Saturday, 8 May 2010

Audio Repair

The "number 2" audio system hasn't been working for the past month or so, but life has been too hectic to sort things out. Things came to a head this week when the XYL wanted to listen to something and couldn't, so she booked me in for some audio repair work!

I should explain ... the "number 2" system is what you might call classic British Hi-Fi (except for the Sony CD player used as source). There's a Quad 33 Control Amplifier and matching FM Tuner, exquisite Goodmans LS3/5As, a collection of Quad II power amplifiers (which don't get used very often) and a pair of Beam Echo Avantic DL7-35 amplifiers, one of which is at the heart of the fault that set the XYL complaining.

[The "number 1" system is what you might call "pro audio", based on Genelec 1031As - to be honest, I prefer listening to the number 2 system!]

The fault was a loud intermittent buzzing, which sounded like a supply fault to me - but all the mains cabling seemed OK - so the left channel's amplifier was removed to the bench...

The DL7-35 is a "commercial version of the Mullard 20-watt amplifier of which full details were given (...) in three articles in the Wireless World by W. A. Ferguson and D. H. W. Busby" (extract from a review in The Gramophone, October 1956). It uses an EF86 Pentode input stage, ECC83 as phase splitter, driving a pair of EL84s in Ultra-Linear configuration. Expecting that I would need to look inside the amplifier, I took off the top cover and connected an input and a load...

(you can't see from this angle [as it is hidden behind the choke] but I had previously replaced the output stage biasing cathode resistors (470 Ohm), along with new capacitors under the chassis)

The amplifier worked perfectly in a one hour soak test - so this must have been a mains wiring fault/intermittency, as I first suspected (indeed, some of the screws in the Bulgin AC supply plug were loose - they were closed onto solder-tinned wire ends and this is known to be bad practice). So - what started off as anticipation of a pleasant few hours messing with a valve amplifier, turned into a trivial opportunity to dust and photograph some of my collection!

Here's the pair of DL7-35s in situ atop the "Billy" corner unit (that loosely inspired last week's "Shack Furniture" project)...

Apart from the extraordinary levels of dust (HI HI), you can see the new phono input sockets I fitted (in addition to the original phono input connector, there used to be a paralleled "TV Antenna" type input socket and an input potentiometer, all of which I scrapped in the original restoration). The chassis of both amps has lost a few chips of paint here and there and the rust worm has made its presence known - but it isn't bad enough to re-finish the amps. To the extreme right, you can just see the first of the line of Quad IIs, which are on display along the rest of the bookcase.

So - now that's sorted, I wonder what to do with the rest of the weekend.

...-.- de m0xpd

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