Rotel Repair

Back in the mists of time (1997) when compact discs ruled and I had access to second-hand CD shops I blew a portion of my student loan on a cheap(ish) Rotel stereo amplifier, Technics CD player and KEF Coda 9 floor standing speakers (the KEFs were particularly spectacular with built-in rear-ported passive subs).

My first hi-fi, student days.
My first hi-fi, student days.

A few years ago, after importing it to Botswana (thanks, Mum), the amplifier started giving me gyp and it got consigned to a cupboard, then I felt the need for noise again and decided to try and fix it.

But back to my hi-fi history… the equipment lived with me through my 2nd and final student years in a couple of Victorian-era houses (oh, the joy of suspended wooden floors – sorry guys), but I left them behind when I emigrated.

When my mum came to visit for the birth of my daughter in 2003 I was able to persuade her to pack the amp and CD player, sadly not the speakers (10.5kg…) though.

They ran paired with some no-name Chinese speakers for a couple of years but something went wrong and the amp started producing terrible hiss and unpleasant crackling until the speakers died.  On autopsy the capacitors on the speaker crossovers had failed, whether this was caused by the amp, Chinese quality control or capacitor plague I don’t know.

When I started in Gaborone I noticed a set of Jamo speakers for sale in Game, at a mildly silly price.  They sat there for months, un-moving (people here like their home-cinema systems with blingy flashing lights, not hi-fi separates), until soon after Christmas I saw they had been put on special… finally, a manager had realised they were dead-stock.  I swooped.

And the amplifier ran fine for a few days before starting to play silly buggers again, finally dropping the left channel completely.  I did some reading and found various possible causes: diodes, capacitors and so on.  Time for some exploratory surgery.  Armed with a circuit diagram I opened the belly of the beast and started multi-metering everything, until I found diode D601 that seemed not to be doing what it was supposed to.

A local electronics store luckily kept stock of the 1N148 diode so I bought one, then tried to fit it… after managing to briefly melt the solder holding the diode while burning my fingers I decided electronics repair was not my thing and went to the professionals, who tested the diode themselves and found it to be fine… so I came home, played some music with the covers off and found it to be… fine.  Props to Maja Enterprises on Lejara Road, Broadhurst (opposite and up a bit from Barclays) for not laughing, and for having an astounding range of electronic components and a shop largely full of knackered cathode-ray TV sets.

So, how to put it back together with all those different length screws? Luckily I had put the top and bottom screw sets in separate bags, but hadn’t made a note of which lengths went where.

Luckily the bottom screws were all in prime number combinations with 3 long, 5 medium, 2 short and 2 fat.

A visual inspection of the relative distances from cover to receiving hole made it fairly obvious where each type goes, so screw it:

And then the ones for the top cover:

So, there you have it, in the event you take the screws out of your Rotel Ra-931 amplifier that’s how to put them back again.  I assume you know how to hold a screwdriver, and not to leave the amp plugged in while you do so.

Circumstantial Frippery

In diagnosing the fault and researching this post I downloaded some of the manuals for posterity:

KEF Coda 9 brochure from the KEF website.

Rotel RA-931 technical manual  – I have a local copy but for some reason it won’t upload to my host.

And a couple of irrelevant excerpts from the KEF installation manual:

All that verbage boils down to “I fitted 18 screws after a miraculous cure.”

Author: Michael

Parent, husband and civil engineer born and raised in Britain before emigrating to Botswana. Interests in construction, information technology, fitness, mechanics and mapping, among others.

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