2SA798 - defective/fake/replacement

• Galaxy3000

  Posts 25

  Joined: Nov 11th, '16, 19:24

  Jan 15th, '17, 10:42

  Just want to report the problems/experiences I had with the 2SA798:
  1 of 3 of the 2SA798 I received with the kit was defective, one transistor inside was working the other not (happily I tested them before soldering in)
  
  I decided to take the plunge and ordered two replacements (about 13€ for both) on ebay - The 2SA798 I received from Hong Kong are FAKES. 2PNP's with common base instead of common emitter. (asking them for refund)
  
  Then instead of buying another overpriced 2SA798 I decided to replace it with 2 matched 2N3906 - WORKS perfectly on the Handclap!!! I bought 20 of them but already found 2 matching within the first 4 (difference of 2 in hFE / 1mV in Vbe - pretty good).
  Should work on HH and Crash circuits too! On non of the circuits they are carrying a audio signal btw. just ENV!
  
  So, if anybody else has to replace a 2SA798 I highly recommend to take 2 matched 2N3906 instead!!! according to this site: http://yusynth.net/Modular/EN/ARPVCF/ 2 matched BC557 should work too! Only drawback is that the legs need to be bended to the right position!

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KlangGenerator

Posts 373

Joined: Aug 8th, '16, 16:36

Jan 22nd, '17, 18:10

Galaxy3000 wrote:Just want to report the problems/experiences I had with the 2SA798:
1 of 3 of the 2SA798 I received with the kit was defective, one transistor inside was working the other not (happily I tested them before soldering in)

I decided to take the plunge and ordered two replacements (about 13€ for both) on ebay - The 2SA798 I received from Hong Kong are FAKES. 2PNP's with common base instead of common emitter. (asking them for refund)

Then instead of buying another overpriced 2SA798 I decided to replace it with 2 matched 2N3906 - WORKS perfectly on the Handclap!!! I bought 20 of them but already found 2 matching within the first 4 (difference of 2 in hFE / 1mV in Vbe - pretty good).
Should work on HH and Crash circuits too! On non of the circuits they are carrying a audio signal btw. just ENV!

So, if anybody else has to replace a 2SA798 I highly recommend to take 2 matched 2N3906 instead!!! according to this site: http://yusynth.net/Modular/EN/ARPVCF/ 2 matched BC557 should work too! Only drawback is that the legs need to be bended to the right position!

Mind sharing how exactly you put those two 2n3906 in the pad?

The SA798 pins should be BCECB. The 2n3906 is EBC. So according to this I put my two 2n3906 together like this:



Unfortunately it doesn't work like this...

Edit: It works perfect! There was another bug causing the HH not to work.

Last edited by KlangGenerator on Mar 11th, '17, 10:10, edited 1 time in total.

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Galaxy3000

Posts 25

Joined: Nov 11th, '16, 19:24

Jan 23rd, '17, 16:38

I'm sorry to hear it didn't work for you - the configuration looks good! I did it exactly the same way.

well, I always avoid to get as close to the transistor housing with the soldering iron and leave at least 10mm of the legs above the board because transistors easily get damaged by overheating.

Do you have a possibility to test the transistors? most multimeters have at least a diode-checker, u could check the '2 diodes' of the 2N3906 according to this drawing:



the multimeter should give u a value in mV or V then (about 680mV) for both diodes - if either check gives no value then the transistor is defective

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