auxiliary components
Posted: 22 Jul 2021, 17:30
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.
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: - 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.
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.
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: - 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.