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).
- 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.
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.
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.
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.
The case & front panel need to be removed….
I found a NOS Noble (Japanese) dual-gang A50kΩ pot (audio/log taper) with switching.
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.
Secondly, the shaft is cut down to length to match the old one – using the Dremel with mini-cutting wheel…..
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.
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!
DIN to RCA cables, similar to this one, are needed for connection to non-Quad amplifiers….
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.