Parasound 1200 II left channel static and....

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A

AJ Bertelson

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Joined 2003
Yep, I actually cleaned all those off after I posted the picture.
http://webpages.charter.net/ajbertelson/diyaudio/cleaned.JPG

Looking at the picture though it appears that there is just a little bit of flux on the center power mosfets, I will try to remove that tonight.

Since I am new to transitors do I just touch the ground to P4 spot(it says G on the diagram so I assume this ground) and start looking for continuity across the circuit? A beep being good of course, and an even stupider question, should the amp be turned on when looking for continuity?

I did play around with measuring the amp last night with the continuity check and noticed that both banks power mosfets were about half the voltage compared to the working channel.

Thanks again for the help all. Also John I saw in a previous thread that you didn't work for parasound, you are just the designer :cool:
 
john curl

john curl

Member
Joined 2003
I am a design consultant for Parasound. I do not actually go to work at all, except from my apartment. This is why I am not aware of many details about many Parasound products, especially products more than 10 years old.
As far as repair of this unit is concerned, it might be more than you can handle. I would look for a skilled electronic tech, to help you. I don't even like to repair these designs, and it is MUCH easier for someone who knows the unit, inside and out, to repair it.
 
Spiny

Spiny

Member
Joined 2008
allthingsspiny.co.uk
When using a DMM please observe the following (I'm sure others will add their points)

Measure resistance (Ohms) and continuity with all power to the circuit OFF. If the Amp has been on wait a minuite for the power supply to discharge.

Measure Voltage with the power on, be careful and set the DMM to a high range (200V+) before measuring, reduce the range if necessary to get a more accurate reading. Be careful. shorting tracks, transistor Pins etc. is all to easy with probes when measuring voltage. :eek:

Measure current (Amps/milliamps) with the power on. Measuring current is more difficult and the meter needs to be placed inline at the point current is to measured, this often means unsoldering parts, adding wires and connecting the meter to the wires. Again a high range and with a problem circuit expect to replace the meter fuse

Use continuity / diode measurement to measure the Mosfets, you should be able to find the blown ones (Short circuit/low resistance) do this with the power Off.

alan

Edit, Can I endorse john's observation, this is a job for someone who knows their way round a circuit.
 
A

AJ Bertelson

Member
Joined 2003
So was the cleaning no sufficient? I was going to have someone look at it this week to see what they can figure out. Should I post a youtube video so you guys can hear the static I hear? Its only there at medium to louder volumes but the bad channel seems much quieter then the working one when inputs are set the same.
 
A

AJ Bertelson

Member
Joined 2003
I got a report back from an electrical engineer that looked at the diagram and poked around in the circuit.

I spent some time looking at the schematic and the circuit, probing around where the transistors connect from the top of the left side board. I did this without taking things apart other than the cover. I put the cover back on and left it where I found it on your desk for now. I have some ideas, but this circuit will be quite a challenge to repair. I'll provide you with what I think right now, so you can try some of this on your own if you want. I don't know that I'd have time to do the work it will take, and replacing parts is a bit of a crap shot. I could help you with the attempted repair if you order replacement parts, and take all the boards apart again when they arrive - so basically I could help you with the soldering work when all the parts are laid out on the table.

So far, I am able to conclude that the amplifier was being used to drive too much of a load or shorted out accidentally on the left side. The short may also be on the board. It appears from the pictures that many of the transistors have been replaced in the past. The burned flux and thermal paste has to be cleaned away especially between the pads where the transistors solder, to avoid shorting out those connections. It looks like, based on the "cleaned.jpg" file in your diyaudio website that you've done this. I'm guessing that may have been done after the damage occurred though, so the original contamination may have contributed to the damage.

At first, I suspected the transistor Q123 would be the culprit given the damage to the trace right there. However, probing it with an ohmmeter reveals it is no different than its neighbors. The readings were not way off, but somewhat different than the right channel counterparts (Q223, etc..). I just compared measurements on the other channel since they are essentially duplicates.

The real damage that I found were the two FETs Q121 and Q115 (I believe you're calling them center transistors). FETs are like transistors, but a bit more sensitive and usually the first thing to blow when overloaded. There is a path between 2 of the terminals on a FET that should be very high (20 Megaohms), and it was on the working right channel. The left channel FETs were reading about 20 kilohms for that same path, which is very bad. I would suggest replacing Q121 and Q115. The parts are 2SJ200 and 2SK1529. I would purchase spares in case they blow up again when replaced. It could be that there are other problems in the circuit and the damaged FETs are just a symptom. I am hoping that it was just a temporary condition leading to the destruction of those parts and the trace. To repair that trace, I would lightly sand away the green layer of the board, to expose more of the copper trace, then solder a short wire jumper to reconnect the trace. Use a large gauge wire (14) since the trace is so large to handle high currents. You will want to use lots of solder wick/braid to disconnect the bad FETs and remove the goober.

I believe it is possible that the other transistors are operational still but they may have been somewhat damaged. The readings that I got on those were different than the other channel, but in the right order of magnitude. I would order many of the S2C3519 and 2SA1386 parts as well because we may find they need replacing. It may not be cheap, but I would get enough spares to replace all these parts several times in case they get blown up again when replaced.

You should be able to look up the part numbers on Google and find distributors that carry them.
Click to expand...

getting closer? I can easily pull out and resolder in new mosfets but is this just the game that we play of replacing a part to try and figure out where the other one failed?
 
nigelwright7557

nigelwright7557

Member
Joined 2008
murtonpikesystems.co.uk
nicoch46 said:
ask if for test the amp can work with only single pair output transistors

if yes buy one pair and two of Q121/Q115 and see if works;)

a new Q111/112 not so bad
Click to expand...


What I do with a new amp is not connect the output transistors but feedback the output of the VAS back into the ltp.
Then I can prove the driver is owrking ok before risking output transistors.
Its also a divide and conquer strategy that works well.
 
A

AJ Bertelson

Member
Joined 2003
I finally go around to getting some replacement FETS and soldered them in.

No go and it didn't fix it. The green light went on but it would go into protect mode after about 5mins, same thing it did before I replaced the FETs.'

It currently will not go into protect mode and the lights no longer light up.
 
Cobra2

Cobra2

Member
Joined 2002
Paid Member
Hello!
I'm also in need of help... -bias-adjustment on one ch is "dead" -no reaction on pot, (have swapped pot too) measure minus 3-4V ! (and not mV).
There was a pcb-error on mine, shorting the mica-cap C111, took a while before I found the error(connected to Q109-collector in both ends). But the 10pF cap is not the problem here...
(Right ch is ok).
OpAmp(servo)swap did not make any change

Arne K
 
O

outfire

Member
Joined 2018
please let me revive this thread instead of a creating a new one. one of my channel blew a fuse. there are two fuses but only one blew. however, it is so old that it words are fading away that is not viewable.Does any one knows what are the internal fuse typed for 1200 mk2?
 
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