[Premium-Rx] Cubic HF-1030 repair and observation

[VK3XYZ]

Hi all,

I recently purchased a Cubic HF-1030 and thought I’d share some info on the
source of a fault and on the heat generated by the receiver.

 

When it arrived the receiver exhibited strange behaviour. Below about 5 MHz
it wouldn’t receive stations, even local AM stations, and above 5MHz it
appeared to sometimes work, receiving stations that were not on the
displayed frequency but showing good sensitivity. These stations would drift
in frequency over time and then just disappear.  The symptoms seemed to
suggest a synthesizer problem.

 

One LO is fixed at 40MHz and the other tunes between 40-70MHz. The 40MHz VCO
was locked, the 40-70 MHz LO output showed that it was not locked, as
expected - it was drifting terribly.  I suspected the worst and started to
look for some of those now impossible to find Plessey modulo dividers and
other ECL ICs.

 

Further investigation revealed that the actual problem was a 22V supply
rail for the VCO. This is created from a discrete voltage doubler circuit.
It uses a 74HC904 with all outputs in parallel driving a 2n2907 and 2n2222
in push pull to a capacitor/diode voltage multiplier. The output of this
multiplier then powers a LM317L. The problem was the 2n2907 was faulty. This
didn’t give sufficient drive to the multiplier to get above 22V and so the
LM317 regulator wasn’t regulating. The output voltage would vary from 7 to
11V.  The problem looks like it might be fairly common as the PCB showed
signs of these being previously replaced. 

 

While repairing the doubler I also noted that the receiver gets extremely
hot.  Output from a full wave rectifier (18-20V, haven’t measured it as yet)
goes to a series pass regulator for 12V with the pass transistor bolted to
the back panel, and this powers the receiver. Two of the plugin modules use
7805 regulators off the 12V rail and the others use a handful of 78L05
regulators. The two modules using the 7805 were also quite warm.

 

Powering the receiver off 15V DC showed it was drawing 1.9A at turn on and
1.6A after the reference heater stabilised.  I replaced each of the two 7805
with a switch mode board replacement that uses a TI TPS5430 and the current
reduced to about 1.3A steady state, saving 5W of heat.  This has reduced the
temperature of the back panel, I can at least touch it now when running off
mains but I think there are still some efficiencies to be made, potentially
a “display off” switch as the 7 segment LEDs draw up to ½ amp, or perhaps a
dimmer of some sort? Another saving may be to change the toroid transformer
to 12V rather than 15V, should save a few watts of heat in the series pass
regulator as long as it doesn’t drop out .

 

So, if you come across a HF-1030 that exhibits peculiar PLL behaviour then
I’d suggest you check the 22V VCO rail and voltage doubler circuit first.

 

Cheers

Greg