Wednesday, April 02, 2008

Quad FM1 - FM2 revision notes

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.

The notes below also include the results of a lot of experience with these two tuners in particular.

But first a WARNING: valve equipment contains lethal exposed voltages: in the case of the Quad valve tuners, there are a couple of hundred volts DC.

Don't attempt any internal measurements or repairs unless you are aware of all the appropriate safety precautions. Dada Electronics accepts no responsibility.

1. Replace all the resistors attached to the HT line with 1 watt carbon.
These will typically have drifted higher in value, and the larger the value the larger the drift.

2. Replace all the red Hunts capacitors in the (larger) IF/audio section.
These will typically have increased in value by 200% or more, or decreased in value by 50% or more. Replace with ceramic.

3. Replace all the electrolytics that are easy to replace. Replace the main PSU capacitor if you get funny noises, or low or unstable HT voltage, or it shows up badly on an ESR meter: otherwise leave it alone.

4. Don't disturb anything inside the (smaller) RF section if you can help it, regardless of the above. Otherwise you are up for an RF realignment at least.

5. Check the anode and cathode voltages of each valve against the voltages shown on the circuit diagram. If the anode voltage and cathode voltages are both out, replace the valve. If just the anode voltage is low, check the grid voltage of the next stage: if it is high, you have a leaky coupling capacitor between the two, so replace it.

6. Always use the same values, or as close as possible, when replacing components, except that you can double the values of the electrolytics apart from the main PSU electro. Always observe the correct voltage ratings for capacitors.

7. Don't attempt any kind of alignment unless: you really know what you're doing; you have the necessary equipment, i.e. an oscilloscope and an RF sweep generator; and you're aware of, and prepared for, the consequences of damaging a tuning slug, which are fatal for the tuner. The alignment instructions in the owner's manual are barely adequate even as a last resort, and won't give you minimum distortion, or decent stereo.

Dada Electronics Australia specializes in valve and solid-state tuner
alignment: We also refurbish and align the sterep decoders that go with the FM1 and FM2.

Esmond Pitt
Dada Electronics Australia
Copyright (c) Esmond Pitt, 2008. All rights reserved.


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