[Test-Equipment] HP 8640B Final Amplifier Replacement??

[Steve Harrisonusa]

> Hello!
>
> There is some hope :-)
>
> http://www.shelbrook.com/~ve7ca/Tst864.htm

Thanks, Jose; I'm aware of that article, which appears to deal only with
using a MAV-11 for the preamp amplifier. It doesn't seem to say that a
MAV-11 has actually been tried for the final amplifier, however.

There are several reasons I am doubting that a MAV-11 will work well as
the final amp:
1. the HP manual indicates the final amp has a gain of around 16 dB;
2. the manual indicates the final amp's output power is +19 dBm, and I
can't see that the final amp is ever turned off while the generator is
powered up;
3. the harmonic content of the HP final amp is very good, indeed, at +19
dBm output power, far better than I've achieved with MAV-11s.

Yes, a MAV-11 will allow one to get the generator back "on the air" at
reduced output power; but before I did that substitution, I was wondering
whether anybody had come up with a "more powerful" solution that would
restore the output power, and output harmonic level, more closely to
original specs.

Motorola used to have some hybrid amplifiers that I rember were speced to
go down to as low as 5 MHz, like the ones in the blue rectangular plastic
packages. I no longer have my Moto catalogs so can't check. I seem to
rember that those hybrids that did go down to 5 MHz, may also have had a
high frequency limit of only 40 MHz; I just don't rember. The attractive
thing about those hybrids is the gain, per module, is around 15-18 dB; and
the 1-dB compression output power spec is generally over +23 dBm. Both
specs are ideal for the 8640, presuming the bandwidth is adequate.

But I also seem to recall that when I chopped tops off some of those blue
rectangular plastic packages, I found tiny toroidal baluns being used for
at least the output stages if not also the inputs. Those transformers
would certainly limit frequency response drastically; and IIRC, they were
one of Motorola's "bragging" points when they introduced the hybrids. So I
strongly suspect there is no one hybrid out there that'll cover 400 KHz
through 550 MHz, as required by the 8640B.

Thanks,
Steve K0XP