Have a question about Variac diode in a HW24T/C528 Hand held.

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- Jamie
  
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posted
17 years ago

Sun, Apr 22, 2007 4:23 PM

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?

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- A
- Arfa Daily
  
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posted
17 years ago

Mon, Apr 23, 2007 1:13 AM

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

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- Fred McKenzie
  
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posted
17 years ago

Tue, Apr 24, 2007 12:32 AM

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