[Premium-Rx] Raven Research Multicoupler R1110 - a warning to all owners

[Michael O'Beirne]

Gents

I reported a problem with the PSU of my Raven Research Multicoupler and
asked if anyone had a circuit since there is none in the manual.

I am pleased to tell that two of our clan responded, David Schofield and Len
Sparrow.  David mailed me a scan of his manual, but like mine it had been
sourced from Telford Electronics and had no details of the PSU.

Len had the same fault as me and had traced the circuit of his PSU module
and kindly sent it.  The PSU is clearly not made by Raven.  It's a large
board designed for many variants, it is largely unpopulated and has many
links.  It's quite straightforward using an LM317T series regulator and the
usual feedback components to give 24 V.

The regulator is mounted on the plated steel subchassis under the PCB.  When
I removed the PCB I found that the area of metal under the IC was badly
corroded, as was the steel securing nut and all three pins of the IC, and as
I moved the IC, the pins fell off.

Under the regulator is the usual PTFE insulator and the fixing screw is
insulated from the IC by the usual TO220 washer.  However, the washer was
badly discoloured and fell to bits as I carefully removed the regulator.
CLUE!

The failure of the washer would have lead to DC leakage from the metal
chassis to the body of the IC effectively creating a battery and causing
corrosion from one of the poles - the chassis.  Not sure how this lead to
corrosion of the IC's pins and no doubt some clever cloggs here will tell
us.

Len had also separately reached this conclusion.

Pat, G3YFK, kindly send me a 317T as both my local Maplin stores had run out
(stock control ???) and today I did some surgery and all is now well.  So a
big thanks all round.

I strongly suggest that all owners of this excellent multicoupler should
open up the cover, inspect the regulator and take appropriate action.  BTW
the main RF PCB is a work of considerable beauty.

For those who like numbers, the overall gain from in to out is about +0.5dB.
The bandwidth at the -1dB points is 77.5 kHz to 59.177 MHz and at the -3dB
points is 59 kHz to 73.333 MHz.  Measured with an HP8640B at HF and an
HP651B on the 50R output for LF and all referenced to -50dBm measured on a
Racal-Dana 9301mV meter.  This is far better than the spec of 1 to 40 MHz.

The IP3 minimum spec of +38dBm and the IP2 minimum spec of +80dBm are too
high for me to be able to measure with confidence that I won't blow up
something.  I did once burn out the 1dB section of an expensive Marconi UHF
attenuator with the full umph of a R&S power signal generator.

73s
Michael
G8MOB