The first phase of the
rebuild was to remove all of the unnecessary
components from each channel – for instance the
play/record switches, all of the wiring to the tape
section, the filter switches, the erase head
oscillator. The idea being to end up with just an
audio input, loudspeaker output and mains power
supply to each channel. Once this was all removed
the amps were checked over and powered up to test,
considering they were running on the original valves
and components they worked well from the start. The
sound still had a lot of ringing at certain
frequencies (most likely caused by the old ceramic
integrated components in the various filters) and
there was a lot of hum in places. However,
everything seemed to be functioning well, the
classic warm valve midrange was definitely in there
somewhere.
|
Looking at the right channel's amp. It all
looks a bit of a mess in there, it will take a
lot of work to rebuild them. |
Some components on the feedback
circuits were originally combined into modules,
small sealed components which would usually contain
an RC (resistor/capacitor) network. These were fine
in 1963 - however, in those packages were ceramic
capacitors and resistors of dubious origin, both of
which are not something you want in audio circuits
if you're working on the basis of good sound. So
these would next be replaced with same value
discrete components.
The layout of each channel was
simple, everything hardwired with most components on
a tag board. However, both channels weren't laid out
exactly the same because the left channel had the
bias oscillator fitted. The next stage was to remove
this and to change the layout so that everything was
as close as possible on both channels.
I rebuilt the amps in two stages,
firstly renewing the components and
neatening/renewing the wiring. I left most
components the same value but increased the cathode
bypasses to 100uF and increased the coupling
capacitors to 0.1uF - this would solve the lacking
bass, bringing the low frequency -3dB point from
50Hz down towards where it should be on good audio
gear, ideally around 1Hz.
|
A quick Photoshop edit of the original
schematic to remove any redundant components -
compare it to the original in part 1, a lot has
been removed. Click for larger version. |
With the new components in place,
the amps were again powered up - the difference was
like night and day. Bass was definitely there now,
the sound was far cleaner, low midrange like velvet.
Imaging was perfect, I don't think that I've ever
heard a transistor amplifier that could image that
well.
|
The left channel rebuilt, lots of wiring
to connect - that's power supplies, speakers,
tone circuits, ground etc. |
On to part 3
