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Old 23rd December 2010, 01:41 PM   #1
Dr Zeus is offline Dr Zeus  United States
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Default LA Sound Sunset 100S

Ive had this amp off and on my workbench for the past few weeks; poking through I originally found the TIP2955 output had failed in the right channel. After replacement, the channel looks clean on the scope w/o a load. As soon as I attach a test speaker the channel gets distorted.

Ive seen this before with other amps, where the driver transistors had malfunctioned, however this amp has proven to be a little stuborn and I'm unable to locate the faulty component(s).

The output that was replaced was Q8R (Top of photo, left output). From what I can tell, all the smaller transistors test OK, and some I've even swapped left-right channel. Not one found yet to be the cause.

I think otherwise, this amp should be a pretty quick an in-expensive fix

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Last edited by Dr Zeus; 23rd December 2010 at 01:43 PM.
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Old 23rd December 2010, 03:43 PM   #2
Dr Zeus is offline Dr Zeus  United States
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FWIW, I own this amp... and there are several good parts made by ST (TIP3055 / TIP2955 outputs) and MOSPEC (U16C20C, and U16C20A Rects), and IRFZ44 fets which I can likey use for other amps needing repair i nthe future

Is this amp worth anything or should I part it up?
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Old 23rd December 2010, 05:29 PM   #3
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The parts are only good for experimentation. You can't (shouldn't) use used parts in an amp you're being paid to repair. I'd suggest that you repair it.

Do you have a clean signal on the output of all of the op-amps?
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Old 23rd December 2010, 08:04 PM   #4
Dr Zeus is offline Dr Zeus  United States
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This amp has 3 opamps; KIA4558P and they are 8 pin. Referencing non-bridging speakerterminals, the outputs of the opAmp in the center of the amp are not clean while a test speaker is at the terminals. The output pin 1 also has -.25vDC on it, while the input pins 2 & 3 have 0vDC and no audio signal.

All opamp power pins have +-14vDC.

Im thinking I want to try swapping this opamp with one from a different location.

Last edited by Dr Zeus; 23rd December 2010 at 08:09 PM.
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Old 23rd December 2010, 08:31 PM   #5
Dr Zeus is offline Dr Zeus  United States
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Well, swapping opAmps didnt help. The center opAmp location seems to be having all the trouble.
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Old 23rd December 2010, 09:09 PM   #6
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The op-amp is working properly.

Virtually all amps use differential amplifiers. Most use 4 transistors but this one uses the differential amp's differential input instead of discrete transistors. The signal doesn't look like audio because it's only driving as hard as it needs to to make the feedback signal look like the input signal. The first schematic on the 'Audio Troubleshooting' page has one example of this type of circuit. Navigate to and open the following file and you'll see that the output of the IC looks nothing like the input (pin 5) or the feedback signal (taken from the speaker output, pin 6).
repair_tutorial/miscellaneousstuff/orion/orionhcca250R/orionhcca250rWaveforms/orionhcca250raudiowaveforms.htm

I think you'll have to check each of the transistors. There appears to be only 5 or 6 that you may not have already checked or replaced.
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Old 23rd December 2010, 10:12 PM   #7
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Yea, Unclemeaty, break any/all bad habit's when you start repairing other peoples stuff. I refuse to half-*** a repair, I go as far as de-fluxing my repairs, for that factory fresh look.


Iv got a little list that I use as my final QC sanity check, you should make one for yourself! Mines got things on it like, check any joints I touched, re-flow "wrinkled" solder joints, Highest quality parts money can buy, silver bearing solder for anything in the audio path, never "down grade" op-amps, never socket a op-amp DEFLUX!DEFLUX!DEFLUX! I cant stand flux coated joints... Looks so unprofessional..


These are just a few of my personal guidelines iv set for my repair standards, they mainly come from tube amp building, minus the op-amp bits...
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Old 23rd December 2010, 11:19 PM   #8
spooney is offline spooney  United States
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Quote:
Originally Posted by unclemeaty View Post
FWIW, I own this amp... and there are several good parts made by ST (TIP3055 / TIP2955 outputs) and MOSPEC (U16C20C, and U16C20A Rects), and IRFZ44 fets which I can likey use for other amps needing repair i nthe future

Is this amp worth anything or should I part it up?
I can't bring myself to part out anything. I just leave it sitting until I get the itch or find the parts to work on it again.
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Old 24th December 2010, 04:11 AM   #9
Dr Zeus is offline Dr Zeus  United States
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I retract my last statement for parting this amp for repairing others. i think this amp is close to being fully repaired, while some amps in my collection are in some cases heavilly damaged, missing unknown pieces, and terminas removed. I dont want to use parts that are not reliable, but I will in testing. We've all made a few new parts go puff from time to time I'm sure, but it is less painfull if the parts were free...

To be honest, I've seen a bunch of cool/neet stuff, and a lot of junk in the past month or so. Im not sure about you guys, but in my area it seems every shop has a pile of amps just waiting to be fixed. Athough I do now have a little $ in my Paypal account, I consider that $ for the hobby and not for profit. I think I'm broken even actualy, and if including my time its not worth the trouble. This is a hobby that I've found some people which I provide a helping hand, an alternative to the $100-200 fees from some of the real shops. If an owner doesnt want to pay much then theres not a whole lot I can do... like the owner of the Alpine 3545 doesnt even want to pay the $30 for new rectifiers - I'l probably just fix it and give it back to him anyways because he gave me a power supply.... New parts.

Every amp I've sold has only had new parts, and only two amps I returned to collectors were not 100% reliabe (Rodek was one of them and an Orion 2250 that took an MPSA07 from an Orion 2150)

Thanks guys, I'l keep it clean.

As for this amp, its a mystery what is wrong with it. I pulled -all- transistors for both L and R and I cant find any problems. I' work on it some more after the hollidays.

Happy holidays!

Last edited by Dr Zeus; 24th December 2010 at 04:18 AM.
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Old 24th December 2010, 07:00 PM   #10
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Quote:
Originally Posted by 807tubenut View Post
Yea, Unclemeaty, break any/all bad habit's when you start repairing other peoples stuff. I refuse to half-*** a repair, I go as far as de-fluxing my repairs, for that factory fresh look.


Iv got a little list that I use as my final QC sanity check, you should make one for yourself! Mines got things on it like, check any joints I touched, re-flow "wrinkled" solder joints, Highest quality parts money can buy, silver bearing solder for anything in the audio path, never "down grade" op-amps, never socket a op-amp DEFLUX!DEFLUX!DEFLUX! I cant stand flux coated joints... Looks so unprofessional..


These are just a few of my personal guidelines iv set for my repair standards, they mainly come from tube amp building, minus the op-amp bits...

Nice to see others with the same ethics I have. Plus I spent almost 20 years working on PG amps and they had to be cosmetically perfect so they could be flipped and placed under plexi for viewing. Clean, clean, and then clean again. All that gold plating has to shine like a mirror. Heaven forbid that you slop a solder joint, as gold plated boards show no mercy and hide no mistakes...

Unclemeaty, I have a stack of dead amps I keep, mostly cause it costs out the arm to have them buried nowadays, but they also provide hardware parts that are non-existent in the real world. Every once in a while I use them to find my way on badly destroyed amps.
To be honest the amount of work required to remove old parts and then re-install them and hope for the best is just not worth that much effort. Its like doing two repairs instead of one. Even if I must wait for the postman to bring the part its such a time savings to use only new parts and work the the repair job only once. I hope you save yourself some time by doing things the easy way and time savings way. You will have more time for other amps waiting for the mailman, and you will rest better at night knowing only the best of yourself and your parts went home with the client....

Last edited by 1moreamp; 24th December 2010 at 07:12 PM. Reason: spelling
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