Function of the suppressor capacitor on Akai RTR

[PJay1966]

Hi All

Forgive me if this basic but I am still learning about the finer points of reel to reel tape recorders. I have an Akai 4000DS MK2 and its working very well after a strip down and relube of the main mechanical parts. Record is excellent and playback superb but there is still one last problem. When I turn the player on I get a pretty loud crack through the amp and the vu meters peg, once that it is over it works perfectly. I have been doing some research and read that the suppressor cap can cause this but I am a bit unsure what the actual function of this capacitor is. Wondered if someone can educate me and also comment on whether this could be causing the problem?

And finally if it is advisable to replace it I would be grateful for advice on where to buy a replacement.

Thanks guys!

[Analogue man]

The suppressor capacitor is used to suppress the spark generated in the mains switch when the device is turned on or off, in the case of the 4000DS there should be one across the mains switch and one across the end of tape sensor microswitch. They are normally wired in parallel with the switch contacts.

[DMcMahon]

Checking the schematic the suppression device is CR1, which is a CR snubber, which is a suppression capacitor of value 0.033uF in series with a 120 Ohm resistor, this is connected across the mains power switch SW6.

As Graham explained this is to suppress/quench/snub the spark/arc across the switch contacts, this is to reduce the wear on the switch contacts and reduce/stop the EMI/RFI noise that can induce the crack in the amplifier circuitry and also nearby equipment such as radios.

These snubbers can sometimes fail, if they go open circuit then they will no longer provide any suppression, sometimes they may go short circuit, then the switch is effectively bypassed all the time.

The schematic shows the same snubber is also connected across the tape tension switch SW5, i.e. there is only a single snubber in this case as far as I can see.

It is certainly worth fitting a new snubber both to hopefully fix the issue and provide long term reliability of the part.

Here is one example of a suitable part :-

https://uk.farnell.com/roxburgh/re12...-20/dp/2336107

There is a high handling charge for low value orders which may make it too expensive, if so try Googling sites such as eBay to try and find affordable part.

Note that this part is listed as 33,000pF which is the same as 0.033uF

David

[Radio Wrangler]

If there are any other parts for other jobs you can aggregate to make up the order to a point where handling charges are more palatable, that gives you one way around it.

Farnell are a very professional supplier with a huge catalogue also they take care that their stocks are genuine and get their supplies direct from the manufacturers. Dispatch and delivery are quick.

There is one other thing to beware of with Farnell... They have an affiliate distributor in America, called Newark. Some parts in the Farnell catalogue are kept in the US. If you put any of these into your shopping basket, your bill is hit with a substantial surcharge (£17, I think) so be careful to avoid them. Sometimes it's the only way to get something obscure, or else it's something you want a lot of (Their US warehouse prices are often lower) and can make a saving to offset the surcharge. The surcharge is one-per-order, not one-per-line at least.

If you need parts not common in the UK, you can als try Mouser or Digikey. Mouser seem friendlier than Digikey, and theur website is easier to use.

David

[peter_sol]

please bear in mind that the click and so called surge may be perfectly normal and not a fault.

[PJay1966]

This is really helpful thanks everyone. Just had a look on eBay and there’s lots selling X2 safety capacitors would this be suitable at the equivalent 33nf rating?

https://www.ebay.co.uk/itm/114580210...mis&media=COPY

[DMcMahon]

Yes 33nF is the same value as 33,000pF & 0.033uF

Your link is just the suppression capacitor without the resistor so not a CR snubber but it would be OK to use. You could add your own resistor in series with it.

It is X2 safety rated which is important and Metalised Polypropylene (MKP) which is good.

There is at least one example on eBay of 33nF + 120 Ohm snubber.

David

[PJay1966]

Thanks David, I have had another look on eBay and found the correct spec 33nf+120ohm spark quenchers. I have bought some and will install them into the deck when they arrive. I really like this Akai so I think its worth doing both just to be on the safe side. Now I just need to work out how to get behind the power switch!

[Radio Wrangler]

X2 is a safety rating. That doesn't slow down component fakers one jot. Some will put those markings on anything. Other suppliers are perfectly fine. The problem is how do you make sure you're buying a good part. The tests to verify that an X or Y rated capacitor really is such involves testing several to destruction. So while known trustworthy sources are more expensive they can save risk and trouble.

If the snubber across the switch fails shorted, you just wind up with a recorder stuck on. If the snubber fails open circuit you get sparking eroding the switch contacts away. No risk to life or limb but a nuisance. However as X class had been mentioned, you ought to know what they are and the issue of fakes floating around.

David

[PJay1966]

Very helpful information and I am grateful for members on here for helping me with my education! I have bought the correct components from a trusted seller I have used before so hopefully they should be good!

I have to say having started out restoring valve and hybrid amateur radio gear I am finding working on R2R's really interesting and a bit addictive I already have two! Might have to sell some radio gear to make space for more!

[ms660]

I don't know what version the OP has, but just to note that according to the schematic I'm looking at there's no snubber fitted across the mains switch in the 220/240 VAC version.

Lawrence.

[DMcMahon]

Originally Posted by PJay1966

Now I just need to work out how to get behind the power switch!

From memory all the ones I have worked on (Universal with multi voltage mains selection switch) the snubber is physically connected across the tape tension switch SW5 (near the Supply spool area) which electrically also connects across the power switch.

David

[DMcMahon]

Originally Posted by ms660

I don't know what version the OP has, but just to note that according to the schematic I'm looking at there's no snubber fitted across the mains switch in the 220/240 VAC version.

Lawrence.

Yes the snubber is only across the tape tension switch, also both lines of the mains are switched through 2 pole SW6 switch, as opposed to 1 line in most of the other variants.

David

[PJay1966]

Mines a UK version, it doesnt have the 50/60Khz cut switch on the front and has loads of stickers inside saying "UK". So no snubber on the mains switch, easy fix then!

[DMcMahon]

So does your machine have a 220V/240V mains voltage selection switch ?

David

[locknut]

Originally Posted by peter_sol

please bear in mind that the click and so called surge may be perfectly normal and not a fault.

I've had experience of a few of these over the years, recently acquired another DS mk2.

They have all had an audible pop on the output at switch on and the jump of the VU meters, I don't think it's abnormal.

[PJay1966]

Originally Posted by DMcMahon

So does your machine have a 220V/240V mains voltage selection switch ?

David

No there isn't a mains selector switch on my Akai.