Silvertone 7140

[zenithfan1]

That is one awesome set!

[miniman82]

Finally mostly nearly almost done unpacking the TV related stuff, that means it's time for an update!


Got some time this weekend to pull the chassis back out of the cabinet (no CRT's died in the back of the U-Haul truck on the way here thank GOD!), so I put it on the bench and brought it up with the variac. First I removed the horizontal and vertical deflection tubes, so I could reform the power supply caps. That went well and nothing got hot, so next I scoped the horizontal oscillator to make sure a waveform was there to drive the HO tube (it was), so I put all the tubes back in and brought it up...


Nothing.

It was absolutely dead.

Started digging, and there's voltage to the vertical oscillator. Strange it should be running, wonder if there's bad caps in it. Twiddled the LIN and height controls because sometimes after sitting there's high resistance in the elements that can keep it from working and with a BANG and a SNAP, the vertical oscillator started running. This was a smokers set unfortunately and it's very dirty and dusty, yet another chassis I'll end up pulling all the pots out of for a scrab and deoxit. Dammit. Well for now I had vertical sweep, which on this set is powered by B+ not boost, so I moved on.

Still no horizontal, though I can hear it sort of trying to spool up- at least when the dirty HO tube's socket doesn't decide to not power the filaments. Note to self, schedule socket cleaning...grrr...

Looked at the HV wiring on the 3B2 and 1X2 focus rectifier, and it's toasty. All cracked and mangled. Possibly horizontal is getting dragged down by HV leakage? Took me 3 hours to get the HV cup apart on this because like a CTC-4, there's a giant doorknob built into the bottom of it. The tin can lid of it has to come off the bottom first, and it was soldered together. Took a torch to get that apart. Finally get to the guts of it only to discover that the rectifier socket had been replaced with a ceramic one at some point in its history, and it's way too big for the little metal plate it's mounted to. Now it's all deformed, and I gotta pound it back into shape.

Get the old socket removed and the pie plate straightened out, but I don't have any octal sockets that fit in the smaller than normal hole. Piss, this just ain't my day. Thankfully I had won an Ebay auction for a replacement one not long ago, so I stuck that in with some new Belden 40kv anode wire. Let's try this again: only 5kv...

OK, not good. Either the flyback is bad, or I got some kinda serious unknown drain on HV somewhere. I already knew the CRT tested good from when I picked it up, so I knew that wasn't the problem.

Oh BTW, I discovered at some point that the selenium LV rectifiers were past their prime, so B+ was seriously sagging as soon as the tubes started drawing current from the power supply. Those got replaced with silicon along with some new electrolytic caps, I'll add series resistance to it later.

Only other thing that can drag down HV is the shunt tube, a 6BK4. So I disconnected the CRT anode lead and the plate cap of the 6BK4 and try one more time before assuming the worst- Bingo, 29kv shows up on my meter! OK cool the 6BK4 grid is probably too high, so I put a meter on it. Sure enough, it's at 210 when it should be around 135 VDC. HV pot doesn't do anything either, so I trace it out.

The 6BK4's grid receives its marching orders from a HV bleeder string that starts on the focus rectifier, goes through some very high megohm resistors, then the HV pot gets a chance to shunt some of that voltage to ground, then there's a 1-meg to the 6BK4 grid. So logically if the pot does nothing and the tube is stuck fully on, there's gotta be an open somewhere either on the pot or the 1-meg going to the grid. Read through the pot and it actually came back nice and smooth with the correct resistance, so I look at the 1-meg going to the grid- it's drifted up to 14 meg!

After replacing the 1-meg grid resistor on the 6BK4 I now had a full bright raster with controllable HV, so I set the pot to around 23kv and called it good. Now all the major parts have been verified as working, so I can sit down after the ETF meet and start ordering all the caps this thing is going to need to be reliable.

Also need to build a new HV cage for it, we searched the house it was in but didn't locate it.

That being the case, I pulled the CRT and took a shot of the backside of the chassis. CRT is a Sylvania made tube according to the EIA code (if that's what the 312 on the label is anyway), and has the original Silvertone brand label on the bell. First color set I've ever owned to still have all of it's original tubes in it, gotta be some kinda miracle. Enjoy the pics.



Here's a link to a high-res shot of the chassis: http://miniman82.4t.com/images/Silvertone/image1.JPG

The CRT:

[ohohyodafarted]

Congrats on getting it going Nick. The chassis reminds me a lot of MOTO chassis, a gobbldygook hodgepodge nightmare. Not nice and orderly like a RCA. God knows why they work at all, but they do.

[dieseljeep]

According to the code-date stamp on the label, the CRT is from 1960. Sears Silvertone replacement CRT's were always sourced by Sylvania.
The original CRT was probably an RCA.
The original owner might've had a maintenance agreement from Sears. That would explain all the Silvertone branded tubes.

[miniman82]

Thanks Bob, it is a mess isn't it? lol

Dave, I assume you're looking at the numbers '016' on the label? I guess that means 16th week of 1960, so it probably is a replacement. What makes you think the original was an RCA? The other Sears set I got from the same house is just like this one, all original and untouched (CRT replacement aside). The owner was very proud of his Sears brand items, and probably had service agreements on all of it.

Next task will be carefully replacing the CRT base with a new one, don't suppose anyone has a spare Silvetone base for me? That's gotta be a rare item, if it even exists. Probably just put a generic one on there, anyone got one with no markings that could be donated to the cause? I'll look for a brightener at the museum, but don't know if they have one.

[dieseljeep]

Originally Posted by dieseljeep

According to the code-date stamp on the label, the CRT is from 1960. Sears Silvertone replacement CRT's were always sourced by Sylvania.
The original CRT was probably an RCA.
The original owner might've had a maintenance agreement from Sears. That would explain all the Silvertone branded tubes.

It seems, RCA made all the 21AX CRT's. Sylvania might've just regunned them or if it was a new tube, Sylvania bought it from RCA and relabeled it.

[miniman82]

To add to the mystery, this tube has no neck weld which would tend to indicate it was rebuilt. But the base did have a 274 EIA code on it which is RCA.

[dieseljeep]

Originally Posted by miniman82

To add to the mystery, this tube has no neck weld which would tend to indicate it was rebuilt. But the base did have a 274 EIA code on it which is RCA.

Every early color set that I've seen, that used a 21AX CRT, that CRT had a EIA 274 on it. That was a GE, Hoffman, Motorola and an Admiral. It had the OEM brand name on it, with a 274 EIA.
IIRC, Sylvania was the main supplier to Sears for their replacement tubes. The OEM tubes were made by several different Mfg's.

[miniman82]

Chassis is now moved to the designated resto bench in the garage, where recapping can take place. I've already stuffed the can 'lytics and replaced a few wax tubulars, but there are many more discrete 'lytics and other assorted caps that need to be rooted out before I fire it up again. I also need to figure out how exactly to connect the horizontal deflection coils to the test jig: the chassis has a center tapped setup where the center tap is cap coupled back to the flyback, while the test jig doesn't have a center tap at all. I can probably do one of two things, either leave the CT out or add one to the jig's yoke and see what happens. If I can't get it going, I'll just have to wait till cap replacement is done then put the CRT back in.

Eric: the replacement base worked flawlessly, thank you very much. I was even able to use a brass wire wheel to remove the 21FJP22 mark on it, it disappeared leaving shiny black plastic behind and you can't even tell it was ever replaced.

Here's where I'm currently at: