Radford SC24 has developed a hum

[RojDW48]

I have had this old pre-amp for ages and ages - in the last few days it has developed a volume dependent hum in both channels - I estimate around 80Hz and it comes and goes (but doesn't disappear). Not the amp - I have swapped pre-amps and it's fine. Symptoms ring any bells? I am not totally incompetent with valve based audio, but a bit of a novice with solid state.

[thermionic]

Hi.

If it comes & goes, first I would check the grounding connections inside. Also, if I remember correctly this pre amp uses individual colour coded plugs & sockets on the pcbs to connect the wiring harness to the boards. Check these for tightness and cleanliness.

Also check the tightness and cleanliness of any grounding connections to the chassis.


Good luck & keep is informed.


SimonT.

[Trevor]

i would check all electrolytics in the power supply sound like supply ripple
Trev

[Techman]

Originally Posted by RojDW48

it has developed a volume dependent hum

I would think that this rules out the smoothing electrolytics.

It may sound like 80Hz but it's probably mains hum.

I always find the problem is the same with many of these old pre-amps, and that is that the bent round tags that hold the metal backs of the pots to the alloy/monkey metal main part, get an oxidised coating between the dissimilar metals. Sometimes the main earth relies on a soldered connection to the metal back covers of these pots. Get a screwdriver and press it on the tags of each pot and try to bridge the actual joint and see if it alters the hum level. Obviously you need to be careful not to let the screwdriver slip and short out any live parts. You may be able to get away with a quick fix by crimping them up a bit more with some strong pliers, or you may be better replacing the pots or re-working the earthing system - that is of course, if this is in fact what is causing the hum in this instance.

[dave cox]

Have tried switching between the various inputs ?
Does it make any difference ?

Could be an earth issue, as already mentioned.

The main supply capacitors are not going to be highly stressed in the unit, so they may still be OK. My experience is that the smaller PCB capacitors are more unreliable so I would be checking the operating point of the various stages in case they being effected by a leaky DC block. Another possibility is an OC bypass capacitor, there are a stack of these in the input and tone control units.

dc

[RojDW48]

Many thanks gentlemen. I pulled all the circuit boards out, cleaned the contacts and stuffed them back in again - likewise all the ground connectors I could find - and it seems to have done the trick! I am very fond of this old pre-amp - like most of us I expect, I like to play with all sorts of old things and the amazing range of adjustments on this unit means I can always approximate equalization - even for the most ancient of audio sources. I am greatly relieved it was a simple task (for a simple soul). That sort of complexity in the attached pic just gives me nightmares.

[thermionic]

I’m so pleased to hear that you have (hopefully), got this sorted out!


I still have an SC24 that I use occasionally along with many other Radford Amps, so I’m getting accustomed to their quirks!

As you say, the SC24 is still a very useable amp, especially as you can vary the input sensitivity of individual inputs, and it has a decent tone control section, too. The independent headphone amp is also a nice facility.


Cheers. SimonT.

[Trevor]

I have 2 of them also for the time a very advanced piece of kit
it belongs to a period of real design and innovation rather than generic op-amp designs that seem to be made these days
Trev

[GW4FRX]

Slightly off-topic but the slider controls on my recently acquired example are a little stiff and feel as though some lubrication is required. Any recommendations?

[thermionic]

Are you referring to the front or rear panel sliders? They are both of a different construction. Usually Servisol 10 switch cleaner is sufficient to clean and lubricate sufficiently.

Don't be tempted to use WD40, or similar.

[RojDW48]

One of my reservations when I bought this pre-amp many years ago was the reputation of slider pots for deterioration and noise....but they were and are immaculate in operation. All other vintage items with similar controls I have come across have been in an awful state. I wonder if Radford made their own? Yes, if you need to service them, Servisol would be a first choice.

[GW4FRX]

Originally Posted by thermionic

Are you referring to the front or rear panel sliders? They are both of a different construction. Usually Servisol 10 switch cleaner is sufficient to clean and lubricate sufficiently. Don't be tempted to use WD40, or similar.

WD-40 is an invention of the devil. According to its MSDS it's merely a combination of white spirit and mineral oil, neither of which I'd allow within fifty yards of any electronics.

It's the front sliders which are a bit stiff. I'll try some Servisol 10.

[pipeline1979]

Originally Posted by RojDW48

Many thanks gentlemen. I pulled all the circuit boards out, cleaned the contacts and stuffed them back in again - likewise all the ground connectors I could find - and it seems to have done the trick! I am very fond of this old pre-amp - like most of us I expect, I like to play with all sorts of old things and the amazing range of adjustments on this unit means I can always approximate equalization - even for the most ancient of audio sources. I am greatly relieved it was a simple task (for a simple soul). That sort of complexity in the attached pic just gives me nightmares.

Hi there,

I'm so glad you posted this because I've been having what sounds like very similar troubles with mine. I acquired it a few months ago and noticed these hums that come and go so I sent it off to be looked at. They replaced the power supply caps and cleaned some bits but it has since come back.

I think I'll give what you tried a go now.

Cheers!

[RojDW48]

Mine is still nice and quiet (a relative term, of course, with which my wife would tend to disagree!). Good luck.

[pipeline1979]

I've had a poke around with the oscilloscope in there. There seems to be a point near the power switch which links the "tone amp output" to the slider controls/tone control module.

If I play some test tones into say, the tuner channel I get a flat response.

If I play an approximately 2mV rms selection of test tones into the 'Disc' channel I don't get a very flat response at all - much bigger LF outputs. I think the RIAA equalisation is in the 'low level' boards on the other side, so it should be flat here - but it's not.

Am I right in thinking I should change the electrolytics in these boards first and see what happens? The only thing that's making me doubt this is that it's hard to get a very smooth input down at that level - when I look at it at the input the trace is a little fuzzy.

[pipeline1979]

Ive done loads of fiddling with mine, including breaking the now irreplaceable mains switch and changing all the electrolytic caps.
On the plus side the headphone output is now stronger, on the minus side it still has an endemic amount of hum. By which I mean if I have nothing input wise connected and listen through the headphones there's a steady low level hum.
Can somebody tell me how theirs is grounded if they have one too? When mine came back from being repaired with a new main cap the safety earth to the plug was on the bottom of the chassis behind the transformer. Only the transformer grounds and the return from the pilot lamp connected to it. On the back, where there is a ground post, most of the input signal grounds and two long wires from the balance control ground and a ground near the 'switched output' pins joined up.
So there were two chassis connections.
On the circuit diagram I have, the only connection to chassis from the circuit side of things is through the rectifier board to the negative of the power supply cap. Also, the instruction leaflet tells you a hum may result if, both the power amp and the preamp are connected to earth and theyd rather it was the preamp.
To further complicate things, mine had a thick screened power cable going from front to back for the mains switch, the screen of which was soldered onto the upper switch board only.

At the moment, I have a twisted pair going to a new switch on the front and its not screened, should it be?
The connection to safety ground is the same as before.
My input signal grounds all go straight onto the upper switching board. (The record player is now grounded through its mains plug.)
Everything else goes to ground through the output signal cables to my sta15.

Also, the 'speaker off' switch either makes the hum louder or just makes the music more faint. And there are big thunks if you switch channels.

So, I think I'm saying, surely that's not how it's supposed to be?

Apologies for the rant, but Im running out of ideas.

[RojDW48]

Sorry to read you are still getting problems. Have you tried dis-connecting grounds to eliminate the possibility of a loop? When my Radford things went wrong beyond my limited ability to fix them I was lucky enough to be in touch with Mike Davis (ex Radford and later, Woodside) and he always did a fantastic job. He has passed the baton on to Radford Revival - http://www.radfordrevival.co.uk/ in Cornwall - Steve and Will Moores, very nice, helpful chaps (they are building brand new STA 15s and 25s!). Could be worth a phone call - they are quite happy to talk.