RCI Resistor arrays

[brandon7861]

Does anyone know the internal configuration of the resistor array marked RA3 (100k/200k) in RCI radios that have varactors on the tuning cans like the RCI69VHP, RCI6300 and DX2517? The RCI part number is RCS0670025.

The reason I am curious is because I am trying to figure out how the band-tuning varactors on the 69VHP get their voltage with the band switch set to low (if there even is voltage??). With the switch set to high, the mode switch voltage at the 12MHz oscillator turns on D149 via L46, but in low, that diode is not conducting. Do the varactors operate with 0v in low or is something supplying voltage via RA3?

I'm not fixing anything, no radio in front of me, just saw that and got curious because I never saw a varactor in circuit at 0v before. I'm missing something.

 

[nomadradio]

If I read the schemo right, that network serves to reduce the control voltage driving the track-tune varactors. The tuning voltage that drives the VCO also controls track-tune varactors in the receiver's 2nd RF and the transmitter's mixer output. The track-tuning varactors D18/D19, D99,100 and D148 all run at a higher frequency than the VCO. Should need less tuning-voltage change per MHz than the VCO varactors.

Or I could have this all wrong.

73

 

[brandon7861]

Thanks for the reply Nomad. I highlighted the trace supplying these varactors in green, and it appears they do not track with the VCO. I cannot find any connection between the PLL/VCO voltage and this trace. It appears to only have power when the band switch is set to high (orange trace), but if the switch is set to low, I dont see where the varactors get their voltage from.

I am thinking the varactor that tunes with the PLL is internal to the "VCO" block (not shown) and these tracking varactors outside the VCO have only two states, one for the high and low bands.

 

[nomadradio]

Sure enough. The VCO module is a "roach motel" for the tuning voltage coming from pin 7 of the MC145106. Tuning voltage goes into the module's pin 5, doesn't come out.

More than once I'll get asked about some section of a radio that seldom breaks. I'll have to excuse my lack of knowledge, and point out that what I know best is the stuff that breaks most frequently. This part of the 6900 circuit board never breaks, knock on wood.

Looks as if the bandpass circuits' varactors just get a binary control. A fixed voltage to raise the resonant frequency for the high bands, zero Volts to lower the resonant frequency for the low bands. Not exactly track tuning as I fooled myself into thinking. The 2950-type radios and their descendants do work that way. This one doesn't.

73

 

[brandon7861]

Sure enough. The VCO module is a "roach motel" for the tuning voltage coming from pin 7 of the MC145106. Tuning voltage goes into the module's pin 5, doesn't come out.

More than once I'll get asked about some section of a radio that seldom breaks. I'll have to excuse my lack of knowledge, and point out that what I know best is the stuff that breaks most frequently. This part of the 6900 circuit board never breaks, knock on wood.

Looks as if the bandpass circuits' varactors just get a binary control. A fixed voltage to raise the resonant frequency for the high bands, zero Volts to lower the resonant frequency for the low bands. Not exactly track tuning as I fooled myself into thinking. The 2950-type radios and their descendants do work that way. This one doesn't.

73

I appreciate the second set of eyes. I thought all these radios with BPF varactors tracked, | guess not.

Thanks!