UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Success Stories

Notices

Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

Reply
 
Thread Tools
Old 8th Jun 2019, 11:17 am   #1
GW4FRX
Pentode
 
GW4FRX's Avatar
 
Join Date: Aug 2005
Location: Shrewsbury, Shropshire, UK.
Posts: 144
Default Radford SC.24 control unit

Somewhere between a restoration and a rebuild, this Radford SC.24 was non-functional and very tired when bought from an estate sale a couple of years ago. I had owned one together with an SPA.50 power amplifier in the 1970s and liked them but reliability was not their strong suit. The SC.24 was intermittently noisy on one channel and the power amplifier demolished driver boards (and on one occasion its power supply) on a regular basis. I hoped to be able to restore both to as-new condition or better.

Both units contain a multiplicity of Lockfit transistors, chiefly BFW5x and BFW8x in the SC.24. Interestingly these are not shown as such on the schematics and one of the recurring themes of the restorations was that Radford schematics do not reliably reflect the reality of the hardware. It was decided to replace all the Lockfits with more modern devices in the shape of BC107, BC177 and BC109. Given that a few of the electrolytics were visibly leaking and checks with a component tester showed that others were either leaky or well down on capacitance, it was also decided to replace them all with modern Vishay components. The major circuitry of the SC.24 is laid out on a total of six PCBs of which one is the +60V regulator and the other is a combined headphone and line-amplifier board. This made progressive substitution and testing simple.

The original voltage regulator in the SC.24 is a simple Zener/pass transistor arrangement which provides nothing in the way of current limiting or protection other than a 500mA fuse. On the basis that a failure might have potentially serious consequences, it was decided to fabricate a new regulator PCB using a TL783 keeping the original as a spare. Perhaps not surprisingly the new regulator proved to be considerably less noisy than the old one and to exhibit better line and load regulation.

Changing transistors and electrolytics on the PCBs was tedious rather than difficult. Removing Lockfits inevitably leaves large holes in PCB pads and even with care there was some lifting of tracks and pads. After completion of the replacement work, the boards were carefully cleaned with propanol and spot repairs made as required with thin wire bridges and Araldite, after which they were conformally coated on the track side. This strategy worked very well with the RIAA and tone-amplifier boards but the headphone and line-amplifier board proved to be considerably more mechanically fragile. It was resolved to repair it but also to fabricate a new board for this function, again keeping the original as a spare. Apart from mechanical reasons for this, it was noted post-repair that Ts4 and 4a in the original were running undesirably hot. The circuit arrangements are such that these devices are dissipating almost 180mW which for TO-18 devices without heatsinking is not considered wise; in passing one wonders whether the original BFW57 was really up to the job. It is planned to use BSV17-10 and BSV64 TO-39 transistors for Ts4-6 in the replacement with a small finned heatsink applied to Ts4 and 4a.

Removing the motherboard for cleaning and replacement of the main reservoir capacitor and a degree of rewiring is easy enough and also allows access to the front-panel switch assembly. This was very dirty and required liberal use of propanol-and-toothbrush and Servisol to clean. The tone-control sliders are mounted on a sub-panel and can be easily removed, cleaned and replaced. In the original the switched mains supply is connected directly to the mains switch via a twisted pair and led over the motherboard to the transformer. This was replaced by a new cableform containing the switched and unswitched supplies and the feed to the front-panel pilot light, all executed in 7/0.2 PTFE wiring and run beneath the motherboard in cable sleeving. This made for a much neater (and safer) assembly.

The interconnections between boards and modules in Radford amplifiers of this period are made with a multiplicity of PCB pins and jumper wires. This no doubt made for ease of assembly but the result is not particularly neat. It was resolved to form them into twisted pairs and cableforms where possible. A major problem presented itself at this stage, in the form of a number of pins in the PCB which proved to be loose in their mounting. It was initially thought that these were dry joints but in fact the associated PCB pads were lifting, presumably due to rough handling during assembly or a subsequent repair. All were inspected with an 8x loupe and repaired as necessary, after which a thin layer of Araldite was applied to assist in retention and the track side cleaned and conformally coated.

The mains connection to the SC.24 was originally made with a captive lead. At the risk of being accused of sacrilege or worse, it was evident that if the mains transformer was moved slightly further into the chassis there would be ample room for an IEC mains socket. It seems that Radford had made provision for two different transformers in production since there were two additional mounting holes available which were ideal for the purpose. The transformer was duly moved, the IEC socket fitted and a degree of re-wiring carried out.

Along the way a good many minor faults were cleared up. These included an open-circuit upper connection on one of the balance control sliders, an electrolytic capacitor on the headphone and line-amp board which had been inserted the wrong way round at manufacture, some decidedly loose inter-board connections and a variety of others. All these minor snags were eventually cleared and the unit fully reassembled for soak-testing and measurements. So far everything appears to be working correctly and the unit ‘sounds’ very pleasant indeed. When the headphone and line-amplifier board has been replaced it is hoped to be able to carry out some formal measurements.
Attached Thumbnails
Click image for larger version

Name:	RIAA preamp.jpg
Views:	159
Size:	108.4 KB
ID:	184670   Click image for larger version

Name:	Tone amp.jpg
Views:	156
Size:	109.1 KB
ID:	184671   Click image for larger version

Name:	JHN production.jpg
Views:	145
Size:	61.4 KB
ID:	184672   Click image for larger version

Name:	1.jpg
Views:	149
Size:	105.4 KB
ID:	184673  
GW4FRX is online now   Reply With Quote
Old 8th Jun 2019, 11:54 pm   #2
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Mt. Maunganui, New Zealand
Posts: 2,356
Default Re: Radford SC.24 control unit

Well, that was a tour-de-force! The SC24 is, I think, both complex and complicated. It had many component parts and the available schematics are not so easy to follow. I suspect though that somewhere there might be another Radford TI that explains the circuitry section-by-section, similar to that which was provided for the SC22.

The SC24 was something of a step-out in British practice at the time. Hi Fi Year Book 1972 shows it at £80.00 as compared with £43.00 for the much simpler Quad 33, which may be used as a reference point. The SC24 used I think 50 transistors as compared with 12 for the Quad 33. The latter of course was designed at a time (1966-67) when the unit cost of transistors was high, and they thus had to be used sparingly. By 1971 that had changed significantly. (As another measure of that change, the Quad FM3 tuner of 1971, in its first iteration, used 14 bipolars (11 of those in auxiliary functions) as well as 2 mosfets and 3 ICs.)

In the 1960s valve era, the Rogers Master control unit was more complex than most, but it did not have a solid-state successor. Rather Rogers pitched its solid-state offerings more at the mid-market level. The Radford SC22 valved control unit was generally simpler than the Rogers Master, so effectively Radford moved quite a step upwards, complexity-wise, with the SC24 solid-state successor.


Cheers,
Synchrodyne is offline   Reply With Quote
Old 9th Jun 2019, 9:11 am   #3
GW4FRX
Pentode
 
GW4FRX's Avatar
 
Join Date: Aug 2005
Location: Shrewsbury, Shropshire, UK.
Posts: 144
Default Re: Radford SC.24 control unit

Quote:
Originally Posted by Synchrodyne View Post
The SC24 is, I think, both complex and complicated. It had many component parts and the available schematics are not so easy to follow. I suspect though that somewhere there might be another Radford TI that explains the circuitry section-by-section, similar to that which was provided for the SC22.
There is a reference to one (TI.C24.970) in the SC.24 leaflet but I've never come across an example.

The poor-quality schematics are no help to the restorer and in the end it was necessary to cross-check the boards against each other and the schematic to be sure that all disparities had been caught. If anyone wants a list I’ll be happy to provide one. All the boards in my example have part numbers suffixed /3 for what that's worth.

Restoring the SPA.50 was entertaining as well…
Attached Thumbnails
Click image for larger version

Name:	2.jpg
Views:	109
Size:	93.0 KB
ID:	184746  

Last edited by GW4FRX; 9th Jun 2019 at 9:17 am.
GW4FRX is online now   Reply With Quote
Old 25th Aug 2019, 7:56 pm   #4
duran
Tetrode
 
Join Date: Sep 2005
Location: West Sussex, UK.
Posts: 50
Default Re: Radford SC.24 control unit

Hello
I just found an SC24 at the local antique market(?), in good condition, and it's WORKING!, but I'd appreciate a parts listing for if/when it fails.
The only issue with the SC24 was nearly all the sliders were stuck, but a squirt of switch cleaner has freed them.
Any suggestions for a lubricant to keep them moving freely?
Many thanks
__________________
Regards

Duran

Last edited by duran; 25th Aug 2019 at 7:57 pm. Reason: spelling, grammar
duran is offline   Reply With Quote
Old 29th Aug 2019, 10:52 am   #5
duran
Tetrode
 
Join Date: Sep 2005
Location: West Sussex, UK.
Posts: 50
Default Re: Radford SC.24 control unit

The sliders on my bargain SC24 are very stiff/noisy and I'm looking to replace them, but I can't find dual 25k LOG/LIN sliders anywhere. Any clues?
__________________
Regards

Duran
duran is offline   Reply With Quote
Old 29th Aug 2019, 4:22 pm   #6
m0cemdave
Heptode
 
Join Date: Feb 2009
Location: Bletchley, Buckinghamshire, UK.
Posts: 720
Default Re: Radford SC.24 control unit

What size are they?

Mouser have the Bourns 60mm type in stock, in mono and stereo versions.

PTA6043-2015CPA223 22k Log single

PTA6044-2015CPA223 22k Log dual

HTH
m0cemdave is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 10:51 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Copyright ©2002 - 2019, Paul Stenning.