Back on this after a short break.

I'm experiencing what Gary Appel described a few posts back. The SL1 VCO module on an extender (per prior photo) can be adjusted (two potentiometers, one controlling detector voltage into the VCO, other controlling pretune DAC into the VCO) to lock across it's working range (the output of the 20-30 MHz signal at 11707A output), however, once back in position (no extender), the wandering problem returns. The reactive parts (varactor, trimmer cap and solenoid inductor) are "up close and personal" and possibly affected by proximity to the enclosure walls.

One observation (admittedly with the VCO module on it's extender) is that the VCO control voltage (coming out of SL1 phase detector, which goes to the SL1 VCO) seems to be working within a range of about -3.2 volts (below which the PLL never locks) and -1.2 volts (above which the PLL never locks. Not sure if this is a problem in the phase detector or perhaps the VCO is far off it's normal range due to (absence of) enclosure effects.

The adjustments section (5-33) begins with "set frequency to 5.55 MHz and adjust (two potentiometers on the VCO) for 0 volts at the test point (VCO control voltage) - this was not possible in any setting of the two potentiometers on the VCO module. Although I could find settings (of the two VCO module potentiometers) that would maintain lock across the 20-30 MHz range (11707A output port reflecting the composite low-frequency synthesizers), this was done with the A19 VCO board on an extender. When lock fails, the phase detector digital logic waveform (a duty cycle which gets smoothed into a continuous tuning voltage) ends up at 0% or 100% duty cycle. All of this provides a nice "moving parts" view of a PLL, but is indicative of a component-level problem probably on the phase detector or VCO module.

Advice received at this point will guide my next attempt (boards back in their shielded enclosures and test points accessed from the motherboard rather than equivalent points on the two boards themselves (the advice re measuring an enclosed test point is appreciated!)

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On March 1, 2025 4:43:06 PM "AArnaud via groups.io" <a.cabiyaud@...> wrote:

According to the manual they're actually both electrolytics.
Thanks for you kind word!

Can you post a picture of these capacitors?

-Dave

--
Dave McGuire, AK4HZ
New Kensington, PA

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On 3/1/25 11:57, AArnaud via groups.io wrote:

Hello everyone,
I'm glad to report the HP 3403C works again!
I've replaced the duos of LDs A/D chips, but this was not enough to revive the unit. It kept blinking +1 although it had lost its tendency to sometime display garbage.
I turns out I lost the +12V supply rail which I previously had. On the test point of the supply had a dead short. Taking boards out I found that it was on A7 (the board between the converter and the motherboard). Looking at it I found that both C1 and C2 looked really weird has they had a ball of solder on them and you could see inside of them.

That solder-looking ball is metallic tantalum that melted and burst through the outer shell of the capacitor. This is a classic failure mode for a shorted tantalum capacitor, usually the "dipped" variety.

I've taken both them out and although C2 is apparently healthy the C1, which is the smoothing cap of +12V, was shorted. Good thing it was regulated.
With them out the unit works again and is not too far off reality aswell. Very happy about it.
My beginner mistake was that I made the power supply measurements with basically all of the boards out. However I think the new chips were necessary as the display was not working even when A7 was out.
I'll order new capacitor for that board, and i'll also recap the power supply aswell.
Here it is reading the probe test of my oscilloscope together with its friend the Racal Dana.

An excellent recovery of an excellent instrument. Nice work!

-Dave

--
Dave McGuire, AK4HZ
New Kensington, PA