[Collins] Troubleshooting a R388

[Dr. Gerald N. Johnson]

On 6/1/2012 11:16 PM, Wally Klinger wrote:
> Hi Jerry:
>
>  >From your message below.....
> 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.
>
> ---- Copy that! I switched down to BCB and voila! the plate came up.
>  >
>  > 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.

The bias on that grid is also from the oscillation and the low voltage 
indicates very low amplitude of oscillation.
>
> I put my counter on pin 5 the plate of the oscillator, and I saw RF to
> coincide with the manual.

In some circuits adding plate to ground capacitance, the counter cable 
might cause it to oscillate. You need to know the amplitude which should 
have a peak voltage equal to the bias on the mixer tube. You can try the 
counter on the mixer grid, if it doesn't count there it would indicate 
the 100 pf coupling capacitor or its connections is open.
>
> Does the RF output from the Colpitts Oscillator couple via the plate, or
> how does it feed the mixer? Like I said, I put the counter there and I
> measured Rf that changed as I tuned the band switch.

It couples to the mixer grid through a coupling capacitor. C136.
>
> I will try to locate the grid and cathode caps on the schematic and the
> radio. The olive colored moisture paint/varnish makes troubleshooting
> interesting!

They are C165 and C166. Check the screen bypass, C164 also.

This is an electron couple Colpitts oscillator. The oscillator "plate" 
is the bypassed screen, then the electron beam passes mostly to the 
plate which has RF voltage but has some isolation from the oscillator so 
changing loads has less of an effect on the oscillator frequency. An 
open screen bypass will lower the RF voltage at the plate, or a leaky 
bypass will lower the screen voltage and lower the RF voltage 
everywhere, which is hinted at by the very low grid voltage.

The counter is probably way more sensitive than the mixer grid.
>
> Appreciate your insight Jerry!
>
> 73--Wally W9BEA


73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.