Help! frustrated with 58 Space Command

[Tubejunke]

I have a 1958 Zenith Model C4007R Space Command with the 18C24Q chassis. The set has virtually no vertical sweep and only a hum for sound with no signal. I figured electrolytic trouble for sure. Well, I disconnected the leads to the cans and did an ohm meter "kick" test and found nothing unusual. No low resistance readings like I expected. I replaced several, but not all of the electrolytic caps and the set has not improved at all. I have replaced around 6 of the white tubular paper caps. I keep thinking I should easily run across some component with a way off reading like a shorted cap or something open but I just havent hit it yet. There are 2 black beauties in the vertical section that I havent touched. I dont think the small value caps would cause the serious ills of this set but I may be wrong. One thing I want to note is that there is a 5U4GB in place of a 5AU4. The set is producing DC, but could that tube cause some kind of short? A site that has sub info did not list the 5AU4 as a sub for the 5U4. They both have the same pin out so there should not be a short. I guess I'm hoping that some good folks out there are familliar with this chassis and its charicteristics. I know Zenith used this and variations of this for a few years. I notice that some scematics call for 5AU4 and others 5U4. There has to be some difference. I should be getting at least some life out of this set by now. It is more than half re capped. It will be a good one once I get it staightened out since it looked like the back cover had never been removed and all stock tubes. I figure low mileage. I also need a remote....
Thanks on advance!!

[Eric H]

Lets start at the beginning, you need to go back and replace the filter capacitors, the hum in the sound indicates they are not doing their job.

Not every electrolytic in the set is a filter for the low voltage power supply. typically capacitors C1 & C2 will be your main filter caps, they will be connected right after your rectifier tube or Selenium. (you do have a schematic?) I would start with them.

You can't test them with an ohmmeter, the normal failure mode of an electrolytic is to go open or lose capacity or be slightly leaky, not a dead short.
It doesn't take much to make your meter jump, especially if it's digital but that doesn't mean the cap is good, they can also leak between sections and cause all sorts of problems.

Step two, get rid of the black beauties, they absolutely will and most likely are the cause of your vertical failure.
Start by replacing every small (.001 or bigger) cap attached to the vertical osc tube and go from there, the ones with pf values are likely ceramics and will be OK.

Of course it couldn't hurt to check some voltages and see if you find something way out of whack. You could have an open resistor or bad vertical output transformer.

The vertical osc/output tube itself could be bad of course but the odds are against it, sub it anyway if you have one, swap with another from a different section if there's another with the same number.

Eric

[Chad Hauris]

Personally I like to do voltage checks rather than component checks. Improper DC voltages in a circuit will point you in the direction of the bad component. A lot of the time the old electrolytics are fairly good, but it is the paper caps that need to be replaced. Shorted bypass caps can damage other parts such as resistors...this often results in no plate voltage to a tube. It only takes ONE bad cap in the "wrong" place to completely disable a stage.

Remember that power supply filter problems will cause hum in the speaker even if the volume is down. If the hum can be shut off by turning down the volume the trouble is likely somewhere else. Maybe a cap shorted in the sound IF or discriminator stage and took out a plate load resistor, causing a stage to be inoperative.

A trick for vertical testing is to take 6.3 vac filament voltage (the ungrounded side) and hook up a .047, or .1 or so 600 volt cap. This prevents high voltage from going back on the filament lines. Take the other end of the cap and hook it to the grid of the vert. output stage. If you get vertical deflection then the output stage is ok. Then move it back to the grid of the oscillator stage. If you get deflection here, the tube is getting voltage but there is a problem with the feedback loop causing the oscillator to not oscillate. This method has helped me find vertical problems that appear tough at first.

[Tubejunke]

Thanks Chad for the well informed information. If I were you I would probably be watching this set right now. I really like your info about bypass caps ect.. I will pay mor attention to them. The fact that I tend to overlook the "black beauties" is probably the source of most of my frustration. I do know that neither of the two BBs in the vertical section do not read correct value on my digital meter. It seems like they are .0068 and measure maybe .02 or so.

Thanks also to Eric who always seems to be there. Both power supply filters are in a can that shares sections with the vertical output and horizonal osc. I have replaced all but the 80/400 which is one of the power supply filters. When I talk about doing a kick test I always break out my analog meter just for the record. All the electrolytics in this can had a good strong kick that would hold the meter pegged for a second and then go to high resistance. Coming back to Chads ideas the "hum" is adjustable with the volume control. I think my ignorance of the nature of the black beauties may be the whole thing. I guess the unused look of this set threw me off a little. I didnt expect to have this much trouble getting a little life out of a chassis you can eat off of and has 90% factory tubes. I pretty much planned on making this my first total recap and I guess the plan is working if nothing else by force. The process has been touch and go as far as parts. The local electronics suppliers dont cator to well to the high voltage, antique electronics crowd. Most act as if it is rediculous to restore old tube equipment. Last visit the counter man refered to us as "die hards" as if we are just to stubborn to get rid of the junk and go solid state. Huh! Solid state! What a laugh.....

[Chad Hauris]

I have personally gone to mail order to get all my caps and stuff for repairing these old sets. There is only one local electronics distributor left in this area and rarely have any of the high voltage caps, HV anode wire, belts, etc. Also I try to salvage as much as I can from junked modern equipment. Computers and computer monitors will often have a lot of higher voltage electrolytics and power resistors. One time I completely re-caped a 1940's GE radio using only parts from computer monitors. Worked great.

[Jeffhs]

Originally posted by Tubejunke
I have a 1958 Zenith Model C4007R Space Command with the 18C24Q chassis. The set has virtually no vertical sweep and only a hum for sound with no signal. I figured electrolytic trouble for sure. Well, I disconnected the leads to the cans and did an ohm meter "kick" test and found nothing unusual. No low resistance readings like I expected. I replaced several, but not all of the electrolytic caps and the set has not improved at all. I have replaced around 6 of the white tubular paper caps. I keep thinking I should easily run across some component with a way off reading like a shorted cap or something open but I just havent hit it yet. There are 2 black beauties in the vertical section that I havent touched. I dont think the small value caps would cause the serious ills of this set but I may be wrong. One thing I want to note is that there is a 5U4GB in place of a 5AU4. The set is producing DC, but could that tube cause some kind of short? A site that has sub info did not list the 5AU4 as a sub for the 5U4. They both have the same pin out so there should not be a short. I guess I'm hoping that some good folks out there are familliar with this chassis and its charicteristics. I know Zenith used this and variations of this for a few years. I notice that some scematics call for 5AU4 and others 5U4. There has to be some difference. I should be getting at least some life out of this set by now. It is more than half re capped. It will be a good one once I get it staightened out since it looked like the back cover had never been removed and all stock tubes. I figure low mileage. I also need a remote....
Thanks on advance!!

I would also get rid of that 5U4GB and put the right rectifier tube (5AU4) in its place if your set calls for a 5AU4 on the tube layout chart in the cabinet, and/or on the schematic if you have one. I am amazed your set works at all with an incorrect rectifier, even if the pinouts of the two tubes are identical. If the 5U4 or any of its successors (5U4G, -GB, etc.) were meant to be used as substitutes for a 5AU4, it likely would have been listed as such on the website you mention or any tube substitution manual.

As to a replacement for the remote, I'd keep an eye out on ebay. Many times older remotes are listed there for very reasonable prices; some of them are even shown with a "buy it now" link below the opening bid. If you find one with such an option, I'd say grab it. Also, you can place an ad in any one of many antique radio/TV websites, such as Phil's Old Radios, asking for leads to one of these remotes (the site is also used for inquiries as to information or parts, etc. for antique TV sets).

Good luck. Zenith was a very good make of television sets in the 1950s and until they were acquired by Gold Star in the '90s. Your set is from 1958 and should work well for you for many years, once the restoration is complete.

[Jeffhs]

Originally posted by Eric H
Lets start at the beginning, you need to go back and replace the filter capacitors, the hum in the sound indicates they are not doing their job.

Not every electrolytic in the set is a filter for the low voltage power supply. typically capacitors C1 & C2 will be your main filter caps, they will be connected right after your rectifier tube or Selenium. (you do have a schematic?) I would start with them.

You can't test them with an ohmmeter, the normal failure mode of an electrolytic is to go open or lose capacity or be slightly leaky, not a dead short.
It doesn't take much to make your meter jump, especially if it's digital but that doesn't mean the cap is good, they can also leak between sections and cause all sorts of problems.

Step two, get rid of the black beauties, they absolutely will and most likely are the cause of your vertical failure.
Start by replacing every small (.001 or bigger) cap attached to the vertical osc tube and go from there, the ones with pf values are likely ceramics and will be OK.

Of course it couldn't hurt to check some voltages and see if you find something way out of whack. You could have an open resistor or bad vertical output transformer.

The vertical osc/output tube itself could be bad of course but the odds are against it, sub it anyway if you have one, swap with another from a different section if there's another with the same number.

Eric

Eric,

I learned something today when I read your post. Your statement as to the failure mode of electrolytic capacitors cleared up a misconception I've had for years about them. I was always under the impression that electrolytics, especially old ones, do short often, causing a dead short across the AC line. The fact that electrolytics generally open, lose value or become leaky when they fail, rather than short, also explains very clearly to me why my 1951 Zenith H511 radio still sounds good today (no discernible hum) and my 1963 Zenith K731 AM/FM sounds as good today as it did when it was new. Also of note: My late grandmother had a Sears Silvertone table model radio which she purchased in 1936. That radio worked well in its day (so I was told) and still sounded great (no hum or other distortion), with its original filter caps, 45 years and more later. The only thing wrong with it when it was finally discarded a year or so ago was that the line cord was badly frayed (crumbling insulation).

[peverett]

You say that you discarded a 1936 model radio just because of a bad line cord? What a mistake!!!!. The collector marked for old radios (especially pre-war) is much larger than the collector market for old TVs. Just check Ebay out.

[Jeffhs]

Originally posted by peverett
You say that you discarded a 1936 model radio just because of a bad line cord? What a mistake!!!!. The collector marked for old radios (especially pre-war) is much larger than the collector market for old TVs. Just check Ebay out.

I think you misunderstood me, or else I wasn't clear enough explaining the situation with that 1936 Silvertone. I agree with you; throwing a 1936-model radio into the trash just because the line cord was shot is a mistake, but you have to understand I did not discard the set--in fact, I had nothing to do with it. The situation was this. The radio was in a cottage my grandmother had owned; she passed away in 1985. About a year ago, give or take a few months, the cottage was sold. The new owner, who probably, even likely, knew nothing about the value of antiques of any kind, was likely the person who discarded that radio when he remodeled the place. As I understand it, the man discarded everything in the cottage (including that '36 Silvertone radio) when he bought the building and started absolutely fresh.