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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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19th Oct 2017, 9:33 am | #1 |
Pentode
Join Date: Aug 2005
Location: Shrewsbury, Shropshire, UK.
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Radford SPA.50 guru required ;-)
I'm in the process of restoring a much-mistreated Radford SPA.50 and have found some anomalies on the schematic for the M.3297 driver board. The highest resistor number is R29 but both boards additionally carry R30 (2.2R between negative end of C3 and ground), R31 (47R between Ts1 emitter and C4) and R32. This latter is between Ts2 base and collector and in my amplifier the values are different -- 120K on one and 150K on the other.
Looking at the schematic for the later SPA.60 there is a preset in this position. In the even later HD250 service manual this corresponds to P6, adjusted to ‘minimize crossover spikes’. Can anyone shed any light on this? In particular, was the value of R32 adjusted on test and how should it be adjusted if semiconductors are changed? (I've also posted this to the Radford Yahoo group but there's very little activity on there.) |
19th Oct 2017, 9:48 am | #2 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
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Re: Radford SPA.50 guru required ;-)
I'd recommend you contact Radford Revival http://www.radfordrevival.co.uk/. Not only do they hold, as far as I know, the remaining technical literature from Radford Electronics but Steve and Will are also both enthusiasts and electronics specialists.
Cheers, GJ
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19th Oct 2017, 10:53 am | #3 |
Dekatron
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Location: Kington, Herefordshire, UK.
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Re: Radford SPA.50 guru required ;-)
Mike Solomons at London Sound put in the hard yards on these...
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19th Oct 2017, 4:33 pm | #4 | |
Pentode
Join Date: Aug 2005
Location: Shrewsbury, Shropshire, UK.
Posts: 149
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Re: Radford SPA.50 guru required ;-)
Quote:
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20th Oct 2017, 7:17 pm | #5 |
Octode
Join Date: Aug 2006
Location: Stoke-on-Trent, Staffordshire, UK.
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Re: Radford SPA.50 guru required ;-)
i have an sp50 or should i say sp60 in my collection it was purchased as a 50 but after opening it up i found it to be fitted with 2 x sp60 modules and a derated power supply
i corrected the supply and then found a channel to be noisy be aware the correct transistors for these are hard to obtain and are very expensive however having said this the amplifier sounds very nice you will need low distortion oscillator distortion analyser and a scope in order to fix this correctly regards Trev |
21st Oct 2017, 2:10 pm | #6 | |
Pentode
Join Date: Aug 2005
Location: Shrewsbury, Shropshire, UK.
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Re: Radford SPA.50 guru required ;-)
Quote:
Two capacitors have different values from those stated and there's an extra that's not shown on the schematic. Ts3 is slated as a BC108 but is a BC107 on both boards. And so on. No doubt Radford policy was one of continuous improvement. If only they'd documented the improvements as well... |
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21st Oct 2017, 3:03 pm | #7 |
Octode
Join Date: Aug 2006
Location: Stoke-on-Trent, Staffordshire, UK.
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Re: Radford SPA.50 guru required ;-)
i would tend to agree with you Arthur Bailey was the designer look up the 30 watt amplifier world class in its time
Very much a cottage industry more so the transistor designs most of the exotic devices were of RCA manufacture My Radfords I would describe as well home made! Not in the same class as Leak Quad etc this comment does not apply to the valve designs Regards Trev |
21st Oct 2017, 3:06 pm | #8 |
Dekatron
Join Date: Sep 2015
Location: Southwold, Suffolk, UK.
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Re: Radford SPA.50 guru required ;-)
I still have difficulty in even associating Radfords with Semiconductors....
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21st Oct 2017, 3:45 pm | #9 |
Pentode
Join Date: Aug 2005
Location: Shrewsbury, Shropshire, UK.
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Re: Radford SPA.50 guru required ;-)
That's perhaps a little unfair. Contemporary Leak transistor amplifiers had several design shortcomings and were considerably less well screwed together than an SPA.50. I don't know the Quad 33/303 very well but they don't seem to have a stellar reputation for reliability.
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21st Oct 2017, 4:29 pm | #10 |
Octode
Join Date: Aug 2006
Location: Stoke-on-Trent, Staffordshire, UK.
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Re: Radford SPA.50 guru required ;-)
Facts speak for themselves I used to retail all these from the late 60s through to the nineties Leak Quad Armstrong Rogers Radford Exposure Crimson Hitachi Trio etc
And while the Radford figures were very good the actual build quality can be described as woeful just look at the wiring gauge by modern standards very poor But to be honest non of them can be described as of military build Trev |
21st Oct 2017, 5:04 pm | #11 |
Octode
Join Date: Aug 2006
Location: Stoke-on-Trent, Staffordshire, UK.
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Re: Radford SPA.50 guru required ;-)
I can remember receiving a Radford sca 30 1969 the boards were warped ie not flat like bananas in fact
On reflection i think the best build quality was Rogers Trev |
21st Oct 2017, 5:37 pm | #12 |
Dekatron
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Location: Kington, Herefordshire, UK.
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Re: Radford SPA.50 guru required ;-)
You having a laugh, sir? There are probably over half the total production of 33/303s still running sweetly today after three or four times their design life of service. The reason they turn up here so often is that there's hordes of the things about - and a jolly good thing too. Their faults are few and well documented, and the electronic and mechanical design was well ahead of most contemporary units - it doesn't look too shabby even today.
Last edited by Ted Kendall; 21st Oct 2017 at 5:48 pm. |
21st Oct 2017, 6:44 pm | #13 |
Octode
Join Date: Aug 2006
Location: Stoke-on-Trent, Staffordshire, UK.
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Re: Radford SPA.50 guru required ;-)
Well said sir
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21st Oct 2017, 7:27 pm | #14 | |
Pentode
Join Date: Aug 2005
Location: Shrewsbury, Shropshire, UK.
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Re: Radford SPA.50 guru required ;-)
Quote:
Last edited by GW4FRX; 21st Oct 2017 at 7:40 pm. |
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21st Oct 2017, 7:41 pm | #15 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
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Re: Radford SPA.50 guru required ;-)
If I remember correctly there was a failure mode associated with the main electrolytics oozing liquid under thermal cycling and the liquid then dripping onto the pcb and corroding it. The solution (no pun intended) was to invert the capacitors. That generally stopped the oozing, and even if it did occur the liquid would tend to sit as a droplet on top of the capacitor where it did relatively little harm. I don't remember at what stage Quad discovered this fix though. I recall seeing 303s with the original factory-fitted capacitors already in the 'upside-down' orientation.
Cheers, GJ
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21st Oct 2017, 9:01 pm | #16 |
Dekatron
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Re: Radford SPA.50 guru required ;-)
I think that penny dropped fairly early on. In ten years in the Beeb, during which I used and saw used Quad 303s and 405s for everything save banging in nails, I only had one failure, and that was an AM8/16, which was of course based on the 405.
This was a bit awkward, I will concede. In the middle of balancing a session for Listen To The Band, I lost the tweeter in the right-hand LS5/8. I take some pride in the fact that the remainder of the session was useable. The next morning I got on the 'phone to ED and ordered another 8/16, as I wasn't going to risk that happening again without a spare to hand. Did my manager chew me out for that! But this was in the reality-defying days of the short-lived North East Region, which thought that champagne on beer budget could be achieved by running not so much hot as actually smoking... |