Dumont RA-110 from ETF

[wiseguy]

OK
Calm down
Drink a beer or 5
Have Volt/Ohmmeter probe in hand
check all Wire wounds, check Voltages on power supply Caps for a Start
Have another Beer
Visually Check the Caps in the Damper and audio output to make sure they are correct Values and VOLTAGE Ratings
Also Remember that soft Start Circuit with Relay does it operate? I just sent someone here a good one

[holmesuser01]

I was thinking about Candohm resistors. Does the 110 use them? They were all bad in my RA-113.

[Kamakiri]

Well, when I say "start from scratch", I don't say that in a panicked way, just basically means that I pull the chassis and start testing some components. First on my list are the candohms. I replaced them all in my RA-113 as well. Even if they're not bad initially, I've had them fail or drift badly after time.

[bonanzaman]

Still want to shake this guy's hand?

[Kamakiri]

Sure, with a charged capacitor tucked neatly between my fingers

I know what happened here. DuMont caps are the gooeyest things in the world, and my bet is that he didn't realize that the goo on the wires isn't exactly something that promotes solder adhesion. The filter cap connections are fine, along with anything that had a wire connection. When reconnecting everything, I just sanded the old chassis wires with some folded 400 grit paper pulling it forward over the wire, and used some flux. All of the connections are nice and solid now.

With the BANG I heard, I would have guessed that the result would be obvious, but it's not.....at least not to the extent that I've examined it. The chassis is on the bench now, but attending a wedding and getting the old van ready to pass state inspection took precedence this weekend

[Kamakiri]

Well, made *some* progress......

It missed my attention the first time around, but C3A was somehow disconnected. The previous tech used these funky bits of waferboard with copper coating on either side to turn the old positive terminals on caps into mounting points. Not exactly what I would do, but it is what it is.

I saw this pair of filters sitting near the terminal, and they didn't LOOK as if they were disconnected, but apparently they were. They're each 1000 uF 50V. What made me scratch my head is the values on them. I traced out the entire circuit and decided that it must be the replacements for the values that are stated on the Sams, which I can't make heads or tails of. See for yourself.

Anyway, it's connected now and no change in the set. Onward to the candohms.

[bandersen]

You can plug those numbers from Sams into this reactance calculator.

http://www.sengpielaudio.com/calculator-XLC.htm

1.5 ohms @ 120 Hz is about 884 uF which you could round up to a standard value of 1,000 uF. They are used as filter caps on the horizontal and vertical centering controls.

[Kamakiri]

Checked all the candohms and larger resistors (wirewounds). All in spec.

Brought me to thinking about the power relay under the chassis.

When I was cleaning up the mess of connections I discovered, there were 2 terminals on the relay that were bridged with a couple pieces of wire from the multistrand wire going to the relay. Thinking that it was just a sloppy job, I separated the wires, but now I find myself wondering if that was what caused the set to actually work in the first place.

One of the wires that was connected goes to pin 8 of the 5U4s, the others I can't see where they go, but I assume that those two complete that particular circuit. From what I can find, the relay should show 5000 ohms across the coil, I have 3800.

If this relay were dead, and with those strands of wire connected, could this have caused the set to work before, and not now?

[Bill Cahill]

Those caps still look under rated to me. Anything less than 1KV is trouble waiting to happen. One may have blown.
Further, I doubt very much it's the flyback.
Is there a high voltage paper cap across the caps of hv rectifiers? If so, replace. I believe they are rated at 10KV.
Bill Cahill

[jr_tech]

Originally Posted by Kamakiri

One of the wires that was connected goes to pin 8 of the 5U4s, the others I can't see where they go, but I assume that those two complete that particular circuit. From what I can find, the relay should show 5000 ohms across the coil, I have 3800.

If this relay were dead, and with those strands of wire connected, could this have caused the set to work before, and not now?

Makes sense, if the relay or driver circuit were bad, the connection might have been bridged on purpose to provide B+ to the set.

jr

[Kamakiri]

Yep, that was it

Had the chassis on the bench, so on my lunch break I jumpered the connection, and used the variac to power it up.

It's sure an improvement from where I was when I had a picture last, but it's still not right. Contrast control does nothing, and neither does width. I can get it wider, but then I have to max out the horizontal drive and the whole picture darkens up in the center. Focuses fine. Not very bright, but I did notice that the picture is getting better just with tweaking and use, as everything is beginning to "wake up".

Getting there, getting there . Think it's time to re-test tubes before I go further. One candohm was working but was out of spec, so I'll place that on my next Mouser order.

[earlyfilm]

Originally Posted by Kamakiri

. . . . width. I can get it wider, but then I have to max out the horizontal drive and the whole picture darkens up in the center. . . . .

After tubes, the first thing I'd check would be the cathode resister on the horizontal output tube.

If your other checks do not find the problem. . . .

I notice that the Sam's that you are using lumps the 12 inch RA-103 with the 19 inch RA-110 chassis. I see that Rider keeps them separate.

I have no idea what Du Mont changed (other than the yoke) for the bigger CRT, but there is a slight chance that the person who recapped your set used the more readily available Sam's instead of either Du Mont or Rider diagrams and accidentally created an image size problem.

Jas.

[Kamakiri]

This is the relay that was retrofitted under the chassis. Look like it should work?

http://www.digikey.com/product-detai...Q15-1t1-VQ16-c

[earlyfilm]

Originally Posted by Kamakiri

This is the relay that was retrofitted under the chassis. Look like it should work?

The P&B relay is within specs on amperage, under specs on voltage, but probably is OK. (Switching 440 + DC with a 120V RMS AC rating.)

Apparently Du Mont did not publish the pull-in current requirement and that makes it impossible to answer without a test.

What should be a safe way to check the relay is to remove both 5U4's but leave the two 6X4 bias rectifiers in. Unsolder both switched leads. * (The lead going to the 5U4 will have 440 AC on it, so cap it.)

Put a battery and same voltage test lamp in a loop circuit with the switched relay contacts. Power the set up, still with 5U4's out, and remove and replace the 6AL5 power delay tube. The relay should click with each operation, possibly with a slight delay. Test lamp should be on with tube in, and out with tube out.

If relay clicks, but does not light the lamp, the relay is bad. If relay does not click, the trouble probably is in the switching circuit, but there is also a slight chance it could be a bad relay, or the relay coil pull-in rating is incorrect.

With that said, I've never done a relay test on a Du Mont, but have used similar methods on other industrial equipment when I did not have component information or a spare relay. If anyone sees anything I just wrote potentially harming the set, please holler now!

Jas.

* Oops! Clearly an oversight above, hope there are no more.
Before someone points it out, with the 5U4's removed, the only voltage on the socket filament pins is 5 V AC since the 440 AC is on the plates. I would still cap the hot wire before running the test.

[Kamakiri]

Was suggested by a member in PM a while ago (honestly don't recall who) that suggested that I look at the yoke for the problem. Bought a new yoke for the set, and it's sat since about 2 weeks after my last post on this.

Now that I've got a yoke tester and have learned how to use it (very easy BTW), I tested both the new yoke and the old. Looks like that's (at least some of) the issue.

Here's where I'm at. Aftermarket yokes always make me scratch my head, but with the supplied diagrams usually I can figure it out. This one's confusing me a bit.

Below are the pics of both the new and the old yokes. The old yoke has colors red, red/black, green, and tan/black. The new yoke's wiring is green, green/black, tan, and tan/black.

As well the new yoke has caps in it while the old doesn't. Resistors are the same.

Appreciate a little advice