This radio is dual band Heath Kit hand held, Same as the
Standard C528/A
It appears that the VHF Tx has low audio compared to the
UHF., Using my SM, I noticed that I need to turn up the
audio drive that goes to the VCO much more than the UHF
VCO.
Both VCO are of the same design, just using different
size components due to freq differences.
There is a Variac DIODE there being injected via the
Mic audio for the FM deviation.
Both Audio sources for UHF and VHF come from the same
Mic pre amp source using a trimmer from the output for
each band..
The UHF needs only about 50% set point to get the max
5K spread how ever, I need to turn the VHF up to 100% to
make it satisfactory.. This unit has been repair three
times by some one else that has replaced all of the smt
electrolytic caps due to being leaky and thus claiming to
cause the low audio problems. Now I am looking at it.

I guess my question here is this:
Have any of you ever seen a Variac Diode lose/reduce it's
capacitance dynamic range?

--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

"Jamie" t wrote in message
...
This radio is dual band Heath Kit hand held, Same as the
Standard C528/A
It appears that the VHF Tx has low audio compared to the
UHF., Using my SM, I noticed that I need to turn up the
audio drive that goes to the VCO much more than the UHF
VCO.
Both VCO are of the same design, just using different
size components due to freq differences.
There is a Variac DIODE there being injected via the
Mic audio for the FM deviation.
Both Audio sources for UHF and VHF come from the same
Mic pre amp source using a trimmer from the output for
each band..
The UHF needs only about 50% set point to get the max
5K spread how ever, I need to turn the VHF up to 100% to
make it satisfactory.. This unit has been repair three
times by some one else that has replaced all of the smt
electrolytic caps due to being leaky and thus claiming to
cause the low audio problems. Now I am looking at it.

I guess my question here is this:
Have any of you ever seen a Variac Diode lose/reduce it's
capacitance dynamic range?

That's a vari-cap diode ...

Unless it has developed a physical fault such as having gone leaky, which
you could of course easily measure, it's hard to see how a varicap could
lose swing, and I can't recall ever having seen one do so. At the end of the
day, there is nothing special or magical about varicaps. All diodes exhibit
the effect to a greater or lesser degree, as it is purely down to the width
of the barrier layer at the junction, which is in turn dependant on the
level of reverse bias that's being applied. The only thing that makes a
varicap a varicap, is that the doping and construction has been specifically
fine tuned to maximise and linearise the effect.

I think that my first move in a case like this, would be to take some
careful measurements of both the DC bias conditions and AC drive levels at
the diode, specifically using a high z measuring device - ideally a 'scope,
if you have access to one. A x10 low capacitance probe will probably be
needed to stop the input capacitance of the scope and probe lead, from
loading and stopping the oscillator.

I would be very suspicious of board damage - partially eaten and hence high
resistance thru' plated holes, and corroded / partially eaten surface mount
Rs and Cs, if the replaced sm electros really had leaked.

Arfa

--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

In article ,
Jamie t wrote:

Both VCO are of the same design, just using different
size components due to freq differences.
There is a Variac DIODE there being injected via the
Mic audio for the FM deviation.
Both Audio sources for UHF and VHF come from the same
Mic pre amp source using a trimmer from the output for
each band.

Jamie-

You mentioned SMT and the Standard C528/A. Does this mean the Heath was
not built from a kit?

Based on my experience with Heathkits, I'd question whether it had ever
worked correctly if built from a kit. I'd be examining it for incorrect
components, backwards electrolytics, solder bridges, et cetera.

One thought - if the VCO is multiplied to get to the output frequency,
less modulation would be needed for the higher frequency due to the
additional 3X multiplication. The design should have taken that into
consideration. One way would be to use a different vari-cap diode for
VHF, that had a 3X greater delta C/delta V ratio. Is it possible that
the two vari-caps are interchanged?

I noticed that when adding a PL oscillator to an older radio, a series
resistor is used between its output and the radio's FM modulator. In
your HW24T, how are the trimmer resistors isolated from the varicaps?
If by a high-value resistor, could the VHF resistor have gone high in
value? Or be the wrong part?

Fred