[Engineer[_2_]]

On Nov 21, 6:34*am, Patrick Turner wrote:
I wrote before about my old HP606A 6 band oscilator with AM
modulation.

The switchable 10dB stepped output attenuator is enclosed and buried
in metalwork and too hard to pull out to examine. No need, there's
nothing wrong, and its quite nicely designed for constant resistance
50 ohms output R and compensated loading on the 6CL6 output tubes.

Operating the concentric small knob on the attenuator switch gives you
fine adjustment of any chosen level of the switch between that level
and no signal. The actual adjustment to the sig level is by adjusting
the Vdc bias on the 12B4 modulator with a pot so that the 6CL6 can be
run and normal idle full signal with 4mA idle current, or cut off, so
no signal appears.
But if the internal or external modulation is set for 100%, the sigal
becomes much over modded when one reduces the carrier level with this
fine adjust vernier knob to alter Vdc bias.

So the level must be chosen carefully, preferably without using the
vernier at all, which is possible if the radio under test has a vari
cap coupling to its antenna, and the signal must be monitored with CRO
because the RF level meter gives different mod % reading for different
ranges of F for the same % of modulation.

The 606A has an interesting feature in that it has a 6AW8 differential
amp which allows dc control to calibrate output levels and also to
amplify the modulation signal and have a detected AF signal applied as
negative feed back to keep the modulation envelope linear. The gain
reduction is 1/65, so that if I had 3% AF envelope THD at 75%
modulation, with NFB this almost completely dissapeared to a value of
about 0.05%.I tried shunting the FB, and the modulation linearity was
poor above 66%, and of course it only took a tiny amount of drive to
get high modulation.

Thus its possible to get full 100% mod with under 1%THD at below 1kHz.
But above 1 kHz, and by about 10kHz, the modulation cannot be
increased to 100% despite all that NFB, more like 50% before THD sets
in bad by 20KHz.
The distortion is a slew rate type and I could not find the cause. The
internal detector workings looked fine.
But the *AF BW of a receiver can be tested using 50% mod when the
detected AF to 20kHz is way under 1% and quite negligible if the
incoming modulating from a low THD source is used. With carrier at
455kHz for IF testing it is a very good tool to use, and if an AM
radio reproduces an clean AF wave from a 100% modded incoming carrier
then the radio is usually very good sounding. The audio energy lessens
above 1kHz and its rare 100% mod occurs. If the AM radio is flat to
say 5kHz, with -3dB at 7kHz and fast roll off above, then usually the
radio sounds extraordinarily good compared to the usual crap where its
-3dB at 2 kHz, with rough sounding THD&IMD products replacing the real
source HF content.

The 606A will help keep the workshop warm in winter.

My own SE pentode AM sig gene is 20db simpler and does most of what I
want but not any short wave and the short wave gene I made with a 6BE6
and about 60% AM, very rough, is rather primitive and a bit harmoniky.
The 606 would be better.
But an afternoons work to change my own gene to have audio FB and a
differential amp drive for the grid modulation should reduce the
approx 1.3% THD at 90% mod I now get to very low levels indeed, and
push the envelope to real 100% mod without quitting at 95%.

As it is, my own gene gives wider BW with 95% mod than the 606.

Because the 606 goes to 65MHz, its possible to make an FM gene and
then double the F in an external box, and cover the FM band. But I
probly won't get around to it, and my home brew stereo FM gene in a
metal cigar box is quite good enough. The BA4014 runs off 1.4Vdc, no
need for 15Kg and lotsa toobes and volts.

My own AM gene can be set for 455kHz and I have a switched in group of
7 x 65V zener diodes used as varicap diodes set up so they don't
conduct with a variable Vdc bias which changes their capacitance
nicely. So the gene produces a wobbulated 455kHz signal, +/- 44kHz,
and you see the IF selectivity shape while adjusting it if wanted.
I guess the 606A could be made to wobble its oscillator F about +/-
100kHz quite easily at about 47MHz carrier, not much C change is
involved.

I think I know why aircraft were so slow to get off the ground in
1955. Their electronics were so heavy.

And generally, even with their constant smoking, young blokes were
fitter in 1955, and because there was so much luggin 'evvy gear round.

Nowdays, nearly all don't smoke but they are mostly overweight, and
fobidden to carry more than 16 Kg.

Patrick Turner.

Just a coincidence, but I'm in the process of troubleshooting a 606B -
very similar to the 606A but with a solid state P/S - the rest is
tubes like the 606A. It won't oscillate on any band! So far the P/S
checks out (300, 200 and 25 VDC), although the 25 VDC rectifier diodes
seem to get a bit warm - must check that. I'm about to tackle the RF
oscillator section. I have the manual, actually for both the 606Aand
606B - the latter downloaded from an HP site, the 606A a paper copy
that came with the 606B unit - not the same, the "B" has a tube-based
P/S. The thing is very large and heavy! It's built like a brick
outhouse and takes up a huge amount of bench space.
Cheers,
Roger