Beam Echo Avantic SPA11 restoration. Help please!

[Radio Wrangler]

Originally Posted by ChrisMarsden

Here is the datasheet for the resistors, i have the RCC050 1/2W type. What do you think?

https://docs-emea.rs-online.com/webd...6b81539afb.pdf

The datasheet says they are solid carbon... well that gives too limited a range of resistance, so most of them will be powdered carbon mixed with a powdered insulator in various proportions and pressed together.

Carbon composition resistors exhibit a phenomenon called 'Excess Noise'. ALL resistors make noise by virtue of the effect on electron motion which makes them resistive in the first place. The level of this noise is set, would you believe, by one of the quantum-mechanical constants of our universe. This is an absolute floor you can't beat. It's set by the random vibration of all particles due to temperature and it's why things like radio telescope front-enf amplifiers are refrigerated.

Does it matter at audio?

Yes, the noise voltage scales up proportionally to resistor value. In some places it sets a limit to how low you can get the background noise hiss. This is for the best resistors with pretty much zero excess noise.

Some resistors have excess noise, carbon composition being the worst offenders with excess noise around 20dB greater than the level of natural thermal noise at that temperature and resistance value. 20dB means 10 times the voltage which means 100 times the power. Is this noticeable? Not in a lot of tolerant places. very much so in plenty of touchy places.

Carbon film resistors are not as bad as carbon composition resistors for noise and for long term stability. Back in the sixties Philips ones acquired a good reputation for being noticeably more reliable and quieter than the more common carbon comp ones.

Metal oxide ones are quite decent. Metal film are the stars. Good long life expectancy and almost no excess noise. Wirewound are OK too but tend to be limited to lower values, and their little bit of inductance can be troublesome at radio frequencies.

If I'm going to the trouble of replacing parts, I'm not a stickler for originality, I use metal film types and where higher power is needed. wire-wound types.

Modrn resistors are made of far better materials, so smaller parts get higher power ratings, because they can be run to higher temperatures without risk of failure. In vintage equipment the hotter parts might be close to wiring. So the usual ploy is to buy higher power rated parts such that the body size of the new part is similar to that of the old part and this means the surface temperature will come out the same.

Sometimes people fit higher voltage rated capacitors just to get them in a case size more like that of the old part. It makes them easier to fit and they don't stick out like sore thumbs.

Would the original designers of classic amplifiers have gone for the components we have now if they'd been available? Most certainly.

For some reason there has grown a belief systeM that all parts impart their own characteristic 'sound' and carbon resistors sound sort of carbon-y, metals ring like bells or something. It all gets a bit silly. People forget that these things are handling ELECTRICITY and the signals are not mechanical. It's a bit like where the witch-doctor explains that the root of the wotsit plant is shaped like a particular organ of the human body and therefore ground up bits of that root are good for treating ailments of that organ. The logic is a load of cobblers. Sometimes the root may do something useful, other times it doesn't, some times it's just fatally poisonous. The shape has no connection. But expect to be the next sacrifice to the gods if you try telling the witch-doctor that

David

[GrimJosef]

Originally Posted by Radio Wrangler

... The logic is a load of cobblers. Sometimes the root may do something useful ...

Both of these things are true.

Just to add another dimension to the debate though, we have to remember that listening goes on in the brain. The ears are simply a channel through which the signal passes before the serious processing begins. Our brains are very susceptible to suggestion. So although the sound in the air may well not be affected by the type of capacitor that we know has been fitted to our amp, the sound in our head can be powerfully and absolutely genuinely affected.

So don't be too quick to tell people that they can't be hearing what they're hearing. You might as well tell someone with a sick relative not to worry because worrying won't do any good, or tell a diner who doesn't like brussels sprouts that there's nothing wrong with them and he should shut up and clean his plate. In the end, after all, we are just talking about entertainment .

Cheers,

GJ

[Diabolical Artificer]

"Some say polyester are for guitar amps - something to do with high distortion. is that nonsense? " Yes, but with the caveat of what GJ said above.

You want something like this - https://www.ebay.co.uk/itm/Axial-Ele...ReUras2v6a02dQ though any electrolytic made by Panasonic, Rubycon, Nippon chemicon, Sony etc will surfice. One word of caution though, if these are NOS, they may need a bit of reforming. I might have an NOS one in the shed and will reform it for you if I can find one.

Andy.

[ChrisMarsden]

Originally Posted by Radio Wrangler

Metal oxide ones are quite decent. Metal film are the stars. Good long life expectancy and almost no excess noise. Wirewound are OK too but tend to be limited to lower values, and their little bit of inductance can be troublesome at radio frequencies.

If I'm going to the trouble of replacing parts, I'm not a stickler for originality, I use metal film types and where higher power is needed. wire-wound types.

Modrn resistors are made of far better materials, so smaller parts get higher power ratings, because they can be run to higher temperatures without risk of failure. In vintage equipment the hotter parts might be close to wiring. So the usual ploy is to buy higher power rated parts such that the body size of the new part is similar to that of the old part and this means the surface temperature will come out the same.

Right, I'm sending the carbon's back - you've convinced me! Please can you suggest a metal film part that will work well form me. The originals were 1/4w. There's an article on the web that's cited often:

https://www.justradios.com/resistors.html

It says that the small body types should be avoided. I'm a little nervous that I'll order the wrong part with an insufficient rating so a link to a Farnell part would be appreciated.

Originally Posted by Diabolical Artificer

"Some say polyester are for guitar amps - something to do with high distortion. is that nonsense? " Yes, but with the caveat of what GJ said above.

You want something like this - https://www.ebay.co.uk/itm/Axial-Ele...ReUras2v6a02dQ though any electrolytic made by Panasonic, Rubycon, Nippon chemicon, Sony etc will surfice. One word of caution though, if these are NOS, they may need a bit of reforming. I might have an NOS one in the shed and will reform it for you if I can find one.

Thats very kind!

[Radio Wrangler]

I use Farnell as my main supplier of small components. They have metal film resistors from a variety of makers. 'Multicomp' is their house brand Vishay is a firm that's been taking over all sorts of smaller firms. You'll also find Welwyn and Holsworthy.

Pick any. They are all fine parts. They all perform well and are very reliable. By dealing with a commercial/industrial distributor, you pay normal commercial prices, not boutique prices.

Use 1/2 watt parts to replace the original 1/4 carbon composition ones.... generally double the old power rating and they'll run cool.


Most metal film resistors are sold at 1% tolerance. It just comes easily with this process. It was difficult making old type carbon resistors with any accuracy, so they graded them. Buy 20% resistors and you could bet that all the more accurate ones had been creamed off and flogged as 10% and 5% parts!

With 20% resistors having a wide spread, each 10:1 (decade) range in resistance was divided into six values. Make a resistor randomly, and there was always one value (+/-20%) that you could flog it as.
10% needed 12 values
5% needed 24

These values are geometrically spaced the ratios between values are the 6th root of 10, the 12th root of 10 and the 24th root of 10. This explains the weird-looking standard values. There is maths and logic behind them!

With 1% resistors, there are 96 available values per decade. The spacing between values is the 96th root of 10. And to make sense they need an extra digit in their value. If you didn't know this, they'd look extra-weird. So some people use 1% resistors but use the E96 family values nearest to the popular E12 values to keep down the number of types they use. Other people buy 1% resistors made and marked with E24 or E12 series values. It saves confusion.

So if you're replacing a 1.8k resistor for example, you'll find 1% parts at 1.8k, but you'll also find 1.78k being the closest 3-digit E96 series value. Either will be fine. If your 1.8k original was 10% it could have been within 180 ohms high or low right from new.

If you do the maths, you'll find that 1.8k resistors should be a more complicated number and it got rounded to two digits as 1.8k. Round it to 3 digits and you get 1.78k.

David

[Michael Maurice]

One thing to watch out for is the voltage rating. I use Vishay or similar 2W metal film types because they are rated at 500 volts. The also look a bit like the originals.

[Diabolical Artificer]

Had a look Chris but only have up to 100u in axial, but have several low voltage radial's, but these will look silly.

Forget Farnell, every MF R has a £15.95 extra delivery surcharge. This is a real b****y pain with Farnell, you find the part you need but get hammered by this extra.. Rapid do metal film R's like this one - https://www.rapidonline.com/vishay-p...-of-10-62-6340
or this - https://www.rapidonline.com/vishay-p...-of-10-62-6742 As Michael says you may as well get 2W. What values are you after?

Andy.

[Beobloke]

Originally Posted by Diabolical Artificer

Forget Farnell, every MF R has a £15.95 extra delivery surcharge. This is a real b****y pain with Farnell, you find the part you need but get hammered by this extra..

The £15.95 surcharge only applies to items that are US stock. Ther are plenty of metal film resistors stocked in the UK that do not attract this surcharge.

[GrimJosef]

Yes. The first thing I do when I'm searching on the Farnell site is to tick the box which says 'No Delivery Surcharges'. It's a shame there isn't some way of locking this preference into my searches.

Cheers,

GJ

[PJL]

There is a picture here of my SP21 with replacement 2W metal film resistors.
https://www.vintage-radio.net/forum/...d.php?t=102751

I only replaced the out of tolerance ones (that was all of them).

[Studio263]

Originally Posted by ChrisMarsden

How did you clean the aluminium? it looks like new my amp is much dirtier internally.

With a damp rag and normal kitchen degreaser - nothing special. I'm not that far from you so you are more than welcome to come and have a look / take notes. My advice remains not to replace anything that isn't actually faulty - a resistor has to be a long way out in a design like this before you notice anything.

I'll PM you with my contact details later.

[Diabolical Artificer]

"The £15.95 surcharge only applies to items that are US stock." Are you sure? I got caught when I miss bought some heatshink which came from their Lille depot in France.

"The first thing I do when I'm searching on the Farnell site is to tick the box which says 'No Delivery Surcharges'" Didn't know you could do that, ta.

When looking for R's for Chris the first 5 pages of results had the surcharge; I've found in the past that usually "normal" stuff comes first, IE low price, available etc and the surcharged bits come last, therefore I presumed there were no R's to be had without the SC. Whatever, I gave up. This is an annoying aspect of Farnell, it's actually just as cheap to use Digikey and just as quick without all the traps, surcharges, non availability of parts etc. Rant over.

Anyhoo, you can understand folk using ebay for single or low quantity components, but you do pay over the odd's.

Let us know the value's you need Chris, will try and point you in the right direction.

Andy.

[Beobloke]

"The £15.95 surcharge only applies to items that are US stock." Are you sure? I got caught when I miss bought some heatshink which came from their Lille depot in France.

Yes, I'm sure.

As an example, go to Farnell and enter "MFR4-100KFI" into their search box. It should come up with two results - stock number 1100015 and stock number 1833313. Both are exactly the same item (a Welwyn MFR4 100kOhm resistor) but the latter is a US stocked item and thus elicits the £15.95 surcharge.

[ChrisMarsden]

Hi, I've started replacing some of the coupling capacitors. On closer inspection the actual value fitted at C2 is a .02uf which disagrees with the schematic that shows .05uf. Its counter part on the other channel is the same type and value.



Should I replace with .05uf value as per the schematic indicates or with a 0.02uf part as fitted? I've no idea if this is factory fitted or some later modification but i will say that the cap barely fits between the tag rows. Its an RS part as are the 32uF PS smoothing cans.

[Radio Wrangler]

I'd go with the schematic to start with.

Guessing what the load impedances on it (beyond the right edge of the schematic you posted) as maybe 50k in total 1/(2*pi*F*C) gives 159 Hz as the 3dB point for bass cutoff with 0.02uF which might be a bit disappointing. Even if my impedance estimate's out by 2:1 then it looks like a mod to do some rumble filtering.

Back in the day, Radiospares (now RS) were distributors to the repair trade and rather too expensive for equipment manufacturers to use. They'd have bought directly from capacitor manufacturers. You might find the odd RS part in manufactured equipment to get around some shortage, but manufacturers wouldn't stomach their mark-up for long.

David

[ChrisMarsden]

Posting the full schematic.



hmmm, maybe this will be easier to see:

[Edward Huggins]

Now I have seen the full circuit again (and for the first time in over 40 years) it reminds me this was optimised for a ceramic/crystal PU input. In terms of matching peripherals, I'd suggest they might be a Decca Deram (50mV) or say the Sonotone 9TA/HC cartridge and a pair of high sensitivity reflex speakers e.g. Tannoy Mercurys @91db. I bet it will sound great.

[Radio Wrangler]

Not far off what I'd reckoned.

0.047 is what I assume you've got. Should be good.

David

[ChrisMarsden]

Originally Posted by Edward Huggins

Now I have seen the full circuit again (and for the first time in over 40 years) it reminds me this was optimised for a ceramic/crystal PU input. In terms of matching peripherals, I'd suggest they might be a Decca Deram (50mV) or say the Sonotone 9TA/HC cartridge and a pair of high sensitivity reflex speakers e.g. Tannoy Mercurys @91db. I bet it will sound great.

Wow! glad i could revive your memory!

I'll probably only use it with my EAR834 + Lenco turntable.

Originally Posted by Radio Wrangler

Not far off what I'd reckoned.

0.047 is what I assume you've got. Should be good.

Yes, thats what I've got.

[Radio Wrangler]

Finding a Deram to go with it would be rather difficult.

If used, I suppose it'd end up being used with a magnetic cartridge and a little outboard RIAA preamp.

David