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 Post subject: Philco 48-1000 - Brightness Failed
PostPosted: Oct Tue 10, 2017 2:17 am 
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I have a recently-restored Philco 48-1000 (see the restoration topic/saga here.) I completed the set electronically earlier in the year, and it has been played almost daily for several hours ever since. Now the inevitable has happened, it has broken, and I need to be a TV repairman again. The symptoms are:

- picture is very dark
- Background knob does nothing - rotating it to the stops either way has no effect whatsoever

Clearly the brightness/background function has failed. I'm going to poke around in the circuitry and do some Googling, but I wanted to get feedback from the experts here about how to proceed. Any advice you have would be appreciated.

-Steve


Attachments:
File comment: On the TV, it is very very dark. This photograph doesn't show it too well, but it is indeed very dark.
Too Dark.jpg
Too Dark.jpg [ 98.08 KiB | Viewed 1475 times ]
File comment: The Background knob controls the brightness, but now it does nothing. Rotating it has no effect at all.
Background Knob.jpg
Background Knob.jpg [ 80.34 KiB | Viewed 1475 times ]
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 Post subject: Re: Philco 48-1000 - Brightness Failed
PostPosted: Oct Tue 10, 2017 3:24 am 
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Location: Near Brandon, Iowa
From what I recall of my B&W TV repair interest days (about 50 years ago), the brightness control acts on the CRT cathode; that is, it adjusts the amount of current flowing through the tube's cathode to ground. High current flow=bright picture; low current flow=dim picture. Or so my memory would have me believe. Perhaps you have a burned out pot or fried part in the brightness control circuit? Time to bust out the documentation...


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 Post subject: Re: Philco 48-1000 - Brightness Failed
PostPosted: Oct Tue 10, 2017 10:03 am 
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Location: Long Island, N.Y.
Try a cooling fan on the HV cage if the picture goes dark as the set plays. These early Philco's have the same issue as the early Hallicrafters with the picture getting dark when the set heats up.


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 Post subject: Re: Philco 48-1000 - Brightness Failed
PostPosted: Oct Tue 10, 2017 8:52 pm 
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Location: Beaver Falls, PA. USA
I don't have the schematic handy, but on most older sets the brightness control varies the bias on the CRT grid. A few voltage checks around that circuit should pin it down.

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 Post subject: Re: Philco 48-1000 - Brightness Failed
PostPosted: Oct Wed 11, 2017 3:32 am 
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This set has the "background" control on the cathode. There's a 100K resistor between the arm of the pot and the cathode, and a 0.5uF cap to ground (but through a 10uF). I'd suspect that resistor and/or the capacitors.

The schematic and service information is on the ETF website: http://earlytelevision.org/tv_schematic ... hilco.html

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Mark Nelson
A collector of TV signal boosters and UHF converters -- God help me!
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 Post subject: Re: Philco 48-1000 - Brightness Failed
PostPosted: Oct Wed 18, 2017 3:34 am 
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UPDATE. Today I replaced R337, and R342. Both had previously (months ago during the restoration) measured out of tolerance, especially R342 which measured up around 850k ohms out of circuit. (Note: though out of tolerance months ago, the set *did* work very well, so I left them.) I also swapped out the 7C5 Video tube for a fresh one. Now the situation is worse - no picture at all. Before, it was dark but the picture *was* there.

To be perfectly clear: There is no picture displayed, but the picture tube lights up and the retrace lines are visible. Also, there is a bit of whine internally, reminiscent of when the horizontal frequency was all off early in the restoration when I was trying to just get a picture.

It seems to me that the picture tube is working and getting a signal, but that whatever is supposed to be putting the television picture into that signal does not have enough whatever - voltage, amplitude, attitude - to generate a bright picture. I suspect the video amplifier circuits/tubes and will be looking at those next.

I checked R344, the 100k resistor, and in circuit it measures within 10% so it looks ok. The brightness pot itself is also working - the multimeter shows it varying from 0 to 250k as I turn it.

If you have any guidance, I'm all ears.


Attachments:
File comment: All of the components around the brightness control have now been replaced, most during the restoration over the last couple of years, R337 and R342 just today.
Brightness-related resistors.jpg
Brightness-related resistors.jpg [ 123.13 KiB | Viewed 1161 times ]
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 Post subject: Re: Philco 48-1000 - Brightness Failed
PostPosted: Oct Wed 18, 2017 4:28 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 673
Location: Crystal Bay, NV
Check the voltages on the crt grids and cathode (yellow, green and orange on your schematic). You should find something wrong there.
======
Ron


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 Post subject: Re: Philco 48-1000 - Brightness Failed
PostPosted: Oct Wed 18, 2017 4:40 pm 
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Ron - as I haven't checked voltages too often, and this is the picture tube (they don't make these anymore you know), I want to be exactly sure I know how to check it. I assume that it is with one end of the multi-meter attached to the chassis/ground, and the other probe to the yellow/green/orange wire. Using DC voltage on the MM.

Also, what are the proper values? What are bad values? They are not indicated on the schematic.


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 Post subject: Re: Philco 48-1000 - Brightness Failed
PostPosted: Oct Wed 18, 2017 5:07 pm 
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Location: pensacola fl
check the voltage on the center terminal of the background control. It must go to 0 at max. You could check that the resistance goes to 0 measured from ground terminal. The way it works is the higher the voltage at this point the darker the screen up to and including picture tube cutoff. C333 if open will not allow the dc restorer to develop the grid voltage and the screen will be dark to off. If the 6AT6 or its socket is bad same result no or very low picture intensity. If you do not have a capacitor checker bridge in another cap in across it to see.


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 Post subject: Re: Philco 48-1000 - Brightness Failed
PostPosted: Oct Wed 18, 2017 5:18 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 673
Location: Crystal Bay, NV
Voltage checks are one of the best tools you can use, because it checks things in operation.
Yes, most check are taken relative to chassis ground.
The typical values are often given along with the schematic in accompanying material.
The value on the cathode (yellow) is probably in the range of 0 to 30 volts.
The value on the control grid (green) should be from 0 to 20 volts less than the cathode (yellow).
The accelerator grid is probably much higher, perhaps from 100 to 300 volts.

After you check and report your readings, I will take a few minutes and pull out the data sheets that will tell how you are doing. OK?
=====
Ron


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 Post subject: Re: Philco 48-1000 - Brightness Failed
PostPosted: Oct Thu 19, 2017 2:08 am 
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Location: Woodinville, WA USA
TahoeTV wrote:
The typical values are often given along with the schematic in accompanying material.
Some manuals show the voltages on the schematic, others in a separate voltage table. Either way, look carefully at any little footnotes at the bottom of your document. They usually tell you the correct line voltage setting (often 117VAC) and other test conditions, such as "X control turned all the way clockwise," etc. In a few cases, the measurement may be made with the negative voltmeter probe connected to some place other than chassis ground, and that will also be highlighted with a little footnote.

It's best to use a variac to set the line voltage at 117VAC or whatever else is specified. If that's not possible, measure the line voltage at your house and be aware that your measured voltages may be somewhat higher than what the manual shows.

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html


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 Post subject: Re: Philco 48-1000 - Brightness Failed
PostPosted: Nov Sun 05, 2017 10:00 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1269
Location: Phoenix, Arizona
You have a significant amount of smear in the picture which tells me poor low frequency response in the video circuits.

Video goes into the CRT on G-1 and brightness is a dc bias on the cathode of the CRT.

If you could generate a window test pattern you could trace the video amp and see where the square wave video window gets tilted.

If the IF alignment were messed up it could also mess up the low frequency response.

How well does it sync up for vertical hold.

Jim


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 Post subject: Re: Philco 48-1000 - Brightness Failed
PostPosted: Nov Sat 11, 2017 11:43 pm 
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Posts: 404
I've just spent some time with the TV after a couple of weeks. What I have to report:

- ARCING - There was a squeal, like when the horizontal frequency was way off. I just happened to look into the HV cage and there was arcing below the 1B3GT HV Rectifier tube. The arcing was exactly at point "2" on the schematic. Interestingly, the arc seemed to be between the socket of the tube right where R101 is soldered in, and the ceramic support for the tube. Strange - you can't arc to ceramic. I put some wood between the tube and the base, and it now runs with no arcing. Still no picture though.

- TUBES - I swapped out all tubes in Sections 1, 3, 4 and 5 (all except audio, which is working fine). The only two tubes I did not have spares for are the 6BG6G H Sweep Output, and the 6J6. But both of those are NOS tubes I installed in place of the originals that were in the set when I got it, so they are not old. (It's because I've already replaced those tubes that I have no spares). There was no improvement, I replaced all of the original tubes.

- EXCESS VOLTAGE? - After I replaced the last of the tubes I ran the set to make sure it was where it was before - sound, glowing picture tube raster, but no picture. That was indeed what I got. Interestingly, when I turned off the set then turned off the Variac, the GFI tripped on the outlet. This plus the arcing in the HV circuit makes it seem to me that excessive voltage is building up. Any thoughts? Note: it could be a Variac problem, the switch on it is not working quite right.

- VIDEO OF CURRENT STATE - see the latest video HERE. It shows the picture tube glowing with the raster visible, very very very dimly. It's rolling because I've fiddled with the vertical and horizontal hold knobs. Note: the picture is steady when viewed directly - videoing it with my phone makes it look like it's fading in and out. It does not fade in and out. It is a very, very dim raster, with some rolling that varies when I turn the H and V hold knobs.

- CHECKING VOLTAGES - You all have recommended that I check voltages while the sets running. I intend to. I hesitate because I've twice been shocked badly by this thing, and I've fried a multimeter. Sticking probes into it while it's on is daunting. I will get to it, wish me luck.


Attachments:
File comment: I took the chassis, picture tube and speaker out of the cabinet for testing. What a chore!
Back on the bench.jpg
Back on the bench.jpg [ 165.36 KiB | Viewed 653 times ]
File comment: There was arcing from the point where the HV cable is soldered under the socket, to the ceramic support post. I put wood in between to prevent the arcing while I test.
Was arcing.jpg
Was arcing.jpg [ 186.83 KiB | Viewed 653 times ]
File comment: The point marked "2" at the lower right is just about where the arcing was occurring.
Test Point 2.jpg
Test Point 2.jpg [ 118.95 KiB | Viewed 653 times ]
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 Post subject: Re: Philco 48-1000 - Brightness Failed
PostPosted: Nov Sun 12, 2017 12:50 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 673
Location: Crystal Bay, NV
First of all I must say that is a very clean chassis.
The arcing you report is a common problem. The voltage is about 7,000 at that point, and although it is not lethal (only about 100 microamps current), it can give you a small burn and cause you to jerk your hand and cause other injuries.
Ceramic is a very good insulator, but it can be contaminated with dust, dirt and whaterver. Clean everything with alcohol. Any sharp solder connections on the resistor can cause the air around it to conduct (yes, air will conduct) and arc. There are paints/cements that can be used to stop the arcing, but I think that cleaning and smoothing any connections will do the job.

The real danger is in the other power circuits (300-400 volt range). When using a probe, keep your other hand away from the chassis and any other metal surface.

Don't give up on the voltage checks. Go slow and use only one hand.
I use an old VTVM (vacuum tube volt meter) which is practically burn-out proof. Never try to make voltage measurements around the 1B3 tube, the top of the 6BG6 or some of the connection on the flyback transformer. Best to stay way from the high voltage cage if you can.

Forget the Variac, it is not needed now. The idea of voltage building up does not fit this situation.
=====
Ron


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 Post subject: Re: Philco 48-1000 - Brightness Failed
PostPosted: Nov Sun 12, 2017 4:54 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 673
Location: Crystal Bay, NV
I looked at the video, but I really didn't see any crt brightness.
One thing you might be overlooking is the ion trap on the crt neck. This must be moved around until you find the best brightness on an a full raster. Have you done this?
=====
Ron


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 Post subject: Re: Philco 48-1000 - Brightness Failed
PostPosted: Nov Sun 12, 2017 6:57 am 
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It might have started arcing because there's no beam current in the CRT to draw the HV down, brightness goes down HV goes up, at least some.

Do you have a CRT tester to be sure the CRT hasn't opened up on one of the elements? and have you checked to make sure the HV is actually reaching the Anode on the CRT?

And as Ron mentioned, perhaps you bumped the Ion trap while removing the CRT?


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 Post subject: Re: Philco 48-1000 - Brightness Failed
PostPosted: Nov Sun 12, 2017 7:54 am 
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Also doesn't this model use an electromagnetic ion trap? Is it possible that coil opened up or it somehow got disconnected?


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 Post subject: Re: Philco 48-1000 - Brightness Failed
PostPosted: Nov Sun 12, 2017 10:57 pm 
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I have not, ever, moved any of the components on the CRT around. They have all been solidly in place since I got the set, and I was able to get great performance out of it, so there was no need. This latest problem presented itself without the TV having been moved or anything, so the tube wasn't bumped. After I assembled it in the cabinet this spring, it worked flawlessly for 4 or 5 months, then one day it was very dim, the next day, and still, there is no picture at all.

One of my fears here is that it is the CRT that has gone bad somehow. I've not worked on the tube, and don't want to screw it up. I'd also like to keep THIS particular CRT. As this is the set my family had back in the day, a different CRT would make it much less the "same" set. Of course if it's dead I have no option.

I do not have a CRT tester. How would I go about checking the CRT, without risking damage to it?


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 Post subject: Re: Philco 48-1000 - Brightness Failed
PostPosted: Nov Sun 12, 2017 11:06 pm 
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Measuring the voltages at the CRT socket and seeing how the voltage at the cathode and grid vary with the background control will tell us if the CRT is being driven correctly. If it is not being driven correctly then the CRT is probably OK and you can proceed to track down the cause of the bad drive.

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 Post subject: Re: Philco 48-1000 - Brightness Failed
PostPosted: Nov Sun 12, 2017 11:40 pm 
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Thanks Tom - how can I do this? I have a high voltage probe, pictured below. Where do I clip the black wire, and where do I put the probe? Clip to chassis, probe at point two - i.e. the pin under the HV rectifier where the wire leaves to go to the CRT? I should measure 7300 right, like it says on the schematic, correct?

Also - I measured R101 in circuit and it was high but not way way high. It is also not shorted. Though the arcing was right next to it it seems to be ok. Should I try replacing it? Would that be a possible cause?


Attachments:
HV probe.jpg
HV probe.jpg [ 169 KiB | Viewed 592 times ]


Last edited by sdyer on Nov Mon 13, 2017 12:08 am, edited 1 time in total.
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