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A very useful feature to have in any integrated amplifier is a preamplifier to amplifier connection point. It is easy to do so with a single line that is attached to the buss that connects the preamplifier (or tone amplifier) output and the power amplifier's driver input. The buss point is typically found on each channel terminal of a single pot balance control. With no source signal present, it is possible to use the amplifier portion of a receiver to amplify another signal, perhaps that of a volume controlled source. Depending on the termination impedance of the substituted source, any signal on the buss that originates from the source preamplifier will be squelched. |
The main advantage of the single line is that it does not add an additional set of contact points to each channel. Since any contact in a circuit will cause intermittence, there is no reason to add some where none existed before. Such is not the case with the Scott receivers that are the subject of this modification. It is possible to convert the Scott receivers that are equipped with an accessory outlet to a single line buss, and indeed, such a conversion would improve the performance of these receivers since it would eliminate the extra contact points that the Scott design places in the circuit.
There are a number of options that can be explored when implementing a modification of the circuit. The main advantage of the conversion that I describe in this article is that it supplies a truly separate preamplifier output to amplifier input that allows the receiver to be used as a tuner-preamplifier, and as a power amplifier; each with its own purpose.
A second, also valuable connection method retains the record buss (accessory out), and converts the accessory in jacks to the amplifier driver inputs. This configuration retains the versatility of a second record buss that can be used to feed a second record line or perhaps, to reduce the circuit impedance to attenuate the line amplifier input. The amplifier input can also be attenuated in the same manner.
The modification that I describe here can offer neither of these adjustment points. As is the case with the existing configuration, which places the contact points between the record buss and the tone amplifier, the modification is a circuit break between the tone amplifier and the amplifier driver.
Configurations of the Scott R74S Accessory Circuit.
There happens to be no good reason for Scott's configuration of the accessory circuit. With the exception of the monitor, all of the signals on the record buss were routed through a connection that was often problem-prone. The accessory circuit may have been designed to permit a quadrial circuit to be added to the receiver once the format caught on. It never did.
As you will discover upon reconfiguring the circuit, there's no need for any extra wiring: The existing wires are sufficient. There's plenty to spare. The diagrams above demonstrate actual differences. For economy, the jumper used between the the tone amplifier and the driver is also used between the record buss and the tone amplifier. The leads that connect the accessory jacks to the record buss, when relocated to the tone amplifier have five inches of spare lead. This can be trimmed, or left intact and dressed around the existing wire bundle that the leads are a part of. The lead from the accessory input terminals, once relocated from the tone amplifier input to the driver input, has seven inches to spare.
Layout
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In the image above, the record buss is to the left, the tone amplifier is positioned to its right, and the input to the driver amplifier is above the tone amplifier.
There are four solder points per channel. These points are illustrated above.
1. The record buss (both channels indicated by a yellow dot).
2. The tone amplifier input (both channels indicated by a yellow dot).
3. The preamplifier output (left and right channels as shown).
4. The amplifier driver input (left and right channels as shown).
Scott connected its circuits with Belden solid core steel wire with different coloured sleeves. Scott used different colours to distinguish its circuit paths. All wire groups used solid colour sleeves for the left channel and striped sleeves for the right channel. Scott wove the signal leads with black sleeved Belden wire that were connected at ground points at one termination only.
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A close-up of the work-area |
This modification affects a purple, an orange and a yellow wire set. In the R74S stock configuration, these colour coded wire groups connect the following:
A purple wire pair attaches the record buss to the accessory out jacks.
An orange wire pair attaches the accessory input jacks to the tone amplifier
inputs.
A yellow wire pair attaches the preamplifier outputs to the driver inputs.
During the process of detaching and reattaching wires, observe the following:
Establish the exact termination points using a DVM when attaching the record buss to the tone amplifier. The channel termination points shown in the diagrams above are exact. When viewing the receiver upside-down from the front, the terminations read:
[LR] [RL] [RL].
Cut the wires one centimeter from the terminal. The remaining portion of the wire sleeve serves as a channel tag, where a striped wire indicates the right channel. These tags also serve as flags, making it easier to find the terminals to make attachments.
Make certain to attach the black ground wires woven with the yellow wires to the nearby ground terminal. You can identify the correct terminal by observing the terminal that has black sleeved wires attached to it, by measuring each terminal's impedance to ground, or by observation:. The terminal is positioned to the left of the tone amplifier input terminals. There is no reason to make a ground connection to the other wire groups as they are connected to ground at the accessory jacks.
After modification the colour coded wire groups connect the following:
A yellow wire pair attaches the record buss to the tone amplifier.
A purple wire pair attaches the preamplifier outputs to the accessory
out jacks.
An orange wire pair attaches the accessory input jacks to the driver.
Since Scott's build style was entirely influenced by their instruments division, build quality in the R74S is exceptional. The p.c. boards are made out of 3/32" thick glass-epoxy board, thick traces, and wirewound termination points are standard. These terminals are substantial in build and are excellent hosts for wires that are attached with solder.
One important, final step in this modification exercise is to restore the contact surfaces of the four accessory jacks to their proper function. In all instances, this task includes polishing the conductors and retensioning the jack's inner sleeve. It is useful to clean and retension all of the other jacks, and to renew the contacts in other circuit areas at this time. All these tasks must be done topside, with the aluminum cover removed. The cover slides in and out along a track and should not be pulled upwards.
By removing the top cover, you are exposing some of the finest electronics of the twentieth century to the open air. Since the Scott R74S has a sealed top cabinet, there will be no dust on any of the components. The R74S should provide service a century from now. I am assuming of course, that there will be something on the radio that resembles speech or music.
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