auxiliary components

[Fonotec]

Feedback board, A0060, for the GO741, is one of the circuits that has soldering posts for adding an auxiliary component around one of the main ones so as to fine-tune the performance of the network being used by a given sub-circuit of the amplifier - in this case, the Feedback sub-circuit. The value of the auxiliary component remains 'undefined' until a calibration of the boards for a given chassis is carried out.

The cards to be designated for one or the other stereo cutting channels are assembled into the chassis and then, after a test configuration is established, specific measurements are taken while the chassis is receiving the +/- 40 V from the dual PSU (which the chassis regulates at +/- 15V for parts of the circuit) and, by using a decade resistance or decade capacitance box, the precise value needed to be added in parallel with the main component is determined.

In the attached photo, two calibrated Feedback boards are shown, side by side. One has a Gul ('yellow') sticker, and the other, a Grøn ('green') sticker. The boards with the Gul stickers from this set must be installed together in one of the GO741 chassis, and the boards with the Grøn stickers must go in the other chassis because each chassis's boards are calibrated to work together so intimately, in order for the two amps to behave identically, that the necessary fine-tunings of each chassis' ear-marked boards are essential to the chassis-group to which they were calibrated and, in some cases, a sub-circuit with intermingled cards may not work at all.

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R22 on both cards is a 180-Ω-nominal 5% tolerance resistor. (4-color band: Brown, Gray, Brown, Gold)

R22a on the A0060 card with the Gul sticker on its edge handle - now visible from this alternate vantage:

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- is a 182-Ω-nominal, 1% resistor, which gives the R22 network a net-89,7 Ω. Since the soldering posts for R22a are in parallel with R22, the selected resistor (Brown Gray Red, Black, Brown) has lowered R22 (which started out as '180E') by 90 Ohms (which is much lower than by 5% (i.e., - 9 Ohms) - the 'low'-side tolerance of the 180E resistor).

The A0060 card with the Grøn sticker on its edge handle is a 221-Ω-nominal, 1% resistor, which gives the R22 network a net-99.2Ω.
This selected resistor (Red, Red, Brown, Black, Brown) has, here, lowered R22 (which started out as '180E') by 80.8Ω.


Clearly, the auxiliary component soldering posts are not there to help one get the resistance for R22 to a specific value, such as '180E', to compensate for the 5% JEDEC-normalized tolerance of the standard part. Rather, 180E is used as a resistance that is high enough for the circuit so that by adding more and more resistance in parallel (across the posts for R22a), the ideal resistance - for the parts on the specific board and also the parts that the specific board signals to and from on one or another board in the same set of chassis boards - can experimentally be determined and easily added without disturbing the factory welds.

[Fonotec]

The Monitor board (A0051) takes the signals from either the Feedback coil or the Pickup channel and performs the inverse of the IEC record equalization so that the signals can be heard as intended. (The 75 µs time constant for monitoring is switched off, and the same equalization section is defeated from the cutting signal, whenever the chassis is in 'Test' mode (by pushbutton-activation from the front panel).

Although the ideal values for the resistance and capacitance of each section of the IEC equalization can be stated unambiguously, these boards each have auxiliary component soldering posts for adding parallel capacitance to the factory-stocked capacitors so as to fine-tune the responses of each section when used by the specific components on the board and when that board interacts with the other boards designated for a given chassis.

These boards have rather different auxiliary capacitors of the de-IEC filter from each other.

It's noteworthy that, on the board with the Grøn sticker, the 2N409 JFET was replaced at some time with a 2N5638 JFET. This TO-92 JFET has over-all specifications that are similar enough to the original TO-18 (can) JFET as to be a suitable substitute part, but some of the operating qualities differ.

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[Fonotec]

These Input boards (which are of an alternate version of A0052) have only a few, slight differences between them - the 'A'-chassis board has a little relay (circled in gold), which was added merely as way to repair a small section of the circuit trace (which for ergonomic reasons necessitated moving one of the factory-installed through-hole resistors to a pair of soldering posts used normally by an auxiliary resistor for that part), and, also, a small diode (circled in orange) of a different model is used on each board.

The filter sections for the IEC record equalization on both boards have identical main and auxiliary components as the attached image demonstrates. The relevant time constants were written by hand during a repair session.

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[Fonotec]

These are the new Input (A0089) boards that were packed at etec when Torben built the new set of GO741 circuits using NOS boards and components from inventory.

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The A and B boards have the same value auxiliary capacitors, as was seen in the vintage Input (A0052) boards.

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I agreed that we should wait until after New Year for the shipment. Of course, I tested them on arrival, and they performed perfectly as seen on SpectraFoo's FFT spectrum analyzer. The Power Units for the chassis were also rebuilt, and the input transformers were bypassed.

[Fonotec]

These are the new Monitor (A0051) boards.

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They have different components on the soldering posts next to two sets of 'starting' capacitances, as was seen in the previous example of the vintage Monitor (A0051) boards.

[Fonotec]

The boards in the new set of amps have an A or a B written on the handle. This is the A amp... Notice the current meter, input transformer, and Power Unit also bear the chassis-letter...

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