Quad 33 restoration (Pt II)

Next, the Amplifier boards are removed for modifications:

  • Reduce input gain by 50-60%, to suit higher output from  modern digital sources (Roe MK2 and Takman REY metal film resistors, change values to increase feedback, output stage).
  • Replace electrolytic capacitors (Elna Silmic II).
  • Renew transistors (3 per channel, BC550).

Also:

  • Existing Hunts green metallised film coupling capacitors (3 per channel) are not replaced – film and foil would be better sounding, but space is too tight and we are on a budget here!
  • Only one input coupling cap (C400, 0.68uF) has been replaced with a slightly larger value 1uF Wima MKS4 cap from my parts stock – which should also slightly increase LF bandwidth.
141115pb144856_

Removing the plug-in Amplifier Boards, L & R channels.

pc250131

Amplifier boards – before

pc280275

Amplifier boards – after

Disc (phono) preamplifier board:

  • PSU dropper resistors increased to suit higher 16vdc supply.
  • Replace transistors (4 x BC550).
  • Electrolytic caps replaced (Elna Silmic II).
  • PSU decoupling cap C313 increased to 470uF – should improve LF bandwidth.
img_0003

Phono preamp board – before

OLYMPUS DIGITAL CAMERA

Phono preamp board – after (C313 needs to be hot-glued sideways onto board).

p1130320

Phono preamp board – re-installation

p1160325

Phono preamp board installed – it’s a tight fit (on the right)

Disc Adaptor board:

  • Increase gain of M1 position (optional) by changing feedback resistors R105/106 and R107/108 to 470Ω and 220Ω.

Tape Adaptor Board:

  • Replace transistors (2 x BC550).
  • Replace carbon film resistors R203/204 and R215/216 with metal film (Roe, Takman REY or similar).
  • Replace output coupling caps with larger value (increased from 0.33uF to 1.5uF, Panasonic metallised polyester) to increase LF bandwidth.
  • Replace input coupling caps with larger value (increased from 0.33uF to 0.68uF Panasonic metallised polyester). These were the largest physical cap sizes that would fit in through the rear hatch.
  • Adjust the 3-position, input and output sensitivity screws to suit your system levels.
OLYMPUS DIGITAL CAMERA

Tape Adaptor Board – completed (note ‘sensitivity’ setting screws for Record and Replay).

OLYMPUS DIGITAL CAMERA

Tape and Disc Adaptor Boards – re-installation.

Volume control:

After almost 50 years the volume pot on this Quad 33 is giving trouble – the shaft bushing is very worn and loose.  Signal ‘drops out’ occasionally, at low volume settings especially.

p1020298

Original volume pots – note the very tidy Quad wiring and early ‘ribbon cable’.

The case & front panel need to be removed….

img_0132

I found a NOS Noble (Japanese) dual-gang A50kΩ pot (audio/log taper) with switching.

img_0319

Quad British pot (top), Noble pot (below).

The first problem….
The new Volume Pot is Japanese, so its ‘bushing’ is metric 10mm, slightly bigger diameter than the ’60s British 3/8 inch (9.525mm).

The chassis hole is enlarged using a Dremel tool with grinding stone.

img_0971

Secondly, the shaft is cut down to length to match the old one – using the Dremel with mini-cutting wheel…..

img_0324

The ‘flat’ on the new shaft is not in the right place for the knob markings – grind the new shaft, with some trial and error.

img_0323

Finally complete – the new pot with re-soldered wire connections.  As the new pot is a bit smaller, the wires are a bit stretched.  Not quite as tidy as Quad’s original work!

img_0325

Interconnect cables:

DIN to RCA cables, similar to this one, are needed for connection to non-Quad amplifiers….

OLYMPUS DIGITAL CAMERA

OLYMPUS DIGITAL CAMERA

5-pin DIN plug .

Although fiddly to solder and unsuitable for large wires, in my opinion, DIN connectors are superior to RCA connectors for signal fidelity – minimal metal-mass, silver-plated pin connections, ultra short signal path and no mechanical joints or threaded surfaces to distort the signal.

OLYMPUS DIGITAL CAMERA

3.5mm TRS plug for iPod, etc.

quad-33-block-diag-110217

Quad 33 – amended block diagram.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s