I'm working on a Zenith K412G. Power enters the Selenium Rectifier, but doesn't exit; thus the radio does not work. My research on the rectifier indicates that I need to replace it with a working Selenium Rectifier or a Silicone Rectifier. Where would I find the necessary replacement? These attachments are: the schematic and parts list of the Selenium Rectifier. As always, any assistance is greatly appreciated.

Thanks,
Mike

Just use a 1N4004 (4002,3,5,6, etc) Diode. Banded (positive) end to the circuit, negative to AC.

Mike

Add an extra series resistor of 47 ohms. This will make up for higher voltage out of a silicon diode. Agree with fifties but use 1N4004 through 1N4007, not 1N4002 etc. Lower numbers are lower voltage rated.

Even Radio Shack has these diodes.

Unusual to have no voltage out of a selenium. Usually it's low. Make sure a series resistor isn't open.

Mike...

Look for a PM from me.

Norm is right in his observations.

Rich

Thanks for the correction, Norm; I wasn't aware of the lower voltage ratings on lower numbered Diodes myself, obviously.

Fifties and Norm, thank you both ....... will check the resistor and am off to Radio Shack tomorrow. Rich, got your pm and will study it tonight. To everyone, I'm blown away with the fast and accurate response to all my emails!

Mike

I replaced the selenium rectifier with the 1N004 silicon diode (1N004 was the only one at my Radio Shack). My schematic had the diode going into a 4W 180 ohms resistor; I replaced this resistor with a 5W 200 ohms resistor. Between the diode and 5W resistor I place a 1W 47 ohms resistor as Norm suggested----is this overkill? My measurements are 122 ac going into the diode 106 dc coming out (before and after the 47 ohms resistor) and 87 dc leaving the 5W resistor. I also have 7.2v going into the first heater plate of tube 3V4. All seems within reason. What I don’t have is any tubes lighting---tubes all checked fine...so it is something else. Work had been done on the radio before me. For example, the CLR (between tube 1U5 ans 3V4) has been changed (see my attachment). I don’t know why and I don’t know how to check it. My understanding is these parts don’t go bad as a rule so I want to re-attach it---problem is the part is marked 1 4 2 3 6 5 and the schematic shows B A C and A B C. Perhaps I’m wasting my time----your thoughts.

That should not affect the tube filaments.See if you have voltages at the heater pins.

Tube filaments in tubes of this type do not light brightly enough to see easily.

Steve Chambers

Agreed you will not see the tubes glowing. Now, does the set play at all? If not, check the plate voltages as shown on your schematic and let us know what you find.

Almost anytime you supply more than 1-1/2 volts to the tubes, you can see some dimly lit filaments, although in a completely darkened room, even with battery power.

Best bet is to check their heater voltages with your meter. If one shows it's voltage per the schematic, they most likely all are OK, given that they are wired in series.

Re the couplet, yes, components in them can and do go bad, although not as certain as wax or Electrolytic caps do. You might verify that the path from the input grid of the 3V4 to ground is within 20% of what the print shows for that resistor in the couplet (my old eyes can't quite read it). I recently had to jump two Couplet-ed resistors in an RCA portable tube set B/C they had changed value beyond tolerance and were affecting it's performance.

After you have verified the heater voltage(s), you might check those on the plate circuits.

The radio doesn't play at all. I do get static when I touch pin 2 of 1U5 and the voltage reading is 18V. Here is the voltage for the heaters with their correct voltage in parentheses. 3V4 (6. 7.2, 1R5 (4.2) 4.8, 1W4 (2. 3.1, 1U5 (1.5) 1.5. The plates: 3V4 (85) 78, 1U5 (15) 17, 1U4 (84) 68, 1R5 (75) 65. All the readings appear exceptable. It's a small radio and difficult to get into some areas. I keep coming back to the couplet area with lots of ????

Thanks for looking at my post,
Mike

Check the Primary and scondary of the OT.
I have had many of those go bad in this type chassis.


Bob T.

Joined: 04 Oct 2011 16:49
Posts: 67
Location: Petaluma, CA I replaced the selenium rectifier with the 1N004 silicon diode (1N004 was the only one at my Radio Shack). My schematic had the diode going into a 4W 180 ohms resistor; I replaced this resistor with a 5W 200 ohms resistor. Between the diode and 5W resistor I place a 1W 47 ohms resistor as Norm suggested----is this overkill?


A basic understanding of Ohms law and power formulas would answer this question.

Bob T and Peter,

Thank you both for your input. I'm in Kentucky for the summer and the radio is in California--I will check out the transformer when I return in the autumn. In the meantime I'll "bone-up" on ohms law.

Mike