Adcom GFA-585 Blown 20w 4.7ohm on soft start board

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ben62670

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djk

djk

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Joined 2001
Those eBay parts are counterfeit (no Motorola parts made after Motorola was sold to ON Semiconductor in 1999), buy from Digi-Key.

Use MJE15032 and MJ15033 for drivers (plastic parts mounted next to outputs), I have got away using the MJE15030 and MJE15031.
 
lgreen

lgreen

Member
Joined 2003
Ben62670- You cannot be sure those are not fake, although probably they are ok. EDIT- I See that I posted simultaneously with the one above and have to yield- they are prob. fakes.

Have your heard of ONSemi? They make MJ21194 and 21193 hint- why don't you register and see if provides any benefits? (such as free samples...ouch did I say that?)

Learn how to match the output transistors
Check the driver transistors to see if they are also gone.
 
Cloth Ears

Cloth Ears

Member
Joined 2001
Hi Ben,

I dealt with John Walton of Tech-DIY when I was looking for matched Mosfets. I see that he has:
MJL21193 + MJL21194 - On Semi -Pairs $10.40
MJL21193 + MJL21194 - On Semi -10 Pairs $91.50

I don't know whether these are matched sets or not, but you can always ask (my Mosfets were all within .1%).

Always worth a try...
 
anatech

anatech

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Hi Ben,
Don't abuse the samples program. You won't get enough for matches anyhow so it's a wasted effort.

Just simply buy 10 of each from Digikey, Newark or Mouser. If you think about it you could buy 25 of each (getting a full rail and guaranteeing some good matches because they come from On Semi that way. This will be well less than the cost of a bad repair that will need to be redone at your cost. The drivers djk suggested are fine, buy 5 of each to try and get the gains as close together as you can. Actually, I should check. You may have TO-3P's for drivers in that.

So Ben,
You have to buy bulk. I have to buy bulk. You can't skin things right to the bone and do a good repair. The up side is that you will have some excellent outputs for another repair or some projects. I'm actually try to save you money and time here, even though it may not sound like it.

Do you know how to match them? You can build a jig to test them. You have the meters and you just need a power supply and some resistors.

-Chris
 
anatech

anatech

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Hi lgreen,
everyone- Sorry about bringing up the samples, I did feel guilty about suggesting it...
Click to expand...
The samples should be used for developing a product like the projects we see here form time to time. Most times a repair doesn't need a value worth misusing this program or more parts are required than they will ship. Another waste, as often you want parts from the same run. I'm just hoping "we" can control ourselves. On Semi's parts are not expensive either.

Hi Ben,
Okay. An ohmmeter is useless for this. They are easy to test though. Your first mission is to hunt down "how to measure transistor beta". Feel free to clear up anything that isn't clear once you find a procedure. Steve Bench's site may have something there.

-Chris
 
anatech

anatech

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Hi Ben,
For power transistors, yes - worthless. You can try to match signal transistors with it.

Build the jig for measuring bias current. You will learn a bit about transistors in the process. This is something you will use throughout your time on the bench in the future.

-Chris
 
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ben62670

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Joined 2006
I have searched for literally hours. I have read tons on transistor testing. Almost all is on little ones. Can I use 12dc (car battery steady voltage) to test them? Also on the resitors I have seen10ohm, and 100ohm resistors used. They don't specify how many watts. I can't find Steve's website except for an old aol site, and a site on tube amps. My head hurts. I need to sleep. :bawling: :smash: :dead:
 
anatech

anatech

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Hi Ben,
Sorry.

Essentially, all you are doing is running some amount of base current into the transistor (or out as the case may be) and measuring the collector current. The ratio of the two will give you the beta. You need to try and keep the emitter - collector voltage constant and run the part at a realistic temperature. I go as far as to run the parts at the voltages and currents they will be used at when matching them, four at a time on the same heatsink. This is overkill.

A 12 V battery should be fine. I honestly didn't think the information would be that hard to find, but I guess it's how you search. My wife is much better at finding things than I am.

-Chris
 
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ben62670

Member
Joined 2006
anatech said:
Hi Ben,
Sorry.

A 12 V battery should be fine. I honestly didn't think the information would be that hard to find, but I guess it's how you search. My wife is much better at finding things than I am.

-Chris
Click to expand...


I hear ya there. My X used to find things for me all the time. I have been pretty ill the last couple of days. I took a break. Well not much of a break. I have 1 1/2 and 3 1/2 year old daughters.
 
anatech

anatech

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Hi Ben,
A few search results from here : http://www.google.ca/search?hl=en&q=%2Btesting+%2Btransistors&btnG=Google+Search&meta=
http://www.anatekcorp.com/qdmmvom.htm
http://www.elexp.com/t_test.htm
and
http://www.kpsec.freeuk.com/components/tran.htm
covering testing these with a meter, not matching.

If you search here : http://www.google.ca/search?hl=en&q=%2Bmatching+%2Btransistors&btnG=Search&meta=
http://sound.westhost.com/transistor-matching.htm
From our friends at passdiy, http://www.passdiy.com/howto/matching.htm for mosfets.
http://www.elecdesign.com/Articles/Index.cfm?AD=1&ArticleID=6251

Personally, I match for hFE when using BJTs. Normally the emitter-base drop will be close and is not as important as the current gain.

-Chris
 
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