[time-nuts] Re: Power supply buzzing in HP 58503A GPS receiver.

[Tom Knox]

I would bet it is the cap and possibly semiconductor on the front right of the main board, not the main supply.
Hope that helps.
Cheers;
Tom Knox
SR Test and Measurement Engineer
Phoenix Research
4870 Meredith Way Apt 102
Boulder, Co 80303
Formerly of:
357 Fox Lane
Superior Co 80027
303-554-0307
actast at hotmail.com

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________________________________
From: Jim Lux via time-nuts <time-nuts at lists.febo.com>
Sent: Monday, September 11, 2023 2:20 PM
To: time-nuts at lists.febo.com <time-nuts at lists.febo.com>
Cc: Jim Lux <jim at luxfamily.com>
Subject: [time-nuts] Re: Power supply buzzing in HP 58503A GPS receiver.





Instability in a switching regulator faced with a capacitive load isn't unusual.  So if the mains supply is a switcher, it might not be stable into the actual load. So a input capacitor (or the equivalent input capacitance from the input of a downstream switcher) that changes, either up or down,  can destabilize the upstream switcher.

I'd point out that this is something that is quite tricky in real life - tools like LT Spice, or the tools provided by the mfrs can help, but ultimately, it's "breadboard it and measure it." Especially since capacitors have significantly wide tolerance bands.




On Mon, 11 Sep 2023 09:10:08 +0100, Adrian Godwin via time-nuts <time-nuts at lists.febo.com> wrote:

I don't have a 58503A but I do have some RFTG-u which I understand to be
quite similar. I'm not sure if that extends to the power supply
architecture. Do they just have an integral mains power supply before a
Lucent DC-DC converter ?

I had one fail due to a tantalum capacitor downstream of the DC-DC
converter. It didn't shut down the DC-DC but it did cause it to take a
heavy incoming current resulting in the literal melting (and subsequent
mains breaker cutout) of a cheap power brick that was being used to provide
24V.

So, the mains part of the supply might be running at an overcurrent without
shutting down.

Replacing the tant (and the power brick) fixed it without any other damage.


On Mon, Sep 11, 2023 at 5:31 AM Ed Marciniak via time-nuts <
time-nuts at lists.febo.com> wrote:

> I don’t have one of those units, but switching power supplies under
> certain load conditions can have subharmonic oscillations that happen to
> fall in the audible frequency range. Absent an overload from a shorter
> capacitor or similar, I’d take a hard look at whether there are capacitors
> that no longer meet their ESR specifications and/or have cold solder joints
> effectively removing them from the circuit.
> ________________________________
> From: Wilko Bulte via time-nuts
> Sent: Sunday, September 10, 2023 3:57:06 PM
> To: Discussion of precise time and frequency measurement <
> time-nuts at lists.febo.com>
> Cc: Wilko Bulte
> Subject: [time-nuts] Re: Power supply buzzing in HP 58503A GPS receiver.
>
> Wild guess here, this sounds to me like a coil "whistling". Maybe there is
> an inductor or transformer that lost its potting compound, or has somehow
> come loose from its fittings?
>
> best,
> Wilko
>
> > On 10 Sep 2023, at 21:24, David Kirkby via time-nuts <
> time-nuts at lists.febo.com> wrote:
> >
> > After about 6 months of being powered off, I switched on my GPS receiver
> > yesterday. The PSU is buzzing quite loudly. Sound file at
> >
> >
> https://www.kirkbymicrowave.co.uk/noisy-PSU-in-HP-58503A-GPS-time-and-frequency-standard-recorded-10th-Sempember-2023.m4a
> >
> > I thought that one of the 100 uF 400 V capacitors after the rectifier
> might
> > be bulging, although I was unsure. The capacitance measures okay, and the
> > ESR is below that of anything I can buy that will physically fit. Same
> for
> > the 47 uF 400 V capacitor. These were both measured out of circuit, using
> > an HP 4284A LCR meter at 120 Hz. I chose that, despite the fact that we
> > have 50 Hz mains here in the UK, as data normally seems to be provided on
> > capacitors at 120 Hz.
> >
> > Has anyone experienced this before? I believe that someone wrote here, or
> > might have been elsewhere, that the GPS receiver was pushing one of the
> > supplies to the limit, and another one was not loaded enough. I know
> there
> > was some mention of converting it to run from a linear supply, although I
> > think that would be physically larger, so not fit in the case.
> >
> >
> > --
> > Dr David Kirkby Ph.D
> > Kirkby Microwave Ltd
> > Email: drkirkby at kirkbymicrowave.co.uk
> > Web: https://www.kirkbymicrowave.co.uk/
> > Telephone 07910 441670 (UK) or +44 7910 441670 (international)
> > Registered in England and Wales, company number 08914892.
> > Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3
> 6DT
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