Recapped the Admiral! - Still having issues... - Videokarma.org TV - Video

RitchieMars · #1 06-22-2010, 07:46 PM

Awhile back, I had talked about my Zenith television and it became clear to me at that time that I should start by learning to work on my radio's first. So, I did finally work up the nerve and read up on it and bought the capacitors I needed to do the Admiral and my Zenith H724. The Zenith came out fine, louder and seems to pick up stations better now that I've added an external antenna. The old Admiral, however, remains problematic.

All of the wax paper capacitors and the electrolytic capacitors in my great-grandmother's 1942 Admiral 4215-C7 are new and installed according to the factory schematic. Trouble is, I had to swap out the rectifier tube with another 6X5 out of a derelict chassis I had sitting around to get it to work. This rectifier tube worked fine for awhile until today when I let the set warm up, the tube suddenly got bright and went out, arching and whatnot. I'm not quite sure if it's simply that the tube was already going bad, or if something inside my chassis is wrong and caused a short. The electrolytic caps associated with this tube are both 20uF and I know that I could have safely went with 40's. It did have a slight buzz when you turned the volume up, but it was a louder buzz when switched to phono. While the set did run, and I allowed it to warm up and run for periods no longer than 10 or 15 minutes at a time, and I did notice that the rectifier tube and the output were both rather hot. Up to this point I was almost able to pick up radios signals, but I had to squeeze wires to the antenna loop with my fingers to actually hear anything, even with the volume all the way up.

I had a replacement audio tube ( 6SQ7 ) on the way because one of the two seemed to glow very weakly, but now that doesn't matter since my rectifier went out... =( Any ideas?

Just to give you a general idea of what I'm dealing with, my Admiral's tube compliment includes a 6X5GT as the rectifier, a 7A7 as the IF amp, 6SA7 1st Det. Osc, 6SQ7 2nd det. osc, another 6SQ7 1st audio, a 6J5GT as the 2nd audio, and then a 6K6GT as the output tube.

Reece · #2 06-22-2010, 08:57 PM

How bright a tube filament glows is usually not important. Some are dimmer than others by design and work fine. 6X5's can be problematic if there's a heater to cathode short. You may just have had a bad tube. Before you plug a new 6X5 in there you want to look for any excess load in the B+ circuits of this set: any partial shorts to ground or resistors in the plate circuits of tubes that have dropped in value (usually they go up.) Some tubes will get very hot, too hot to touch, in normal operation, but their plates should never glow red which means they are drawing too much current. How does the transformer feel under load? It may get quite warm, but not so hot you can't put your hand on it.

Also be certain that electrolytics are connected correctly with respect to polarity.

With two 1N4007 silicon diodes, you could operate the set without a 6X5 for testing purposes. The cathodes of the diodes (end marked with a band) would go to the tube pin where the tube cathode was connected, and the other ends to the plate pins.

I couldn't find the schematic for this set on the web but if it has a loop antenna, both wires to the loop have to be connected to complete the tuning circuit. The fact that stations were very weak may just be because someone played with the alignment of the set before you got it. Sometimes a person will "tighten all those loose screws" on the I.F. transformers and the tuning capacitor in an attempt to "fix" the radio.

RitchieMars · #3 06-22-2010, 09:52 PM

Yeah, I only figured that one dim tube may have been faulty because the other which was the identical type, was a bit brighter. Even when switched, the dim tube remained dim no matter which slot it was in. I don't have a tube-tester yet so it was all guesswork. The bad rectifier was much more obvious, and the first one was causing the transformer to start getting hot as the set warmed up. When I swapped that 6X5 out for another, it ran fine. All the tubes simply glowed orange at the very top, the rectifier and the output tubes were the hottest and brightest but no glowing red plates anywhere. The transformer at this point would only get just slightly warm on top, and I could feel this sort of vibrating quiver coming from it whenever I touched it or turned the knobs on the front of the radio. I've never actually "felt" the current running through one of my radios so I didn't know if that was normal or not.

The weak reception probably was partly my own fault, as I do recall trying to turn at least one of those screws back about a year ago when I first pulled the chassis out for cleaning. I'll have to go back and realign everything once I'm sure it's running like it should be.

Here's a scan of the schematics I acquired, although there is a few minor differences I've noticed. For one, my tone control is the on/off switch rather than the volume knob. The radio had a lot of repair work that I had to sort out which included an added Mallory 2x8uF wired parallel as a 16uF which was used to replace one of the bad 20uF's in the multi-section can. All that's done away with now, of course. I don't much care for the cheap-looking hand-painted resistors, either.

radiotvnut · #4 06-23-2010, 01:28 AM

You want to be careful with that rectifier tube, as a shorted tube (or any other short in the B+ line) can fry the power transformer. Your 6X5 could have failed on it's own; or, it could have failed due to an excessive load somewhere in the radio. Since you changed all of the capacitors; you'll need to make sure there are no frayed wires that are touching the chassis or other parts. The same goes for any component leads. Also, make sure that no solder balls or any other conductive objects are in the chassis. I'd also suggest placing a fuse in the center tap of the HV winding on the power transformer. A fuse on the order of 250-500 mA should work. Most of these old radios were not fused and a fuse would have saved many a power transformer.

Reece · #5 06-23-2010, 12:17 PM

This set was made at a time when the industry was changing from older style resistors to the striped ones familiar today. The older resistors were a bar of resistive material with wires wrapped around the ends and were all factory painted like those in your radio, and are read "B.E.D" or Body, End, Dot: the body color is the first digit, the end the second digit, and the dot is the multiplier.

wa2ise · #6 06-23-2010, 05:52 PM

I've heard that there was a rectifier tube that was failure prone, with heater-cathode shorts. Think it was the 6X5, but I'm not sure if that was the one.

And how bright a tube heater looks to be doesn't really indicate the health of the tube. It depends partly on how much and how close the ends of the heater wires are to the end of the cathode pipe, some tubes the heater wires stick outside, some the ends are inside the pipe. Has no real impact on how hot the cathode inside where the grids and plate action is.

jr_tech · #7 06-23-2010, 06:09 PM

"I've heard that there was a rectifier tube that was failure prone, with heater-cathode shorts. Think it was the 6X5, but I'm not sure if that was the one. "

I think that is the case... I have read of numerous 6X5 failures (mostly shorts). Restorers seem to favor the "X Plate" design.

http://www.antiqueradios.com/forums/...64698e2acbeb0c

jr

leadlike · #8 06-23-2010, 08:57 PM

Yup, 6x5 tubes are known for this-I had one fail on a Zenith. When they go, they can take out the power transformer-best to get your set fused. Your set should play really well once it gets aligned. Did it have a built-in antenna inside of the cabinet?

Reece · #9 06-23-2010, 09:17 PM

The way some get around the 6X5 problem is to wire a couple of silicon diodes under the chassis and put a dud 6X5 tube in the socket for show, disconnecting the appropriate wiring to the tube. This has the additional benefit of reducing the heater load to the transformer.

bandersen · #10 06-23-2010, 10:17 PM

I recently had a 6X5 blow out too - just like you described. I replaced it with diodes as suggested and it's working fine now

RitchieMars · #11 06-24-2010, 04:39 PM

Wow, I would have never had guessed that this set used a tube that was faulty by design. I would like to get a NOS 6X5GT, but if I can't trust it then I guess I would rather get those diodes instead. It sucks since the vacuum tube design is what draws me to these radios, and in this case it seems like I'd have to make at least one small concession for reliability purposes. The diodes are certainly way more affordable than the tubes. I rather like the idea of using the diodes and keeping the tube installed with the heater glowing just for looks. How would I do that? Or, would it be best just to leave it out all together?
http://tubedata.milbert.com/sheets/127/6/6X5GT.pdf ( 6X5GT )
Referring back to my radio's schematic ( 3rd post in this thread ) why is it that pin 1 isn't even mentioned? Several pins in my radio have wires running to them that aren't covered in the schematic. Isn't it leaving out the path that heats the filaments?

bandersen · #12 06-24-2010, 05:15 PM

I think if you fuse it properly you can still use one without fear of blowing the xfrmr. Here's some food for thought: http://antiqueradios.com/forums/view...&highlight=6x5

I've had a crazy idea for a while. Remove the base from a 6X5. Wire in a couple 1N4007s to rectify and a red LED to simulate the filament. Then remount the base and no one's the wiser

RitchieMars · #13 06-24-2010, 07:06 PM

Hm, that "Copper Cap" tube replacement is pretty neat. Much pricier than the diodes, of course. I can get those diodes AND a NOS 6X5GT for under $7. I think I'll get some fuses and resistors while I'm at it. I really would like to simply run the set with the 6X5GT, but I know for a fact this transformer has had a few close calls already. It got toasted once before by the first bad rectifier and the bad capacitors, and it started to overheat slightly the first time I tested it right after I recapped it. Then I put in the 6X5GT and it ran well until the other day minus weak reception. The same RCA I borrowed that tube from has a compatible transformer if need be, but I'd hate to have to take that thing out and sort out the wiring on such a vital component.

RitchieMars · #14 06-26-2010, 06:03 PM

I've got those diodes on the way but I've also read about putting a dropping resistor to keep the B+ from going up? A guy in one of those threads mentioned a 6X5WGT which seems to be a military grade 6X5 and it's an X-plate like those that are generally regarded as the best variation. I might like to consider that option in addition to fusing my transformer which I should do with either option, I would think.

Reece · #15 06-27-2010, 09:21 AM

Revisting all of the 6X5 discussions over on ARF, I think I'd go with diodes and just leave the rectifier heater glowing for looks. If my transformer were running hot, I'd leave the 6X5 heater disconnected. With diodes, as long as the B+ measured at tube plates is within the tube ratings as determined from a tube manual or a lookup site like

http://www.nj7p.org/Tube.php

you should be OK. If you need to drop voltage, just figure by Ohm's law. From tube data, add up the plate currents of all the tubes, figure how many volts you need to drop, and R = E/I. Then to figure the size of the resistor in watts, P = E x I. I would double this wattage rating to make the resistor run cooler.

As to tube socket terminals not shown on the schematic, sometimes a particular terminal is not used on a tube. That leaves the socket terminal free for the manufacturer to use as a tie point in the wiring.