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.

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.

Is there actually a fault? Transformer-rectifier supplies have ripple.

On 3/17/24 18:48, Cursitor Doom 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.

Isn't that what you'd expect? The storage capacitor gets topped
up twice per mains period. In between top-ups, it's the sole source
of the output current, so its voltage drops until the next top-up.

You could measure the rate of voltage drop and check if it has the
expected slope for the output current and the storage cap's value:
dV/dt = -I/C.

You could also check if the pass transistors still have enough
voltage across them at maximum current and just before the next
top-up. Do you see ripple on the regulated output?

Jeroen Belleman

On Sun, 17 Mar 2024 11:33:12 -0700, John Larkin <jl@997PotHill.com>
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.
>
>Is there actually a fault? Transformer-rectifier supplies have ripple.

Yes, it's a fault alright. The ripple is superimposed on the output of
the RF generator; it's impossible to ignore.

On Sun, 17 Mar 2024 20:20:58 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:

>On 3/17/24 18:48, Cursitor Doom 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.
>
>Isn't that what you'd expect? The storage capacitor gets topped
>up twice per mains period. In between top-ups, it's the sole source
>of the output current, so its voltage drops until the next top-up.
>
>You could measure the rate of voltage drop and check if it has the
>expected slope for the output current and the storage cap's value:
>dV/dt = -I/C.
>
>You could also check if the pass transistors still have enough
>voltage across them at maximum current and just before the next
>top-up. Do you see ripple on the regulated output?
>
>Jeroen Belleman

Not only am I seeing ripple on the regulated output, I'm seeing it on
the final output of the signal generator this PSU powers.
It's quite a beefy PSU, but as little as 90mA draw gives rise to an
unacceptable level of ripple which permeates through the whole of the
downstream circuitry.

On 3/17/2024 1:48 PM, Cursitor Doom 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?

You sure you got your probe division/scope division configured congruently?

On Sun, 17 Mar 2024 16:05:46 -0400, bitrex <user@example.net> wrote:

>On 3/17/2024 1:48 PM, Cursitor Doom 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?
>
>You sure you got your probe division/scope division configured congruently?

Yes, it's on 1x @ 5ms.
The only thing I can think of is some sort of issue with a rogue
feedback signal which has ripple on it and is causing the PSU to (mis)
regulate accordingly. But I have no schematic and there's none
available online either, so I can't check for that.

On 3/17/2024 3:47 PM, Cursitor Doom wrote:
> On Sun, 17 Mar 2024 20:20:58 +0100, Jeroen Belleman
> <jeroen@nospam.please> wrote:
>
>> On 3/17/24 18:48, Cursitor Doom 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.
>>
>> Isn't that what you'd expect? The storage capacitor gets topped
>> up twice per mains period. In between top-ups, it's the sole source
>> of the output current, so its voltage drops until the next top-up.
>>
>> You could measure the rate of voltage drop and check if it has the
>> expected slope for the output current and the storage cap's value:
>> dV/dt = -I/C.
>>
>> You could also check if the pass transistors still have enough
>> voltage across them at maximum current and just before the next
>> top-up. Do you see ripple on the regulated output?
>>
>> Jeroen Belleman
>
> Not only am I seeing ripple on the regulated output, I'm seeing it on
> the final output of the signal generator this PSU powers.
> It's quite a beefy PSU, but as little as 90mA draw gives rise to an
> unacceptable level of ripple which permeates through the whole of the
> downstream circuitry.

OK, we need to know more. Your scope picture shows about
1 volt ripple (assuming an X1 probe). Is that correct?
(As I see the image it indicates 500mV per division, but
the units - mV - are an assumption on my part as the image
is not 100% clear.) 1 volt ripple measured across the
filter caps is not a problem.

Next, what is the DC level on the input to the regulator,
and what is the DC level on the output of the regulator?
The ripple should be eliminated by the regulator if it's
working properly. Can you descri9be the regulator circuit?

Ed

On Sun, 17 Mar 2024 16:49:52 -0400, ehsjr <ehsjr@verizon.net> wrote:

>On 3/17/2024 3:47 PM, Cursitor Doom wrote:
>> On Sun, 17 Mar 2024 20:20:58 +0100, Jeroen Belleman
>> <jeroen@nospam.please> wrote:
>>
>>> On 3/17/24 18:48, Cursitor Doom 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.
>>>
>>> Isn't that what you'd expect? The storage capacitor gets topped
>>> up twice per mains period. In between top-ups, it's the sole source
>>> of the output current, so its voltage drops until the next top-up.
>>>
>>> You could measure the rate of voltage drop and check if it has the
>>> expected slope for the output current and the storage cap's value:
>>> dV/dt = -I/C.
>>>
>>> You could also check if the pass transistors still have enough
>>> voltage across them at maximum current and just before the next
>>> top-up. Do you see ripple on the regulated output?
>>>
>>> Jeroen Belleman
>>
>> Not only am I seeing ripple on the regulated output, I'm seeing it on
>> the final output of the signal generator this PSU powers.
>> It's quite a beefy PSU, but as little as 90mA draw gives rise to an
>> unacceptable level of ripple which permeates through the whole of the
>> downstream circuitry.
>
>OK, we need to know more. Your scope picture shows about
>1 volt ripple (assuming an X1 probe). Is that correct?
>(As I see the image it indicates 500mV per division, but
>the units - mV - are an assumption on my part as the image
>is not 100% clear.) 1 volt ripple measured across the
>filter caps is not a problem.
>
>Next, what is the DC level on the input to the regulator,
>and what is the DC level on the output of the regulator?
>The ripple should be eliminated by the regulator if it's
>working properly. Can you descri9be the regulator circuit?
>
>Ed

I'm unable to answer very much until the next time I have an hour to
spare to look at the issue again (which will probably be next Sunday).
In the mean time, I'll be noting any questions such as yours and will
answer them all in a single post after the next inspection. It's a
pity I don't have more free time available for this, but that's just
the way it is at present. :-/

In sci.electronics.design Cursitor Doom <cd@notformail.com> wrote:
> On Sun, 17 Mar 2024 16:05:46 -0400, bitrex <user@example.net> wrote:
>>
>> You sure you got your probe division/scope division configured
>> congruently?
>
> Yes, it's on 1x @ 5ms.

The question, which appears to have flown straight over your head, was:
Does the scope setting match the probe you are using?

I.e., is it a 1X probe (or a switchable probe on the 1X setting)?

Is the probe 1X, 10X, or some other X?

On Mon, 18 Mar 2024 01:23:18 GMT, Bertrand Sindri
<bertrand.sindri@yahoo.com> wrote:

>In sci.electronics.design Cursitor Doom <cd@notformail.com> wrote:
>> On Sun, 17 Mar 2024 16:05:46 -0400, bitrex <user@example.net> wrote:
>>>
>>> You sure you got your probe division/scope division configured
>>> congruently?
>>
>> Yes, it's on 1x @ 5ms.
>
>The question, which appears to have flown straight over your head, was:
>Does the scope setting match the probe you are using?
>
>I.e., is it a 1X probe (or a switchable probe on the 1X setting)?
>
>Is the probe 1X, 10X, or some other X?

DC coupling, with zero somewhere low on the screen, would be more
meaningful.

We don't know if that is 1% ripple, or 50%. Or what the DC voltage is.
Or what the regulator drop is.

I like digital scopes, that flag zero and the trigger point.

On 2024-03-17, 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:

100Hz triangle wave 1.1V peak to peak. Assuming a 50Hz supply, that
on the input to a regulator is to be expected, on the output not so
much.

Possible open ground connection of a 3-terminal or shunt regulator,
especially if the output voltage is high,

> What could cause ripple to arise when even very light loads are
> applied to the output of a pretty substantial linear PSU?

Very light loads? all I can think of is open circuit input filter
capacitors.

--
Jasen.
🇺🇦 Слава Україні

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.

On Sun, 17 Mar 2024 21:37:15 +0000, Cursitor Doom <cd@notformail.com>
wrote:

>On Sun, 17 Mar 2024 16:49:52 -0400, ehsjr <ehsjr@verizon.net> wrote:
>
>>On 3/17/2024 3:47 PM, Cursitor Doom wrote:
>>> On Sun, 17 Mar 2024 20:20:58 +0100, Jeroen Belleman
>>> <jeroen@nospam.please> wrote:
>>>
>>>> On 3/17/24 18:48, Cursitor Doom 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.
>>>>
>>>> Isn't that what you'd expect? The storage capacitor gets topped
>>>> up twice per mains period. In between top-ups, it's the sole source
>>>> of the output current, so its voltage drops until the next top-up.
>>>>
>>>> You could measure the rate of voltage drop and check if it has the
>>>> expected slope for the output current and the storage cap's value:
>>>> dV/dt = -I/C.
>>>>
>>>> You could also check if the pass transistors still have enough
>>>> voltage across them at maximum current and just before the next
>>>> top-up. Do you see ripple on the regulated output?
>>>>
>>>> Jeroen Belleman
>>>
>>> Not only am I seeing ripple on the regulated output, I'm seeing it on
>>> the final output of the signal generator this PSU powers.
>>> It's quite a beefy PSU, but as little as 90mA draw gives rise to an
>>> unacceptable level of ripple which permeates through the whole of the
>>> downstream circuitry.
>>
>>OK, we need to know more. Your scope picture shows about
>>1 volt ripple (assuming an X1 probe). Is that correct?
>>(As I see the image it indicates 500mV per division, but
>>the units - mV - are an assumption on my part as the image
>>is not 100% clear.) 1 volt ripple measured across the
>>filter caps is not a problem.
>>
>>Next, what is the DC level on the input to the regulator,
>>and what is the DC level on the output of the regulator?
>>The ripple should be eliminated by the regulator if it's
>>working properly. Can you descri9be the regulator circuit?
>>
>>Ed
>
>I'm unable to answer very much until the next time I have an hour to
>spare to look at the issue again (which will probably be next Sunday).
>In the mean time, I'll be noting any questions such as yours and will
>answer them all in a single post after the next inspection. It's a
>pity I don't have more free time available for this, but that's just
>the way it is at present. :-/

Were you measuring across the filter cap terminals? No other scope
ground connection?

RL

On 18/03/2024 6:47 am, Cursitor Doom wrote:
> On Sun, 17 Mar 2024 20:20:58 +0100, Jeroen Belleman
> <jeroen@nospam.please> wrote:
>
>> On 3/17/24 18:48, Cursitor Doom wrote:

<snip>

>>> 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.
>>
>> Isn't that what you'd expect? The storage capacitor gets topped
>> up twice per mains period. In between top-ups, it's the sole source
>> of the output current, so its voltage drops until the next top-up.
>>
>> You could measure the rate of voltage drop and check if it has the
>> expected slope for the output current and the storage cap's value:
>> dV/dt = -I/C.
>>
>> You could also check if the pass transistors still have enough
>> voltage across them at maximum current and just before the next
>> top-up. Do you see ripple on the regulated output?
>
> Not only am I seeing ripple on the regulated output, I'm seeing it on
> the final output of the signal generator this PSU powers.

That sounds as if the linear regulators aren't regulating.

You need to compare the voltages applied to the inputs of the
regulators, which can be expected to show appreciable ripple with the
nominally regulated output, which shouldn't. Any residual output ripple
should be a couple of orders of magnitude smaller than the input ripple.

> It's quite a beefy PSU, but as little as 90mA draw gives rise to an
> unacceptable level of ripple which permeates through the whole of the
> downstream circuitry.

It may be "beefy" but it doesn't seem to be working as it should.

--
Bill Sloman, Sydney

On 3/17/2024 9:23 PM, Bertrand Sindri wrote:
> In sci.electronics.design Cursitor Doom <cd@notformail.com> wrote:
>> On Sun, 17 Mar 2024 16:05:46 -0400, bitrex <user@example.net> wrote:
>>>
>>> You sure you got your probe division/scope division configured
>>> congruently?
>>
>> Yes, it's on 1x @ 5ms.
>
> The question, which appears to have flown straight over your head, was:
> Does the scope setting match the probe you are using?
>
> I.e., is it a 1X probe (or a switchable probe on the 1X setting)?
>
> Is the probe 1X, 10X, or some other X?
>

Always good to thoroughly consider user error.

I wonder if Cursitor Doom has heard of "MF Doom"? "Do you see the
perpetrator? Yeah, I'm right here.."

<https://youtu.be/gQtKJbptcns?si=Rd-UMvGUG4MeADmM&t=94>

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.

In article <ugr00jprjhs5ktshfqhm9j65kluariaj1h@4ax.com>,
Cursitor Doom <cd@notformail.com> wrote:
>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.

If the shield was open, then the return current for the RF would probably
have been forced to flow back to the source via the power-supply ground
paths (back to the power supply, and then through the ground wiring to
the source circuitry). Any impedances along this ground path would
have created some voltage drop, and variations in the power supply
currents (e.g. ripple) would have created a varying voltage which
would have been impressed on the output.

On Sun, 24 Mar 2024 18:20:58 +0000, Cursitor Doom <cd@notformail.com>
wrote:

>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:
<snip>
>>
>>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.

Good to hear you've tracked down the problem.

Actually measuring the output ripple of a functioning linear
can be an issue, as you're often expected to measure microvolts
on a DC level repeatedly - requiring carefull decoupling, probe
technique and sometimes external amplification, if the spec is
typically silly.

Suddenly you will become aware of all of the unshielded sources
of low frequency interference in your work area.

RL

On Mon, 25 Mar 2024 10:18:07 -0400, legg <legg@nospam.magma.ca> wrote:

>On Sun, 24 Mar 2024 18:20:58 +0000, Cursitor Doom <cd@notformail.com>
>wrote:
>
>>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:
><snip>
>>>
>>>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.
>
>Good to hear you've tracked down the problem.
>
>Actually measuring the output ripple of a functioning linear
>can be an issue, as you're often expected to measure microvolts
>on a DC level repeatedly - requiring carefull decoupling, probe
>technique and sometimes external amplification, if the spec is
>typically silly.
>
>Suddenly you will become aware of all of the unshielded sources
>of low frequency interference in your work area.
>
>RL

Indeed. Anyway, if they hadn't made it such a bitch of an area to
inspect, I'd have spotted the issue at the outset.