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  #1  
Old 02-10-2019, 02:33 PM
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AlanInSitges AlanInSitges is offline
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Somebody give me a little nudge? Philco with no video

I've hit a bump repairing an early-60's Philco. It seems like there is enough info here that it should be evident where I need to look, but I'm not seeing it.

I have a raster with no video or snow, with bad v. linearity and, like a foldover about 1/4 of the way down the screen:



If I inject video at point B, the grid of the video output tube,



I get a clear, sharp, and completely geometric image with no foldover line, that is to say it's not just video that appears but the vertical sweep issues seem to disappear as well when a video signal is injected:



I have verified that the video detector is OK, or, at least checking like a germanium diode with a 2.65v drop one way and open the other.

I have tried an IF signal on the grid of the 2nd IF as well as the 1st IF tubes, with no visible effect (full disclosure my IF source is variable with a tiny analog meter to set the frequency so I'm pretty much throwing rocks in the dark, though sweeping up and down around where it should be is giving me nothing at all).

Plate voltage on both IF tubes is about 25% high, but then so is the B+ - that 145V source at point 47 measures 182V. That 2nd IF cathode is right on the money at 1.5V.

All the resistance measurements for V1 and V2 according to Sams are pretty close.

To complicate things, when I put my meter probe on the plate of V2, the screen darkens quite a bit and the vertical foldover/linearity problem becomes more pronounced:



When I put my meter probe on pin 1 of L7, at the output of the detector, the screen goes black and the vertical shapes right up, looking pretty much like the crosshatch above but without the, um, crosshatch.

As I said, I feel like I should have enough information to know next steps, but I don't. It seems like the weird vertical problem should be a clue to the lack of video, given that it goes away when pretty much anything happens on the grid of the video output tube, including touching my meter probe to it.

I don't know enough about IF and RF to understand how my probe may be affecting the behavior of the circuit, especially on the plate of V2.

Can anyone point me in the right direction? (Edit: V1 and V2 are new)

Thanks.

Last edited by AlanInSitges; 02-10-2019 at 02:40 PM.
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Old 02-10-2019, 04:54 PM
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The most likely suspect would be "M4" , but you say you checked that . I presume all the voltages around the IFs are within spec ?
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Old 02-10-2019, 05:40 PM
zeno zeno is offline
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The vert is probably not "right" & is oscillating way off freq without
sync. Just a red herring IMHO. Deal with that after the video is fixed.
Detectors went a lot up to the mid 60's sets & if we suspected it just
changed it, they were cheap........
The high 145 V makes no sense unless it comes from a voltage divider.
With set running pull the 2nd IF tube & see if it causes noise in
the pix as the pins make/break contact. Work back or forward from
there.

73 Zeno
LFOD !
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Old 02-10-2019, 09:59 PM
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Nudge

I assume you are using the 1077PAL as your IF source...Have you tried connecting it to the first IF of another set that HAS working video and IF to confirm your IF injection source works? Also sometimes the analysts don't have accurate IF freq calibration or a set does not respond where you think it should. I often sweep through the entire dial range. Set the output as high as it will go when injecting IF...If it is too strong it won't do damage and you can turn it down, but if the output ain't set strong enough diagnosis gets tricky.
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Old 02-11-2019, 04:57 PM
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Thanks Tom and and guys.

I tried zeno's suggestion of pulling the 2IF tube, and...it doesn't have any visible effect at all.

I've re-checked M4 and it does ohm out as it should. I don't have a replacement readily available so I guess it's time to take apart another set and pull one out. This relates back to the other half of the problem, I have nearly a dozen non-working TVs here, none of them are useful to check the IF frequency (which is coming from a Philips 5509 btw, the 1077 is...not working yet). Since the (er, a) problem seems to be between V2 and the grid of V3A, I checked continuity of all the coils along the way, checked to make sure C22 isn't shorted, and verified the resistance of L7 and L6. Nothing off. I re-checked the resistance for V1 and V2 against the Sams and it's all as it should be.

One thing that't curious is the high plate voltage on those IF tubes. The main B+ at the choke is only 5V high, and the big dropper resistor is a little bit low but not much. It's weird to see that voltage way up there, like the tube isn't conducting at all.

At this point I don't know where else to go aside from the diode, and haven't checked the new IF tubes. That's first on the list for tomorrow, followed by changing out M4, and then maybe subbing a tuner somehow if I can find one that works.
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Old 02-11-2019, 05:43 PM
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Another thing you can try is (IIRC you have a scope) to inject signal at one point and follow it down stream with a scope.

Part of me wonders if the IF tubes are biased right...I once had a portacolor with a tin whisker shorted AGC pot where the IF grids seemed reasonable but were actually biased to cutoff. It took manually biasing the AGC to get the set to poorly pass signal, then I shifted focus to the AGC and it started working good after I got the AGC sorted.
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Old 02-12-2019, 09:06 AM
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Quote:
Originally Posted by AlanInSitges View Post

I have verified that the video detector is OK, or, at least checking like a germanium diode with a 2.65v drop one way and open the other.

I don't know enough about IF and RF to understand how my probe may be affecting the behavior of the circuit, especially on the plate of V2.
Results when testing that diode with a meter depend somewhat on the type of meter, but this probably is a germanium diode and so the voltage in the forward direction should be more like 0.25 to 0.5 V.

Is your meter a VTVM or a DVM? A typical DVM or VOM meter will add capacitance to the point the probe touches. This effects the IF/RF level. The usual VTVM has a 1M ohm resistor in the DC probe. This resistor isolates the probe tip from the capacitance of the meter and the leads, making the capacitance loading much less.
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Old 02-12-2019, 02:19 PM
Phil Phil is offline
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I would bet on a bad diode. They more often than not seem to be bad. The voltage drop you indicate in the forward direction is many multiples of normal. I would also suggest using a scope to see if you have video at the stage prior to the detector.

I have used both 1n5711 and BAT41 diodes in such circuits with good results.
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Old 02-15-2019, 12:34 PM
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AlanInSitges AlanInSitges is offline
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Well guess who's got working video!

Kinda.

I swapped out the detector diode and have some snow on screen and in the audio. I can't get anything from the tuner though, nor feeding IF, and the screen's just gone black so will have to track that down before I can go any further with the rest of the set.

But hey, little victories.

Thanks everyone.
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Old 02-28-2019, 03:47 PM
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Bringing this back to life. The thread, not the TV.

So, I have a bit of snow on the screen. It doesn't look like regular snow, but still, it's more than a white raster I had before now that I changed the diode.

I've been all through the IF stages without seeing anything amiss, and I'd like to try injecting a signal at the various stages and see if I can pinpoint where the failure is.

When I connect the RF output of my Philips generator to pretty much anywhere in the IF strip, the screen goes black. I suspect it's either impedance or capacitance adding to the circuit that's causing this to happen.

So, are there any best practices for coupling this thing to say the grids of the IF tubes?
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Old 02-28-2019, 08:00 PM
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Going black is likely signal overloading. Does your generator have an attenuator or output level control that you can turn down?

My B&K in lieu of the correct missing cable has an x10 scope probe connected to it's RF out which I use for the injection connection...Output ain't as strong at max as it could be (and sometimes I wish I had more gain), but it never blacks things out...

Capacitively coupling your generator to prevent DC grid bias changes from probe connection is advisable too.
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Old 03-05-2019, 09:21 AM
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Thanks for that Tom.

I've been round and round with this a few different ways, with a scope probe, with a coupling capacitor, disconnecting the IF cable from the tuner, and just get no sign of anything.

I decided to take a working set and try the generator on that to narrow down the problem. I'm using a solid-state Sony TV-110. Feeding an RF signal from the Philips generator (a UHF channel 26 aprox), I can tune it on the Sony no problem, so I know the generator's RF output is working.

I've followed the instructions, such as they are, in the Philips manual, for injecting IF, which says to connect the RF lead, without matching transformer, through an isolating capacitor, to the input of the first IF stage, and tune the RF output to the receiver's intermediate frequency, which in this case is 38.9MHz. For reference:



Connecting the Philips generator's RF output to the Sony's IF input, I get no sign of anything on the screen except for some faint snow. Now, I know this IF input is working because I can put the tuner back on it and it works as expected. I have tried using a scope probe on X1 and X10 in place of the generator's RF lead, I have put a couple of different values of capacitor (68nf, .1mf) in series with the input, and sweeping from the bottom of the generator's range way up to the top, there is absolutely no sign of any signal - this is the same behavior on the Philco set that is the subject of this thread.

The generator has two low-band (which include IF) presets, a high-band preset, and two UHF presets. I've tried on both of these low-band presets with the same non-results. The UHF preset, as mentioned before, works as expected.

It is looking to me like maybe the generator isn't working on the low band. Is there another way I'm not thinking of to verify its operation (my scope only goes to 20MHz)? It sucks not being able to trust your tools.

If I'm unable to to verify that the generator is working correctly, I'm thinking about trying to patch the tuner IF output from the Sony to the Philco's IF input. Is there anything I need to keep in mind about that approach? Just strip the ends off of some coax and go to town?

Thanks again to everyone who's been helpful here.
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Old 03-05-2019, 07:18 PM
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Quote:
Originally Posted by AlanInSitges View Post
Thanks for that Tom.

It is looking to me like maybe the generator isn't working on the low band. Is there another way I'm not thinking of to verify its operation (my scope only goes to 20MHz)? It sucks not being able to trust your tools.

If I'm unable to to verify that the generator is working correctly, I'm thinking about trying to patch the tuner IF output from the Sony to the Philco's IF input. Is there anything I need to keep in mind about that approach? Just strip the ends off of some coax and go to town?

Thanks again to everyone who's been helpful here.
No problem.

If you supply a higher freq carrier than designed into your scope you can still glean some info. You won't be able to see individual cycles of the carrier wave or measure it's frequency (unless you have a digital frequency counter built into your scope), but you will be able to see it as a nice fat trace...The thickness of which will be modulated by the AM video carrier. You can check carrier presence, amplitude and modulation.

May want to clean the switches that control the output mode/frequency...Old push buttons can get tarnished contacts that won't work in some positions till cleaned.

Things to be aware of in tuner patch overs: If one set is hot chassis or has the chassis at some DC potential connecting the sets together could cause damage if you don't connect one and only one set to a line isolation transformer. Also, many sets especially tube era sets instead of having a discrete B- lead on the tuner instead used the shield on the IF coax as the DC return path for the B+...If you unhook the IF lead the tuner may loose power (unless you clip the IF shield to it). Lastly, if the set with the good tuner has its IF disconnected it will have no signal and may react to that by setting the tuner AGC line to a less than ideal value.....If you can navagate those pitfalls then using another sets tuner as an IF source is possible.
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Old 03-12-2019, 04:32 PM
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AlanInSitges AlanInSitges is offline
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Who's got a Philco with picture and sound?

THIS GUY!

After Tom's guidance about the generator, I decided to take it apart and see what could be amiss. And boy did I get a surprise. This thing had clearly been dropped, or kicked, or maybe fell off a building at some point. The RF PCB has several cracks in it and what looks to be a clumsy attempt to repair it. Also all the screws were missing inside. No wonder it only worked halfway.

I thought for a few minutes about whether I wanted to take that on, hopped on eBay and found a newer all-digital Philips multisystem generator with a guarantee, and the seller accepted a 2-digit offer. It arrived today from Poland, double-boxed, wrapped in plastic. Not five minutes after I opened the box I had it connected to the IF input on the Philco, punched up 38.9MHz, and a picture on the screen. Modulating a signal from a DVB tuner I have a decent picture and sound. So the IF section is good!

On the other hand now I have to fix a broken tuner that has never tuned a channel in its entire life (these were UHF-only, even though they had a VHF tuner). That should be a blast!

Thanks again to everyone for the patient help (Tom especially). With luck I won't be back to ask more questions before I'm ready to show off my first TV project.
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Old 03-12-2019, 09:45 PM
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Glad to help.

One bit of advice on VHF tuners: in US designs (don't know about EU stuff) they turn into an extra IF stage to pass the UHF signal...
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