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Old 3rd January 2012, 06:32 AM   #1
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Default reference picasso class A issues

i have a 1996-97 polished aluminum soundstream reference picasso class A amplifier which was working fine. it didn't wow me so i decided to replace all capacitor except for the ceramic ones, 24 -diodes 5402, 24 -1ohm resistors 1watt, and all transistors, tip102, tip107, ndp7050 (replaced with ndp7060), and the biasing pots. As for fep16dt, fen16dt, mc7815ct, and mc7915ct i removed them from the board but decided to not replace them because i couldn't find replacements.

the caps was replaced with same value, the ultra low esr caps i replaced with the panasonic ultra esr caps, and the rest of the caps with replaced with audio grade caps elna silmicII and nichicon. the 8 polypropylene caps were replaced with solen audio caps 4.7uf 400v.

i touched up all the vertical boards on the amp 6 total with solder.

after all my hard work and spending over $300 in parts i was finished.

i now am going to adjust bias to 20mvdc but am not able to moving the pot to its minimum the bias is at 14mvdc and at its maximum it is at 19.6mvdc. each pot doesn't get affected by adjusting the other. so if i leave one pot at 17.6mv and adjust the other pot the first pot doesn't get affected it stays at 17.6mv. across the 1ohm resistor it is average of 8.8mv. the pots i purchased are 500ohms and is marked with 501 but the original part is marked with 500. i tested both pots and at its minimum they are both about 500ohms. thinking the pots might be the cause i replaced one of the pots back to the original pots and problem still remains the same. from my notes the bias adjustment should be able to adjust from 0 to over 100mvdc but should be set to 20mvdc +/- 5mv. what might be the cause of this.

i decided on setting all pots to 17.6mv and now its time to test all outputs.

all channels work and play music but for some odd reason on the front left channel of the amp the level is double the loudness compared to the other 3 channels. what can be the cause of this? would it be the 1 channel being faulty or the 3 channels being faulty. all 4 channels play without distortion and before i replaced all the parts on the amp all channels played the same no sound level differences.

please help. this is idiotic and stupid of me replacing parts on the amp which i know nothing about.
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Old 3rd January 2012, 07:22 AM   #2
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Hummmm... have you checked the gain pots and the switches like Perry suggested?????

The switches are the most likely suspect, but I have seen the gain pots become un-aligned with one another after some un-knowing hands had been inside the amp before myself and they for whatever reason pulled out the shaft that connects them together and accidentally left one gain pot out of alignment with the others......

Other then that I would check for DC voltages on the op-amps inputs and outputs, and make sure the lower 15 volt rails are properly balanced and equal
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Old 3rd January 2012, 08:53 AM   #3
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how to perform this test "Other then that I would check for DC voltages on the op-amps inputs and outputs, and make sure the lower 15 volt rails are properly balanced and equal" sorry you are speaking to an idiot with no electronic circuit background.
the switches looks new like someone changed them i can tell by the soldering job and flux residue on them.
do you think the problem is with the 3 channel or with the 1 channel. i assume it is with the 1 but not sure.
how about the bias pots why am i only able to adjust from 14 to 19mvdc and this goes for all 4 channels. would the ndp7060 cause this?
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Old 3rd January 2012, 03:30 PM   #4
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Quote:
Originally Posted by russdeanlee View Post
how to perform this test "Other then that I would check for DC voltages on the op-amps inputs and outputs, and make sure the lower 15 volt rails are properly balanced and equal" sorry you are speaking to an idiot with no electronic circuit background.

Have you read Perry's web-site yet? It will tell you how to read voltages on op-amps to see if any of them have failed

the switches looks new like someone changed them i can tell by the soldering job and flux residue on them.
This is how they looked originally so don't go by flux contamination as to weather someone before you has replaced them. The switches on all of these SS amps are a very big problem, so don't over look the possibility that they might have dirty or bad contacts

do you think the problem is with the 3 channel or with the 1 channel. i assume it is with the 1 but not sure.
I am fairly sure you have a single channel issue.

how about the bias pots why am i only able to adjust from 14 to 19mvdc and this goes for all 4 channels. would the ndp7060 cause this?
No the mosfets in the power supply have really nothing to do with the bias controls. What are you rail voltages ??? take the black probe of your voltmeter and connect it to the center tap of the transformer and then with your red probe measure each supply rail both positive and negative should be equal. The only reason I can think of for the bias not being as high as spec is because it is either limited by the factory or some supply voltages are not correct
Really please study Perry's tutorial it will inform you about what you are doing and how to go about what you are doing. It could have saved you much money so far and a lot of un-needed replacement work. Plus now that you have pretty much gutted the amp your problem is still present and in your mind you are now in doubt of the work you preformed and the new parts you installed. It is very important to follow a procedure, and the most important thing was to figure out what was wrong in the first place then replace parts as needed. Once the problem has been dealt with all of the new things you wanted should have been a piece of cake. But with all of the upgrades and such they confuse the original issues you have now. Sort of a catch 22 you are in now...

Read Perry's tutorial... Take it at you own pace... It will show you how to do all of what you posting about.
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Old 3rd January 2012, 07:41 PM   #5
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the rail voltage is 23v and -23v, the one channel that is louder than the rest sounds as there are more high frequency sound coming out of them compared to the rest i'm testing the amp on a bench with a 4ohm 5 1/4" focal midrange speaker.

what must i do to properly balance the lower 15v rail.

yes i do have perry's tutorial i purchased it about 6 months ago. there is tons of reading that i haven't really read yet. i should probably start.

thanks for helping i really appreciate it.
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Old 3rd January 2012, 08:52 PM   #6
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"what must i do to properly balance the lower 15v rail."

What are the lower rail voltages? There are not going to be the exact same but should be very close < within a few milli-volts of each other >

For the channel to have off frequency response and less volume I am thinking either the FEB is defective or the signal from the front end is bad to begin with, but without a definitive testing of the switches those would be my second best guess's at this point.

by chance what are the DC offsets of all the channels at the speaker terminals especially the off channel ? This is often a good indication is something is off spec in the circuitry.
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Old 4th January 2012, 08:00 PM   #7
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lower rail voltage are +/- 15
dc offset is

FL: 5.0mv
FR: 1.5mv (playing louder than the rest)
RL: 1.5mv
RR: 3.0mv

all op amps has +/- 14.57v (off 12v power supply)

after really listening to it the one channel playing louder than the rest sounds the best in my opinion and 4 of my friends who i asked to listen to it the loudness sounds as it is double the loudness of the rest of the channels.
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Old 4th January 2012, 08:04 PM   #8
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Is there a switch for high/low RCA input?

If so, set it to the position where the amp produces the highest audio output for a given input level. Does that allow all 4 channels to play at the same level?
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Old 4th January 2012, 08:09 PM   #9
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can you get a hold of jaime and ask if he has 4 feb boards and the two pre-amp (i think thats what they are) boards in stock i would like to purchase them. for the 1996-97 reference picasso class A polished aluminum amp.
is it posible the resistors near the rca to go bad i check a couple of them and the some of the value are incorrect would i need to lift up one leg before testing?
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Old 4th January 2012, 08:45 PM   #10
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how did you know i was an idiot Mr. Babin. the amp operates at same level now i had one of the switches on the balance side so i correct all switch to correct location and amp works fine now. thanks i was so close to ordering all parts on the amp to renew the entire amp.
another question is i heard about people doing op amp swaps with the amp i would like to install the bur brown op amp can someone reference me to the correct ones to work with this picasso class A.
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