[ARC5] Deaf BC-454

[Brian]

Hello all you helpful folk,

I have checked all the resistors, including the screen Voltage divider chain – all OK. I checked all the fixed capacitors and replaced all the faulty ones. One at a time, I replaced all the tubes – none had been cold, and none was excessively warm. 
I’ve loosened off all grounding screws a couple of turns and retightened, and with the potted capacitors, loaded the screws with Almanox before reaffixing the caps – so, I know all grounding points are OK.
I have set the HT at 170 V and the screens get 73 V. Current drain is about 35 mA. Operating the RF gain control gets all the cathode Voltages rising in step.
For a signal generator, I use an HP 8640 B and for an audio level meter, a Marconi terminating output Wattmeter.
I measured along the chain from antenna input right to output of the 3rd IFT. All the results show this set as being attenuated between 40x and 5000x. For instance, when I apply 1 V of 1415 kHz 30% modulated by 400 Hz to pin 4 of the 2nd IF amplifier, I get 4.6 mW AF output; the op and maintenance manual says that 110 mV should give 10 mW AF out. So, attenuation is 10 log 10 / 4.6 + 20 log 1000 / 110 = 3.37 + 19.17 = 22.5 dB down.
The audio output waveform from the detector to the output side of the AF output xfr is 400 Hz blips spaced 20 ms.
In all my repairing of Command Rxs, I haven't met such a deaf one before. Usually when I've aligned the IF strip, the rest is pretty close. But not this time. Pain.

73 de Brian, VK2GCE.

From: J Mcvey 
Sent: Monday, September 26, 2016 12:17 AM
To: Brian ; arc5 at ix.netcom.com ; ARC-5 
Subject: Re: [ARC5] Deaf BC-454

The AGC is effectively shorted out at max volume on the BC-454. 

I once encountered strange issues with an ARC-5 radio, which was traced to a mis-wired front panel plug-in.
The gain control ground ran through the audio output winding!  This caused poor gain, distortion and oscillation.

Did you restore the "capacitor pots" ?
don't remember ever finding a bad half watt resistor in one of those radios, just bad "paper caps" in the "pots"
I cut them open with a pipe cutter and replace them with ceramics, then solder the pot back together. Looks "legit".

The 7K /10W wire wounds are often bad, which provides screen voltages could cause gain issues too.


If you have a scope and RF generator , you can check divide the problem in half by checking the IF chain.

Maybe it's just a bad RF or MIxer tube? 
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