convinced the set was a good restoration candidate and went through the laborious process of labeling all wires and removing the PCB. Although I don't typically like doing this, I shotgunned it, meticulously replacing every cap, resistor, and couplates using a kit I bought from Crist Rigotti a while ago, figuring I should just replace it all now and not have to pull the board again.

I put the board back in afterwards, cleaned up some of my electrolytic replacements, and was hit with a wave of disappointment. No high voltage, and a redplating horizontal output tube. Turns out a coil in the horizontal circuit was open, I had apparently damaged it some time while handling the board - no matter, I have the PCB from the parts chassis! This brought HV back, but not vertical. I tracked THIS down to the vertical linearity control - one terminal is entirely missing, and it appears the previous owner put some tape on top to disguise this? :nono: I'm not sure how they bypassed this as there was not a fixed resistor anywhere and it was apparently just left open. I have not yet replaced the control, instead opting to substitute it with a 390 ohm resistor for the time being. I will replace it with a proper pot once I get the chassis fully working.

The issue I am currently battling resides in the 160V B+ rail - something somewhere loads it down to around 20V as the set warms up and draws enough current to cause the voltage dropper resistor to get in the 300 degree range. I also get a blank raster with no tuner activity, which I believe is related to this problem, as the IF board uses this 160V B+ rail. Has anyone else experienced this problem? I am pretty sure I got all my wiring right, but I will double check it as well as double check all the components coming off the 160V rail. I've verified the replacement filter cap is fine and if I disconnect the wire from the PCB, the voltage remains stable (if a little high as there is less current draw).

If anyone could point me in the right direction on this I'd be eternally grateful...this is the last issue before the set should be fully working and I can start on the cosmetic work and de-rusting. Thanks! :scratch2:

Sounds like good progress so far. My approach would be to start with resistance checks from the tube pins. Should be a chart in Sams.

The vertical linearity pot is SUPPOSED to have only one terminal. The wiper and bottom terminal are grounded...They are grounded by internally connecting them to the metal case of the pot that then is riveted to the grounded metal chassis...most pots of that cosmetic style were made that way...it saved production time by eliminating 2 hand wired solder connections...the set uses couplets for the same reason. If you haven't rotated the pot through its range or sprayed cleaner in it you might want to try it. It could as easily be the pot is good but has a dead spot as it could also be the pot is dead open.

I tend to unhook loads from shorted B+ rails to track down shorts. The lowest resistance path to ground is often the culpret... thought some shorts only happen when warm so sometimes I simply see which load says the B+ when connected.

Just looked at my parts chassis and that pot only has a single terminal as well, not sure how I noticed it before...thanks for clearing that up. Will definitely try using some contact cleaner, I hope the pot is good after all. Would save me the trouble of drilling it out. :scratch2:

As for disconnecting loads, the 160V rail only splits off to the IF board and one point on the PCB. Disconnecting the PCB point (as described in the OP) gives me a stable voltage, so it is a problem on the PCB; however, this point measures 10K to ground, and this is correct according to the official Philco service manual. My current theory is something shorts while heating up, or is at least excessively leaking electrically. Could be a defective component as there is not much I have not changed, but I'll likely figure this out once I start taking stuff out of circuit. Hopefully it's not something super difficult to find.