[Milsurplus] BC-342 Help Needed Again

[BSugarberg]

Hi Joe,

I am looking at a copy of the BC-342 manual TM 11-850 (June 4, 1942).

Paragraph 47 - Voltage Checks - Page 36:

d: Another important but often overlooked point is that the heater
circuits in all of these receivers are connected in series-parallel.
The nine tubes are grouped as follows:

1st Det and 2nd RF
RF Osc and 1st RF
1st IF and 2nd IF

2nd Det, CW osc, with a 60 ohm resistor R47, all in parallel, and
the combination in series with the audio amplifier tube.

Thus, if one heater in any of the 1st three pairs goes open, the companion
tube, while still operative, will show no heater voltage.

The 4th group is more complicated.  If the audio tube heater goes open,
no voltage appears on either the 2nd detector or CW oscillator tubes.
However, the heaters of either or both of the latter can go open, but a
reduced voltage still appears across the audio tube because the 60 ohm
bypass resistor remains in the circuit.  The resistor is added to the
parallel heaters to equalize the current distribution in the network
formed by the three tubes; the audio tube takes 0.7 ampere, the other
two together only 0.6 ampere.

I hope this helps.

73, Bruce WA8TNC
================
carolew wrote:

> OK, it definitely seems like a heater-voltage problem. The 6F6 (2nd
> Audio) and 6R7 (1st Audio) stay stone cold. The rest of the tubes get
> warm. In reviewing the schematic, I don't see how I can have heater
> voltage on every tube except the 2nd Audio and 1st Audio. The wiring
> seems intact. The two bypass caps (C-78 and C-79) are new.
> 
> Any ideas? Any suggestions on troubleshooting? There appears to be a
> broken wire on the back of the middle section on the Off-MVC-AVC switch.
> Does the filament chain connect to that switch?
> 
> As you can probably tell, this is the first BC-342 that I've really had
> to dive into. It's also the first time I've had trouble with any
> boatanchor filament system.
> 
> Again, thanks in advance.
> 
> Joe Connor