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SX-1080 High End Muffled / Protection Circuit Comes On at High Volume

Awesome

Thanks again Mark!

I'll finish up what I have here tomorrow night and hopefully the 2SA979's will be in my mailbox soon so I can finish up.

Once I get all this done I guess the procedure is to just plug 'er in and measure the center voltage and idle current?

Hopefully I don't spark anything else.
 
Grog said:
Thanks again Mark!

I'll finish up what I have here tomorrow night and hopefully the 2SA979's will be in my mailbox soon so I can finish up.

Once I get all this done I guess the procedure is to just plug 'er in and measure the center voltage and idle current?

Hopefully I don't spark anything else.
Click to expand...

USE THE DIM BULB TESTER, AND DO A TRIAL RUN SETTING THE IDLE CURRENT TO TEASE OUT ANY HIDDEN DEMONS, RETURNING THE IDLE CURRENT TO Z E R O AFTER DOING ANY PARTICULAR CHANNEL.

to NOT ZERO IT means disaster when plugging in to the outlet without the DBT in the way. it will be way too high!!

i think you have some dim bulb reading to do, and questions to ask FIRST!!!
 
probably 100w but you may want to get an assortment.

It all depends upon how power hungry the receiver model is, when it is just sitting there not driving speakers.
 
Ok

I've been reading all kinds of threads about these dim bulb testers. From what I gather, all it does is tell you if you have a short?

I don't understand completely how that relates to whether I borked the repair - if I plug it in and another couple resistors decide to combust how does the DBT warn me that's about to happen?

I'm not totally comfortable with electrical wiring. It concerns me there are so many versions of these DBT's.

I'm going to continue reading.
 
If the unit connected to the bulb draws a modest amount of power, the bulb glow dimly and the unit gets most of the power it needs, most means that the line voltage "at it's plug" is like a severe brownout - say 100 volts. Maybe enough that the protection circuit may not recognize the unit is turned on. It is enough to do a trial idle current set, to tease out any faults in that circuit. The trial idle current MUST be turned back down to zero, because leaving it and plugging into the power line directly could have the idle current disastrously high.

When the unit tries to draw MORE power, which is what happens with destructive faults, the bulb glows brightly and the unit is starved down to just a few watts of power - limiting the de$truction.

The bulb is inserted in the power line EXACTLY like a fuse or circuit breaker. Lots of interpretations on how to do that.
 
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My 2sa979's must have taken a left turn at Albuqurque

Still waiting on them. All other parts in.

I still can't find a thread with a simple diagram with how to make the dimbulb tester for Dummies.

I see lots of different ones ...
 
it's very simple.

a plug

a socket

one wire (neutral / white ) goes from plug to socket

the other wire (hot / black) goes from the plug, into the bulb, then out from the bulb to the socket.

The bulb is put in line EXACTLY like a fuse.

Some guys use a 3 prong plug and socket, ground (green) all exposed metal of the tester, and connect the plug and socket grounds.

FAR more fancy construction details than electrical wiring details...
 
Well - good news and bad news

OK I built my DBT ( good news ) - plugged it in, switch to the off position. Plugged in the receiver.

Bad News - Turned on the power. Light went bright, dimmed, then came bright again and as I was turning off the receiver resistor R11 smoked on the AWH-071 I just rebuilt. It was the one that was damaged the most last time and I just replaced it.

I had a 100 watt bulb in the DBT. Nothing else looks damaged or smoked or heated up. I'm hoping the DBT did its job and saved my parts.

So I shut it all down.

Let me know where to go from here my friend.

The wind just blew out of my sails.
 
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edit:

<screech> sound of stylus being dragged across record.

All that bupkus was for the power supply board...

The AWH-071 is the power amp....
 
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Ok

I'll get at it.

The last order I made I bought 6 or 8 of everything. I'll check everything and report back.

CAP C18 and diode D9 on what board? Those aren't on the AWH-071? You must be referring to another board as there is no Pin 28 on this board?
 
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I replaced all electrolytic caps in the receiver including the filter caps but I didn't touch the tuner board caps.

After I did that the receiver worked perfectly and sounded beautiful then someone told me to check the center voltage and idle current. That's when I had the accident and fried the resistors on the AWH-071 board.

The list of parts you had me replace were in Post # 83 of this thread and here is your list:

Q1 2sa979 dual transistor pnp bcecb 100v 50ma 0.4w 250hfe 150mhz
you have this on order

Q2 2sc1775a npn to-92 ecb 120v 50mA 0.3w 160hfe 100mhz
512-KSC1845UBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.05 ea

Q3, Q4 2sc1885 npn to-92 150v 0.1a 0.75w 150hfe 200mhz
512-KSC2310YBU TO-92L ecb 150v .05a .8w 100mhz 40-240hfe $0.11 ea

Q5 2sa912 pnp to-92L ecb 150v 0.1a 0.75w 150hfe 200mhz
512-KSA1013YBU to-92L ecb 160v 1a .9w 50mhz 160-320hfe $0.17


Q7 2sb536a pnp to-220 bce 120v 1.5a 20w 250hfe 40mhz
863-MJE15033G pnp to-220 bce 250v 4a 50w 30mhz 100hfe $1.25

Q6 2sd381a npn to-220 bce 120v 1.5a 20w 250hfe 45mhz
863-MJE15032G npn to-220 bce 250v 4a 50w 30mhz 100hfe $1.07

Q8 2sc869 npn to-92 bce 50v 30ma 0.2w 35hfe 150mhz
512-KSC2310YBU TO-92L ecb 150v .05a .8w 100mhz 40-240hfe $0.11 ea

d1 & D2 (D6 & D7 too, but not needed?? cheap get em...) 1s2471
512-1N4148 Diodes - Small Signal 100V Io/200mA

D3 & D4 SIB01-02
512-UF4004 ultra fast rectifier 1amp 400v $0.13 ea

Caps:
C1 & C2 2.2uf 25v low leakage:
647-UKL1H2R2MDDANA 2.2uf 50v this is original
they use two to make a non-polar 1uf cap, so:
667-ECQ-E1105KF is 1uf 100v poly cap non-polar
get two, to do the other side so any potential improvements in sound are matched


C11 0.22uf 25v low leakage, to stacked poly metalized film:
667-ECQ-E2224KF 0.22uf 250v


150k multi-turn trimpot - offset (like 1980's)
652-3296P-1-254LF Bourns Trimmer Resistors - Multi Turn
3/8" 250Kohms Sealed

AND get an insulated screwdriver to adjust this one!!!

100 ohm single turn trimpot - bias(like 1980's)
652-3386H-1-101LF Bourns Trimmer Resistors - Single Turn

R7 R8
2.7k 1w 5 % metal film resistor 594-5073NW2K700J $0.16 ea

I triple and quadruple checked all polarities and positions of transistor legs.
 
heh heh, beat me to it, I was just about to post/quote the list....

Where did / what did the trimpots set at?

100 ohms was to zero?

150k to max. Halfway would have been ok, not catastrophic...

gotta think on it, make a print, mark it up....

gimme a day, look for any inadvertent shorts

what about the output transistors, and did you check all the resistors when rebuilding

this post is now complete.
 
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Sounds good.

Oh and we replaced the following per Post # 75:

R11 100 ohms: 71-CCF07-J-100

R22 160 ohms: 71-CCF07-J-160

Cheers!
 
Don't know how to answer this:

"Where did / what did the trimpots set at?

100 ohms was to zero?

150k to max. Halfway would have been ok, not catastrophic..."

I just soldered in the new pots. I didn't really fiddle with them I spun around the little screws to check how they felt but other than that I didn't set them? Not really sure how without powering up?

Also - "what about the output transistors, and did you check all the resistors when rebuilding "

If you could clarify what I was to do with the output transistors that would be cool. Do you mean the ones attached to the big black heatsink?

I checked all the resistors on the board and posted the results, but I didn't think to check them all after re-soldering them. Is that the routine?
 
Obviously, since the same thing happened again, whatever caused it wasn't replaced or corrected when we replaced a lot of the components.

Thus I have to "play head games" with what didn't change, the original accident, starting with a working circuit.

testing output transistors:
http://www.audiokarma.org/forums/showthread.php?t=43186
Yes, the ones attached to the big black heat sink.

The settings of the trimpots had to be made "safe" before any power was applied, in probably being rushed, I apparently overlooked this. The settings from the factory are usually at the 50 % point (but you "spun around the little screws", WHERE were they left??), but this might be too high for the idle current pot setting. The offset pot wasn't as critical in that circuit, since even the minimum setting wouldn't have been destructively low - there are other resistors in that circuit as well.
water under the bridge.... onward.....

checking resistors after resoldering isn't necessary. I meant checking while replacing parts and before powering it up.
 
We must have made progress because there is a difference in the symptoms. Here is some insight for you to consider:

Original Accident:

After touching the legs of Q2 ( got a spark ), and while the receiver was still on, R22 fried red hot. I turned off the power switch then turned it back on right away and then R11 fried. Also, R7 and R8 heated up. I then quickly shut the power switch off.

Last Night:

A slightly different thing happened when I turned the receiver on this time after the repair. This time, only R11 started smoking.


Question: Would R11 fry if I left VR2 at too high a setting? How does one safely apply power to the receiver in order to have the time to spin the pot back to safe levels?

I do know I turned VR2 a few full rotations - it is the one with the little brass screw and I was looking for the stop to check and see if it was half way. I never did find a stop and I lost track of how many turns I made. I turned VR1 each direction to do the same thing but I put it back where it was.

Perhaps not knowing where I left VR2 perhaps I should just yank it out and put another new one in and start over? ( I have 6 spares ). Maybe I shoulld yank out both of them. It'll only take a minute.

I'm going to disassemble everything again anyway to get those heat sinks out and test those output transistors.
 
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Update

I've replaced both VR1 and VR2 and R11 with new components and I have not touched the pots.

I am now embarking on trying to figure out the output transistor test.

I'll post back the results.
 
Results of tests of Output Transistors

I only checked the output transistors that are attached to this repaired AWH-071 board. I did not check the ones attached to the good AWH-071 on the other end of the heat sink.

I didn't de-solder them and remove them from the heat sink - I did this with them still mounted. I attached the leads to the pins sticking up from the foil side of the board. They are clearly labelled BCE etc.

Using my mulitimeter on Diode Test mode and using your format from your post # 11 of 09-08-2005 from that thread you pointed me to about testing transistors here are the measurements:

First the PNP 2SB701 Transistors:

Top One:

black lead to base, red lead to collector, result = .565
black lead to base, red lead to emitter, result = .622
red lead to base, black lead to collector, result = OL
red lead to base, black lead to emitter, result = OL
black lead to emitter, red lead to collector, result = OL
red lead to emitter, black lead to collector, result = OL

Bottom One:

black lead to base, red lead to collector, result = .560
black lead to base, red lead to emitter, result = .616
red lead to base, black lead to collector, result = OL
red lead to base, black lead to emitter, result = OL
black lead to emitter, red lead to collector, result = OL
red lead to emitter, black lead to collector, result = OL

Next the NPN 2SD737 :

Top One:

black lead to base, red lead to collector, result = OL
black lead to base, red lead to emitter, result = OL
red lead to base, black lead to collector, result = .600
red lead to base, black lead to emitter, result = .635
black lead to emitter, red lead to collector, result = OL
red lead to emitter, black lead to collector, result = OL

Bottom One:

black lead to base, red lead to collector, result = OL
black lead to base, red lead to emitter, result = OL
red lead to base, black lead to collector, result = .598
red lead to base, black lead to emitter, result = .635
black lead to emitter, red lead to collector, result = OL
red lead to emitter, black lead to collector, result = OL

TAG! You're IT! :D
 
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