For these Zenith AM/FM sets that use selenium rectifiers I have merely swapped in a 1N4007. I upped the filter cap working V to 160 or 200. I have not found an issue. I think that the chassis is hot because it is an AM-FM radio and using caps to make it a "warm" chassis may greatly affect FM performance. I have 4 of these Zenith sets, the H725, the 7H820, and 2 from the early 1960s. All of these sets have interlocks. The newer sets have no exposed screws, and I think that the knobs are captive by their clips and very difficult to completely pull off (tools required) I also have an RCA 8X71. All of these are hot chassis sets. IMHO, it is best to move the power switch to the hot side, add a polarized plug and use screw covers on any exposed screws on the older units.
Someone made mention of the lack of 3 pin grounded outlets on older houses. All US wiring has the "neutral" at ground potential. On all but the oldest outlets (Pre WWII), even ones without grounds, the holes are not equal in size. The longer slot is neutral (or is supposed to be).
Yes, some bad things are done by previous owners of houses. My house was built in 1954 with the original "Romex" brand NM cable with only 2 conductors, no ground. Most of the outlets were 3 prong. New 2 prong outlets are available at the big box stores and electric supply houses. When I changed my service from 60A to 200A (I did this myself and passed inspections from both the electric company and the inspection service directed to me by the town), I replaced all outlets on the original circuits to 2 prong outlets (no ground) with the larger prong being neutral, and replaced all metal switch and outlet plates with plastic ones, in order to pass Underwriter's inspection I had added new circuits for the kitchen, refrigerator, office, TV and room AC units, as there was only 2 lighting circuits and one appliance circuit which supplied kitchen outlets, refrigerator and washer. I may have overwired a little, but I would rather be overwired than underwired. Now have separate circuits for Refrig, Washer, total of 3 Kitchen Counter circuits (One outside outlet shares a kitchen circuit), Utilty circuit in basement separate from the washer. All new living space circuits for general use are AFCI protected, original circuits are GFCI protected. Dryer and range circuits are 4 wire 120 / 240V with seperate neutral and ground. Inpection agent was very happy.
In addition to incorrect polarity, one of the more dangerous things that some that think that they know do is to tie the neutral to the ground pin on these 2 conductor, no ground wire type circuits. If the neutral circuit breaks, the ground now gets hot. (Splices do go bad.) this is the switched neutral on a hot chassis writ large!
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
The radio is here! Unfortunately we're busy tonight, so pics will have to wait until morning - I haven't set eyes on it yet, it's still in Mrs H's car. Sorry folks
Many moons ago (1963), Dad bought Mom a V-M (Voice of Music) Stereo phono that had a 3 tube "SE" Amp2 50L6s and a 12AX7, 2 detachable speakers, each containing 2 6" speakers. The VM Changer contained an Astatic ceramic cartridge.The power came from a voltage doubler circuit and the output transformers were larger than normal. This set sounded rather nice. Unfortunately, when the family house changed hands, the stereo, which had deteriorated and was relegated to the basement, was lost. The large output transformer, large speakers and about 160V on the plates of the 50L6s was the key. A similar formula (About 130V on the plate, 8" speaker and more iron and copper than normal in the OPT) contribute the great sound of your Zenith. I have an RCA 1R81 "Livingston" with power transformer and 245V on a 6V6 output tube and an 8" speaker. This does not sound much better than the Zenith H725 at a reasonable volume. Only at high volume toes the Livingston outperform the H725.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
Hello MrFixr,
We must think alike " I upped the filter cap working V to 160 or 200."I use Nichicon 100ufd at 200 volt capacitor to replace Zenith's normal 80ufd .
Sincerely Richard
P.S. I agree with that Zenith really does sound very nice .
Had a little fun with the Zenith today.. I'll get pictures up here later. The first thing I tackled was documenting everything with pics, then carefully removing the chassis. One of two cord drives broke while I was looking it over, but I got that fixed already, and have the tuning mechanism running much freer already.
The radio has a date stamp under the chassis declaring 10 Jun 1954, so it is a later model. The OP tube is a 35C5, so the schematic I have is not necessarily correct (it lists 35B5).
All the valves seem to have good filaments, but there is a selection of makes, RCA, Ratheon, Philco and a lonely Zenith, suggesting some replacements have been made.
There are also a number of paper caps, and two black-beauty types, so there's a bit to go through. Should I stuff? The main PSU filter is 80,40,40,40 uF. Haven't decided on the path forward with that yet.
I spent a bit of time on the Bakelite case, and have that looking really nice already.
I have a couple of these Zeniths and one DuMont battery / Line power 4 tube portable with the original can caps and they actually work. Of course, others were bad.
Forgot to post this in previous posts. There was one of these or a similar Zenith model, chassis only in the Clinical Chemistry lab of a major Jewish hospital in Brooklyn, for years. I used to service this lab for the company that I worked for. Chassis and knobs only. Someone likely donated it to the lab after shattering the Bakelite case. Again, this is a hot chassis set. I warned them about this; it never seemed to bother them. They kept it on a classical music FM station. Again, was a great sounding radio!
As to stuffing, that is a personal choice. I don't know how valuable the radio is. The only time I stuffed caps was on my RCA 6BT Farm set, and mostly because these caps had loose paper tubes with disks as end caps. Popped out and back in really easily. Also stuffed on my Philco 20.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
As part of other errands this morning, I stopped at the hardware store and bought replacement grommets. $6 out of the door for a pack of 6!!! fortunately I had another two as eight were required. Am I being tight, or is that a ridiculous price for small pieces of rubber.
I removed the speaker, and gave the chassis a good cleaning - it needed to come out anyway to access the grommets on it's mounting plate. and got it all back together OK, except the dial cord for the tuning indicator has also broken, so will replace that later. I was more or less expecting this, since the other cord let go at first sight.
Once it was back together, I arranged the bench carefully, put knobs on the chassis, and hooked up the dim bulb tester, double double checking the chassis connection and plug orientation. I applied power, and the bulb lit fairly brightly at first, but steadily faded to the dimmest of dim glow. At first, nothing, but eventually a hint of received buzz (not power supply hum) and a station on AM. As it warmed up further, there was a pretty good reception across the range. Switching to FM it made all the right noises, and tuned in stations, although it does seem in need of alignment and not delivering the best audio - then again, FM reception in the work area basement isn't the best.
Anyway, lots of progress, and definitely a very viable project. Time to replace the paper caps and bring this up properly. I have not decided what to do about the four section filter cap yet (80, 40, 40, 40 uF) and although it works for now, I think it should be replaced as it is going back to our friend.
Ed;
I used to get packs of grommets from some auto parts stores, it's been a while but they were less then half that much, for more of them. Grainger and McMaster-Carr are two other places to find stuff like that. Getting back to the hot chassis design, I think if it works well on an isolation transformer then you probably could rewire a chassis to make it float, basically for shielding and RF grounds, the trick is to find the minimum value of capacitor that will couple the chassis, and B- together, but block most of the voltage.
Regards
Arran
I think if you went to Homeowner #&!!, they would probably be just as expensive. I have been going to my hardware store since I was 13. That's 57 years. Family owned, has passed through generations. Not much more expensive than Home Depot or Lowes, often, same price, sometimes cheaper. If I go to Home Depot, gotta park, there may not be a handicapped spot (and I never take the last one; there are people much worse than me), grab a crippie cart (unless it's the last one), get halfway to the hardware section where the battery dies on the cart, then go on a scavenger hunt for the part or for help.
OR
Pull right up to the hardware store, 5 steps to the counter where there are a minimum of 3 people plus the owner, all who know my name, in case I need help finding anything. And they usually have almost any hardware, plumbing or electrical sundry that I need.
As for floating the chassis, there may be a real issue with that. You HAM types, please chime in, but unless you make some sort of bus to transfer each ground connection to, and very near the original grounding point, the FM section alignment may be greatly affected. I guess that this is one advantage of PC boards, which Zenith eschewed, and no one else used until about 1956 or so. The FM Section of this set is tuned by variable inductance, like a car radio instead of by variable capacitor.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
Thanks for the info - I'll look around. It's not a big deal, but good to know I can plan ahead for some less expensive grommet stocks
And per the comments from others who have worked on or own one of these - it does sound good, even in electrically original condition. Nice to know that it works "as-is" with no big mysteries to solve, or missing parts to find.
I wasn't sure about the styling at first, but it has rather grown on me.
I have the same set. I don't think it is an H725 but it is very similar. I believe that this is 7H820. It has 3 bands- AM Broadcast, the original 43-46 MHz FM band that was moved to make way for TV, and the current 88-108 MHz FM Band. The chassis is similar to the H725 except for the RF and OSC Mix tubes. Both are great sounding radios.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
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