AMI CD100 AMP keeps blowing up

[jdemarti]

Ok, I am having some trouble with a few AMI CD100 amps.  I am not sure what I am doing wrong.   I have an amp for a CD100a.   The amp had the common shorted transistors, so I would change the transistors out with good ones.  I am using the right kind of transistors, in the right order and installed correctly.  Then, when I turn on the jukebox I would hear a pop and then a burning smell.  Come to find out the new transistors would short out then cause a resistor on the driver board to burn out. usually it is resistor R34. It has happened to me with several different cd100 amps and on different jukeboxes so it is not a single amp or juke causing this problem.

The latest issue happened today.  I had a perfectly working amp.  I put it in a cd100a jukebox yesterday and it played fine. Today, I went to turn on the jukebox and then the pop and burning smell happened. It worked fine yesterday; it had new transistors!  Any ideas?  Someone has to have seen this issue before. It is happening way too often to my amps.

Thanks

[ami-man]

Hi,

You didn't buy those transisters that were on Ebay in the USA did you?

Are the mica washers ok? it could be that the bias is out.

More information please.

Regards
Alan Hood
ami-man
UK

[jdemarti]

Yes, all the transistors that I purchased were from Ebay. Some were sellers from USA and some were from Hong Kong. I tried transistors from different sellers. Even tried ones that had a different 4 digit sub-number on them (they were all 2n6284 and 2n6287 but had a different 4 digit number).  I have been using new MICA washers.  Not sure what you mean by bias is out.

Thanks

[poolman]

Hi Just had the same problem, or nearly the same. My amp has bad crackerling in the left speaker, so swaped it over to the right side to determine if it was the amp or output transformer and it proved to be the left side so brought new 2n6284 and 2n6287, fitted the 2n6287 first, ok, but still crackerling so fitted the new 2n6284 powered up loud pop as usual then bang, the 8 amp fuse blow on the driver board. So checked the other 2n6284 as I brought two of each and it has a 2k short across the base to collecter where as the good one working side is 10k so I have order 2x new ones from Farnell perhaps the answer is dont buy ones if you dont know were they are made as there is a lot of counterfit parts on the internet.
Poolman

[Ken Layton]

If there are bad transistors or open resistors on the driver boards, it could also take out your newly replaced output transistors.

[jdemarti]

Update

I ordered new transistors from Mouser and they seem to work fine.  Apparently, the 2n6284 and 2n6287 transistors I bought on Ebay are crap and do not work with ROWE amps. Lesson learned.

[Ken Layton]

If you bought them on ebay, they were probably counterfeit parts.

[jdemarti]

Ken, you are probably right. I have bought the transisters from a seller in Hong Kong. Have not had any luck with them so I will be staying away.

[lilshawn]

if you cut the top off the can o your ebay part, and the orginal part you will probably find the transistor wafer in the ebay part is 1/2 the size.

http://www.cdkands.com/counterfeits.html oh and this gets me too
 

also... there is a small board on the underside of the heatsink (single screw with a spring holding it down) with some diodes on it (3 i think... they are for heat compensation) sometimes if you have a loose joint or cold solder joint there it will cause perfectly good transistors to pop on you.

be sure to use new heatsink grease on them when you put it back on.

[jdemarti]

WOW!  Thanks for the article.  That was really interesting. I did notice it was strange that the lettering on the transistor wiped off very easily.  So lesson to me is to stick with the reputable distributors and stay far away from those Chinese imitations.