A-1020 transistor replacement - can I do it?

[mhjmhjones]

Ok, I got my semi-dead A-1020 back from the shop today (see here for backstory: http://audiokarma.org/forums/index....diagnose-and-or-restore-my-a10-20-amp.749783/ and also here: http://audiokarma.org/forums/index.php?threads/yamaha-a-1020-needs-tlc.635631/page-6).

I did not get it repaired as the repair estimate was $250. The guy at the shop (Quest for Sound in Bensalem, PA, USA) told me 4 of the transistors are bad. To repair, he said that all 14 transistors inside that amp would need to be replaced.

So I am now thinking about doing the repairs myself. My experience with electronic repair is: I successfully rebuilt some AR-3A speakers (rebuilt the pots and replaced the caps). Is transistor replacement basic? How involved is it? I am handy and have soldering iron, multi-meter and decent tools.

Obviously, I am in the early stages of info gathering here. Calling Bratwurst7s (James)!

 

[Bratwurst7s]

Did he say exactly which transistors are bad?

Working the amp in general was a good experience. I don't recall that there were any seriously difficult areas to get to. What I mean is that there have been several projects that I've worked on now where there was a cap etc that was mounted in an area where it was impossible to get to the soldering points without removing a/the pcb. If you decide to work on it I don't recall any areas where I had to stop and ask "WTF! How am I going to get to that and change it?"

I would have to pull out my scribbling and remember if there were any transistors that I couldn't find subs for, but as I recall I eventually completely rebuilt the amp section and was able to find good parts for all of it. In any case the last I heard the amp I worked on is still working, so that's a plus.

Lets start with this. Find out which transistors are supposed to be definitely bad, and we can talk about it from there. And I'll have to pull out my paperwork and re-familiarize myself with the unit. It's been a couple of years now and heck, I'll be 60 at the end of this month.

Cheers,
James

 

[mhjmhjones]

He didn't say which transistors were bad. So I guess my first steps are:
1. Figure out what a transistor is/looks like.
2. Learn how to test them.

Thanks James!

 

[Bratwurst7s]

I don't recall, but did you download the service manual from the AK database? You will need it and it identifies the different transistors on the schematic, with a small drawing of each type that includes the pin layout.

As I do recall we are not supposed to post direct links to ebay here, but if you search the 'bay for this...
"12864 Mega328 ESR PNP NPN Transistor Resistor Diode Capacitor Mosfet Tester"

You should see a nifty small component tester. There are a number of different types of these little things. The one shown should be the type that comes with mini-grabber test leads. For the price they are invaluable. They can test pretty much all small components, transistors, diodes, caps etc and will make your life a lot easier doing this project.

Cheers,
James

edit: http://www.audiokarma.org/forums/index.php?threads/25-component-tester-too-good-to-be-true.596851/

 

[Bratwurst7s]

Also, read through my thread. You'll see that I was in much the same position as you when I started on the amp. Well, I knew what a transistor looked like but didn't know how to test them, and next to nothing concerning troubleshooting. But the photos will help you get an overview of things. With help from others I was able to go through the amp. If you can solder you should be able to fix the amp too. And if Avionic is here you have the best advice available on hand.

Making a dim bulb tester will be helpful...
http://audiokarma.org/forums/index.php?threads/dim-bulb-tester-build.758607/
http://audiokarma.org/forums/index....udio-gear-eh-heres-the-tools-you-need.333423/

Give me a few days and I'll make a list of transistors and substitutes, and links to Mouser. You should count on replacing all of the diodes also. And there will probably be some fusible resistors that are bad.

Cheers,
James

 

[clinic-audio]

Hello

You better don't touch anything inside if you don't know how a transistor look like . You can easy make mistake more costly than expected and "do it yourself" is not a good idea in my point of view .
If you need to know which sub you can use , send us a list of defectives transistors

 

[zaibatsu]

To echo what James said, don't do anything with power going to the unit until you've built yourself a DBT.
Until then, you can do plenty of testing without any power to the unit, and it's best to fix as much as possible in that state anyway.

You can use the diode / continuity test function on your multimeter to test transistors.
http://audiokarma.org/forums/index.php?threads/bipolar-junction-transistor-testing-basics.43186/

Once you get a feel for how to test a PNP and NPN transistor (two types which together form a complementary pair), you can exploit the fact that you can often test for dead shorts in-circuit (saves you having to remove everything). EchoWars explains this in detail in his thread. This doesn't tell you if transistors are working, but it will make it easy to weed out most of the dead ones quickly.

If the main output transistors are blown (the biggest ones on the heatsink), you can replace them all temporarily with a 470Ω 2W resistor across the B/E terminals (the outer two) as a placeholder while you fix the rest of the amp. Those transistors are the most expensive generally, so it's nice to leave your new ones out until you know everything is back to normal. This doesn't apply to every amp, but it should be fine on this one. The goal is to get the relay to click when powered on (with the DBT). Once the relay clicks it's safe to put the main output transistors in (never without the heatsink!).

If transistors blow, it's common for the fusible resistors (little green upright ones) to burn out as well, inspect all these first, and make sure you look at every angle. These must be replaced with "flameproof" types, generally 1/4W in that amp. If you're lucky the diodes might be fine, but basically, it's rare that a bunch of transistors blow without other components going with them (at least a few of those resistors). If the main output transistors have blown, also test the large white cement resistors (out of circuit).

Finally, do plenty of reading here, particularly threads where avionic helps others revive their Yamaha amps (A-?00, A-?20, M-60, M-80, M-65, M-85) etc. Most of the info applies to your amp too. If I read all of those before ever picking up a soldering iron, I would have saved myself a lot of time and heartache.

Good luck!