Paul's Repair Bench

48-1000 Restoration Part 2

2009-12-06T05:47:00.000-08:00

Before I got down to the restoration business, I took a couple of videos. The first one showed that the set was in a working state before I got my hands on it. Plus the video shows some of the interesting features of the set. If I screw up royally......

The second video shows the underside of the chassis and my strategy for replacing the canned electrolytics.

In general, the strategy is to replace only a few parts at a time, starting with the main filter electrolytics, then power up the set momentarily to verify that things still work properly (or actually expecting an improvement). Since the Sams schematic and parts list I had listed the electrolytics first, I just followed in that order. It wasn't long before this well-thought-out plan turned to crap.

Here we have the original power supply section. C1 is the combination of the "center" of the 3 cans and the cap that was "added". The known leaky overheating capacitor is C2, which is below C1, closest to the edge of the chassis.

As each capacitor was disconnected, I'd check its value with my DMM. The clip mounted cap measured pretty close to its 30uF rating. It was the one of the few that would do so. Most would not even register a value since they had so much leakage.

As shown in the video, the actual replacement parts can go elsewhere - where the wire from the can cap actually leads to. C1 could go right to the rectifier socket.

The overheating capacitor, C2, had 4 sections. One was the filter immediately after the main filter choke. The second section filtered the power to the horizontal sync, tuner and IF strips. The third filtered power to the horizontal oscillator and the last section was the horizontal output cathode bypass.

By the time the 4 sections of C2 had been replaced, the set did not fade out after prolonged playing and seemed a bit more stable.

One section of C3 filtered power to the Vertical Sync Amp. In order to connect the new cap, one lead of each of the resistors of the plate load divider needed to be removed from their terminal strip and I tested them.

The 68K resistor measured 123K, far beyond an allowed 20% tolerance. I had purchased all of the high value resistors (470K and above) and didn't expect the lower values to be going bad.

At this point, whatever order I had was thrown out and it became a "restore each circuit" strategy, starting with the ones that were most suspect. Since I was already working around the sync amp circuits I finished replacing what was necessary.

Philco 48-1000

2009-11-29T15:47:00.000-08:00

It's been a long time since I posted here, just no time for hobbies & such. But I put aside the Tele-Tone without getting too much into it because there's something else taking up most of my bench space and I'd rather it be elsewhere and in a more working condition.

I've had this Philco 48-1000 that I got at the now defunct Norton Flea Market for a little more than 20 years. It's always "sorta" worked, in that it would be OK for 15-20 minutes then the picture would gradually fade away. While it was working, the pictures were fairly decent.

But it's gotten progressively worse, meaning it's only OK for about 5 minutes now, so it's time for a recap. There's LOTS of caps, though.

All of those brown cylindrical parts are capacitors. In addition, if you look at the top of the chassis,there are also 5 other round objects (that aren't tubes) that are a different type of capacitor and will be replaced in a different manner.

First, a few tidbits of information about the set.

Year of manufacture - 1948
Number of tubes (including CRT) - 26
Number of semiconductors - 1 (a Sylvania 1N34 to be exact)
Power consumption - about 250 watts.
Channel coverage - 8 VHF channels set up at time of purchase with specific coil sets loaded into the tuner - this set covers channels 2,3,4,5,7,9,11 and 13
High Voltage Supply - about 8000 volts.
Receiver IF type - Split sound - separate IF strips for video and audio

After counting up all the electrolytic and paper type capacitors a total of 72 capacitors were ordered. In addition, I figured most of the high value resistors (470K and over) were probably bad as well, so 20 resistors of various values were added to the order.

Fortunately, parts have gotten much smaller in the last 60 years.

In any event, I'm fairly well along in the part replacement process, I decided I should at least start the blog and catch up to where I'm at over the next days. There have been quite a few surprises along the way, most of them not too pleasant.

Preliminary steps

It had been at least 9 months since the set was last powered up, at that time there was no audio and I could have sworn I discovered an open primary on the audio output transformer. About 2 months ago I brought the set up slowly with a variac (about 5V every couple of minutes) and monitored the power supply voltage and power consumption. Since there are no longer any VHF analog signals around, I used a DTV converter box.

The first thing that surprised me when I got to around 90VAC was hearing very scratchy audio. Wasn't expecting that. By the time I reached the "nominal" supply voltage of 115, bad things were starting to happen. The picture which was out of focus and very dim with significant blooming as the brightness control was advanced to begin with was fading away to nothing, the power consumption was rising quickly (from 230 watts up through 250 watts), and the power supply voltage was falling (from a high of 350 to about 320).

After shutting down I started checking some of the tubes. The 1B3 high voltage rectifier tested very weak (even though I had replaced it only a couple of years ago and the set has very little use), so I had to liberate the 1B3 out of the TeleTone (oh well). A quick check showed the picture was much brighter now and there was no blooming, but it still faded away quickly. While inspecting the chassis I nearly burnt my hand on the capacitor between the power transformer and filter choke in the lower left hand corner of the chassis top picture. With my new multimeter with an infrared thermometer function, I measured the capacitor temperature at 150 degrees. Apart from feeling really stupid that I let it get to this point, I'm even more surprised that the cap didn't go boom. That explained the power consumption rising and the power supply voltage falling, which also caused the picture to fade out. This capacitor is the first filter off the low voltage rectifier.

In the next installment of the blog, we'll actually get to work.

Revisiting the Philco

2009-01-13T08:29:00.000-08:00

A couple of years ago I measured the power consumption of the Philco using my Watts Up? power meter. Last week I did the same measurement with the repaired Philco.

The top trace shows the power consumption going up the longer the radio is on, until it's been on about 3 hours. The bottom trace shows in comparison a steady power draw, much lower than before. The little drop at the end is where I turned on my oscilloscope, which is on the same outlet so the line voltage drops a bit. I wonder where the extra 10 - 30 watts was going.

Anyway, that's as good an indication as any that things are working right.

Next Victim

2009-01-07T18:06:00.000-08:00

Satisfied that the Philco is working OK, it's time to turn my attention to something else in the pile.

I got this TeleTone TV about 10 years ago (maybe longer) and when I got it - it was totally dead. After replacing a couple of burned out resistors in the power supply and heater string, there was sound but no picture to speak of. Nearly all of the filter capacitors are bad, so I replaced a few of them and tacked some temporary replacements in other places, the result was a fairly bad picture.

My only goal was to determine if there was any life left in the 7JP4 CRT - they are now almost impossible to come by, so I just tacked in what I had lying around parts-wise.

Satisfied that I had something to work on, I ordered some replacement parts, but soon after I moved and the TV got buried somewhere. It's been at least 7 years since it got turned on, so I powered it up through the iso-transformer and variac. I got intelligible sound at about 85VAC and a visible picture at about 100VAC. In some ways it was better than the picture I got before as there were no hum bars but it was nowhere near as sharp (sharp being a relative term)

It's very unstable, requiring almost constant adjustment of the hold controls and the picture is much too large.

This set is interesting in construction, the RF/IF/Video/HV and audio sections are on the bottom chassis while the deflection circuits and the CRT chassis is mounted on the side of the cabinet. In the upper left is the antenna, which essentially is 2 "tape measure" type reels.

This is the bottom of the sweep chassis, the largest of the brown "paper" capacitors are rated at 6000V and are likely bad, I have replacements now. The other problem is the high voltage dividers for the focus and deflection voltages. The design itself is inadequate, the resistors are not designed to withstand the voltages they are being subjected to and their lives will be very short. A proper repair would require the use of special high voltage resistors ($$)

My next step is to replace the high voltage capacitors in the deflection circuits and see what happens. My guess is that the high voltage is being dragged down, causing the picture to be larger and dimmer than it should be. We'll see how wrong I am.

Success

2008-12-09T18:18:00.000-08:00

Last week, the Philco got powered up with the chassis still on the bench and it got a final cleaning of the tuning capacitor with a bit of alcohol and the bandswitch got a spritz of contact cleaner. After working the tuning cap the noise related to the tuning control movement is gone, there's still a little bit of noise in the bandswitch but it's too crowded in there to think of disassembling the switches.

The SW bands are off a bit calibration wise, it also can't tune to the highest frequency (22MHz) without breaking into oscillation. Of course now the SW bands are mostly dead anyway because of the sunspot cycle.

On the lookout for some 7B5's to replace the weakened ones.

Finally the chassis got put into the cabinet and everything got secured. Ran some test tones through the AM transmitter and all sounds well. I tried to record a video of the radio in operation, but the camera doesn't handle that type of recording well without overloading.

Philco surgery

2008-11-30T18:08:00.000-08:00

The repair parts came in on Friday, so Saturday I got the soldering iron out and proceeded to get those bad parts out of there. First were the capacitors across the line. One was already blowed up, so out came the second one.

As you can see, I pulled the one still there away from the terminal strip to show the wax it left behind. That cap was not long for this world. Here's the replacement.

Here are the original "blowed up" cap, the new replacement, and the other cap about ready to go. You can see printed on the new cap all sorts of regulatory agency approval markings from UL, CSA, VDE and many others. These caps are specifically designed for being placed across the AC line and if they fail will fail safely (not go BOOM). Now the new safety caps are in.

You can see another cap that was replaced, the white braided thick "wire" is actually a power resistor, and you can see what it did to the old capacitor. Nice burn marks.

And now you can see a recurring headache that popped up throughout the re-cap process. The new parts are so much smaller than the originals that in many cases the leads aren't long enough for the parts to go in the same positions.

Now that I had the radio relatively safe, I wanted to check the bias voltages on the output tubes. I suspect that the open bias resistor I found earlier would cause a problem, and it did. Instead of -20V bias on the 7B5 grids I had 0. So instead of a plate voltage of 295, the tubes were drawing much more current than they should have and the plates were down to 220. Very surprised something didn't melt in there.

The next step was to replace every resistor and capacitor in the output stage, as the resistors that hadn't opened were drifting way up in value.

After all that was done, the bias was checked again and it was almost dead on -20V. Plate voltages were up around 310V, a little high but OK, those tubes took quite a beating and are probably very weak now.

That was enough for Saturday, by this time the radio was playing again. Sunday the rest of the capacitors got replaced. These were a little deeper in the chassis, and some surprises were lurking as well.

A "resistor to nowhere". Finally everything was done.

I did not reinstall the wiring to the phono socket because it's too crowded in that area and I never foresee using a phonograph with this.

Now playing on the bench. There's still a couple of minor issues to take care of. The tuning capacitor is dirty so there's much noise when tuning the shortwave bands, the calibration is also off by a bit. But the AM section sounds OK.

Philco 53-960

2008-11-23T11:02:00.000-08:00

It's been a long time since I've been able to visit the bench, it's just been too busy. But instead of one of my eBay finds, one radio that I've had for a long time is starting to sound poorly so it takes a spot on the bench.

The Philco 53-960 is a 9-band (AM - 8 SW) table model receiver with 8 tubes. I like it mostly because of its large 10" speaker with a decent 7 watt push-pull audio stage. Saturday nighs I'll play oldies through it with my SSTran AM transmitter. Somehow, Sinatra sounds better through something like this.

A couple of weeks ago, it started to sound very distorted as it warmed up, so last weekend I brought it down to the cluttered workbench and took off the back.

I let it play on the bench for a while with an o'scope probe connected to the speaker. It was starting to sound a little distorted, but with the scope connected I tried to see just what kind of output power I could get, so I cranked the volume. BANG!! The radio kept playing, but I pulled the plug but quick. The chassis had to come out to assess the damage.

The capacitor in pieces and the other one still in one piece are connected from the AC line to the chassis. The unfortunate thing is that my connecting the scope probe ground to the speaker probably brought about this capacitor's demise (alarm bells should have gone off in my head when I got a spark connecting the scope ground), though any of these type capacitors (called bumblebees or 'Black Beauties') should always be replaced on sight as their chance of being good after 50 years are near zero. A look at the rest of the chassis shows a few more of these "bumblebees" (7, I think).

But this failure probably had nothing to do with the distorted sound, so with a just-obtained Sams schematic in hand I made some resistance measurements to the tube sockets. How about that, a problem in the output stage.

The arrow points to a nearly-open resistor that helps set the bias for the two output tubes. That's probably a good a reason as any for the distortion. Most every other reading seemed to be within reasonable limits.

The next step is to order the replacement parts. All of the 'bumblebee' and paper/wax capacitors will be replaced as a matter of course, since they can all be thought of as "time bombs" now. Instead of the 'bumblebee paper' capacitors across the AC line, special "across the line" rated capacitors (with so many regulatory agency approvals there's barely enough room on the part to list them all) are available which are designed to fail safely - like not go boom. Not that it really matters, since there's plenty of room under this chassis, but the modern replacements are much smaller and longer-lived than the originals.

First victim selected

2007-04-01T20:23:00.000-07:00

I remember the very first portable radio I had, an orange Stromberg-Carlson about 1955 vintage. I've been looking on and off for one on eBay and found one like it recently. There were two different versions, the one I had only used batteries, the one I got from eBay could also be plugged in.

My first experiments with it yesterday yielded no sound of any kind, so today I brought it down the the bench to do some quick checks on it. I was surprised when I checked it tonight; while there's some nasty hum, I could hear stations on it. I'd never trust running something like this unattended while plugged in, but it might be interesting putting some batteries in it.

I'm turning in for the night then in the next few days I'll get some pictures and more details posted.

Back to the bench

2007-04-01T18:32:00.000-07:00

Way back before the interwebs became popular, I used to spend my spare time fiddling with old electronics - defined by me as something that had glass bottles known as tubes in them, whether they lit up enough to be seen or not.

I've got a bunch of stuff in the cellar just aching to be put up on the bench and I'll see if I remember enough about this stuff so I don't electrocute myself or the nosy cats (the wife knows enough to stay away).