It has a Selenium Rectifier which shows high resistance in both directions. How large a diode should I use to replace this with? the resistor R25 rated 22ohms 1 w reads 29 ohms should I replace this also with a 22ohm This is the first radio I've worked on that has a Selenium Rectifier. Thanks
Whatever you do, don't install the diode according to that schematic, since it shows it to be reverse of what it should be (they mark the anode end with the + sign).
You can use a 1A 600V silicon rectifier. Change your 22 ohm resistor out and use a 72 ohm about 3 watt resistor. A 5W would not hurt.
Brenda
Looks like a trip to Frey's to get the parts I have a 35 ohm 5 w that I got from there so I should be able to at least get another 35 ohm to series if not a single around 70 ohms and there's a lot of room underneath.
The tab is marked with a +
looking at the schematic as it is there should no current flow? so the band on the diode should be toward the 35b5?
Eric
That is correct. Except for the part about no current flow. There would indeed be none through the tubes.. but it would place a negative voltage on the positive sides of the filter cap(s).
As I remember selenium bridge rectifiers, the "+" sign was in fact at the positive output. They used the wrong symbol for it, but the "+" sign is at the right place, which in case of the selenium element would be at the Cathode (negative). However a single diode, of course, installs with its triangle to the AC input and with its bar to the output. We just have to make sure we do understand the marking. If it is a regular today's diode there will be no "+" sign, but the Negative (the bar) will be marked with a bar and this is what connects to the load, and the unmarked end connects to the AC.
I would never count on resistor to do the "fuse" duty, though there are those "fusible" resistors today.
Install a properly sized fuse if you want to protect it and calculate your resistor power dissipation according to what it needs to dissipate safely.
BTW, even though there's been quite some time since I held a selenium rectifier in my hands, it does not exhibit the same resistance as a regular diode. Thing is, there are several selenium diodes in series and they have a large Vf in the first place so the battery in your meter may not be able to bias the selenium diode into conduction, and the leakage of selenium diodes is higher, so you will have some finite reading in revers too, hence the high resistance in both directions. You may test it using a transformer and a load, and then see if the proper DC voltage develops.
I got that from a book "Elements of Radio Servicing" from the mid 50's. I will try putting a little voltage across it as I have a variac and some time as I've got 2 bad tubes 19T8, 12BA6 and a 50b5 in place of the 35b5 so I've ordered 3 tubes. Also when I first took the chassis out and flipped it to recap the .05uf at the base of R25 22ohm one end was blown off.
Thanks Brenda for the diode on the radio explanation. I looked at the schematic and said to my self, "self this is backwards". Of course I'm wrong often so kept silent.
Jerry
A friend in need is a pest! Bill Slee ca 1970.
(This post was last modified: 12-11-2013, 04:54 PM by jerryhawthorne.)
Finally with the Holidays over and the deep freeze fading I got the replacement tubes put the diode in place of the selenium rectifier and powered up the set .21amps draw. FM works but I get nothing on AM other than static except for about 5sec it played then went to static a couple of times. Unfortunately the print off Nostalgia leaves a lot to be desired as to resistor and cap values and the colors on the resistors have faded so some are hard to read I don't have vol 19 so I'm working on getting a better print. I did the voltage readings and almost all are within 15% with the following exceptions. See attachment, which I hope come out
Not sure where to look next I have been looking for extra grounds and shorts, have retested tubes all for not... Any Ideas on what areas to concentrate on?
Eric
I used detox to clean the switch I might be able to use an emery board or a small piece of scotch brite if I can get it in there. I have removed one of the IF's T6 I think (my print is bad) it is connected to the 19T8 as it is listed as the am detector, The coils were good. should I look at the 12BA7 circuit? I don't want to open any coils I don't have to as I'm afraid of damaging one where there was nothing wrong.
