Quad 44 Amplifier Problem

 The Quad 44 is actually a pre-amplifier and is used with a main amplifier to provide input selection, matching and tone control.
The unit is British designed and as such it has some recognisable quirky British characteristics, such as needing two different Allen keys of almost, but not quite, the same size to remove the knobs and hence the front panel.
A stock fault is faulty selection of inputs and tape monitors.

This article describes a method of (possibly?) fixing a selector problem without buying a new part.

 For some reason best known to the Quad designers their Quad 44 uses an electronic assembly to select a particular input, rather than a rotary switch.
If you compare your equipment with the Issue 5 of the circuit diagrams you may notice that Quad originally used two different chips, an SN16862 and a pair of SN16861 to handle the switching. The latter is a 4-input switch and the former, a 2-input switch. There are five possible inputs but an added option is to select either of two monitor positions.
The requirement therefore is to switch one of five inputs plus one of two monitor positions, making a requirement of seven in all.
A later modification dispensed with the SN16862 substituting for this a third SN16861 chip.
I understand from reading various reports that there are several different problems encountered with the selector unit, and more than one explanation, such as switching on account of noise present on the power rails etc.

The problem I encountered was intermittent selection of the top monitor position. This has the effect of significantly attenuating the desired input.
I see from irate users that often an equipment is repaired by a dealer or even by Quad themselves, only to misbehave later, in the same way.
The problem seems to be a failure in an SN16861 chip.
As the fault worsens indeed noise may trip a switch, but the latter is not the root cause.
Each chip has four inputs and if one of these is connected to a positive voltage for a short period it will cause that input to latch its related output. The latch will remain set until another input is selected, when this will latch and the other to unlatch.
Daisy-chaining is possible via pin 16 of the chip, so that in a two chip system selecting any of eight inputs will work just like it did in a single chip.
Quad use another feature. By connecting pin 6 of IC101 to an input of the IC102 chip, one can select one of two monitor positions in conjunction with a selected external equipment such as a radio.
Selecting one of two monitor positions will not reset the selected input. The latter will remain latched. If a different monitor position is selected this resets any existing monitor position when a different input is selected.

My fault was due to internal leakage inside the SN16861 spuriously selecting an input. This generally cannot be unlatched as long as the leakage persists.
I also found that when a chip is cold the leakage is worst; in fact after say 30 minutes leakage can drop to the extent the frozen input will be re-latchable, and everything will work normally.
This, I suspect, may lead a repairer to believe he has fixed the problem.

To tackle a selector problem. First one should identify exactly what's going on. To simplify matters you can unplug the module from the mainboard and apply 12 volts across the blue and green wires (blue positive and green negative).
The module can then be operated independently of the equipment which can be left unpowered.
In the unlikely event there's a power supply problem, this will be readily determined as one can use a variable power supply to test the selector module. Typically, when you apply 12 volts to the module you will see around 100mA of current drawn.
If you reduce the voltage you'll find around 9 volts or less will result in the buttons not working. Don't increase the applied voltage beyond about 16 volts.
If a problem is apparent the module should then be detached from the front panel, after determining which input or inputs is faulty. This will be the one whose LED is permanently on despite which button is pressed.

Next you will need to examine the printed circuit in conjunction with the circuit diagram and determine which pins of the relevant chip need to be modified.
From the circuit diagram you can see the associated inputs and outputs corresponding to the four elements of the switch.
These are (for the SN16861) Pin 4 and 11; Pin 3 and 12; Pin 2 and 13; Pin 1 and 14.

The plan is to cut the connections to the faulty input and its associated output then connect an unused input and output pin in their place.
In my example the fault was a leak at Pin 2 of IC102 causing its output at Pin13 to permanently latch.
I cut the connections to Pin 2 and reconnected the track to Pin4, leaving Pin 2 unconnected also cutting the connection to Pin13 and strapping this to Pin11, leaving Pin13 left unconnected.

You should find that each SN16861 has an unused input/output.
IC100 if it's an SN16861 will have two spares and IC101 will have one spare.
If you have an SN16862 at IC100 you can use the spare input/output at IC101.

If you have two or even three bad inputs for Radio/CD/Disk/Tape1/Tape2, you may still be able to make a modification if you have three SN16861 chips as there are three spare input/outputs available.
Unfortunately there is only a single spare input/output in the Monitor selector.

Please don't contact me for more detail as I am assuming that you have enough technical know-how to determine any modifications yourself. If not then do not attempt it.

I assume also you can find the relevant circuit diagram on the Net... several are available free of charge.

Post Script:

Alas, after successfully testing the repair, a few days later I tested it again and found the chip had degraded even more and the internal fault had rendered another section unusable.

I checked in my box of old TV semiconductors (I never throw anything away) and was very surprised to find an SN16861 chip. I seem to recall buying this to repair one of those small portable TV sets from the 1980s. The chip was possibly used on the front panel or conceivably in an early remote control.

The old chip worked like a dream after putting back the original tracking....

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