QX-646 Part ID help

A friend gave me this Pioneer QX-646a. He said that he got smoke and one of the channels went dead.
I found this thingy burnt on one of the power amplifier boards. VR2 Semi-fixed 220-B.
A variable resister?
The photo is of the good side.
Any idea where to find the part?
What might have caused it to fail?
Is this thing worth putting a lot of time and money into.
Thank you

fdwrench said:

A friend gave me this Pioneer QX-646a. He said that he got smoke and one of the channels went dead.
I found this thingy burnt on one of the power amplifier boards. VR2 Semi-fixed 220-B.
A variable resister?
The photo is of the good side.
Any idea where to find the part?
What might have caused it to fail?
Is this thing worth putting a lot of time and money into.
Thank you

Click to expand...

That is a 220 ohm trimmer resistor. I am not sure if MarkTheFixer helps on the quad units, but it's best to wait for him, or another knowledgable poster, to reply before even powering the unit up agian. A $2 repair can move to $100's in a split second.

They actually did something smart in that amplifier - the 220 ohm idle current adjust pot has a 330 ohm resistor across it, so if the pot opens up the amp doesn't self destruct, so there is hope.

Now, why the pot would self-destruct is another matter, let's hope the stv-3h survived.... (it's on the opposite side of the heat sink)

If you do it yourself, it will be a few bucks to replace all of the transistor in that channel, 6 bucks for shipping, and some time.

You will need soldering equipment and a DMM.

Posting a picture of the destroyed component composed like your first picture would be great... don't need the "paint" add on....


2sa725's are known to fail....

transistor replacements:
2sa725 ecb 35v 0.1a 0.2w 500hfe 100mhz
512-KSA992FBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.05 ea

2sa720 / 2sc1318 ecb 60v 0.5a 0.625w 85-240hfe 200mhz
512-KSA916YTA_Q TO-92L ecb 120v .8a .9w 120mhz 60-240hfe $0.05 ea
512-KSC2316YTA_Q TO-92L ecb 120v .8a .9w 120mhz 60-240hfe $0.05 ea

2sc789 bce 70v 4a 30w 40-240hfe 3mhz
512-KSD526Y npn to-220 bce 80v 4a 30w 3mhz 15-240hfe $0.62 ea

--------------------------
edit: adding capacitors, one board's worth - need x2 for whole unit
c1,c2 1uf 25v cssa 1uf 50v 667-ECQ-V1H105JL $0.38 ea
c7,c8 220uf 25v cea 220uf 25v 647-UPW1E221MPD $0.28 ea
c19,c20 2200uf 25v cea 2200uf 25v 647-UPW1E222MHD $1.12 ea
c21,c22 2.2uf 50v cea 2.2uf 50v 647-UPW1H2R2MDD $0.14
c23 100uf 50v cea 100uf 50v 647-UHE1H101MPD $0.29 ea
c24 220uf 35v cea 220uf 50v 647-UPW1H221MPD $0.44 ea
--------------------------

looking for the pot... horizontal mount/adjust is the complicating factor
don't order till I say ok, we have to find any other destroyed parts.

I
have pulled the pot from the board. I have attached a photo or two.
Is there an easy way to get the wires off of the posts?
Ed

The square posts with the wires going off the board?

They are NOT unwrapped or bothered in any way. They break off short, and then no longer reach. resoldering to the pins has it's own set of problems.

The picture I wanted was of the board that had the bad pot, iiac the second board picture is the good board again? - I am looking for signs of other problems on the bad board.

This was not yet the time to pull off the bad pot.

This is the the board in question.
Ed

Everything else appears undamaged, I noticed the light blue sanyo caps (they didn't "stick out" on the first photo)... thus I added the replacement caps to the earlier post. Still working on the pot...

I looked a little closer and found this capacitor. I believe that it is C8
It looks like it is ready to pop.
I do appreciate all of your help!
Ed

probably not, the flat top would show doming.

the heat shrink covering may have been damaged when new, and subsequent heat allowed it to pull open.

Kind of dropped the ball on this one. It has only 4+ years.
I have been trying to find the pot with no luck. I really do not know what I am looking for.
Markthefixer still out there?
Ed

So far 4 wires have broken off of the square posts. It seems that if you look at them wrong they break. What is the problems with soldering them back?
Ed

Nothing wrong with soldering them back on. The solid wire is very brittle, and the wire wrap is a specialized tool and knowledge level job. If you absolutely have to remove wires, best to unsolder the whole pin from the board. But normally you can get enough slack in the wires by opening up the wire tie downs.

220 ohm 1/2watt
http://www.mouser.com/ProductDetail...=sGAEpiMZZMtC25l1F4XBU2VDyyiMO6RAbKQhnCH8qDs= $9.58ea. HOLD OFF ON THIS Until MTF weighs in on it.

220ohm and 250 ohm trimmers are scarce as hens teeth apparently. I searched Mouser and Digikey and came up with the above. Kind of overkill. Bournes jumps from 200 ohm to 500 ohm. The 500 ohm will work but you'll use only about 1/2 of the travel. If you did get the 500ohm mark it up so you know to NOT go past a certain point (like a redline on a tach.) And if you go out of spec (past normal bounds for a 220ohm) you could do serious damage to the amp. Let's see what Mark says 1st.

Could I make this one work?
http://www.digikey.com/product-search/en?KeyWords=3386G-1-201LF&WT.z_header=search_go

Physically?.....yes. And that was what I was looking for in a 220ohm.
Electrically?.......it's up in the air. Trimmers you try and keep the same or slightly larger value, as you don't know how close to the end of travel the set point is going to be although I would imagine it should be close to the center of the trimmers range. I'm no good with any of the math so I wouldn't know how or what to figure out.

Try Surplus sales of Nebraska. or one of those radio surplus places. they might have something like what you need.

Ordered parts today then took a look at the channel fuses. There was a good 10 amp fuse in 1, (1 is the board that smoked.) no fuse in 2, a blown 1 amp in 3 and a good 1.5 amp in 4. The caps on the fuse holders look like they have been removed many times. Scares me! The main fuse is good.

Ordered the 200 ohm pot. I will give it a try. I do not know how to adjust the pot once I get it installed. Maybe someone can help me out on that.
Ed

Write down what all the fuses should be then go to Home Depot/lowes/ratshack and get some AGC type fuses of the the correct ohms.

Without disturbing the pot on the corresponding mirror (L to Right and front to front or Rear to rear) measure from either side pin to the center pin. It looks like the center pin is common with one side. If that's so measure the other side to the wiper. Then Determine which of the side pins will be common the the center pin on the new pot. Adjust to match.

This is where the DBT comes in HANDY. Bring it up on a DBT and check for shorts, smoke etc. Meter the speaker outputs with it set to DCVolts. DO NOT ADJUST ANYTHING ! Just take readings. If everything looks and sounds ok, turn it off and hold up until i can get zeb over here to double check me.

Mark was concerned about the STV-3. If it opened it could cause the damage found.. The STV should be checked along with the other components in that circuit. The transistors near the trimmer look like they had been messed with as they look pushed in and slightly twisted. The outputs should be checked for condition. Mark will kill me but I would check with the components installed on the board. Check the resistors for value the transistors for shorts between the legs and diodes for continuity in both directions. This is before powering up on the DBT.

Thank you for the help!
This is my first experience with something that does not have vacuum tubes.
The stv-3 seem to be ok checking them in the circuit.
I did pull Q11 2sc289 near the smoked trimmer. It seems to be shorted if I checked it right. With my Fluke 87 diode checker. Black on base to collector .765, to emitter .723. Red on base to collector .469, to emitter .450. Emitter to collector .066 swap the leads it is the same .066. I will check the rest today. I am supposed to be getting the bird ready for the smoker.
Have a wonderful Thanksgiving.
Ed

Ed; read this thread on testing BJT's. http://www.audiokarma.org/forums/showthread.php?t=43186 Pay particular attention to post #11. Then check Q11 again. This is just so everyone is on the same page for testing procedures. You might want to cut and paste the testing format in the 1st para also.

Great thread,

I checked it again and plugged in my readings below.
I also tested with my Fluke MD 23 with lower but similar readings

NPN:
black lead to base, red lead to collector, result = .760
black lead to base, red lead to emitter, result = .719
red lead to base, black lead to collector, result = .469
red lead to base, black lead to emitter, result = .450
black lead to emitter, red lead to collector, result = .065
red lead to emitter, black lead to collector, result = .065