SX-1250 right channel "WHAP" speaker distortion

[Eagle1]

Before checking the power transistor voltages I decide to try flip/flopping the right and left power amp boards by reversing the pre-amp/amp inputs. Right to left and vice versa instead of pulling boards and then reproduced the same music and settings. Fleetwood Mac's "Rumours" played flawlessly, AC/DC's "Who Made Who" produced the bass clip on the right speaker again. So it would appear that both the right and left power amp boards and main power transistors might not be the culprit since the clip is still coming from the right HPM-100 speaker. I even moved the HPM-100 speaker wires from the speaker A terminals to the C terminals on the 1250 and even tried new wire to the speaker. So this has me stumped now. On a positive note engaging just the JBL speakers the 1250 hasn't gone into protection yet and even at a higher listening level than before with the HPM's hooked up to the speaker C terminals. :scratch2:

1250 went into protect mode listening to Peter Gabriel's CD "Shaking the Tree" with the JBL's at -28db. Some of the commons is bass and the dial lights begin to flicker. But even with both of the bass controls positioned all the way counterclockwise on the 1250 it still goes into protect just above -28 db.

 

[markthefixer]

waiting on those emitter resistor voltages.

All you did with the flip-flop of the pre-outs/power-amp inputs is prove the problem is NOT in the preamp, which others were pretty sure it wasn't.

The combination of JBL's and HPM's at the SAME TIME is the cause of the clipping? NOT with the JBL's ALONE, and NOT with the HPM's ALONE??? OR do the HPM's alone clip?


If you want to swap cables around and the HPM's ALONE clip, THIS will be more productive:

FIRST, swap the right and left speaker connections AT THE BACK OF THE RECEIVER; put the speaker cables connected to the HPM that is on the LEFT side of the room onto the RIGHT channel output connections, put the speaker cables connected to the HPM that is on the RIGHT side of the room onto the LEFT channel output connections.

THEN see if your clipping moves.

IF it DOESN'T move and STILL clips on the RIGHT side of the ROOM then the speaker or the wiring has a PROBLEM. Which would explain the unit going into protect.

IF it DOES move and now clips on the LEFT side of the ROOM, then MAYBE you could find the time to measure the VOLTAGE ACROSS THE FOUR EMITTER (0.5 ohms) RESISTORS ON THE RIGHT CHANNEL'S amplifier card.

Or do as Dudeman suggested and swap the two amplifier cards, THAT will exonerate (or indict) everything on the card if there is no change in symptoms.

IF an output transistor is damaged, and forcing the receiver into protection, you may not have much longer before your fiddling around lets out the magic (and expensive) smoke. Right now it could be as easy as replacing a few output transistors. The future COULD be a train wreck, especially when something blows at higher power levels......

 

[Eagle1]

Mark,
I am only running one set of speakers at a time. The clipping occurs with the HPM's only so far. The 1250 does not go into protection with these hooked up. I haven't heard any clip with the JBL's but these cause it to go into protection at ~-28db. I also connected the right HPM with new wiring and still had the same clipping problem.

For fear of the magic smoke, the 1250 was removed from service and back on the bench for further tests. We'll measure the VOLTAGE ACROSS THE FOUR EMITTER (0.5 ohms) RESISTORS on both channels and post and wait for your direction. Thanks

 

[markthefixer]

Glad we got the speaker symptoms straight. While quite significant, they tend to reinforce my suspicions as to the output transistor.

The voltage measurements are done under the same conditions as the idle current - that's really what you are measuring, how the transistors are sharing the idle current between themselves.

A dead or damaged output will read VERY differently than a good one.

 

[Eagle1]

Mark,
Goat67 and I got to measuring emitter voltages across the .5 ohm resistors earlier tonight. Started with the right side first and immediately ran in to problems with the wiring harness to the transitors again with connectivity. Pulled the Amplifier board to check for cracked solder joints/loose pins. All looked and tested good. Problem turned out to be in the blue wire connector. With some careful cleaning and more Deox we were able to get a good continuity reading (with the heat sink removed) between the harness and transitor connections on all four. After re-assembling these are the readings we got, all pretty good we think.
All voltages are in millivolts and averaged to a certain degree due to some fluctuations.
Right side:
Front top: 49.5 Front bottom: 49.5
Back top: 46.6 Back bottom: 46.6

Left side:
Front top: 53.5 Front bottom: 45
Back top: 49 Back bottom: 49

The caps looked good on the right amp board, not saying that age has taken some toll. I didn notice that on the stabilizer board there is a 47u ohm cap in the front center that the wrap is showing very minor shrinkage but more importantly a cap in the center, right under VR1 and VR2 that has shrunk about 1/8" from the top. I have the majority of the cap replacement lists from Mouser that you and Mark have posted for the other boards. But there are a lot of caps on the Stabilizer board that I have seen posted or haven't seen it. If anyone has that list to save me time I would greatly appreciate it when I put in an order.

Unless you advise otherwise I'd like to temporarily hook up the 1250 up to the speakers for a check. I'm in no rush as I'm using my back up receiver for music

 

[markthefixer]

That connector may have been the problem, do the listening tests.

re: 1250 caps, I looked at my lists, which turn out to have transistors only.

There's enough cap parts number lists here on the forum to cover most if not all the power supply caps. Post a list and I'll see what can be done to match up parts, with some voltage margins increased, once I LOOK at it....

 

[Eagle1]

That connector may have been the problem, do the listening tests.

re: 1250 caps, I looked at my lists, which turn out to have transistors only.

There's enough cap parts number lists here on the forum to cover most if not all the power supply caps. Post a list and I'll see what can be done to match up parts, with some voltage margins increased, once I LOOK at it....

Thanks Mark,
I'll give it test in the morning when the neighbors are awake. Steve had a list of 5 of the 15 caps for the Stabilizer board and covered the Circuit Protection board and Power Supply boards which I can work from. I'll post the Stabilizer board electrolytic caps with the Mouser equivilants that I have. I'm learning more than before and having fun doing it thanks to you and Goat67.

 

[Eagle1]

I went thru Steve's cap sheet, he's listed all the caps for the power stabilizer board AWR-106, power stabilizer board AWH-048 and the protection circuit board AWM-091. Mouser has 3 of the cap PN's on backorder and potential ship is Nov. 08 to Jan. 09. The ones on b/o: for the stabilizer board are C1 & C2 220mf 80v (Mouser# 647-UPM2A221MHD), C16 22mf 10v (Mouser# 647-UPW1A220MDD). Power amp boards AWH-048, C15 & C16 470mf 80v (Mouser# 647-UPW2A471MHD). The alternatives they have listed are all non-stocked. Digi-Key doesn't seem to have them but Parts Express lists several per each that might be compatible and in stock. Do you have PN crosses for those on b/o from other suppliers? Thanks. I'll keep investigating.

 

[goat67]

Dude
Here is an in stock for
the 220uf 647-UPW2A221MHD or
an audio grade 647-UKW2A221MHD these are 100v
remember you can go up in volts not down
The 22uf 647-UPS1C220MDD at 16v
The 470uf 647-UPM2A471MHD at 100v

 

[Eagle1]

That connector may have been the problem, do the listening tests.

re: 1250 caps, I looked at my lists, which turn out to have transistors only.

There's enough cap parts number lists here on the forum to cover most if not all the power supply caps. Post a list and I'll see what can be done to match up parts, with some voltage margins increased, once I LOOK at it....

Did the listening tests this afternoon. Since cleaning the connections on the right amp board the sound is much better and the temps are very equal between right and left now and not hot to "the touch". I let the 1250 stabilize for about an hour before increasing the volume before doing any testing. Had no problem with my Fleetwood Mac CD, but the ACDC CD still created the bass clip on the right channel using only the HPM's. Tried the JBL's only, same song and the 1250 went into protection at -25 db. The JBL's have dual 8" woofers so they seem to demand more power. When hooked up to Goat67's Soundcraftman amp they really sang with no problems. We'll probably recheck bias voltages on the amp boards and the stabilizer board voltages. I guess next step after that would be to swap the right and left amp boards to see if the HPM clip moves to the left side.

 

[Eagle1]

Mark,
I figured out why the 1250 was going into protection with just the JBL's. The JBL's have the capability to be bi-wired. I had one set on the JBL's connected to the 1250 and the other set to my Onkyo A/V. The shorting bars were still in place. After removing the speaker wires on the JBL to Onkyo the 1250 no longer went into protection. I guess with them both connected together it dropped my ohms to less than 4 ohms which is not good! I guess a speaker selector switch would be the next mod. Even so this did not cure the right channel clip on the HPM.

 

[markthefixer]

The NEXT step is to see if the right channel output clips on the left speaker as well as the right speaker. Until that time, DON'T swap any boards around in the unit.

SO FAR, you have proven that the ONE observed problem was a result of an external influence, and that you are NOT in total command of what you are doing with the testing.

You have to PROVE to me that the HPM's are BOTH ok.

No clipping with the JBL's is VERY suspicious to me.

 

[goat67]

Mark
I agree with you and that is why we swapped the speakers from right to left.
Replaced speaker wire still stays with the right channel so it is definitely in the amp on the right side.
Swapping pre inputs had no change stayed on right.
Just to be clear it takes obnoxious amounts of volume to get this to happen.
To me it sounds like a cap or transistor is failing it is a quick sound like static.
The only reason I can think of as to why it is not happening on the JBLs is
that they are more efficient and the amp has not been turned
up to the point it would happen.

I do not have any dummy loads yet but am ordering some
it seems to me that to reproduce this it would be better with using the dummy loads.
Thanks for all your help

 

[markthefixer]

Right side:
Front top: 49.5 Front bottom: 49.5
Back top: 46.6 Back bottom: 46.6

Left side:
Front top: 53.5 Front bottom: 45
Back top: 49 Back bottom: 49

When these were done, was the measurement made at each resistor's two leads, even before the solder joint?

I am very suspicious of the left side front top and front bottom readings, they are not working together as the other three pairs are. The splits are front to back, with front handling one polarity of the outputs and back handling the other polarity.

While having the dummy loads (non-inductive) is nice, swapping out the outputs on one channel with a set of MJ21193G & 863-MJ21194G 's and seeing what happens is a lot cheaper. Of course I always keep a few around, to do just this...

 

[Eagle1]

When these were done, was the measurement made at each resistor's two leads, even before the solder joint?

Measurements were made on each of the resistor leads.

 

[goat67]

Mark
I will recheck them. This time I will use 2 meters and get the readings at one time instead of turning on and off.
Unfortunately we do not have the transistors so can not do as you suggest right now.
Could we swap them side to side to get the symptom to move?
It would be a good thing to apply new thermal grease anyway right?

 

[markthefixer]

Mark
I will recheck them. This time I will use 2 meters and get the readings at one time instead of turning on and off.
Unfortunately we do not have the transistors so can not do as you suggest right now.
Could we swap them side to side to get the symptom to move?
It would be a good thing to apply new thermal grease anyway right?

Re-measuring isn't necessary.

Yes, swapping channel to channel would confirm if it is the transistors if the problem moves to the other channel. But it puts the other channel at slight risk. Do the usual precautions when mounting, including checking for shorts to the heatsink with an ohmmeter.

Yes, new thermal grease or sil-pads when installing transistors. If reusing micas be careful cleaning them....

 

[Eagle1]

Re-measured left front and bottom transistor reading using 2 DMM's, your right nothing changed. Still have that 10mV spread. Swapped the left and right amp boards and adjusted to spec. and still had the same 10mV spread on the left front and bottom of the transistor/resistor readings across the resistors. Tried the sound test and bass clip still occurs on the right channel. Goat67 has a quick question, does the left side looking from the front, drive the left channel or right channel? His experience with Soundcraftsmen is that the left side doesn't drive the left channel, but the right. What would you suggest as the next step? Put in a part order?

 

[markthefixer]

The easiest way to find out is WITH THE RECEIVER OFF, connect one ohmmeter lead to one of the emitter resisitors and then probe the speaker connections (no speakers connected, you could turn off a & b and use c) for continuity.

That'll figure it out very quickly.

 

[Eagle1]

The easiest way to find out is WITH THE RECEIVER OFF, connect one ohmmeter lead to one of the emitter resisitors and then probe the speaker connections (no speakers connected, you could turn off a & b and use c) for continuity.

That'll figure it out very quickly.

Tried that Mark to a 'T", but got nothing from either side on the pos. speaker connector. Neg. yes on all. I believe after looking at the protector circuit schematic, power must be on in order to energize the relays in order to complete the speaker circuit.