Parasound 1200 II left channel static and....

[AJ Bertelson]

There isn't a parasound or repair dealer in my state

Here is what I have tried so far or diagnosed
-Shake it and smacked it, no effect
-checked and removed fuses and no luck, fuses are good
-I can hear a relay click over when it starts up
-The amp doesn't go into standby
-The right channel works and it will work as loud as my ears can handle my speakers
-The left channel will work at lower volumes but when a high pitch tone hits then static will appear. When I start to increase the volume the channel becomes nothing but static.

I have access to a digital multimeter and I think one of the transistors has went out but I need help on figuring out which multimeter went bad. I set the dmm to a continuity check and got resistances across across the legs.

here are some pictures of the amp, they are 1600x1200
http://webpages.charter.net/ajbertelson/diyaudio/top01.JPG
http://webpages.charter.net/ajbertelson/diyaudio/top02.JPG
http://webpages.charter.net/ajbertelson/diyaudio/transistors.JPG
http://webpages.charter.net/ajbertelson/diyaudio/centertransistors.JPG
http://webpages.charter.net/ajbertelson/diyaudio/transistorscloesup01.JPG
http://webpages.charter.net/ajbertelson/diyaudio/transistorscloesup02.JPG


This is my first time working with amps and I know I am playing with fire but I was wondering if I would be able to get some help in figuring out what went wrong. I am trying to learn about this stuff more.

 

[nigelwright7557]

There are a number of approaches depending on what equipment you have.

A circuit diagram would help no end.
Then its just a logical step through the amp to find a voltage amiss.

As a last resort I have been known to remove all the transistors and test them out of circuit, not a task for the faint of heart as it can damage the PCB if not done with great care.

While the transistors are out you can check resistor values and look for shorted capacitors.

 

[john curl]

Get schematic from Parasound Richard@Parasound. com

 

[AJ Bertelson]

Parasound sent me a schematic
http://webpages.charter.net/ajbertelson/diyaudio/HCA 1200.pdf

Also I forgot to post the picture of the bottom of the amp
http://webpages.charter.net/ajbertelson/diyaudio/transistors.JPG
The top of the picture is the problem channel

So at what point to I start at? I checked the continuity, with the amp turned off last night of all the trans
Also I recorded temperatures of the transistors with my laser thermal probe last night and what I got from the failed channel was a temperature range of 72-76 while the working channel average 102-106. So it appears that power is not getting to the transistors, or at least my process of elimination has somewhat confirmed this.

So now that I have a roadmap of the amp, what is the best place to start? Since the right channel works I assume there is nothing wrong with the power supply section. I can confirm this by checking the six leads that lead from the power supply to the amplifier. I am guessing that the brown and black wires are the grounds so I set my dmm to DC volts/amps and put one lead to the ground and the other to either the white or the orange, correct? This will confirm if power is actually reaching the amp.

How do I determine what leg is the ground on the transistors though?

 

[john curl]

Is this really the schematic? Looks too old, to me.

 

[john curl]

YUP, wrong schematic. They gave you the 1200 schematic, not the 1200 MK2 schematic. Sent proper schematic to SY (moderator) Hopefully he will post it somewhere.

 

[AJ Bertelson]

http://webpages.charter.net/ajbertelson/diyaudio/HCA1200II.pdf

I believe this is the correct one.

 

[john curl]

This is the schematic you want IF you have an HCA1200 MK2.

 

[john curl]

This unit APPEARS to have been repaired before, and the two center complementary power mos fets near the center, MIGHT be the problem, along with any output part.

 

[PH104]

It also looks like a Holco resistor in the feedback (R115). If this is a mod, you might also carefully inspect the circuit board in that area.

 

[john curl]

Holco is normal feedback resistor.

 

[nicoch58]

burn trace PCB on Q123

 

[john curl]

You bet! Repaired by solder droplet.

 

[nicoch58]

Q123 not in short ?

 

[john curl]

Who knows?

 

[john curl]

BAD REPAIR JOB. CLEAN FLUX, MAKE BETTER CONNECTION AT Q123.

 

[AJ Bertelson]

So I repaired the burned trace with solder joint/goober

An externally hosted image should be here but it was not working when we last tested it.

SJ did not touch the resistor to the left

No dice, still the same static. Is the burned trace indicative that it took out the entire channel? Also I lost my printout of the schematic but I will print out another one at work tomorrow. After I get another schematic I am going to try out the DMM. What is the process of elimination though that i should start with though? Should I start out with the center mosfets?

 

[john curl]

PLEASE, clean the dirty spots near the power transistors. Do it before, ever asking me anything again about this amp. AND please remember that I am its designer, not its builder!

 

[nigelwright7557]

So I repaired the burned trace with solder joint/goober

An externally hosted image should be here but it was not working when we last tested it.

SJ did not touch the resistor to the left

No dice, still the same static. Is the burned trace indicative that it took out the entire channel? Also I lost my printout of the schematic but I will print out another one at work tomorrow. After I get another schematic I am going to try out the DMM. What is the process of elimination though that i should start with though? Should I start out with the center mosfets?

If its taken out a track then its like to have taken an output transistor(s) with it.
Check the output transistors for shorts.

 

[john curl]

CLEAN FIRST!