Antique Television
This is a 1949 RCA Television, model # 9T240. The 9 means it was produced in or for the 1949 model year. The "T" indicates that it is a television. The 24 indicates that it has 24 tubes. It has a 10" round picture tube (10BP4 - it was called a kinescope). I paid $25 for this at the Salvation Army store in town. I have about $400 of parts into it so far! I think its worth about that after its restored.
If you have any more info or comments on this TV, please email me.
December 2001
I started tearing it apart right after Christmas. The first thing I did was to remove the RF unit (tuner) and thoroughly cleaned all the contacts in it. Curiously, it uses a belt to drive the fine tuning cam. The cam lifts a small core in a coil. I was able to find a replacement belt at Waterbury Electronics - its just a VCR drive belt and it works perfectly.
After lots of searching on the net, I finally found a set of schematics from Sam's web site. I was really surprised that they're still in business and they still sell all of their old schematics. The schematic and technical notes were about $25. Later I found an RCA schematic book with all of RCA's television schematics from '45 to '50 for $10. There were quite a few discrepancies between the two schematics. Most were just different values in resistors and a couple of capacitors. The actual circuit matched some values in the Sam's schematic and some in the RCA schematic. Maybe these were later improvements on the circuit. If anyone knows if there were any "service bulletins", please email me.
To start, I went through and marked down all electrolytic and paper capacitors for replacement. I also marked down the plastic ones (which are really paper inside) and any others that looked a bit off. Those that weren't electrolytic I replaced with the orange drop type of capacitors. All were much smaller than the originals. Some were 1/8 the size! Some values were hard to find, so I had to double up some values by putting them in parallel. Remember to watch your working voltages when you do this! In the past I've seen them explode!
Then I tested all the tubes. About half were bad. I had most of them already that I'd bought at tag sales, etc. The rest I got from www.tubesandmore.com and ebay.
Next I tested the tube pins for the proper resistance values listed in the Sam's data. There were a few that were slightly over 20% off. Then I removed the Horizontal Oscillator tube and SLOWLY powered up the set with a variac. Except for the tubes lighting, nothing really happened. I began testing all the voltages listed in the Sam's data and found the same points that were off with resistances off with the voltage measurements. While doing this I heard some crackling sounds. After some intense listening, I determined that one of the main power resistors was probably breaking down. Testing the B+ voltages confirmed this. At one point all the lights dimmed in the house - so I promptly (read panic) cut the power and did the rest of the testing with the ohm meter. Two of the voltage divider resistors were shorting. Another trip to Waterbury Electronics - the resistors were very odd values, but I was able to put a few in series to make up the right values. By the way, I checked all the parts as I was installing them and none measured over 2% off. Many of the parts I took out were completely open or shorted, and many were just way off.
I powered up again and all the supply voltages were almost right on - none more than 2% off. Most of the voltages looked OK - a few were still off in the video and horizontal oscillator sections. Since I had Volumes II and III of RCA's Pict-O-Guides, I decided to put it back in the case and hook up the Kinescope. It powered up with a bit of a whine from the horizontal circuit - which I later figured out was abnormal. I had remembered from the old days (1970's) that our TV's used to make similar noises, so I thought it was "normal". One thing I discovered is that the tuner and IF circuits must be working well as far as the separator since very clear sound came out of the new Radio Shack speaker.
Anyways, there was very little of what you'd call a raster. I tested the voltages on the Kinescope - the grid voltage was way off at -90V. I needed more information, so I visited Nutmeg Book Shop and found a couple of books from 1949 and 1950 on repairing TV's and radios. Apparently, the 10BP4 tube has a grid cutoff voltage of -45 volts. Meaning the electrons from the gun can't reach the screen when the voltage reaches -45VDC. So, I ripped it all apart again and started looking at resistors in the video circuit. Two resistors were way off tolerance. So I placed another order with Tubes and More and ordered lots of resistors for many different parts of the circuit.
I reassembled the set and there still was no raster. In my investigations, I had come across lots of stuff regarding the positioning of the ion trap. So I kept turning it and up popped the raster when it was in the right place! I was really excited - never mind that the picture was horizontally torn so that multiple images appeared. So, back to the books. The Pict-O-Guide kept pointing to the Horizontal Oscillator circuit - but it only gave a short description of what was going on, so I still wasn't sure if that was the problem, but after more reading I decided that that circuit was the first place to look. Again, I replaced all the resistors in the oscillator circuit, and since there weren't too many in the amp circuit, I decided to replace those too. I was getting nervous handling the Kinescope each time. Turns out there were some resistors really off there too.
I reassembled everything again, turned on the set and heard - nothing. All the tubes were lit but the oscillator was now very quiet and I thought, "I really @*&$%#something up now." Then suddenly on popped the picture - I adjusted the vertical hold and it was almost clear. Some adjustments to the focus control and the lines were sharp as could be. Very cool!
Now for some of the vertical adjustments. The vertical linearity was WAY off. All scrunched up at the bottom and spread apart at the top. After playing with the height and linearity controls with my grandmother's big old deco mirror propped on a chair, I got that looking acceptable - but not perfect. The bottom portion of the picture is chopped off. I returned to the horizontal adjustments from Sam's. The horizontal drive, linearity don't appear to do anything. They probably have to be cleaned (I hope). The horizontal width only works a little - doesn't seem to do too much but it works better than the other controls. The width control is in the high voltage section of the set and is a transformer with a brass core adjustment.
One other thing I did that makes me cringe - I did think about it for almost a day before I did it. One horizontal adjustment, T3 is on the side of the chassis. Since I don't have a TV repairman's bench with extra Kinescopes, the only way to do that adjustment is by drilling a hole into the side of the cabinet. So, as much as I hated to do it, I drilled the hole - in the wrong place. Measure twice, drill once! The second hole was right on and the first hole was very small anyways - just to make sure it was in the right place. The good news is that I hooked up the oscilloscope and was able to properly adjust horizontal section perfectly..
And finally, here's what the picture looks like as of February 2002:
September - November 2002
Now that the gardens and the yard are squared away, its time to refinish the cabinet. My boyfriend Mike found a high quality non-polyurethane varnish. I refinished my piano with polyurethane and have been unhappy with the way it turned out. It just didn't have the depth that the old varnishes have. The cabinet was in such bad shape, I decided to start off with sanding. The veneer wasn't too thick, so I had to be careful. I'm quite happy with the look of the finish, I just need to complete it with a finial polishing to bring it to a high luster.
I've also started to replace the remaining resistors and a few remaining paper capacitors. If I find all of them, there shouldn't be any remaining paper caps left in the set. I have left most of the ceramic and mica caps. I'd say at least half of the resistors I'm replacing are at their extreme tolerances or way out of tolerance. Very few are right on their original values - but I doubt that these parts were right on their marked value even when brand new. Curiously, the original specs call for 20% tolerance on most of the components, and today I can't seem to find any new parts more than 10%. Many of the parts I've purchased are even 2% tolerance. I'm hoping that this will contribute to the stability of the picture later on - I'll keep you posted. I've also found another error in the Sam's schematic - its in the vertical circuit.
December is only one week away and I hope to have it fully reassembled and recalibrated for the holidays - I need to reclaim my dining room!
Well, getting ready to go over mom's for Thanksgiving, I reassembled the set and powered it up (against my better judgment - but I wanted to show it off). Good thing I installed an in-line fuse. It blew.
December 2002
Well, After Thanksgiving, I disassembled the set yet again. I replaced the few remaining resistors in the AGC circuit. I had intended to slowly power the set up again while monitoring either the power supply voltages or currents to figure out which part of the circuit was "shorting". I had forgotten that I hadn't checked the fuse in the set - so I took it out and it was not blown. I suddenly remembered that I had placed a 1A fuse in the Variac because I was previously working on a small radio. Turns out there was nothing shorted after all. So I reassembled the set, turned it on. Up came the raster but there was no sound and no picture. The raster looked kind of washed out too. I suspected the AGC circuit was causing this. I tried some signal tracing but could only get a signal if I turned the RF fain on the generator up to maximum. And even then the sound wasn't that great. So, I take the set apart again (I'm becoming a pro with the CRT!) and begin tracing the 1st an 2nd IF stages. Actually, I traced the 3rd IF stage too, but everything seemed to be in order. So then I started to retrace the AGC circuit - that too seemed to be in order. I followed the AGC line back to the video amplifier and it occurred to me that C52 (Sam's) at 270mmf (270pf) was replaced. Originally I had replaced it with about 8 ceramic caps in parallel because I didn't have a 180pf capacitor. I thought I had put a 180pf cap in parallel with a 100pf cap, but what I thought was 180pf was really 1800pf! So I out came that and in went a proper value cap.
I first disabled the horizontal oscillator and tried running the set without its CRT. Sound came out, so all seemed to be fine with the RF and IF after all. I reassembled the set and on came the squealing of the Horizontal Oscillator. Obviously (see above) there was no Horizontal lock. Multiple horizontal images appeared. It kind of gave a torn appearance. This is obviously an oscillator at the wrong frequency. No amount of adjustment would fix it either. Not wanting to disassemble it yet again, I double checked the components on the tank circuit and grid circuit. Wouldn't you know it - another component that had the wrong value. R100 is supposed to be 2.7M but I had put a 4.7K in series with a 500K! I must have picked up the 4.7K instead of the many 2.2M resistors that I had! Luckily, the horizontal oscillator is easily accessible from the bottom, so I only took out the CRT and was able to solder in the correct resistor. Of course, when I turned it back on I couldn't sync it because I had totally messed up the adjustments. But now the picture looked like a typical out of sync rapidly rolling screen. After a few very quick adjustments, I was able to get it to lock. After adjusting the vertical linearity and height, I followed the Sam's instructions for aligning the horizontal oscillator, but the drive and width controls don't seem to have any affect. After readjusting the horizontal peaking on the scope, the top part of the picture bends to the right. I can't seem to get it straight - however I've read that too strong of a signal can cause this. I wonder if this will cause the unresponsiveness in the horizontal adjustments too. I'll have to try attenuating the input signal, or better yet, connect it to the antenna.
Overall the picture is VERY stable. It locks in really well after changing channels. The vertical no longer "slips" when it runs for a while. The vertical linearity is now right on and very easy to adjust. The picture is very clean and crisp. No retrace lines are visible anymore, however I've read that modern transmissions include lots of data in the blanking pulses, and the old sets expect nothing there, so retrace lines are normal on old televisions except with home recordings.
The only thing left to do is to align the IF tuning sections. While it does work fantastic with a VCR, it needs a full alignment for off the air reception. I have until the end of 2008 to finish - after that there will be no more analog signals broadcast!
12/11/02