[time-nuts] 5370B Input Board Repair [WAS: 5370B Question / help needed]

Didier Juges shalimr9 at gmail.com
Wed Jul 19 13:35:25 EDT 2017


That is very good information. I will add your email to the 5370 page on my
web site.

Didier KO4BB


On Jul 18, 2017 4:02 AM, "Thomas Allgeier" <th.allgeier at gmail.com> wrote:

> Hello All Again,
>
>
>
> I’ve got my 5370B going now and in the process made a “discovery” which I
> thought might be worth sharing:
>
> The A3 input board is a through-hole PCB with a few SMD capacitors and
> resistors on the reverse of the “switch area”. It turns out that on my 5370
> (2410A00777) these components are not soldered, but fitted with conductive
> adhesive. I first thought it was solder with a black coating but under a
> microscope it is clear that it is not solder at all. Most probably it is a
> mixture of epoxy and silver particles, or a similar compound. So no going
> over joints with a fine iron…
>
> Inspecting all this carefully under the microscope I discovered that the
> “joints” on 2 resistors (R23 and R56) had cracked. As you know this board
> gets heat from the hybrid amplifier IC’s and due to the way the board is
> mounted to the front panel I guess it sees thermal stressing when the
> instrument warms up and cools down. While this obviously lasts a long time
> it looks that on my unit the adhesive has eventually cracked in places.
> (Vigorous switch activation and pressing / pulling on the switch handles
> also won’t be helpful in this respect…) One of the resistors just fell off
> at the slightest touch with fine tweezers.
>
>
>
> Anyhow, after removing the offending components, cleaning the pads of the
> adhesive, and soldering replacements in place, we have a perfect 99.9x ns
> with the 10 MHz on the commoned inputs. Happy days!
>
>
>
> So if any of your 5370’s have the kind of intermittent fault I described
> (and one or two other people seem to have reported) or instability that
> seems to originate from the A3 board – check the joints around the SMD’s.
>
>
>
> I wonder why / how it ended up having the adhesive instead of solder –
> were earlier / later instruments the same, or was this a build change
> introduced at a certain period?
>
>
>
> Hope the above is of help to somebody else,
>
> Thomas.
>
>
>
> Message: 1
> Date: Sat, 15 Jul 2017 21:54:47 +0100
> From: Thomas Allgeier <th.allgeier at gmail.com>
> To: Discussion of precise time and frequency measurement
> <time-nuts at febo.com>
> Subject: Re: [time-nuts] 5370B Question / help needed
> Message-ID:
> <CAOT1cQLOjZ0=Xs_+nQ+MzAODboZG5ar0f8G_PydxJ=Oj6grB8g at mail.gmail.com>
> Content-Type: text/plain; charset="UTF-8"
>
> Hello Folks,
>
> I'm coming back to this after a few days of prodding etc. and there has
> been some progress. The fault has been tracked down to the A3 input board
> and it points at a crack in the board or a joint. Here is the sketch:
> After a good clean of the switches and re-seating all boards etc. the 5370
> sprung back into life with almost-in-spec performance of 102.xx ns with
> it's own 10 MHz. Trouble is it didn't last - after a while and triggered by
> sliding some switches around it went back to the 14 ns. Fiddling with the
> BNC's may have the same effect, i.e. there is a mechanical element to it
> all.
> Turns out that running it with the front panel removed I can make it go
> from 14 to 102 ns by slightly bending the A3 board, certainly while it is
> cold. After a while this trick doesn't work anymore, my suspicion is
> whatever crack/gap is causing the trouble has expanded too far to close it
> tight. After cool-down we're back to square one.
> I notice there are a few SMD components on that board, right in the middle
> where it would bend most - basically on the reverse side of the switches.
> Capacitors and resistors I expect. I wonder if it is worth going over the
> solder joints of all these carefully.
> What has stopped me so far are 2 questions:
> Am I the only one with this observation or has anybody come across this
> before?
> Secondly the solder on these SMD's is coated with a black substance.
> Clearly this could be removed somehow but it is probably there for a
> reason. Has anybody re-touched joints like these before?
> On an instrument like this it is very much a case of "proceed with care"
> and I'd hate to do more damage by rushing in.
>
> On a related subject: last year there was a discussion over redesigning the
> input board(s) for the 5370 / 5345. Did this get off the ground? If my A3
> packs up completely...
>
> Thanks in advance for any suggestions,
>
> Thomas.
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