Adding Composite Input (Capacitor Question)

[ARC Tech-109]

Reason for the sync sep chip is so I can use non standard or poor sources and maintain a good lock, I also run everything on a master BB source and an external/internal sync switch was in the works.
Shading seems to defiantly be in the XZ demod section. I thought it was input video related but noticed on some scenes things appeared to have this sort of trailing color smear along with very intense reds and blues. The 6MK8 checks out and I have several new examples of this tube along with the 6HS8 demod. I can turn the color control all the way to zero and even short the pot to ground and I still get a shading that varies with the tint control. If I pull the 6HS8 or 6GH8 3.58 osc tube the shading goes away, where or how this is leaking in has me stumped.

[kf4rca]

Defiantly, you say? That shading is definitely something coming across on your supply rail. Most likely your demod and 3.58 oscillator tubes share the same supply source. Scope it out. You most likely have a bunch of electrolytics that are weak in that set.
I want to see a scan of the schematic of the video detector and the first video stage. Can you post that?

[ARC Tech-109]

Originally Posted by kf4rca

Defiantly, you say? That shading is definitely something coming across on your supply rail. Most likely your demod and 3.58 oscillator tubes share the same supply source. Scope it out. You most likely have a bunch of electrolytics that are weak in that set.
I want to see a scan of the schematic of the video detector and the first video stage. Can you post that?

All the old 'lytics were disconnected and subbed with radial leads of the same or higher values but maybe it's a case of ESR, I will add some .005uf bypass caps on the boards where the B+ comes in and see how that goes.

I can post the schematic sure but will have to take hi-res pics to do it, would you like an email?

[kf4rca]

You can try that. Those call letters at Gmail.Com would be good. But if its a very large file, it won't make it thru the email system. Can you convert it to a pdf? I think Linux can do that.

[ARC Tech-109]

I will snap several detailed sections so you can see what's going on here. Give me a few days to get this all together. This morning was our official fishing season opener and a total bust for me. Cool & windy across Knife Lake.

Added some bypass caps to the DC feeds going to the various sections but the shading remains, going to put another day or two into this before I sideline things for a while as I'm getting burned out and need a break. I am starting to wonder if my add-on caps are bad/leaky/high ESR as I'm getting a very well defined 120hz hum, the hum bars are very square and distinct not rounded like a bad capacitor normally shows so later this evening I'm going to cliplead a couple of known good caps in and see where that goes. Never noticed this until the AGC issue was ironed out as it was masking things with its own issues. Set uses a full wave bridge not a doubler and a capacitor input choke so maybe there's some resonance with a failing cap, the original "cans" are out of the circuits and only plugging the holes and I'm using what was supposed to be low impedance Rubycon 450V photoflash caps (EC series) but they're now 20 years old having been installed the summer of 2006. I don't see more than a slight bit of ripple on the post choke side on the scope, manual shows 2V of ripple on the main 410V source to be acceptable so what I have is far below this. A possibility is heater/cathode leakage in V703 which is the AGC/sync sep 6GH8 so I will also sub this with another... I have over 50 new & used pulls to choose from.

On the plus side of all this the IF sections appears to be on the money and the picture is about as good as I remember it minus the 120hz bending. Vertical is running a bit short despite swapping new tubes but overall I'm happy with the progress and may stay with the RF distribution using the CATV head-end Blonder Tongue modulator, its a CAMS-60 MTS modulator with an IF loopthru and I was able to use the IF from this to verify the Magnavox. Not exactly outlined in the service procedures but it works. I feed the modulator directly from a Microtime T100 TBC so everything is about as clean as an actual broadcast.

[kf4rca]

Those were some very large files. Surprised they made it thru the email system.
Attached is a drawing of the connection I would use. You should go in right after the video diode. Disconnect the anode of the video detector diode and connect a 2.2 uf capacitor. Then connect your video source to that. Be sure to terminate into a 75 ohm resistor.
That's a very old set. You probably have some other problems. I hope you have a good supply of tubes as they are drying up!

[ARC Tech-109]

I measure my tube inventory by the 25 gallon tote however my stock of 6LQ6's are getting thin. I was a Motorola radio tech for the better part of 25 years and accumulated a lot of RF service tubes along with the usual 6GH8s and related low signal TV tubes which go back to my youth messing with TV's.

The problem with the Mag is it's looking for greater than 8 volts P-P on the grid of the 6LM8 pentode, the schematic calls out a volt or so but so far experience says otherwise. I made an op-amp based input interface with 10db of gain using an LT1252 video op-amp using the mfr example schematic however it was hitting the rails at +/-12v driving the 1st video amp crushing the peaks.

I may pick up the project again over the weekend if the replacement tuner shows up, USPS promised a friday delivery however that was pushed back to saturday. I used to be a rural route carrier and know low priority packages are not delivered on the weekends around here.
The big picture plan is to get the replacement tuner in place, run thru the IF once more with the marker gen and bring everything back to "stock" as I've made so many mods over the 40+ years I can't remember them all. It's an original family set and I inherited it in early 85 when it was replaced in favor of a 27" Zenith S-3 however this one died from a shorted K/G1 short on the red gun and ran in this condition overnight. It had the A/V inputs and was left on with only black/burst and no one noticed the set was on, Magnavox won the longevity battle.

[kf4rca]

Yes, V5a grid (pin2) is looking for a volt of video. Why do you think the schematic is not correct? Did you see that circuit that goes off to the Brightness control? I would investigate that. Check the plate and screen voltages of V5a. Verify the peaking coil is not open. That would screw up the bias on V5a.
Maybe a problem further down the road.
How many tubes are in that set? Looks like a well designed set to me.
I once worked at a radio shop. Did mostly installation into cop cars. At that time, it was Mocom70, Micor, and Maxar radios. Syntors had not yet come along.

[ARC Tech-109]

No the schematic doesn't reflect the same voltages as the T933 uses the 6LM8 for a video amp and my T940 uses a 6MU8. The DC bias from the brightness pot is present. My T940 print isn't nearly good enough for a photograph.

So the replacement tuner arrived late Saturday afternoon and installed without a problem but now I'm chasing down another that really has me scratching my head. Tuner has two B+ sources, #1 is the +130V source derived from R240 which is an 8200 ohm 1W on the supply divider board and this feeds the plate circuit of the 6HQ5 RF amp, the other is B+ #2 which is 140V supply that feeds the 6HB7 mixer/osc and gets dropped further to run the UHF tuner when selected. I'm finding +235 volts on the #1 B+ source and it seems to be from a lack of loading, the resistor itself checks good on the meter at 8225 ohms. I've swapped three known good 6HQ5 triodes and nothing changes, these are the ones that have the cathode tied to pins 2 & 7 via thin ribbon inside and are easily damaged by careless testing. I know everyone is passing a signal as I'm feeding in a known good IF signal into the UHF port of the VHF tuner and selecting UHF and if I feed from the UHF tuner I get plenty of mixer noise which is not present on any of the VHF channels.
This is a known good tuner so I' at a loss with this. My test IF signal is from a 80's vintage Panasonic 11" color on an iso trans and what I see on this set is perfect, I tapped the IF out of this tuner and feeding a dedicated jack I use for testing.

The original issue with the tuner was an accidental contact of the AC terminal of the little fuse block near the IF input with the tuner IF cable and that popped a number of parts in the tuner, thankfully another member was able to send me a spare replacement. The IF cable is good showing continuity and no shorts, it's passing the test IF to the chassis IF strip and all is well but it's like the RF osc is dead or way off frequency due to the elevated voltage which itself doesn't make sense. All the grounds are good and the AGC bias is right on the mark at 0.45v. According to the schematic the signal path in UHF starts at the little RCA jack on the side and is then coupled thru the RF amp thru the turret, from there it's coupled to the mixer (pentode side) of the 6HB7 which is acting as a second IF amp before it makes its way down the IF strip to the detector. IF AGC action is good and I can easily push that to an overload or cutoff so there's plenty of signal making its way thru and I know the Panasonic IF is spot on as they use a SAW filter to set the response and the magnavox matches this.

I'm at a total loss to why the tuner B+ one isn't being loaded down by the RF amp. Gone thru three known good 6HQ5 triodes and no changes.

[kf4rca]

Check for a broken pin in the socket or an open cathode circuit.
If it worked before, it can work again. An old CE once told me that. One time I pulled a fried rat out of his TT25BL transmitter. He came into the control room and looked at me and said, it's a dirty job, but somebody has to do it.
Probably wasn't a good idea getting him around all that high voltage after he had just gotten back from the Gold Bug Room.

[ARC Tech-109]

Been in a few FM transmitters in my day, had a rookie tech get mad at one and throw a wrench into the power supply not expecting it to come back and hit him in the face, took his teeth out. Yes anything can be made to work again and my philosophy is if it worked from the factory no reason it won't work again today.

So that being said the replacement tuner has resulted in little progress, it does pass a signal from the little UHF input RCA on the side thru the tubes and down to the IF input on the chassis however that's about it. All the solder connections in the tuner itself are good, the 6HQ5 RF amp and 6HB7 mix/osc check good on the tube tester but I bought some NOS tubes on the 'bay and will give them a try upon arrival. My supply of these is limited to a few in a storage tote and while they do check good on a Heathkit IT-17 something may very well be "off" enough to cause a problem. I've had the IT-17 since middle school (1982) and trust it however being little more than a "diode test" of the tube there could be something like a burnt grid wire or a spot on the cathode that wouldn't show on the meter yet could kill the local oscillator. I got the IT-17 new in the box unassembled at the Medina MN hamfest for $10.00

Until then the remaining repairs will be done using an outside IF source provided by one of those tuner units that was part of a Panasonic portable VCR combo, still need to iron out the weird phasing and chroma shading issues that I suspect might be coming from the TAC board. If you remember these had the "Total Automatic Color" feature that basically added more red & yellow giving everything an orange cast, great if you like burnt orange shag carpeting but it's not my favorite and I may just bypass the board all together.

Will post more when I can.

[damen]

The RF amp seems very simple. 8200 ohm resistor from 270 B+ into tuner, then 1200 ohm resistor to the wiper contacts for the tuner drum, then the 6HQ5 plate. If B+ is not being dropped I would suspect those wiper contacts not making connection. Without the 6HQ5 plugged in, is there B+ on pin 5, measured form the top of the tuner? With set turned off, what resistance is there between pin 5 and B+ 1 source ?

[ARC Tech-109]

I get the 1275 ohms between B+1 and pin-5 of the 6HQ5 and without the tube in place I get the full +285 volts of the power supply so there is a path to the plate. The AGC here is about 0.45V and that's consistent with the schematic, both pin-2 and pin-7 show continuity and I get about 55% on the tube tester, not stellar but it should pull some current.

According to the schematic when tuned to UHF the input from the UHF tuner is coupled directly to the grid of the 6HQ5 and from there gets passed to grid-1 of the pentode section of the 6HB7 osc/mix and that plate is coupled to the tuner IF output coil and out thru a .001uf cap. Feeding in a weak IF from an external source gets amplified and sent to the IF strip so I know things are doing what they should which doesn't make much sense considering how little current the RF amp tube is taking.

I have two NOS 6HQ5 and 6HB7 tubes on the way and once verified in the tester they're destined for the sockets, until then I have a color issue to iron out in the 3.58 chroma bandpass amp and this isn't the first time for the problem.

[Electronic M]

If tube subs don't do much and voltage checks don't give an obvious issue (if you have a SS socket saver with test points that usually is the best Best way to verify the socket) then my next step would be to look for carbon comp resistors that have drifted high enough to be out of tolerance (low out of tolerance is usually something else in the circuit in parallel with it confusing the meter).

[ARC Tech-109]

Yeah the one plate resistor has only drifted a few ohms off the mark so I'm not to worried about that, the 8200 ohm metal film on the supply board is a few ohms high as well and the only unknowns at the moment are the tubes themselves and tracking shows their arrival on Thursday... we'll see about that.
Over the years most of my servicing was done on the RCA and Zenith sets so I have very little for Magnavox specific parts like the 6HQ5 despite being a family TV for 56 years, too bad Magnavox didn't use the Nuvisters as I have plenty of them.