B100R pop on shutoff

[g1]

Ooops, I may have to recuse myself from this one, I missed that part where he said "wise".

[52 Bill]

Ooops, I may have to recuse myself from this one, I missed that part where he said "wise".

I always though that you were a real wise guy.

[mtlbasslad]

Some pics



Tried to measure voltage where I thought I should find the -40 rail - disaster, flash & blew fuse. New fuse with no test leads attached - instant fail. I seem to have created a dead short...

Guess I'll have to go through the pain of removing the board so I can see the track side... arrgh

[Jazz P Bass]

Check the output transistors for shorts.

[52 Bill]

Check the output transistors for shorts.

Agreed, they can be checked without removing the board.

[mtlbasslad]

Ok guys, thanks. They are TIP142 & TIP147 - now, here is what I know... base, collector, emitter. Yup, that's all I know (sigh)

Should I start googling 'how to check a transistor'? Any idea from the pic which is which (BCE) & then what?

[g1]

They are BCE from left to right. First check for any C to E shorts or real low resistances.

[mtlbasslad]

Ok I educated myself a bit...

Base to collector & base to emitter read around 600/1200 when I reverse polarity - I understand it should be 600/infinity, but they are in circuit...

C to E reads OK on the TIP142's but near short on the 147's... I think (TO218 case)

Now what?

[52 Bill]

C to E reads OK on the TIP142's but near short on the 147's... I think (TO218 case)

Now what?

It will probably be only one of the TIP147 that is shorted, but because they are in parallel they will both read as shorted. That being said, you will probably want to replace them both as a pair anyway.

What I do, is remove one from circuit and see if one or both are shorted. If only one is shorted, I will power up the amp with a light bulb limiter, leaving the one good one in circuit to see if it will power up without drawing a lot of current. Sometimes it will power up okay and sometimes not. If not then you need to check for shorted transistors in the driver section.

If you don't have one you might want to build yourself a light bulb limiter to save your fuses and new parts.

[mtlbasslad]

Well thanks again guys, looks like I'm in the market for some TIP147's - I wonder what brand SLM put in there? And can I use any generic type?

@52 Bill - yeah, guess it's time I build one (adding to long list of things to do)

@g1 - tractors eh? I miss my 1956 Case 300...

Looks like I've got a major project here, just getting the board out is scary (for me)

I'll let you all know if I can pull this off...

[Jazz P Bass]

[QUOTE=mtlbasslad;384525]Well thanks again guys, looks like I'm in the market for some TIP147's - I wonder what brand SLM put in there? And can I use any generic type?
QUOTE]

OnSemi
Fairchild
ST

It is a popular transistor.

The 'brand' is not all that important.
Not too sure what 'generic' implies.

Stay away from NTE & EBay.

[g1]

You may want to wait til you have them out of the board & recheck before ordering. There is still a chance there is some other short on the -48 rail that you are measuring through the transistors (like maybe bridge rectifier or filter cap).

@g1 - tractors eh? I miss my 1956 Case 300...

That's a nice looking machine. We have a '64 Case 930. But the one that gets all the use is the "cornflakes box" '51 8N Ford.

[mtlbasslad]

@ Jazz P Bass - generic for me means any old brand... for sure I'll stay away from fleabay... what is NTE? Where we are (far from parts stores my best bet is DigiKey $8 shipping) Now I'm trying to figure out if the TIP147 is a TO218 or TO220 case...

@g1 - good advice as usual, thanks. Glad to hear you are a Case man too, although the 8N is a classic. Took me a few years to track down my Case, had been looking at Farmalls...

Oh boy, I have certainly set myself up for getting more use from the Weller soldering station my kid gave me for Christmas...

[Enzo]

You want to TO218. In fact the TO247 would be even better if ther is room, but the stock ones are TO218. (Or possibly the VERY similar TO3P.) TO220 is the smaller size, same size as a 7815 voltage regulator or a TIP42.

NTE is a general replacement line of parts. They make a bunch of parts and have a big cross reference book. When you want some real part, they will look up the NTE part that is closest. I highly recommend avoiding NTE. If nothing else, the "same" part costs several times what a real part costs. The purpose of the line is some local store can carry just the NTE series, and usually provide a part for most of their customers, instead of stocking thousands of real parts from hundreds of manufacturers. But the NTE are not exact, they are not the same part, they are only what NTE thinks will work in your part's place. If all you needed was one diode, and it was a matter of a local NTE or ordering something, well you could get away with it. Where it really breaks down is power transistors. When you have a row of transistors in the output stage of a power amp, you need all of them to be the same so they share current, and track thermally.


Imagine you want to make lasagna. You need some ricotta cheese. You could drive across town to the real grocery store and get ricotta cheese. Or you could drive half a mile to the local 7/11 convenience store and buy some cottage cheese, and it will be "close enough". Well that cottage cheese is the NTE choice, so to speak.


(On the other hand, make some spaghetti sometime, and toss some cottage cheese in it and stir it around. Tastes a lot like lasagna.)

[mtlbasslad]

Hey Enzo, thanks for checking in - boy, I've really got myself in trouble here. Thanks for explaining NTE. When I go to my favorite supplier DigiKey, the TIP147 in TO218 is not available... going to check out the TO3P. As for the 'row of transistors' maybe I should change all 4 ? - they are like $2 each...

Again I love your analogies - lasagna etc...

Wish I could just have you drop by for a BBQ & help me - I had a friend who was an amp guru, alas cancer got him a few years back.

Hope you are well, would love to meet you someday. Hey why not come up to Montreal for the Jazz Festival, you would be welcome to share lasagna wisdom with us.

Cheers, Lorne & Joanie