[sdiy] Opionions on Switchmode PSUs for analog synths?

Richie Burnett rburnett at richieburnett.co.uk
Sat Nov 2 16:35:33 CET 2019

> Can any bog-standard hobbyist or repair girl successfully and safely replace these with similarly marked caps without knowing anything about what the circuit design does or needs in terms of other parameters?

Safely? : Provided they understand the risk of electric shock presented by dealing with electrolytics that are possibly charged to nearly 400vdc. And provided they install replacement caps with attention to getting the polarity correct so they don't vent in your face, etc. And provided they don't compromise any creapage and clearance distances between the live and low voltage sides whilst they're in there doing the repair, then all should be "safe".  (Most repair shops would do earth bond, flash and earth leakage tests on any mains powered kit they repair before returning it to the customer to cover themselves though.) 

Successfully? : This comes down to the choice of replacement components and to some extent the tolerance built into the original design. (Some dodgy SMPSU designs are marginally stable on a good day!)  But in general always replace capacitors with ones with:

1. Equal or greater operating voltage,
2. Equal or greater capacitance,
3. Equal or lower ESR at 100kHz, 
4. Equal or greater temperature rating (105'C)
5. Equal or greater volume (to dissipate heat)
6. A reputable manufacturer. (Rubicon, Nippon chemicon, BC, Panasonic, etc.)

> I wouldn't hesitate replacing electrolytics in linears, but would be reluctant to do so in a SMPS without being more skilled in the field.

Provided you test the power supply on a Dummy load first, (not in some expensive vintage synth!!!), then you haven't got much to loose by trying. But it may well go bang if something is amiss. Just check and double check everything before powering up, wear safety glasses, keep safe distance back, use Variac, etc.

Generally SMPSUs that hiccup (pulsing power light) aren't usually that hard to fix. It's usually due to crap quality electrolytics that have cooked, and the ESR has crept up and up as they've dried out. Often this goes unnoticed until the unit is turned off and gets cold. Then the supply is unable to start up from cold. (*cough Amstrad Sky digibox.) 

The worst SMPSU faults to fix are semiconductor failures, because these often take out various other components too. For example an output side rectifier diode going short circuit will often cause the live side switching transistor to fail. Once the high voltage switching transistor goes short-circuit there is usually enough stored energy in the big 400v reservoir capacitor to ensure that this transistor is blown to pieces, burn out the current sense resistor and often vaporise sections of PCB tracking! Sorting this mess out is a much more complex undertaking. And if you happen to miss one damaged component somewhere during a repair it's not uncommon to be greeted with a bang and blue flash when you power it up, signaling the destruction of the new parts you've just soldered in! It can be a time consuming, expensive process and also nerve wracking if your not used to things going pop when you plug them in!

That's where the SMPSU repair nightmare reputation comes from, and the recommendation to just toss the knackered one in the WEEE bin and install a new one.


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