The Mercury

[Adam]

All working now, although it does seem to show more retrace lines than most sets. And that noisy power transformer still worries me, but it seems to be working ok. Here's a pic of the set working, yet another pic of the bewitched Zenith...

I'd like to find a replacement for that buzzing power transformer (and maybe also the choke coil, it just leaked out a lot of wax and the set was only on for about 30min) anyone know where I might possibly find one? I don't have a part number, but here's a pic off of the schematic which gives resistances between and the voltages at the different taps on the coils...

[Adam]

Here's pics of the chassis:

The part number for the power transformer on the list stuck on the inside of the cabinet is 5000, if anyone can cross reference that to anything.

[Adam]

The horiz sync was weak and I just fixed that by swapping out one of those micas, but after watching it for a few minutes after that the power transformer finally crapped out. When looking for I replacement, I'm not entirely sure what's going on in this power supply, I understand how the 2 seperate rectifier tubes are working off the same coil, but what I don't understand is how this -2.5V source is working: is the center tap on this transformer actually slightly over to one side and not in the center at 0v?
But then if the center tap was off center I don't think the output voltage of the power supply could be made constant, as each time the AC changed direction the voltage going into the rectifier tubes would higher or lower, so then maybe the -2.5v just has to do with a voltage drop across that 10 ohm resistor and that center tap is right in the middle of the coil at 0v, although that doesn't seem like it should be possible to be both 0v and -2.5v, I'm still confused...

[Steve K]

Adam:

The center tap is centered. You are measuring the B+ from ground and, you are right, the tap is seperated from ground by that 10 ohm resistor which makes it negative with respect to ground.

Steve

[Adam]

I get it just because its -2.5v in reference to ground, doesn't mean it can't be 0v in reference to itself. Thanks. I hope I can dig up a replacement after all the work that went into this set.

[John Folsom]

Adam, What is the model number on the Mercury? It looks very much like a '630 clone chassis, and if so, finding a powrer transformer and choke should not be a problem. I can look up the part # crosses if I have the model #.

[Adam]

The model number is 4118, chassis 101. Thanks for looking that up. I pulled out a 630 schematic (I have a 630 in a cabinet that needs alot of work I wouldn't mind parting out to fix this) but I don't think the power transformer from the 630 would work, it uses 2 5U4s in parallel, whereas in this set the secondary is actually tapped in 5 places 360v-175v-0v-175v-360v, and the 5U4 hooks to the 360v, while a 6X5 goes to the 175v. There's alot of similarities between this and the early RCAs, but where it's different is in the power supply. I seem to remember seeing some early Admirals with something like this though.

[Steve K]

Adam:

Some of the early Admirals did have power xformers with seperate windings for a 5U4 and a 6X5. The voltages were a bit lower than what your set calls for but it should still work.

Steve

[Adam]

I was looking through my sams trying to find a part number for this transformer, and it turns out that Pacific-Mercury was using Sears' chassis, here's a pic of the corresponding Silvertone set from the sams. Silvertone model 9123, Sams 76-19, power transformer: Silvertone A1098, Stancor P-8157.

[Adam]

I have my new power transformer, Merit 3053, which crosses to Stancor P8157 which is the unit the sams says to use as a replacement for Silvertone models 9123-9126 which use the same chassis as this set.

Now the sams says when using the Stancor replacement to use a resistor to drop the voltage, and now I see why, the original transformer was tapped at (according to the riders schematic) 360-175-0-175-360, (according to the sams) 390-175-0-175-390 the Merit replacement is 375-325-0-325-375. So I don't see the 375-0-375 for the 5U4 to be a problem, but I'm going to have to drop the voltage comming out of the 6X5, I would assume that the best place to add the extra resistor would be right after the filter capacitors, also I would think a 5W resistor would be enough, but I'm not sure what resistance to use. And I don't want to connect it all up with too little resistance in there and burn something out, and if I just put resistors in there and then check the output voltage with out anything hooked to the power suppy, because it won't be drawing much current the voltage reading I'm going to get will be way higher than it actually will be with everything connected so I can't figure it out that way. Any ideas?

Also for the filaments this transformer has two 5v secondaries and two 6.3 volt secondaries, I can either just use one of each and get 5v@3A and 6.3v@7A or parallel them and get twice the amps, 5v@6A and 6.3v@14A which way should I go?

[John Folsom]

I would locate the resistor right after the 6X5, that way you keep the voltage on the filters to a minimum. The max DC current for the 6X5 is 75mA, so (325-175)/.075= 2000, and 11.5 watts, as a worst case.
At 5000 ohms, 150/5000=30mA, and 4.5 watts. I would try the 5K ohm 5 watt resistor. Bring it up on a variac and monitor the voltage, and adjust the resistor value and wattage as required.

As for the filaments, try to keep the current rating and the actual load close to each other. Paralleling the outputs when you don't need to will make the filament voltage run on the high side. You may have to add up all the tube load currents by hand to determine your actual requirement if Sams does not give it.

Good luck.

[cbenham]

[QUOTE=Adam;1243909]Model 4188. Chassis 101 It uses the 12LP4.

This doesn't even appear in my Sams index, anyone know if there is a sams for this, if so I probably have it, if not does anyone have a schematic or know where to get one?

It is listed as Model 4118 Ch. 101 in the Rider TV index in Vol. 5-1. I can scan this to a .pdf and email it to you in two pieces that will overlap. Just send me a private message with your email address in it.

Also, I have only the cabinet for a SIlvertone 10 inch set that is laid out the same as your set is. The center knob is lower than the other two and the knob functions are just the same as in your pic. Did Pacific Mercury OEM sets for Sears? It sure looks like they did.

Cliff

[Randy Bassham]

Watch that 6X5, they are prone to arcing and shorting, probably responsible for taking out more power transformers than any other tube in old radios. If you're going to use a 6X5 get one of the newer style with the X type plates similar to the internal construction of the 6X4 they are much less likely to short than the older styles especially the ones with the close spaced parallel plates.

[Adam]

Thanks to everyone who posted here for all the help on this. There already is a resistor 100ohm 4W inbetween the 6X5 and the filter caps, so I'll change it out for one around 5k 5W. According to the sams the 5v winding should be at 3A , and the 6v at 9.1A, so I can just use 1 of the 5v windings for 3A, but the 6v will either be low at 7A or high at 14A.

It already has one of the newer type 6X5s in there which I checked for shorts. I think the demise of the power transformer was caused by the people I bought it from plugging it it to see if it works with shorted filter caps. I don't see many of the older type 6X5s, I just looked in my stash of tubes and only 1 out of 5 6X5s I had was the old type, they've probably mostly all shorted out by now.

I already picked up the riders, and the sams corresponding to this is not listed in the index, but the Silvertone models 9123-9126 do use the same chassis, so you can find it in sams 79-16. I'm not sure if it was Sears making chassis for Pacific-Mercury or Pacific-Mercury making them for Sears though. But then this set did use Silvertone tubes.

[blue_lateral]

If it were me, I think I would use two resistors (of half the needed wattage), and put them in the plate lines of the 6x5. Those were/are a problem tube.

I don't have the sams, but usually the 6x5 filament is run from the same 6.3 volt winding as all the small tubes in the set. If this is the case, usually one side of that winding is grounded, and therein lies the problem.

In a typical 6x5 setup, there is full b+ on the cathode, and the filament winding is grounded. A typical failure is a heater-to-cathode short that burns out the power transformer. It wouldn't surprise me if this is what happened to the original transformer.

The main thing is that white goo on the filament has to hold back b+. A 6x5 apparently wouldn't even hold to it's rating, let alone more. IMHO putting more voltage here is a really bad idea.

If the 6x5 has its own 6.3 volt winding that floats above ground, you can disregard all this

John