Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: Cursitor Doom <cd@notformail.com>
Newsgroups: sci.electronics.repair,sci.electronics.design
Subject: Re: PSU Ripple Update
Date: Sun, 24 Mar 2024 18:20:58 +0000
Organization: A noiseless patient Spider
Lines: 51
Message-ID: <ugr00jprjhs5ktshfqhm9j65kluariaj1h@4ax.com>
References: <3haevi1jbhf4poc4s32t99391bq4tqfc42@4ax.com> <dg8gvihqdmceroi3703qv2b4g9ookvli62@4ax.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Injection-Info: dont-email.me; posting-host="c08a9455afafb065202a3accf161b866";
	logging-data="552756"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1/yuHvmzkLK+dPcvA1GkWCSCsdlt5OQWHQ="
User-Agent: ForteAgent/8.00.32.1272
Cancel-Lock: sha1:PV3UmW2CJK8azccfCFlyWrVsqjQ=
Bytes: 3186

On Mon, 18 Mar 2024 07:19:08 -0400, legg <legg@nospam.magma.ca> wrote:

>On Sun, 17 Mar 2024 17:48:07 +0000, Cursitor Doom <cd@notformail.com>
>wrote:
>
>>Gentlemen (and others)
>>
>>I only get a few spare minutes a week to look into this, hence this
>>update. Hopefully my latest finding might ring a bell for some of you
>>and assist in pinpointing the fault with this (linear) PSU.
>>So, I've carried out a few more tests and discovered that there is a
>>total absence of ripple on the storage caps when all the downstream
>>circuitry has been disconnected. So it's totally fine with no load.
>>However, as I re-connect all those downstream circuits, the ripple
>>commences and the more connectors I re-attach, the worse it gets. This
>>is a screen shot showing over a volt of ripple at only about 66% of
>>the full supply voltage applied:
>>
>>https://disk.yandex.com/i/vgxfpXgNp-F4Yg
>>
>>Now I did check to see if there was anything downstream which had
>>shorted or gone low-resistance which could possibly account for this,
>>but found nothing amiss. So the question is:
>>What could cause ripple to arise when even very light loads are
>>applied to the output of a pretty substantial linear PSU?
>>
>>BTW, the bridge rectifiers were fine and have been exonerated from any
>>culpability in this fault.
>
>Did you replace the rectifiers, until something (anything) changed?
>
>The ripple has changed since your last photo, as have your test
>conditions. You still don't indicate a 0V reference, so we can't 
>tell what the % ripple IS. 
>
>This waveform shows equal phase peaks at the expected frequency.
>
>What is your problem?
>
>RL 
>frequency.

I fell into the same old trap as last time and the time before that
and the time before that....
It was nothing to do with the PSU. I eventually tracked it down to a
coax's shield in the RF section which had come adrift. When
re-grounded, the ripple on the output completely vanished. Must  have
been somehow picking it up from the mains transformer despite all the
screening and compartmentalisation in this device.
All that time I wasted on the PSU - just because ripple *has* to be a
PSU problem, doesn't it. Until it isn't, that is.