Alesis RA-100 Repair - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 8th March 2013, 11:24 PM   #1
Kamuy is offline Kamuy  United States
diyAudio Member
 
Join Date: Jun 2007
Location: Curled up inside these nice warm power MOSFET heatsinks.
Default Alesis RA-100 Repair

I recently came across an Alesis RA-100 through a friend. They told me it was malfunctioning, and I am *so* glad I opened it up and poked at a few things with a multimeter before hooking it up and testing it.

After some inspection and testing, the right channel works perfectly fine. The left channel however, has 45~ volts at the speaker terminals. I'm assuming this means a shorted output transistor. The output pair is two Sanken transistors, 2SA1494 and 2SC3858.

So, my question is... What would be best best way to go about fixing this? Should I replace one of those transistors? both? should I check anything else before replacing them to ensure they don't short out again/ensure there isn't a problem elsewhere on the amp board?
  Reply With Quote
Old 9th March 2013, 12:43 AM   #2
GoatGuy is offline GoatGuy  United States
diyAudio Member
 
Join Date: Dec 2012
Location: SF Bay Area
First, fox up a circuit diagram. They're kind of hard to secure.

Second, I don't remember: is the output push-pull on the RA100? I was under the impression that as a "reference amplifier", it was, but where the pushy-pully sections were biased slightly toward conduction. And I believe the RA100 had no output DC decoupling capacitor(s), but instead relied on a balanced +/- 50V power supply.

So... guessing out loud, you could as easily have a shorted output transistor ... as an OPEN output transistor on the other half of the AB push-pull pair! Its true.

And an output transistor - in my experience - that is open is more like than one which is shorted. They tend to go into thermal run-away (or get zapped by some reverse EMF because someone does something incredibly stupid when hooking up speakers) ... and open. Maybe short first, overheat, and internally explode (invisibly to you, of course). Then they're open.

Best of luck.

I'd secure a pair of output transistors though ... both of them ... and do a double-replacement, myself. Not too expensive, and the most probable way to solve the problem.

If the problem is upwind (i.e. leading to the transistors) then again armed with the schematic, you ought to be able to confirm odd voltage readings at various key places in the circuit.

GoatGuy
  Reply With Quote
Old 9th March 2013, 01:20 AM   #3
Kamuy is offline Kamuy  United States
diyAudio Member
 
Join Date: Jun 2007
Location: Curled up inside these nice warm power MOSFET heatsinks.
Quote:
Originally Posted by GoatGuy View Post
First, fox up a circuit diagram. They're kind of hard to secure.

Second, I don't remember: is the output push-pull on the RA100? I was under the impression that as a "reference amplifier", it was, but where the pushy-pully sections were biased slightly toward conduction. And I believe the RA100 had no output DC decoupling capacitor(s), but instead relied on a balanced +/- 50V power supply.

So... guessing out loud, you could as easily have a shorted output transistor ... as an OPEN output transistor on the other half of the AB push-pull pair! Its true.

And an output transistor - in my experience - that is open is more like than one which is shorted. They tend to go into thermal run-away (or get zapped by some reverse EMF because someone does something incredibly stupid when hooking up speakers) ... and open. Maybe short first, overheat, and internally explode (invisibly to you, of course). Then they're open.

Best of luck.

I'd secure a pair of output transistors though ... both of them ... and do a double-replacement, myself. Not too expensive, and the most probable way to solve the problem.

If the problem is upwind (i.e. leading to the transistors) then again armed with the schematic, you ought to be able to confirm odd voltage readings at various key places in the circuit.

GoatGuy

Thanks a lot for the advice.

If I were to desolder the output transistors, and then hook the amp up and do some testing along the signal path to make sure everything looks good, would that be harmful to the amp? Generally speaking, of course, is that an acceptable way to test to see if the issue is the output transistors without buying new ones first?
  Reply With Quote
Old 9th March 2013, 03:12 AM   #4
GoatGuy is offline GoatGuy  United States
diyAudio Member
 
Join Date: Dec 2012
Location: SF Bay Area
Usually "desoldering the output transistors" ... is vexing - but entirely useful for figuring out the root of the problem.

FIRST though... you know not just theoretically how to desolder, but have a bit, and do a good, clean, quick job ... right? If not, you stand a good chance of, well, making a mess of things. Better to do some practice first on a few solder joints you make, then desolder, and unmake. Back in practice.

Then once they're desoldered - without turning on the amplifier - you can determine whether the emitter-to-base and collector-to-base junctions seem to be behaving like diodes (they should conduct in one direction, and not the other). Then you can see if emitter-to-collector is essentially an "open" (same for both PNP and NPN, with either polarity applied to emitter and collector). Ohm meters with needles are helpful here.

Now if it turns out that both transistors appear to be working as above ... one still could be ferdutzed ... but that takes more specialized equipment to ferret out. Work however on the assumption then that something leading into the output transistors is amiss. Simple ...

You have to find the ground-plane first. Shoudln't be too hard.

Then, measure the voltage that was supposed to be applied to each transistor's base pin. Again, easy enough. If the design is conventional, then it should be the same voltage (polarity reversed, of course). If the voltages are wildly different ... well that could be the problem.

Parenthetically - the RA100 is an excellent amplifier when it works. Brilliantly transparent, clean. Heats up those little heat-sinks on the side quite nicely when the power output exceeds 10 watts. Has a high enough amount of negative feedback to be super-linear, and to also show a very significant amount of damping factor (pull-down and drag-to a particular operating point).

The one thing that is dangerous about operating the amplifier without the output transistors ... is only that without the diagram, you can't tell whether you're going to be potentially blowing out other parts of the circuit.

So... back to basics.

Get the circuit diagram.

GoatGuy
  Reply With Quote
Old 9th March 2013, 03:32 AM   #5
Kamuy is offline Kamuy  United States
diyAudio Member
 
Join Date: Jun 2007
Location: Curled up inside these nice warm power MOSFET heatsinks.
Quote:
Originally Posted by GoatGuy View Post
Usually "desoldering the output transistors" ... is vexing - but entirely useful for figuring out the root of the problem.

FIRST though... you know not just theoretically how to desolder, but have a bit, and do a good, clean, quick job ... right? If not, you stand a good chance of, well, making a mess of things. Better to do some practice first on a few solder joints you make, then desolder, and unmake. Back in practice.

Then once they're desoldered - without turning on the amplifier - you can determine whether the emitter-to-base and collector-to-base junctions seem to be behaving like diodes (they should conduct in one direction, and not the other). Then you can see if emitter-to-collector is essentially an "open" (same for both PNP and NPN, with either polarity applied to emitter and collector). Ohm meters with needles are helpful here.

Now if it turns out that both transistors appear to be working as above ... one still could be ferdutzed ... but that takes more specialized equipment to ferret out. Work however on the assumption then that something leading into the output transistors is amiss. Simple ...

You have to find the ground-plane first. Shoudln't be too hard.

Then, measure the voltage that was supposed to be applied to each transistor's base pin. Again, easy enough. If the design is conventional, then it should be the same voltage (polarity reversed, of course). If the voltages are wildly different ... well that could be the problem.

Parenthetically - the RA100 is an excellent amplifier when it works. Brilliantly transparent, clean. Heats up those little heat-sinks on the side quite nicely when the power output exceeds 10 watts. Has a high enough amount of negative feedback to be super-linear, and to also show a very significant amount of damping factor (pull-down and drag-to a particular operating point).

The one thing that is dangerous about operating the amplifier without the output transistors ... is only that without the diagram, you can't tell whether you're going to be potentially blowing out other parts of the circuit.

So... back to basics.

Get the circuit diagram.

GoatGuy
I have some pretty extensive soldering/rework experience, and good equipment to work with so that shouldn't be a problem.

I do understand a fair about electronics and amplifiers in general, I just don't know quite enough to be able to look at the board and determine exactly what is what. I mean, I can tell it's push-pull, and trace the signal path through from input to output and such. I have a good multimeter and a decent analog oscilloscope as well, so testing things shouldn't be an issue either.

The biggest issue here is, while I understand the basics of the pure 'amplifier' part of the circuit, the protection circuitry and biasing stuff I just don't quite get yet, and I don't have a lot of experience working with analog electronics, so I'm doing my best and I *REALLY* appreciate the detailed and helpful responses you're giving me.

I'll look around and see if I can find a schematic for the RA-100 somewhere, and let you know if/when I do.
  Reply With Quote
Old 9th March 2013, 04:14 AM   #6
Enzo is offline Enzo  United States
diyAudio Member
 
Join Date: Sep 2004
Location: Lansing, Michigan
I have to respectfully suggest a different approach. SHorted outputs can leave DC on the output, yes, as could opens on the other side, yes. But there are other ways this happens too.

The way amps generally work is the bases of the output stages are more or less connected together (with the bais circuit keeping them a set amount apart) Then moving the whole mess up and down to drive the output. SO a stuck on or an open at the voltage amp transistor stage could leave the output stuck on a rail just as easily. The whole amp is direct coupled, so just about any transistor failkure throughout the amp could put DC on the output.

Do not use a speaker or load on the output until you are sure there is no DC on the output. DC on the output by itself won't draw any excess current, won;t stress the amp. Putting a load on it will.

Instead of pulling parts and just gong down the rows testing parts, get out your volt meter and start to hunt. You have the other channel to use for reference voltages too. I'd first verify we had both power rails into that channel. I don't know the innards of the RA100, but I imagine to have a common power supply rather than separate. But it could have channel railo fuses, look. In any case a quick check to see that both polarities have full rail voltage present. Then look at the bases of the outputs. If you have that 45v more or less on the bases of the outputs, they they are just doing their job. Whatever you put on the base is what will come out the amp output. If the bases are close to zero DC or at the opposite polarity from the output 45v, then the problem is indeed the output stage.

Nothing external to the transistor can make a shorted one appear not shorted. SO you can go down the row of output transistors checking for emitter to collector shorts right where they sit. If none measure shorted then none are shorted. On the other hand other things in the circuit can make good transistors appear shorted. SO if you find some that measure shorted, THEN pull themout and retest them. If they are indeed shorted, well they had to come out anyway to replace. If they are not really shorted, then you have to find out what parallel path in the amp has the shorted condition.

You need to find out what is wrong, not just start changing transistors.
  Reply With Quote
Old 9th March 2013, 11:19 AM   #7
diyAudio Member
 
east electronics's Avatar
 
Join Date: Jan 2006
Location: Athens GREECE
In my company we repair an average of 400 amps per year of any kind ... in numbers we replace an average of 6000 electrolytics of any size per year and about 200- 300 output transistors per year ( also of any kind plastics or cans )

Today i am 47 and i am working with amp repairs for the latest 30 years In my life i have never never never and never seen an ""open"" output transistor .... only shorted ones ....

I have seen totally open transistors but only in the first or second stages of an amplifier but that also will be very rare ...

puzzled regards
sakis
__________________
SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
  Reply With Quote
Old 9th March 2013, 01:37 PM   #8
GoatGuy is offline GoatGuy  United States
diyAudio Member
 
Join Date: Dec 2012
Location: SF Bay Area
Well, there you are. Sakis of Greece - gives the on-the-ground clue. Personally, I've seen OPEN output transistors some, but mostly shorted/fused. In the old days (with the transistors usually in those TO-3 cans... especially like the interior of the one I've attached ... OPEN was more common that shorted - because any transistor that shorted immediately conducted the big-can capacitor reservoir to ground, through those little thin wires ...which would then like a fuse, "blow open". Just saying. Depends on transistor packaging.

GoatGuy

TO3 can
  Reply With Quote
Old 9th March 2013, 03:45 PM   #9
diyAudio Member
 
east electronics's Avatar
 
Join Date: Jan 2006
Location: Athens GREECE
well the alesis RA100 doesnt feature any cans and that in a way makes your advice /recomentation kinda worthless

Often there is a chance for one to learn only by pointing him to the right direction than explain him bits of history that will not provide solution .

Finally i dont know if this amp is Alesis for real since the built inside is horrible thermals also are bad and RA 500 has a huge book of history with failure ...For RA 100 i dont have that type of statistics and it may as well be better

kind regards
Sakis Of Greece
__________________
SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
  Reply With Quote
Old 9th March 2013, 05:18 PM   #10
Kamuy is offline Kamuy  United States
diyAudio Member
 
Join Date: Jun 2007
Location: Curled up inside these nice warm power MOSFET heatsinks.
As suggested, I checked the bases of the big output transistors (green - 2SA1494 and 2SC3858). The base of the top one is reading +47.6v. The base of the bottom one is reading -47.6v. So I moved back to the next set (blue - 2SA1668 and 2SC4382). The base of the top one read +46.0v, and the bottom one read +46.0v as well. Onward to the next set (red - MPS8099 and MPS8599). The base of the top one read +45.0v, bottom one read +45.0v.

So, at this point, I have a few questions. Considering all three sets of transistors that I tested are PNP/NPN pairs, and taking into account that only the big output transistor bases read + and - 45~ volts while the bases of the other two pairs all read only positive 45~ volts... does that mean the problem is likely with one of these six transistors, or is this more indicative of an issue somewhere else on the board?

Another question, the transistor in the center of all the others, marked in pink, I'm not sure what it's purpose is. Is it part of the biasing circuit, or something to do with the clipping LED possibly?

(This image is large and not perfect, I know, but I figured it may be helpful since I have not been able to locate a schematic anywhere. I emailed Alesis and apparently they have been bought out since this amplifier was made and no long have the schematic either. :/)

Board Image
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Alesis RA-100 - may need a new transistor deniss77 Solid State 26 7th April 2013 10:25 AM
Problems with Alesis RA-500 sippo Solid State 11 15th January 2013 08:00 PM
ALESIS RA 100 - Schematics fosforo Solid State 8 16th March 2012 01:31 AM
Schematic for Alesis RA-100... Ha! Alesis doesn't have it!! crispycircuit Solid State 0 22nd February 2010 09:16 PM
any one know if the Alesis RA 300 amp can be mod ? zuka Solid State 2 14th April 2003 05:58 AM


New To Site? Need Help?

All times are GMT. The time now is 04:50 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2