First, sincere thanks to everyone who contributed in the forum.I cannot start the project without resources from here.

Recently I have finished my KGBH build but it isn't working.The observable problems are the two LEDs connecting 2sa1968 and the one connecting 2sc3675 do not light up as well as those 1968s not heating.Curiously, the amp works for a second every time after i turn the power off,right after the extinguish of the leds connecting the + and - 15v supply. The PSU unit is  measured of giving the right voltage. I once thought of giving up the project but before that I try my luck to see anyone who once encounter the same problem or know anything about it. Thank you.

Are both channels the same in this regard?

If so, have you checked that the LED's are the right way round, and the 100V zener is the correct way round?

Couple of other obvious questions, are you sure all the sand is genuine as that could cause massive issues.  Also are there any shorts on the boards or has the flux been cleaned off?

That indeed is a good point given that the main semiconductors are long obsolete.  My T2 went together during the first builds with fake 3675's and actually produced sparks.  Killed just about every bit of silicon as collateral damage.

Authentic 2SA1968's are very rare and expensive too so one does wonder. 

Thank you for all the reply.

RE Craig Sawyers Both channel failed and the parts are correctly wired and checked for several times.

As yousaid maybe it is probably due to the sands. Maybe a all-around replace of sand is needed,which is the thing i want to avoid the most.

I am in Hong Kong  and in fact stores from mainland china have all the parts i need at a unbelievable low price, 20cent USD for 2sa1968 for example,but the reliability is as low as the price observed from my failure of the project.

$0.20/pc for 1968 are probably fakes.

Also, it would probably help folks to better help you if you posted some pictures.

$6+ seems to be what  2sa1968 are going for now.  And there is no guarantee that even they are genuine.  A 20c part is almost certainly fake IMHO.

I'd say you have some major problems.

buy the voltage tester from ebay and test them.  Over 1000 volts, they are real, 230 volts and they are a massive pile of fakes from dalbani.

 

Curiously，my a1968s in the psu board are working somehow,but a total replacement is seems to be the only solution.

And anyone knows where is the best place to source the sands?Are they still avaliable?thanks a lot for all the advice.

in the power supply they are running at only 400v and much less current.

Yes - Dalbani were the outfit that supplied me with junk  2sc3675's.  When I put one or two spares on the curvetracer they broke down at 400V or thereabouts instead of 900V minimum.  Avoid Dalbani like the plague.

bdent.com seems to have a few in stock. USD14.29 each if you buy 25 pieces or more. You need 28 pieces?

4 hours ago, JoaMat said:

bdent.com seems to have a few in stock. USD14.29 each if you buy 25 pieces or more. You need 28 pieces?

At that rate it may be cheaper to build Kevin's BJT version of the Blue Hawaii from scratch!

Edited April 12, 201610 yr by mwl168

You can also just post pics of the transistors and we can see if they are fakes.  Super cheap and from China will mean they are fakes though. 

https://drive.google.com/file/d/0BwSbnuGaw37aVk9fdTU1SVhYRlE/view?usp=sharing

Left to right: j79,K216,c3675,a1968

there are markings on each transistor but some of them are quite hard to see

a1968 has the cutouts between the pins, so could be real. no cutouts, definitely fakes. although the ones from dalbani had the cutouts and were definitely fakes.

still a good idea to use a tester first.

Edited April 13, 201610 yr by kevin gilmore

Thank you kevin gilmore. i have no idea they are fake or not

Can a ordinary digital muitimeter do the job? Or do i need anything specialized for transitor testing

you need the specialized high voltage transistor tester.  which do not seem to be on ebay at the moment.

If you search Dy294 on ebay you will find a lot of results

Cheaper and easier to just replace the 1968 ccses with 10m90s and some resistors though

Dalbani are crooks, bought 100 or so 1968's few years ago when I had John's old KGSSHV offboards and luckily with the Dy294 on hand they all had a break down voltage of 190ish to 320. Then I bought 10 1968's from a supplier in China for around $40, they tested all fine and dandy and are still in use today in a KGSSHV I helped a friend build. Lots of fake 2SC3675, 2SA1967 and 1968's floating about so have to becareful.

ok，I think i will have my dy294 very soon

Also check the breakdown voltages of  2SJ76/2SK216 while you are it, I had a few that did not meet the specs.

DY294 is great for taking the uncertainty out of the parts quality.

Edited April 18, 201610 yr by b0bb