RadioBanter - View Single Post

October 15th 03, 05:05 AM

(Fred McKenzie) writes:
Thanks to all who replied. It turns out that the one I have is
the 204c. I put two zeners and caps in place of the batteries
and now the +/- voltages are correct. Now the rest of the
trouble shooting begins.

First of all, I am not able to adjust the bias voltage to the
specified level. It stays at around 3v. The oscillator
will work occasionally, and I tried cleaning the switch
to get it more reliable with some contact cleaner and now
I cannot get it to work at all with the case on.

Any suggestions as to some likely trouble spots? There does
not appear to be any damage from the batteries.

Thanks in advance.

cheers, Paul - VA7NT email:

I have an HP204 audio generator, with shot batteries,
has anyone modified one of these to run strictly off
the 120v mains?

Paul-

I don't have any information about the older 204A or 204B. I purchased an HP
204C from someone on E-Bay, that turned out to have badly leaking batteries. I
could tell from the photograph that there had been leakage, so I bid
accordingly! The corrosion was so bad that it would have been difficult to
renovate the power supply. I also took the approach of changing it to an
AC-only power supply.

I was able to find a military manual on the web, TM 11-6625-1589-15-1, for the
HP 204C. I also found a manual for the HP 204C/204D, but it was incomplete.
Together they have about anything you need. I think you can find them by doing
a Google search.

It seems that the batteries are divided into four groups of five NiCd cells
each. They are wired in two pairs, for a plus 13 volts and minus 13 volts
supply. They normally operate in a trickle-charge mode, so each cell is about
1.3 volts. The AC battery charger provides sufficient current to operate the
oscillator and also charge the batteries.

The solution is simple. I used two twelve volt zener diodes, with each wired
in place of a pair of battery packs. The battery charger supplies sufficient
current so voltage doesn't drop below the zener knee of the diodes. Although
twelve is less than the usual thirteen volts, I found a reference in the
manuals that the power supply voltage was rated to be a minimum of plus and
minus eleven volts. Certainly the voltage would have been twelve volts nominal
when operating from batteries alone.

I can't find my notes, but believe the diodes used were 1N4742A, 12 Volt 5% 1
Watt zener diodes. Two are supplied in a pack from Radio Shack, stock number
276-563.

The hardest part was cleaning up the mess from the batteries!

73, Fred, K4DII