Blog posts

The Myth of the Quad II/22/tuner power supply balancing

Submitted by dada on Tue, 02/09/2021 - 15:10

It has been claimed, and indeed I have uncritically repeated it myself in the past, that the Quad 22 and tuners draw heater and HT current from different Quad II power amplifiers, so that the heater and HT loads are distributed.

However reference to the Quad 22 and FM Multiplex schematics shows that this is simply not the case. Instead the distribution is simply such that the heater and HT that comes from the Yellow amplifier goes to power the 22 and also out the Yellow and Black rear sockets, and the heater and HT from the Blue amplifier goes out the Blue socket.

Quad AM2/3 service notes

Submitted by dada on Fri, 11/27/2020 - 14:00

1. Check all resistors attached to the HT line, including the screen resistor to the detector valve, and replace if more than 10% out of tolerance.

2. There are about ten film capacitors. If they are of the red Hunts type, replace them all on sight; otherwise measure and replace when out of tolerance.

3. Valves in these usually last forever: you are unlikely to have to replace them.

4. Carry out the audio-frequency alignment in the manual if you have appropriate equipment, changing the notch frequency to 9KHz as per international agreement in about 1978.

Quad FM1/FM2 service notes

Submitted by dada on Thu, 11/26/2020 - 11:17

Despite their external appearances being so different, the Quad FM1 and FM2 tuners are internally almost identical.

They are both very well-regarded FM tuners and are still capable of delivering a quality FM sound when in good working order. The guidelines for renovating these are basically the same as for any piece of valve equipment.

Bias Stability in a Quad 303

Submitted by dada on Thu, 11/26/2020 - 11:15

I had set the bias in my own Quad 303 in 2006 or 2007 with the aid of a high-quality Tektronics AA501 distortion analyzer and oscilloscope. This technique clearly shows cross-over distortion spikes when present, so it is easy to arrive at the minimum bias setting required to eliminate cross-over distortion. By this means I usually arrive at somewhere around 10-12mV across the emitter resistors.

Practical issues and benefits of two of the Quad 405-1 modifications suggested in Bernd Ludwig's paper on the Quad 405

Submitted by dada on Thu, 11/26/2020 - 11:11

These modifications are not for the inexperienced or those without adequate test equipment. This article is provided for information only. Use at own risk.

This article is an exploration of the practical issues and benefits of two of the Quad 405-1 modifications suggested in Bernd Ludwig's paper on the Quad 405.

Misprint in Quad 99pre manual

Submitted by dada on Thu, 11/26/2020 - 11:09

Several 99pre owners have complained of low volume and dull sound from the phono preamp. We traced this to an error in the user manual on page 10. It shows an example:

    moving-magnet input selected             PH 00

which should read

    moving-magnet input selected             PL 1.00 (or 3.00 or 7.75)

or else

    moving-coil input selected     PH 00

Quad FM3 service notes

Submitted by dada on Thu, 11/26/2020 - 10:55

Quad produced some 50,000 FM3 tuners between 1971 and 1982. So there are plenty of units out there, so it is well worth while developing a service approach. This article is the result of having three customer FM3s on the bench at the same time as well as my own unit, which I’ve owned for 20 years but am only just renovating now.