Radford SC.24 PCB layout issue

[GW4FRX]

One of the PCBs in the Radford SC.24 control unit is referred to as the ‘Headphone & Line Amplifier Module’. In the example I am currently restoring, this board has the reference ‘M.3307/3’ on the track side. It provides headphone and line-level outputs from both audio channels.

There appears to be an error in the layout of this board such that the circuit configuration around C4 and C5 is not identical between the two channels. However, neither channel conforms precisely to the schematic in the ‘Technical Instruction’ B24/970’ which is the only one I have. This contains several mistakes in pinouts and component values and is also reproduced at very small size (it needs an 8x loupe to read it). Scanning it and zooming renders the component designations unreadable.

Reference is made to ‘Technical Instruction C24.970’ which supposedly gives enough information for “…full service of the equipment”. Does anyone by any chance have a copy of this, or alternatively a better schematic for the unit as a whole?

[RojDW48]

I probably only have the same info as you have for mine - but I'll have a look.

[RojDW48]

Just the technical brochure and some rather inferior photocopies of the schematic I am afraid.

[GW4FRX]

Many thanks for looking anyway.

In case anyone else is restoring an SC.24, here are the anomalies apparent on the headphone and line-amplifier PCB so far:

1. The value of R5 and R5a (12k on both channels vice 120k on the schematic)

2. A connection shown on the schematic between C5a and Pin 12 on the board which doesn't exist (the connection in question is to Pin 11).

3. A connection shown on the schematic between C4a and Pin 9 which actually goes to Pin 8.

4. Different polarities of C4 and C4a. These are shown on the schematic as positive to Ts2 and 2a collector but C4 is inverted on the board.

If anyone happens to have access to the internals of an SC.24 I'd greatly appreciate knowing the values of R5 and R5a and also the polarities of C4 and C4a as installed. The board in question plugs into an edge connector and is nearest to the rear of the unit.

[GW4FRX]

Meant to add that R19/19a are 680R on the schematic and 270R on the board.

There may be others that haven't come to light yet

[Trevor]

Does not surprise me at all Radford unlike the bigger players would change values all the time subject to improvement and availability
I have found this to be the case with 3 off Rad amps I have owned
Trev

[RojDW48]

I expect you know Will and Steve Moores (Radford Revival). They are very helpful. Come to think of it Trev, Will made pretty much the same comment about variations in components when he dropped off a repair a few weeks back and took a look at my SC24 while he was at it.

[Trevor]

Given the low volume and I suppose the very basic assembly conditions Radford were incredible . I well remember the controversy in the early 70's of Radfords zero distortion range It kept the hifi mags talking for months
Trev

[GW4FRX]

Originally Posted by Trevor

Given the low volume and I suppose the very basic assembly conditions Radford were incredible . I well remember the controversy in the early 70's of Radfords zero distortion range It kept the hifi mags talking for months
Trev

Well, if anyone would like to measure an SC.24 and SPA.50 when I've finished rebuilding, I'd be delighted to take them for a little journey

[RojDW48]

Originally Posted by Trevor

Given the low volume and I suppose the very basic assembly conditions Radford were incredible . I well remember the controversy in the early 70's of Radfords zero distortion range It kept the hifi mags talking for months
Trev

You might be surprised at the output of Arthur and his pals - IIRC the amps, especially the MA15 were used by studios and concert halls all over the world and Radford's scientific LT/HT power supply was all over the scientific community like a rash! My first job was at the University of Bristol's School of Chemistry - we alone had about 30 or 40.