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    Don Henschel
    Forum Participant

    Possibly this has been discussed on this forum before but being new to it has me wondering if it has been before.
    My situation started with my Marconi C0M 18805 in my 1950 Mercury being run with an 8 volt battery that it really didn’t need. I ran this car with this battery for around three years and in reality I was pushing this radio hard because a properly set regulator to charge an 8 volt battery is 9.2-9.4 volts instead of 7.2!! Other than the radio warming up much faster eventually the fuse blew and would continue to blow if replaced as soon as the switch was turned on. Capacitors run beyond their rated voltage?? Anyway a number of years later I decided to pull the radio and figure out what happen. I pulled all of the tubes and vibrator and the fuse survived. Plug it in and the fuse popped. I replaced the 6X5 with a pair of silicon rectifiers years ago just to reduce power consumption but tested them. My offshore crap multi tester indicates they are good leading me down the garden path to the capacitors. I tested the electrolytic with my Heathkit tester I inherited from my father ( and have to learn how to use it) and due to this I suspected it was leaky when it wasn’t. A friend of mine with the same type of tester said it was very good and had the same kind also very good so I grabbed it. I reinstalled, and pop went the fuse. I disconnected the secondary windings from the rectifiers and the buffer capacitor across the secondary and the fuse survives? The buffer is a bit leaky but not shorted but left it off to test after reconnecting the rectifiers. Pop goes the fuse. Then I disconnected the B+ to the rest of the radio and pop goes the fuse. What is the common denominator in this picture?? I chopped the rectifiers out and all is happy. First of all I was suspecting a shorted transformer making me think OMG! Where would I get this?? Then was the capacitor measuring only 2 inches tall with a 1 inch diameter in a very confined space including under the chassis and all it was, was a lousy multi tester meter not worth the money to purchase!! Now the funny part is on this forum there is mention of avoiding the 6X5 and I have three of them in my hoard. One of them is a 6X5G shoulder top and it has been mentioned that these were the most problems due to shorting out taking out the power supply transformer. So I used my best 6X5GT and now the radio is running on a 7.5 amp fuse where 9 amps is called for. A 6X5 can take out a transformer but so can silicon rectifiers if they are bad. I guess I exceeded the voltage rating on them even though the diode test position indicated infinite resistance one way and 460 ohms the other way leading me to believe they were good. My vibrator and transformer were getting very warm very fast so I was lucky I didn’t destroy them. Now after this long winded story I will get to the point of the title. Everybody is worrying about capacitors being a problem but when I got this radio to run without the B+ short, the audio wasn’t overly strong and became severely garbled when I turned it up leading me to believe we have severe low voltage problems. After using the service manual Don was kind enough to provide me with, I performed voltage tests on all of the tubes as per the manual. Most voltages other than the B+ directly from the rectifie were very low to the point of being less than half including the grid circuits. Now we are testing ALL of the resistors and everyttime I would replace a resistor I would do a quick voltage test and everyone that I had to replace resulted in the voltages being as high or slightly higher as indicated in the service manual. Two thirds of my resistors were way beyond the resistance indicated by the schematic and color bands. I replaced as many as I could find in my meger assortment and the garbled audio is now gone!! I tested all of the tubes and replaced weak tubes with the best that I have, some of which test 90+ percent that I had on hand. The only ones I didn’t have were the 6SK7’s which are boarderline fai to poor but the radio works 200+% better after replacing the many poor resistors. All of them had considerably higher resistance do this is probably another thing that should be replaced while the capacitors are being replaced.

    Jean Marcotte
    Forum Participant

    Now the funny part is on this forum there is mention of avoiding the 6X5 and I have three of them in my hoard. One of them is a 6X5G shoulder top and it has been mentioned that these were the most problems due to shorting out taking out the power supply transformer.

    You can try using a 0Z4 rectifier in place of the 6X5 tube (direct replacement). This tube has been used in many car radios and I have seen schematics where both tubes could be used indifferently. I was curious to see if I could use a 0Z4 in a home radio. I had two testing good so I tried them in a RCA Institutes 6 tubes radio kit and it worked with one tube but not with the other one. I think there is something about high voltage required to fire-up the 0Z4.

    Don Henschel
    Forum Participant

    Yes and that varies quite a bit because I’m currently working on a variation of this Mercury Marconi, another FDC-18805 found in a 1953/54 Ford Meteor but this one has peanut tubes running bias or B+ of 300 volts whereas my Octals are a max of 260 volts. I remember many years ago taking the can off of an 0Z4 and they are a gass rectifier and glow purple like the number of voltage regulator tubes I have on hand used in Ham or amatute radio equipment. I have both shoulder top octal and peanut of these and the Octals look quite impressive lit up. I’m not overly worried about the 6X5 as mine was in use for many years before I put in the silicon but I plucked them out due to being defective. 6×4’s were flamed as well but I had radios work very fine with no troubles with these. The Current Macaroni I’m working on has a 6×4 and of course the buffer across the transformer secondary below the rectifier was croaked with juice weeping out so to test this radio I temporarily yanked the leaker. Another fun one to find. .008@1600volts. I got some 0Z4’s lurking around. I just have to find them.

    Forum Participant

    I htink you are smart to just replace old capacitors, even if they pass a test with a checker when put into use they may fail anyway. so long as the resistors are maked with colors go ahead and put your meter across them. normally as they fail they will drift up to a higher value or sometimes go open or intermittent. if they look blackened thats a sign too.

    normally it is best practice to check resistors when they are not in circuit which means clipping or unsoldering a lead and that will make sure you aren;t getting an incorrect readiing because of some other route the current could take that may lead from one side to the other.

    if there is a separate current path like this it can mess up your measurement resulting in a lower reading that if you unsoldered it .

    I think it is important to realize that there can be no other circuit that can RAISE the value to a higher resistance this is electrically impossible.

    so I’d say unplug it and so long as you can read the color bands just go ahead and do a quick check of each one in circut and try to locate any that are higher in resistance than the marked value.. then if you want ot be thorough sure unsolder one leg to avoid any possible confusion. Id suspect any great big ones first especially if they are blackened, or missing chunks etc. these big ones get hot and the heat causes failures..

    I think you’ll find some radios where they are mostly all ok and others that have issues. maybe some were more reliable or maybe some were stored in places like hot attics or maybe some radios hjust have more mileage on them.

    I remember some conversation around substituting solid state diodess for rectifiers. with a rectifier the current ramps up more slowly but solid state rectifiers are instant on. I remember where someone smarter than me electrically showed how to add some capacitors and maybe a resistor to slow the ramp up so there is a technique for this but I couldn’t remember or explain it to you but I think you’ll find others who may. I may also be confusing replacing older solid state diode stacks I’m thinking of the word .. its a poison, and those types diodes tend to go up in clouds of smoke.. they look like a stack of metal plates… I believe those older diodes also have a ramp up effect.

    Phil .

    Don Henschel
    Forum Participant

    Those were stacked selenium diodes and i remember they used to be common especially in battery chargers. My Macaroni in my 50 Mercury worked like crap even though I tested all of the tubes and chose the best I had in my hoard but there was poor volume, gain, etc. leading me to suspect lousy B+ so I started checking all voltages after Don sent Don a schematic and what an eye opener!! Voltages were way down, 75+% of the resistors I tested were poor to very poor and rooting around in whats left made me discover a number of these were also bitched. As a last resort I was forced to use the best of my poor resistors to get by until I break down and purchase and some of the replacement resistors were slightly smaller than what they were supposed to be but now my audio isn’t garbled and distorted when I turn up the volume like it was before. Very little volume with super bad distortion led me to believe the voltages were way down but now we are a bit on the high side, and the volume control is just slightly turned up on the stronger stations like barely a 1/4 inch rotation. Another forum led me to this place called Sal’s with much better prices on the Capacitors BUT I sent an email with my address to try to figure out how bad the shipping is. His kits are half the price but the shipping is a mystery?? While I was at it I also inquired about what he can do for resistor kits compatible to Just Radios for all sizes because if I’m going through all of this trouble to get what once was easy to get but now isn’t I might as well break down and go for the works. Keep in mind that Just Radios prices are US dollars so $99 beaver skins for a kit vs $45 for Sal’s of the same quantity and type makes me inquire!! $90 for two kits VS $200 for two kits leads me to wonder if I would still save a bundle even after paying shipping and swindle/corruption fees at the border.but we will find out and perhaps if i slap down a sizable order perhaps he can help out.
    On this forum they also mentioned the prices were much better at Sal’s as well granted they get shipping included but at Just Radios yes the shipping is free but the prices are double.

    This was after a google for capacitors and resistors and on this forum the consensus is mainly about the same thing thats on my mind which is where do we get the goodies.
    As for my google and a search for a solid state vibrator, yes solid state vibrators popped up including web sites and forums discussing how to build these and what problems you can encounter such as the fact that there is a delay or off time with the mechanical vibrator but the solid state is too fast or good with NO delay making a way of factoring in an artificial delay required to reduce increased power consumption and stress on the transformer and the B+ can even rise. This was a good search but also absolute crap popped up while searching for vibrators!! I could care less about sites showing what Donovan sang about in his song Mellow Yellow!! I want a vibrator to make my 67 year old radio happy!

    • This reply was modified 4 years, 10 months ago by Don Henschel.
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