QX949 Offer I Couldn't Refuse!

[JJCalvillo]

Tested the rear outputs with the volume off and the unit on AUX. When the Rear Left got to -134mV and the RR was at -60 or so, it went into protect.

Is this a DC offset issue?

Edit: Didn't test the front, which could be higher, or not.

 

[mrk229]

It certainly seems like it might be an offset issue. Was the offset rising over time?
-134 mV is not crazy high, but the offset detection is additive across all four channels. Rear totals together in the neighborhood of -200mV.

Take a look at the front offsets and see where they are.

You said it was distorting, that is a bit worrisome.

 

[JJCalvillo]

Tested posts 13-16 on the Protect board, and those number look like what I'm seeing on the speaker terminals.
13 = -91.6 mV
14 = -94 mV
15 = -141 mV
16 = -86 mV

Schematic has a -13 Vdc at 14 and if I'm reading it right that should be at 13 - 16 as well.
Edit: Added the - I left off in front of 86mV at post 16.

 

[JJCalvillo]

The DC started low and went up over time. The distortion became noticeable just before the it went to protect, sounded normal until then.

Front channels are a very different story.
FL = -.5 mV
FR = -6.8 mV

 

[mrk229]

Tested posts 13-16 on the Protect board, and those number look like what I'm seeing on the speaker terminals.
13 = -91.6 mV
14 = -94 mV
15 = -141 mV
16 = -86 mV

Schematic has a -13 Vdc at 14 and if I'm reading it right that should be at 13 - 16 as well.

That gives us a total of -412mV. Probably enough to trip the protection.

That is different than the -134 and -60 mV from earlier. How stable are they? Can you adjust them near zero?

13 & 14 front channels. 15 & 16 rear channels. They come from the amplifier outputs. Pin 17 is the -13 volts.

Are looking at the same thing QX949A AWM-079

 

[mrk229]

The DC started low and went up over time. The distortion became noticeable just before the it went to protect, sounded normal until then.

Front channels are a very different story.
FL = -.5 mV
FR = -6.8 mV

Our post crossed in the either.

You have the infamous 2SA726's and the also infamous 2SC1451's in the amplifier channels.

How was it behaving before you started your "who let the smoke out" adventure?

 

[JJCalvillo]

That gives us a total of -240mV.

That is different than the -134 and -60 mV from earlier. How stable are they? Can you adjust them near zero?

13 & 14 front channels. 15 & 16 rear channels. They come from the amplifier outputs. Pin 17 is the -13 volts.

Are looking at the same thing QX949A AWM-079

View attachment 3518382

Check out my edit. I left the - sign off post 16's -86 mV.

Our post crossed in the either.

You have the infamous 2SA726's and the also infamous 2SC1451's in the amplifier channels.

How was it behaving before you started your "who let the smoke out" adventure?

The guy told me there were problems including some distortion, which I think is in the Rear Right channel. I head it but it wasn't always there. Guy I bought it from works on gear, and I don't know how good he is. This had been collecting dust for years though.

Do I go through the amps w/caps and transistors or just transistors, then see what's happening?

Edit: Meanwhile I'll try adjust DC offset.

 

[mrk229]

Check out my edit. I left the - sign off post 16's -86 mV.

The guy told me there were problems including some distortion, which I think is in the Rear Right channel. I head it but it wasn't always there. Guy I bought it from works on gear, and I don't know how good he is. This had been collecting dust for years though.

Do I go through the amps w/caps and transistors or just transistors, then see what's happening?

Edit: Meanwhile I'll try adjust DC offset.

So with the -86 we are near -412mV. I edited my post as well. It seems I forgot to add one voltage in the first time around.

For distortion I think the 1451's would be my first choice.

As for how to go about it. I think it depends on how hard it is to work on the amp boards. IMO If it is fairly easy to access, do the transistors first and then go back for the caps if you want. If it is a pain in the butt and you intend to replace the caps then do it all at once. Or third option just replace the two small caps on the input, .22 and .47uF. Those small caps seem to go bad much more often.

 

[JJCalvillo]

Have all the VR 1 & 2s cranked and that gets the - mVs down to about 20 on each channel before it clicks off.

Haven't worked on the amp boards on one of these before so I'll be finding out how difficult it is. Get to that in the morning.

Mom is 98 and going from hospital back to the care facility tomorrow. So I have to be there mid-day to help her through the paperwork for some additional care. So when I disappear for awhile, I haven't given up.

 

[JJCalvillo]

Also, thanks for helping me with this!!!!

 

[JJCalvillo]

Recommended sub for 2SC1451 is KSC1845FTA. Have 1845s not sure if they're FTA. Says the 1451 was a R119 and the 1845 is a TO-92. Does this mean I need to add a heat sync to the KSC1845?

 

[mrk229]

Recommended sub for 2SC1451 is KSC1845FTA. Have 1845s not sure if they're FTA. Says the 1451 was a R119 and the 1845 is a TO-92. Does this mean I need to add a heat sync to the KSC1845?

1. You are welcome.
2. The long accepted sub for the 2SC1451 as used in a power amp is the KSC3503DSTU which is obsolete. (you can get real ones from PL&M on the bay)
3. I have used both the Toshiba TTC004B and the KSC2690 as a sub for the 2SC1451. More TTC004B's than the KSC2690. I have a lot of TTC's

 

[mrk229]

Have all the VR 1 & 2s cranked and that gets the - mVs down to about 20 on each channel before it clicks off.

Haven't worked on the amp boards on one of these before so I'll be finding out how difficult it is. Get to that in the morning.

Mom is 98 and going from hospital back to the care facility tomorrow. So I have to be there mid-day to help her through the paperwork for some additional care. So when I disappear for awhile, I haven't given up.

That is a bit odd. Are the offsets fairly stable around the 20mV value?
With them being that low I don't think the protection circuit should trigger on offset voltage.
It is also not typical for the offset pots to be cranked to one end.
The offset voltage problem seems common to both rear channels.
(sorry kind of thinking out loud so to speak)

What are the offset values coming into the protection board after it trips? 13 & 14 front channels. 15 & 16 rear channels.

Check these voltages on the protection board after it trips. It may give us a clue as to what is going on.



My mom is 89 and while she won't admit it most of the time she needs help now and then.

 

[JJCalvillo]

Offset is still moving farther negative when it shuts down, at a somewhat slower pace. I would turn the VRs and see it go to +50mV, but it would keep heading south. Next turn +30, then +10, and then it would be maxxed and bounce down to -20mV before clicking into protect.

Unit never went into protect until after I created the magic smoke. The problems on the P.S. gummed something up elsewhere.

 

[JJCalvillo]

Q1-4 on the protection board all have 14 to 15 Vdc on their collectors.

Base of Q9 is a little high. 8.5Vdc and it should be 6.8Vdc

Also, it could be the new bulb, but the dial pointer light went dead. I noticed post 9 on the P.S. goes to protection and lamp holder assembly #14. Maybe a coincidence. I measured voltage on the post as -4mV.

 

[JJCalvillo]

Forget the dial pointer, it was on Aux so it was off.

Q1 C is at 15Vdc when the unit is powered on, but after 10 second or so Vdc starts to drop, until it get to about 7.5Vdc and protect kicks in.
Tested Post 1, input from P.S, and the power drop is there as well.

Seems like the P.S. starts out with good voltage but starts dropping soon after power up. We didn't change Q1, do you think it, or one of the other transistors, can pass a quick test, but fails at full power?

 

[JJCalvillo]

So I went back and tested Q2 C on the P.S. and both it and the collector of Q1 are still at 46Vdc when they are supposed to be at 30Vdc.
Then I checked Posts 20/21 next to the emitter of Q1 and they showed the correct 30 Vdc.
How do you have 46Vdc on the collector of Q1 but the correct 30Vdc at the post next to the the emitter?

 

[mrk229]

So I went back and tested Q2 C on the P.S. and both it and the collector of Q1 are still at 46Vdc when they are supposed to be at 30Vdc.
Then I checked Posts 20/21 next to the emitter of Q1 and they showed the correct 30 Vdc.
How do you have 46Vdc on the collector of Q1 but the correct 30Vdc at the post next to the the emitter?

You have that backwards my friend. The collector is the raw VDC input and the emitter is the regulated output voltage.

Collectors of Q2 and Q1 are tied together and have 46VDC the voltage of the rectifier/filter.
The Emitter of Q1 is the regulated output at 30VDC to 20 & 21.
The base voltage of Q2 controls the output voltage of the circuit.

 

[mrk229]

Forget the dial pointer, it was on Aux so it was off.

Q1 C is at 15Vdc when the unit is powered on, but after 10 second or so Vdc starts to drop, until it get to about 7.5Vdc and protect kicks in.
Tested Post 1, input from P.S, and the power drop is there as well.

Seems like the P.S. starts out with good voltage but starts dropping soon after power up. We didn't change Q1, do you think it, or one of the other transistors, can pass a quick test, but fails at full power?

Q1 C where? And post 1 where? I am thinking you are talking about the protection board?

 

[mrk229]

I am going to assume you are talking about the protection board and go from there. Power it up and monitor 20 and 21 on the PS board. It should be a steady 30 volts even after it enters protection. If that is true then follow the wire back from pin 1 of the protection board looking for bad connections or questionable solder joints.

It looks like it goes to the the relays and then to front L power amp pins 15 & 16. From there it makes its way to the PS board.