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Midrange Squeal I have become the owner of a Soundtech PC-1250 Powered Mixing Board, and I am interested in making something more than a boat anchor of it. When I obtained the unit I was told the building it was in had suffered a lightning strike, and it hadn't worked since. I took it home to find out what I could and I am stumped as to how to proceed. When it powers up, it emits a constant midrange squeal that is very loud out of all four outputs: R, L, Mon. A, Mon. B. If it were silent I know a thing or two to try. I don't own a scope. Can someone please help me to isolate the origin of this signal so I can proceed? Have been advised That because all four channels emit the same noise, my problem is likely in the power supply. Have examined the PS board/components. No visible indications of problem I can detect. Large capacitors hold a charge and diodes conduct one direction. Haven't found any cracks or loose joints. I showed this unit to a friend. He agrees it's probably a pwr supply issue, but gave me reason to skeptical of a lightning strike being the cause. Schematic at: https://blu168.mail.live.com/mail/ViewOfficePreview.aspx?messageid=3396c207-e814-11e2-af38-00237de4a318&folderid=00000000-0000-0000-0000-000000000001&attindex=1&cp=-1&attdepth=1&n=1803853789

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6 Suggested Answers

6ya6ya
  • 2 Answers

SOURCE: I have freestanding Series 8 dishwasher. Lately during the filling cycle water hammer is occurring. How can this be resolved

Hi there,
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Posted on Jan 02, 2017

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  • 113 Answers

SOURCE: Behringer EP2500 no output on CHANNEL 1

check all op amps for correct voltage swing +-should be equal,test output of op amps no voltage should be present, use chasis gnd as tester neg.check voltage regulation especially 12v or 15v +-.note some amps show unequal led indications when only 1 channel connected to a load.I will try to get some more info on this specific amp because i don't know this circuit well,keep in touch as you update though.

Posted on Aug 22, 2007

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  • 3 Answers

SOURCE: Pioneer PDP 5030 power supply problem

Ok here's how I manage solve the problem... Having the back removed opens the contacts on the interlock switch so I guess if it powers up in this state it senses the high voltages are exposed and for safety shuts down... After this the unit will not power up and the power led flashes red continuously as described before…
After some thought it occurred to me that there may not be a problem at all, just be a simple reset function for this condition… I managed to find an easy solution to this… Just call up the hidden engineer’s service menu – On 3rd generation Pioneer Plasmas it’s as follows > Power down the screen > Hold down the “Input” and “Vol +” buttons on the front > Power the screen up and the Service menu comes up on the screen > You don't need to change anything just simply power off and from then the screen should operate as normal…It did for me so I’m well happy now!!
I hope this information will help someone else one day – Good Luck!!…Rob UK…

Posted on Jan 04, 2008

Benimur
  • 6966 Answers

SOURCE: no output from power supply

Hi,

I am assuming that you are very familiar with electronic circuitry and components, use of a DVM and a soldering iron. In the absence of a schematic diagram, and since you have perform initial voltage checks, perhaps you can wing it. Your "brick" will have to follow the basic design of an SMPS (switch mode power supply). Simply stated,an SMPS is an AC to DC to high freq AC then to lo V DC converter. Per your post, you have checked that there is DC (B+) from the AC mains, then what is needed is to check if there is oscillation to produce the high freq AC. Often design calls for an optical coupler, a small rectangular looking component with four pins, two at each end. This is nothing more than a LED and a Light Dependent transistor. Its function is to switch/trigger on the oscillation (and therefore start the SMPS producing the lo B+s).

Again, since there is no readily available schematic diagram, might I suggest that you post back the part numbers of the ICs and transistors. What we can do is pull out their spec/data sheets and/or application diagrams. Such would give us a general idea of the working voltages at what pins and what each component is supposed to do. Example, let us say your power supply uses a 817A/0635 opti coupler and/or an SG5841DZ controller IC, then we can search for them as initially described.

Hope this be of initial help/idea. Pls post back how things turned up or should you need additional information.

Good luck and kind regards. Thank you for using FixYa.

Posted on Sep 30, 2008

  • 1 Answer

SOURCE: Denon AVC-A1SE in protection mode

Hi,

After thorough cleaning of the speaker leads, PC Boards etc, I desoldered all the leads of the main two power transistors of the Left channel that had given problem earlier. Switched on the amp and bingo, it did not go in protection mode which confirmed that the two power transitors were the culprit. After checking the voltages and values of some other vital componenets vis-a-vis remaining left channels, it was confirmed that the circuitry was okay. The power transistors were checked again and found to be okay as well. I then took a chance and resoldered not only these two Left channel power trnasitors but also resoldered all the leads of all the other power transistors as well.

The amp was switched on again and this time also it did not go into protection mode. The amp is working fine. Problem could be either dry solder or short of power transistor leads by stray solder waste strand. There was no need to change any part which was great.

Thanks for help anyway.

HJ DALAL

Posted on Nov 30, 2008

  • 211 Answers

SOURCE: Peavey CS-800 PA power amp 1 channel is out

Hi Bill,

It's a log time ago that I repaired those amps., but I give it a shot.
There's a speakerprotection after the finals, what I remember it's a triac (SAC187, or something like that).
You will find it at a small pcb at the back of the amp. Normally it has three legs but the centre is cut away. If it's down you will measure 0 Ohm at the output, also with a switched off amp.

Thats all I can remember, it was about 25 years ago.

Maybe it helps,
Gerard

Posted on Jan 30, 2009

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