[Collins] Troubleshooting a R388

[Dr. Gerald N. Johnson]

On 6/1/2012 9:56 PM, Wally Klinger wrote:
>
> Hi All:
>
> I have a R-388 on the bench that hears nothing.  I have hiss in the
> speaker and thats about it.
>
> I checked and replaced some tubes that came up as weak on my TV7.
>
> I checked the voltages and resistances on the tube sockets.  The one
> thing that I found to be odd is that there is no grid voltage on the
> 1st mixer tube pin7 6BE6.  It should be -1.5 to -4.5 volts.  The
> resistance on that pin is about 316K ohms, about 10K ohms light.

The voltage on that grid is developed from local oscillator drive, in 
this stage from the crystal oscillator. So there's no oscillator drive 
and so no mixing and a dead receiver.
>
> The resistor checked okay, but I went ahead and tapped another
> resistor to ground to see if the tube will turn on, no good.
>
> I also noted that the plate voltage on V103 is zero, should be 210
> volts.

V103 is only used on the lowest three bands, its not used on higher 
bands and the plate is disconnected on the higher bands. Its essentially 
the broadcast band mixer.
>
> I also saw that the grid voltage on V105 is -1 volt and they call for
> -10 to -30 volts.

V105 is the crystal oscillator, so its not oscillating. Could be a bad 
crystal or bad feed back capacitors (grid to cathode and cathode to 
ground), or open switch contact in crystal selection or plate tuning or 
both.
>
> I am going around the top of the radio with tube extenders and
> documenting the pin voltages further down the line.
>
> I also injected 455 KC into the grid of the 1st IF and I heard it in
> the speaker......  Not screaming loud, but I heard it.
>
> Any ideas on where I should be looking/what to examine next?

>
> 73--Wally W9BEA
>
73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.