RCA 1-TP-2E is silent!

[Jonathan]

I purchased an RCA 1-TP-2E from ebay a while ago. I purchased it because I ourchased an RCA ad that featured this radio. It is one of those radios that uses RCA's VS149 4V battery. I get a quiet pop out of the speaker when I turn it on, but that is it. The speaker has a center tap, believe it or not, and it's connected to ground. I ordered the Sams and have the djvu schematic from techpreservation.net (I think that's the site). I'm not sure where to start. Possibly someone could have popped in a 22.5V battery and fried some things? I'm powering the radio with 4V from a bench supply. Anyone have any suggestions?

Thanks.

Jonathan

[kc8adu]

any background hiss with the set to your ear?
touch the center tap of the volume control and see if it hums.
first replace all electrolytics and proceed with the above if it is still dead.

[Jonathan]

I hear no hiss at all, completely silent. Do you think dried up electrolytics would cause this? I believe two out of 3 are in the AF amp. The sams comes shortly, but in the mean time i'll order replacements.

Thanks.

Jonathan

[Jonathan]

Touching the wiper of the volume pot gives me no sound at all at max volume. Once the replacement electrolytics come I'll give it another try.

Jonathan

[Chad Hauris]

We have found several cases of bad germanium transistors in 50's and 60's equipment so be on the lookout for those too.

[Jonathan]

It uses the 2N412 as an oscillator, 2N410 for it's two IF stages, a 2N408 as it's AF amp, and two 2N408's in a complimentary configuration as the audio power amplifier. This is why it uses a center tapped speaker. These transistors were so popular that NTE sells overpriced replacements. There was also a .03uF ceramic cap soldered on the other side of the PCB for some odd reason. The schematic I have doesn't show that. I believe it's used for coupling. I'll do some tracing to figure out why it's there. In the mean time, I'll play with it some more.

Thanks.

Jonathan

[Jonathan]

Welp, replacement electrolytics came today. I installed them and only the AF section works. The 2N408 transistors comprising the preamp and output stages works great. I'm listening to an Alice in Chains CD through it. I connected the audio output of a CD player through a .01uF coupling cap and it sounds pretty good. This must mean the issue is with the IF sections. I don't even get a little hum or static through the speaker with everything normally connected.

When replacing the electrolytics, the .05uF coupling capacitor that couples the RF signal into the RF converter had a broken lead. The ceramic shell cracked and the plate with some ceramic and the lead was broken away from the main body. To me this failure was odd. I replaced it with a .047uF green dipped capacitor.

Secondly, a 12k resistor had a cracked case. One end of it, part of the shell was cracked, exposing the lead which touched the carbon inside. I removed it and installed a newer carbon film 12k 1/4W resistor. This resistor was part of a voltage divider which is comprised of a 12k resistor (the one I changed) ans a 1k resistor. The one end of the 1k resistor was connected to ground, the one end of the 12k resistor was connected to 3.7V (3.7V is created by connecting a 47ohm resistor to the 4V battery).

Anyway, there is .284V at the cathode of the diode according to my calculations. It's just the voltage needed to forward bias a germanium diode, but .284V is at it's cathode and it's anode is connected to the input of the 1st IF transformer and the output of the LO transformer. There is no DC bias where the anode is connected. The Sams says the germanium diode's purpose is "Overload, AVC". AVC must stand for Automatic Voltage Control, but why is this and overload protection needed? Can the LO and RF section somehow generate voltages higher that 3V? Is this clamping any DC voltage to 3V if a DC voltage somehow finds it's way at to the input of the first IF transformer?

Thanks.

Jonathan