rca_14pd8053 Strange noise

[jr_tech]

Since the sound IF signal is taken from the plate of the video output, and the sound is ok, wouldn’t that imply that the video chain up to that point (diode included) is functioning ok?

jr

[Electronic M]

Originally Posted by jr_tech

Since the sound IF signal is taken from the plate of the video output, and the sound is ok, wouldn’t that imply that the video chain up to that point (diode included) is functioning ok?

jr

The diode rectifies the AM carrier to make video and passes the FM carrier for sound....if the diode shorts it won't make video for the video amp but the sound will still flow down the chain normally....heck if the diode opens and passes just enough signal through capacitive coupling to the video amp you could still get sound.... video detector fails are fun.

If you.can inject and watch video on the vid amp side of the diode then you know the diode failed.

[Jon1967us]

Originally Posted by Electronic M

The diode rectifies the AM carrier to make video and passes the FM carrier for sound....if the diode shorts it won't make video for the video amp but the sound will still flow down the chain normally....heck if the diode opens and passes just enough signal through capacitive coupling to the video amp you could still get sound.... video detector fails are fun.

If you.can inject and watch video on the vid amp side of the diode then you know the diode failed.

Should be easy to test the diode under normal conditions, except that it's near impossible to get to. I can't really get behind the board to unsolder the can. Really thinking of a brute force way of getting to it, IE a Dremel or something Would like to avoid spending upwards of 4+ hours unsoldering everything needed to free this board and then resoldering everything...but given the long weekend I just may have to bit the bullet.

I have plenty of germanium diodes on hand.

By injecting the video signal, could I plausibly take the yellow video out from the D to A converter output? would that be the right method? I've never done it before.

I'm aware of the Tube socket adapters. The problem here is that there is a metal cover, so one would need to pry that off to fit an adapter in.

[Electronic M]

Originally Posted by Jon1967us

Should be easy to test the diode under normal conditions, except that it's near impossible to get to. I can't really get behind the board to unsolder the can. Really thinking of a brute force way of getting to it, IE a Dremel or something Would like to avoid spending upwards of 4+ hours unsoldering everything needed to free this board and then resoldering everything...but given the long weekend I just may have to bit the bullet.

I have plenty of germanium diodes on hand.

By injecting the video signal, could I plausibly take the yellow video out from the D to A converter output? would that be the right method? I've never done it before.

I'm aware of the Tube socket adapters. The problem here is that there is a metal cover, so one would need to pry that off to fit an adapter in.

So there are no holes in the metal under the PCB to access the traces? I'm not familiar with that model, but many sets (even some members of the much maligned Philco Predicta family) have some holes under the board where with a really thin iron and the correct angle you can access most of the solder pads.

On sets where you can't get at the pads you need, if you are strategic on which wires you unsolder you can hinge a side of the board up something like 30 degrees to access all the pads by only disconnecting a small fraction of the wires.
Shielding is important in operation...If you damage the can you will come to regret it.

Test adapters are typically the same diameter as the thinnest tube that plugs into the socket so as long as it is taller than the shield there should be no problem using it...Techs needed their adapters to work ANYPLACE the tube would so if a company made one that didn't they'd get some harsh comeuppance from their customers. One thing you can do if you need an adapter fast and have a spare socket of the right type and a dud tube that fits it, is make your own adapter...Carefully bust all glass but the base, cut all leads inside the dead tube as far from the base as feasible and solder wires (3" is probably a good length) from them to the socket keeping the lead order correct.
A lazier way if you have several good spare tubes of the right type that you don't mind killing is to solder thin wires to the top of the pins for test points...If you heat too long the glass will crack and the tube will die of vacuum loss.

Video injection: With tube era test equipment like a B&K 1077 it is usually a matter of connect and done (the equipment has protection/isolation circuits in place to prevent damage).
With a modern signal source like a converter box plug and play can destroy your signal source unless you perform prior due diligence checks and use care.
First check: is your sets B+ and heater system powered through a transformer? If not you need an AC line isolation transformer to power the TV (and only the TV) thru before you can safely proceed.
Next (regardless of the results of the first check): is there a sizable AC or DC voltage between your sets signal ground and your converter boxes ground jack...If yes stop and get a TV service video generator or use a converter box that you don't mind possibly killing.
Now you can connect the outer shield of the converter box yellow video cable to the signal ground. Then use a .1uF 600V capacitor in series between the center hot lead of the video cable and your injection point....The capacitor couples the video signal to the injection point and blocks any harmful DC that could exist on your intended injection point (or from any adjacent point you might accidentally touch the lead to).

Also, bear in mind a converter box has standard video level and video polarity, and some TV sets do not use both at once at any valid injection point...That may result in an inverted monochrome picture, weak or overdriven video, and or no deflection sync (the synch separator relies on the right polarity and a decent approximation of the right level of video to function).
A proper TV test pattern generator for servicing has a video output that is adjustable for both polarities and amplitudes far greater than a converter can produce.
If you encounter a set with 2 video amp tubes the grid of the first may have negative video and the plate of the first (and grid of the second) may have positive video while the plate of the second has negative video again...Or every polarity is opposite. The waveform leaving the plate of a tube is always inverted polarity from that entering the grid. The polartiy that comes out the video detector diode is determined by the (even or odd)number of IF stages (as well as the correct polarity installation direction of the diode). So the number of IF stages dictates whether you get composite video standard polarity (but not necessarily amplitude) at the detector or not.

[Notimetolooz]

Originally Posted by Electronic M

One thing you can do if you need an adapter fast and have a spare socket of the right type and a dud tube that fits it, is make your own adapter...Carefully bust all glass but the base, cut all leads inside the dead tube as far from the base as feasible and solder wires (3" is probably a good length) from them to the socket keeping the lead order correct.
A lazier way if you have several good spare tubes of the right type that you don't mind killing is to solder thin wires to the top of the pins for test points...If you heat too long the glass will crack and the tube will die of vacuum loss.

Have you had good luck doing this? I tried soldering to the leads inside a miniature tube and the solder didn't take. I believe the leads are nickle alloy or something like that which lead-tin solder doesn't work with. Maybe with a special solder, flux and a torch.