1958 Zenith 18C24Q VIDEO DETECTOR DIODE?
 

I am still hoping to find someone familiar with this generation of Zenith chassis. Zenith used this chassis design for many years. I have been squawking about this old Zenith forever, so I am going to just take up where I left off in other posts in regards to repair attempts to try to keep from repeating everything. Anyway, today I used a megger to check the winding insulation integrity of the vertical output transformer in hope of restoring vertical sweep. I didn't find anything wrong with transformer, however, I did find that the vertical hold potentiometer gives erratic resistance readings. Perhaps that will be a lead in the vertical trouble.

NOW, I was wondering if anybody knows much about the video detector diode M2 . I was surprised to find a solid state diode on a 1958 schematic, but there is one there with a couple of inductor coils in the third video IF stage. One of the coils (L6-81mH peaking coil) that is directly connected to the diode is open. I have a volume adjustable hum for sound and I was just wondering what effect, if any, that this bad diode and open coil may have on anything other than obviously the video section. Could it produce the hum if say it was shorted? Also, would it show the same or similar readings as any other solid state diode like a 1N4004? Could I use the 1N4004 as a replacement? I am hoping in a big way that this might be at least a lead in getting somewhere with this old Space Command. This was a re-cap with no positive results and I have never been able to do voltage checks due to the lack of a test crt, so I am stuck with an ohm meter as my only weapon for now. These were supposedly great performers and ther is no reason why I shouldn't be able to get this thing going. I would love to have a large 24" screen, fifties REMOTE set for daily use! :thmbsp:

Im not familiar with that specific set, but I have run accross several bad video detector diodes.
You can NOT use a 1n4004 and it will not test like a 4004.
A 4004, using diode test on a multi meter will read about .500-.600mv, the diode thats in there is Germanium and will read .200-.300mv one direction and open the other direction.
You may be able to use a modern silicon detector diode, the 1n4004 is not even close to high enough frequency to work as a detector.

When they fail they cause a whole bunch of weird problems depending on if they get leaky, short or open. Everything form buzzing and sync issues to poor color performance and ghosting.

I have been using 1N34 or 1N60 as a replacement for these. They are a germanium diode and work just fine.

I just looked on ebay and you can buy these numbers for like 10 for $1

Thanks for that info CT! Maybe, just maybe, this thing is causing ALL of the issues this set has! I just with that sweep issues was on your list! I can't imagine a video detector diode taking out my vertical sweep. At least I might get rid of the buzz. I heven't even attempted to check the diode; I just know that the pigtail coil is open that I think is in series with the diode.

Its easy to check the diode. There is a new 1n60p out that's a shottkey diode. These should last forever as they don't have tHe organic issue. I ordered some for testing. It can cause vertical lock issues not deflection. Best way I can describe a bad detector is the fine tuning is always off and uncorrectable

I've had luck with the 1N5817 and 1N5818 Schottkys as detectors. The crystal radio folks are even using them as AM demodulators, as the 1N34/1N60's are obsolete and the 1N270 is priced higher.

Germanium diodes are always something I save from any junked device.

Cheers,

I will try to get the diode disconnected, on one end at least, for a test. Just out of curiosity I placed my leads across the diode in circuit and read open leads. it looks like it might be a little tricky to de-solder and properly disconnect as the tuning coil that it is mounted on might be destroyed by either heat or by tugging on a lead too hard. I am tempted to just snip a lead and if the diode checks out OK then I can solder the lead back together. Again, since these diodes cause a multitude of problems I was hoping that somehow vertical sweep would be on the list, but I more or less figured that there was no way.

This set has always had me puzzled with the lack of vertical sweep and hum that have been it's symptoms since before the re-cap. I thought that the hum would go away after the power supply electrolytic capacitor replacement. No such luck! I am still searching, but I have yet to find any bad components. I am starting to think about ceramic discs. Most people leave them alone, but any component can be bad. There is one of the little brown jewels with 22 dot 17 marked in blue on it in the vertical sweep circuitry that has wax dripping off of it. Perhaps that is worth looking into. I sure wish that I had the college's LCR meter here at home! I have more than one digital VOM that has a capacitor test, but I have been told that this type of test is not reliable. Hmmm? I wouldn't think that the meter manufacturers would put this capability on most meters if they didn't work.

They are heat sensitive so be careful.

What kind of hum is it? Power supply or a buzz that changes with tuning/programming? If you are having a hard time isolating it pull the last if tube and see if it goes away. It could be you just need to tweak the sound detector coil a tad.

use hemostats or aligator clips close to the body of the germainium diode (same for germainium transistors) when ever soldering, esp with solder guns.

leave the clip on until the residual heat is gone.

Modern DMM type Cap testers check ESR and Value fine, but they dont check for leakage at rated voltage, which is the most common failure on old paper caps.

Hum could be a heater cathode leak in the audio. If you have a scope you could look at the audio from the detector and trace.

Look at the b+ for ripple with the scope. Is it hum or is it buzz?


I had a tough dog hum in a radio once, turned out the radio had the power tranny and the audio out tranny right next to each other another model of the same set did the same thing. It would hum from the instant it as turned on, even with tubes pulled, magnetic field from the power was coupling to the audio. I could min by putting a heavy steel pipe around either tranny shielding it.

Wow! More great info! Anyway, the hum is not tuner adjustable, but is present at all times and is volume controllable.The power tranny is on the opposite end of the chassis, so I don't think that the magnetic field theory will help this time. When I get time I am going to check that video detector diode. Again, the coil that is in series with the diode is open, so we know that there going to be some issue there, just not exactly what so far. I don't think that the video section is going to produce the hum and certainly not the lack of vertical sweep. Bear in mind that this set has been re-capped and was the same before the work as it is now, so that at least should eliminate capacitor issues (with the exception of ceramic discs) and botched work on the re-cap.

I am almost ashamed that I am soon to graduate an electronics degree program, but honestly, they certainly don't teach much of anything in tube type or consumer electronics of any kind. In my last semester (next), they actually have a troubleshooting class which involves signal tracing with a scope, but of course we will be troubleshooting old broken solid state signal generators. I am so sick of turning on LEDs or incandescent bulbs with SCRs, OP Amps, NPN transistors, and relays! There is so much more to electronics! I did get to build a circuit on a self made (from scratch) PC board that is capable of displaying indication of outputs and controlling a stepper motor through a P-basic computer application. I just hope that I will know enough to be a capable electronics tech upon graduation. My GPA is 4.0., but great grades don't change the program. Essentially they are trying to teach you what it takes to land a job which ends up being knowledge of various configurations of induction motors, starters, controllers, PLC programming, etc.., with a foundation in relatively simple series and parallel circuit design and understanding through Ohms law.

We could be much more helpful if we could see a schematic.

If the coil is open you should be able to check the diode in place..?

I tried to take an electronics course in community collage once. I failed it, i just couldnt get into all that digital flipflop logic probe stuff. Diagnostic skills are a type of logical step by step rational thinking, not something that is easily learned at a later age or taught in a class. Thats why they dont bother.

The 1N34 was introduced in 1946, and I believe it was already in use in
TV sets built circa 1950.

The set shown in my avatar (1960) uses a vacuum video detector,
part of the last IF tube (6AM8), but an earlier TV with similar circuitry
and from the same manufacturer (1957) does use a Ge diode.

Motorola started using them in 1947/1948 in the VT71 sets. It's in a plastic tube hiding inside a metal box.
http://www.bobandersen.com/images/7V...tion/23-04.jpg

In regards to a schematic, I am using Sams Photofact Folder, Set 452, Folder 2. I don't know of any way to post it here, but I wish that we could. As far as getting formal training in the electronics field, I took a year of HS electronics back in the early 80s to start with. My how things have changed! Back then we used Simpson analog VOMs and had a room full of donated television sets ranging in age from the early fifties to the mid 70s or so. There was even a Predicta in a storage room. One must remember that back in those days there were still TV repair shops everywhere, even in a small town like I live in. We have ONE left now and frankly I don't know how he keeps the doors open. Generally there is usually a bunch of consoles and old projection sets sitting outside the shop, so I guess the older folks that are still accustomed to that type of service are still a big part of his business. I guess he times his career just perfectly as he started in the sixties, so he is somewhere near retirement age. TV repair is certainly no viable business venture for a younger man.

As far as a college level program goes, I know from talking with faculty that they must constantly prove to the state that their programs are programs that will lead to employment. With that being said, they really can't justify teaching us radio, television, or VCR repair. In short, home entertainment electronics repair is dead from an employment perspective. However, no matter how much things change they must always teach the fundamentals of electricity (Ohm's Law etc..) and the math to go along with it. That aspect I agree with and appreciate. What I didn't necessarily appreciate (Like you) was the semester hours wasted working with Microsoft Basic Stamp and P-Basic pretending to be computer programmers with toy robots. The method to the digital madness is to get students ready to understand ladder logic and PLC programming, but if it were up to me we would be required to take programming classes so we understand WHAT we are doing as opposed to playing hit and miss until somebody in the class (usually a younger student) gets lucky and figures it out and then they pass it around so everyone can finish lab work.

What I love about the whole thing is that no matter how many younger computer/digital thinking individuals I run into, I can still confuse the whole class by breaking out one of the few remaining Simpson analog meters. They don't know what it is often, and certainly have no idea how to use it. I recently brought in an old tube type chassis and even the sharpest knife in the drawer had no idea what made the thing tick, nor did anyone know what I was doing with a megger as I tested transformers.

If it were up to me there would be more of a balance between old school stuff and modern digital crap. For instance, I can not see how we can call ourselves technicians without at least understanding the principles of transmitting and receiving based on something relatively simple like a superhetrodine radio receiver. I think that I would get more out of that then making robots take computer commands and making sure that I know that Busicom and Fredrico Faggin introduced the first microprocessor in 1971. So, I guess I am lucky that I am old enough to have laid my troubleshooting foundations down years ago through my HS and military training. As far as credentials needed to work in the fields being a degree of some sort, I must take whatever is offered these days and try to make it work in my favor. Fortunately there are more and more of what is left of industrial America's industrial base looking for people who understand PLCs, logic gates, and photosensor configurations, and are willing to pay well. I will just have to leave the Simpson meter at home with the tubes so I don't confuse anyone in the logical electronics world! LOL!

I really appreciate all of the input concerning my old Zenith! I have been messing with old radios and TVs for years, but the learning process never stops. I knew about selenium diodes going back to the 40s or 50s, but I didn't know about the diodes that we have been talking about until just lately. Also, and all too often, I do not have needed equipment such as a scope, or in the case of this Zenith I can't do simple voltage checks because I do not have a test crt and have never rigged up lead extensions to hook up to the stock crt in the cabinet

I am bringing back another old thread as I am TRYING to get some things fixed since you can't hardly give away a nice old set it seems. I would like to thin my collection or radios and TVS if anyone can pick up in southern VA. Anyway, in the interim I need to know if I can by a replacement detector diode somewhere besides Ebay. I mean are they still sold by parts suppliers like Mouser or Digikey etc? Any thoughs on this. I have to obtain something to move forward...

Is this what you want?

http://www.mouser.com/catalog/specsheets/1N34A.pdf

Mouser sells a 10-pack for $6.70.

http://www.mouser.com/ProductDetail/...34PYwAg51eESs2

Phil