You are here

My own dogfood

Customising, building or repairing your own gear? Need help with acoustic treatment or soundproofing? Ask away…

My own dogfood

Postby Folderol » Wed Feb 20, 2019 5:23 pm

A while ago, I put up a readers advert asking for an analogue oscilloscope. I was pleasantly surprised to get two offers fairly quickly, and even more surprised that one was at an unbeatable price - free! I was warned that it might have some problems, but the owner said he didn't really know how to use it anyway.

Since then I've been rather busy with various things associated with retiring, but today finally found the time to take in into work (on a non-work day) and sort it out.

This is a Scopex 14D-10V - dual trace 10M bandwidth, from around 1980, so fine for audio.

The loose handle was an easy 5 minute fix once I got the covers off. I was also pleasantly surprised that the interior was remarkably clean. Not a dust bunny in sight!

Next was the problem of the focus control being hard up one end and still not quite focused. It was easy to find the chain of resistors for this, and one had already been replaced. A quick check revealed that the replacement had more than doubled in value. One with a slightly higher power rating but of the correct value sorted that.

After that, it was a slightly noisy Y1 channel which would then slowly drift up out of sight. That was down to a chip that was going for a walk out of its socket!

This was all the easy stuff. The one that kept me occupied for several hours was no sync. Well not quite, if hit with an extremely high level it would just about latch... sometimes.

Cold checks didn't seem to reveal anything, so it was down to trying to trace the circuit. It's a single board covering almost the entire footprint of the scope, and quite well spaced out, but not easy to follow. A bit of a clue was when I realised that the sync level control seemed to do absolutely nothing, then a live check revealed that both ends of the control were sitting at the negative rail voltage!

Easy, peasy you'd think - just follow the tracks. Well yes, but trying to get a clear idea of the relative positions of components and track on opposite sides such a large board is quite difficult - especially when there are control spindles, screening cans etc obscuring the view. I got highly confused when it seemed this terminal just went to a bunch of resistors, which themselves made no sense. They went to various points in the sync source and type switches, but just appeared to tie everything together.

Eventually I spotted an almost invisible track going from the other side of the solder pad and straight to a ground plane. This had fractured right on the edge of the pad. A short flexible link sorted it and the scope now locks correctly on just about any signal :)

A final point... In this sort of case I always make a flexible link rather than a solid one. If a board is flexing and breaking track, it will also break a solid link fairly soon.
User avatar
Folderol
Jedi Poster
Posts: 7921
Joined: Sat Nov 15, 2008 1:00 am
Location: The Mudway Towns, UK
Yes. I am that Linux nut.

Re: My own dogfood

Postby Hugh Robjohns » Wed Feb 20, 2019 5:42 pm

Nice work, and nicely explained thought processes and remedies. :clap: :thumbup: :-D
User avatar
Hugh Robjohns
Moderator
Posts: 22933
Joined: Fri Jul 25, 2003 12:00 am
Location: Worcestershire, UK
Technical Editor, Sound On Sound

Re: My own dogfood

Postby James Perrett » Wed Feb 20, 2019 9:30 pm

Good work Will. I wish my day of fixing things had been as productive.

I went to switch on the big mixing desk this morning and I remember thinking, "there's no hum from the PSU transformer". However, when I switched the monitor amp on there was hum out of the monitors and all the mute buttons on the desk were alight. Looking at the power supply I realised that the +17V light wasn't working. Since I had someone coming to the studio in half an hour I had to do some rapid reconfiguring to wire in an audio interface direct to the monitors.

When the visitor had gone I did some prodding around and there was no voltage on the smoothing capacitors. I checked the fuse between the rectifier and the smoothing caps and it was OK. I then checked the rectifier with a meter and that all looked fine too. Thinking that it looked like a transformer problem I decided to power it up again and see if I could see any volts on the input to the rectifier - and the +17V was back!

All I can think was that it was a problem with the fuseholder or the spade connections to the rectifier - though the thought has just occured to me that, as it is a PCB mount fuseholder, there could be a dry joint on the PCB. I'm just hoping it doesn't happen again at a time when I don't have a chance to look at it.

Then it was back to doing the job I had intended to do - try to get one working Betamax recorder out of 4. But that's a story for another day...
User avatar
James Perrett
Moderator
Posts: 7924
Joined: Mon Sep 10, 2001 12:00 am
Location: The wilds of Hampshire
JRP Music - Audio Mastering and Restoration. JRP Music Facebook Page

Re: My own dogfood

Postby The Korff » Wed Feb 20, 2019 9:53 pm

The exact same thing happened to my Soundcraft 600 a few years ago, James — 100Hz (plus harmonics) hum out of every output, plus all the mute and PFL lights lit up. It turned out to be a dry joint on one of the big reservoir caps in the PSU. (Then when the problem resurfaced a few years later, it was because the *other* reservoir cap had dropped to half its value...)

Hope you can get to the bottom of it!
The Korff
Frequent Poster (Level2)
Posts: 2101
Joined: Fri Jan 08, 2010 12:28 pm
Location: The Wrong Precinct

Re: My own dogfood

Postby MOF » Wed Feb 20, 2019 10:21 pm

Is the title of this thread some kind of rhyming slang or are you a surrealist?
I was expecting to read about some new career path that a disillusioned musician/composer was now taking to fund what is now a hobby or some unique advertising strand to get to pet owners to then sell them your music.
MOF
Regular
Posts: 86
Joined: Thu Mar 06, 2003 1:00 am
Location: United Kingdom

Re: My own dogfood

Postby BJG145 » Thu Feb 21, 2019 10:09 am

Clever stuff Folderol, good work!
User avatar
BJG145
Frequent Poster (Level2)
Posts: 3616
Joined: Sat Aug 06, 2005 12:00 am

Re: My own dogfood

Postby Martin Walker » Thu Feb 21, 2019 12:12 pm

Bravo Will, and an absolute bargain - a long time ago I saw a scope being offered locally for £30, and have always regretted not going for it.

My own recent triumph was repairing an old Echoplex which required a good clean and a collective tweaking of its various preset pots (and tracking down the original manual on line) to return it to its aging glory. A few minutes with a voltmeter showed that the circuits still seemed to be working fine, so I gave it a good clean with special attention to the tape path, demagnetised its heads and other metalwork (wow - I was able to use my demagnetiser for the first time in over 20 years! ;))

I then followed the service manual recalibration advice, first adjusting the playback and record head azimuth (basically twisting each head very slightly with pliers to maximise the playback level of a high frequency sinewave tone), followed by adjusting the record head bias oscillator level preset to ensure maximum undistorted output (the amount of bias required can vary markedly between different tape formulations used in the continuous loop tape cartridge).

Collectively this did the trick, and the owner now says it has never sounded as good 8-)


Martin
User avatar
Martin Walker
Moderator
Posts: 13389
Joined: Wed Jan 13, 2010 9:44 am
Location: Cornwall, UK

Re: My own dogfood

Postby ef37a » Thu Feb 21, 2019 12:49 pm

James, it seems the +17V regulator is not starting up once upon a bluey?

Self warns about this in "Small signal Design" although he doesn't give much in the way of a remedy!

I would suspect a combination of low mains input voltage at the time, res' caps being bottom tolerance and/or a lot of capacitance downstream of the regulator. You could check the input volts to the regulators then the data sheet for whatever they are for their "drop out" voltage.

DC regulator circuits can be a PITA.

Dave.
ef37a
Jedi Poster
Posts: 10053
Joined: Mon May 29, 2006 12:00 am
Location: northampton uk

Re: My own dogfood

Postby James Perrett » Thu Feb 21, 2019 3:39 pm

ef37a wrote:James, it seems the +17V regulator is not starting up once upon a bluey?


The problem was before the regulator - there were no volts on the smoothing cap. The only possible culprits could have been the fuse/holder, bridge rectifier, the transformer (it has a separate transformer for the +17V) or the wiring between them.
User avatar
James Perrett
Moderator
Posts: 7924
Joined: Mon Sep 10, 2001 12:00 am
Location: The wilds of Hampshire
JRP Music - Audio Mastering and Restoration. JRP Music Facebook Page

Re: My own dogfood

Postby Folderol » Thu Feb 21, 2019 5:35 pm

PCB fuseholders are notorious for intermittent faults, ranging from losing their springyness, to allowing oxides to creep round between themselves and the fuse cap - their shape is a very effective moisture trap :(
User avatar
Folderol
Jedi Poster
Posts: 7921
Joined: Sat Nov 15, 2008 1:00 am
Location: The Mudway Towns, UK
Yes. I am that Linux nut.

Re: My own dogfood

Postby ef37a » Thu Feb 21, 2019 11:26 pm

James Perrett wrote:
ef37a wrote:James, it seems the +17V regulator is not starting up once upon a bluey?


The problem was before the regulator - there were no volts on the smoothing cap. The only possible culprits could have been the fuse/holder, bridge rectifier, the transformer (it has a separate transformer for the +17V) or the wiring between them.

Sorry James I must have missed that. I though you had lost +17Vreg' then it bust into life.

But yes likely a fuseholder, something with "moving parts".


Dave.
ef37a
Jedi Poster
Posts: 10053
Joined: Mon May 29, 2006 12:00 am
Location: northampton uk


Who is online

Users browsing this forum: No registered users