help with car amp

Thread Starter

lokeycmos

Joined Apr 3, 2009
431
i have 2 brand new 100uf 25v in my parts bin. .5ohm ESR. havnt put them in yet. i measured emitter to emmiter on Q908 Q907. peaks about 6 v then drops to 3.6v. im kinda catching on here. does that mean an issue with DC-DC converter or could it be something farther down the line shorted out causing power supply output to drop? i noticed the fan isnt spinning either, but i dont know if it just kicks in when it gets warm.
 
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MrChips

Joined Oct 2, 2009
30,707
The prognosis is as follows. There is supposed to be +15v and -15v supply. The -15v supply is working. The +15v supply is not working.

The same IC901 drives both supplies. Q903 and Q909 make up the complementary pair driver for the +15v supply. Q904 and Q910 handle the -15v side.

What we really would like to see is if there are pulses at the base of the driver transistors. This will tell us if IC901 is good or bad.
 

kam-22

Joined Nov 21, 2011
19
yes the fan would start is the unit over heats... have you checked the dc offset on the o/p of both channels? you can short the o/p relay to carry out this quick test.. or check the voltage at the emitter's of the Darlington transistors on both channels...
 

kam-22

Joined Nov 21, 2011
19
lokeycmos...try this site Basic car audio electronics. there's plenty of info on there..

in some case's you can run the power amp stage using a dual psu via disconnect that part of the circuit. and looking at the circuit it's a straight forward amp stage.

It can even be run on +/- say 15 volts...
 

R!f@@

Joined Apr 2, 2009
9,918
Kam, OP is here to fix his amp. Not to modify it.

If AAC members cannot help him to fix it, no other forum can. This I guarantee.

Because I am here and I can fix that thing with in minutes if it was in my hand.
But I am keeping my distance to see how others are doing.
So please, if you can't help, do not try to lure him away.
 

Thread Starter

lokeycmos

Joined Apr 3, 2009
431
on both pins 9 and 10 of IC901 i have 4.66v 36.80KHz.

on the bases of Q903,Q909 and Q904,Q910 i have 4.44v same frequency...
 

thatoneguy

Joined Feb 19, 2009
6,359
i have 2 brand new 100uf 25v in my parts bin. .5ohm ESR. havnt put them in yet. i measured emitter to emmiter on Q908 Q907. peaks about 6 v then drops to 3.6v. im kinda catching on here. does that mean an issue with DC-DC converter or could it be something farther down the line shorted out causing power supply output to drop? i noticed the fan isnt spinning either, but i dont know if it just kicks in when it gets warm.
The ones on the amp measuring 4.5Ω would be I would call a "Bad Cap" then. Put those two new ones in, and recheck voltages MrChips is looking for. Again, may not miraculously fix it, but they aren't helping things.
 

MrChips

Joined Oct 2, 2009
30,707
So it looks like we will have to replace Q905 and Q911, IRFIZ44G, both mounted on the heat sink. What are your chances of being able to purchase those? They run about US$2 each. I even have them in stock. I don't know where you are located.

You can also pull C908 and C925 and see if they are shorting.
 
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Thread Starter

lokeycmos

Joined Apr 3, 2009
431
So it looks like we will have to replace Q905 and Q911, IRFIZ44G, both mounted on the heat sink. What are your chances of being able to purchase those? They run about US$2 each. I even have them in stock. I don't know where you are located.

You can also pull C908 and C925 and see if they are shorting.

ill have to look, i usually get all my parts from digikey. im in minnesota. so could you please explain how you got to this diagnosis? ill learn more that way. ty btw i did replace the low esr caps. no difference. pretty busy today tho.
 

MrChips

Joined Oct 2, 2009
30,707
I cannot give you a complete analysis until we find the fault. However here are some standard procedures for trouble shooting audio electronics.

Step #1 - History
Know something about the history of the equipment.

  • Where was it being used?
  • When was the last time it was working?
  • What was the situation leading up to failure?
  • What were the symptoms?
Obviously, sometimes we have no useful information.

Step #2 - Condition

  • Look for damage, heat stress, bulging, smoke
  • Smell for burnt components
  • Touch for hot spots
  • Listen for strange noises from the equipment or speakers
Of course, trouble shooting over the internet presents an additional challenge.

Look for fuses, switches, power interlocks or lock-outs.

Step #3 - Apply Power

  • Look
  • Smell
  • Touch
  • Listen

Step #4 - Check all power supply voltages

Test with a voltmeter, better still if you have an oscilloscope.

Having a service manual with schematics makes the difference between night and day.

Sometimes some situations will call for a different approach. A common situation is that the AC Line fuse blows every time the power is turned ON. I have an electronic circuit breaker where I can set the trip current. The breaker will trip when the current exceeds the trip current. This way I can see how much current the equipment is taking before it trips and I don't have to keep replacing fuses.
 
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thatoneguy

Joined Feb 19, 2009
6,359
So ..is it time yet ?
Go gor it, if it will prevent useless replacements. I doubt the caps are shorted, they all measured fractional Ω ESR, I suppose going through and DC testing them would find a shorted one.

I'd be for checking continuity in the supply transformer and inductor windings as well.

It's a grab-bag, I'm guessing some kid brought it to the tech school all blowed up, a student replaced the visually destroyed parts, it didn't work, so it's in the closet and now we have it. We do not know if it worked after the last "fix" or not.
 

R!f@@

Joined Apr 2, 2009
9,918
OK then...let me go thru the thread first.

OK lokey, first off try to give me a high resolution picture of the solder side.
The highest u can take. Good lighting and no glare so I can zoom in and see without a problem.
This would speed things up a lot.
Try to take the Picture perpendicular to the board. Complete board is a must. If you cannot try to take the picture in parts, divide the board into 4 squares and take the shots

Without the pic I cannot tell u where to look properly. I need the pic to compare to schema.
Upload it to file hoster like photo bucket and attach the link ONLY. DO NOT POST THE PIC.

Next tell me the current status of the amp.
1. Is it possible for u to apply power and take voltage measurements.
2. If not explain why.
3. Have u found any faulty parts. If so list them and tell me where it is located.
 
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Thread Starter

lokeycmos

Joined Apr 3, 2009
431
ok, im working on the pics. im goona have a friend host the high res pics on his ftp site so you can download and zoom in in windows. something wierd is going on here C561 and C560 are the 2 10v 100uf electrolytics with bad ESR that i replaced with 25v 100uf. in the parts list they show up as 2 different part no's. i cant even find c561 in the schematic. ill post pics as soon as i can.

yes i can power up and take measurements. i did just pick up a scope on craigslist. model 564 tektronix. please dont ask me to use it. to be honest i have no idea how to operate it. it doesnt have a probe. im using a bnc to coax adapter and a piece of stripped coax as a probe. dont get me wrong i would love to know how to use it, but trying to save that for another time.
 
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