SA 9500II Issue: New Thread Old Problem: I Need Help Please!!!!

[KingBubba]

I have been working on my amp and have been successful in that it will come out of protection and, when fed a source, it makes audio.......sort of. The issue is the right side is way way softer than the left side.

I have been using the amp-in input to feed both sides; bypassing the internal preamp of the 9500II, and voltage on both left and right inputs on the left and right cards are the same. Using a 1K test tone set at 0 amplitude on my TrueRTA creates an output of 495mv (1/2v) on the left side and the right side shows 20mv at the same input level.

If you would, please read my other thread, on page 5, after I shouted "It Works" in post 70 and then realized I wasn't out of the woods yet. As stated in the other thread, I have tested all components on the right board and they show to be good. Actually, I have done it twice now. Everything tests fine, diodes, resistors, transistors.

Today I checked the power input voltage and it is a solid 53v on both amp boards; so power supply does not seem to be the issue. I tested the A + B switch and found all as it should be.I am stumped.

Help please, if you are able, because at this point I don't know where else to look. The problem has to be in the right amp board AWH-052R, but for the life of me, I don't know what else to look for.

 

[markthefixer]

Did you replace any of the original resistors? What values and color codes do they show?

There are only three electrolytics on that board, make sure by ohmmeter that c15+ is connected to R2 and
c4+ is connected to the r5/r4 junction and
c5+ is connected to r8
c5- IS grounded

be sure C5 isn't shorted - maybe swap it to the other side?

the AC gain is set by the ratio of R9/r8 divider as long as C5's contribution is negligible.

right now it may not be clear if one channel's gain is low, or the other channel's gain is high.

It may help to run the voltages on all the transistors a/b so to speak.

 

[KingBubba]

Mark, all capacitors met with the specifications you requested. I have another C5 spare and swapped it out for the hell of it. It made no difference. Right side transistors have now been tested. They all seemed to be within correct voltages. They are as follows:

Q6
C -51.56
B -1.16
E -.60

Q5
C +51.4
B +1.09
E +.53

Q4
C +1.1
B +46.4
E +47.05

Q2
C +7.08 (a touch high)
B -46.9
E -47.5

Q3
C -1.16
B -46.9
E -47.5

Q1 - (a double transistor)
Both collectors -46.8
B +.58
Both emitters +.001

Other than Q2 collector, these readings seem reasonable.

It will be tomorrow before I disassemble the left side. Any thoughts for working on the right side any more?

I replaced two resistors and they tested as spec: for R1 at very close to 330kohms and R3 at very close to 62Kohms. As far as the left side gain being to high, all I can say is that the left side sounds and reacts to source as I would expect it to. It seems right in line with how it used to react to source before I got into it. Tomorrow's testing may show us something, but my bet is on the left side being where it should be.

 

[KingBubba]

I just noticed that a small chip has cracked off the horseshoe shaped sensor diode that attaches to the output's heatsink. It does not appear to be enough to cause a problem. This did however lead me to wonder what the real purpose of this component is and how it works. I have been very careful of these but not careful enough. I may have torqued the screw to tight, causing the crack. Is there a way to test this device and the big question is, could it have an effect on the gain from this amp board? Is it possible that this is causing my problem?

 

[markthefixer]

The horseshoe device is the stv-4h bias diode, developing about 2.1v across itself, varying (decreasing) with temperature.

IF this component opens up the outputs would immediately destroy themselves. "smoking ruin".

You missed or didn't understand my last suggestion,

It may help to run the voltages on all the transistors a/b so to speak.

instead of a/b I should have said left / right. as in compare the left and right channel voltages on each transistor.

That - dash placeholder is driving me NUTS, if you insist on using it, use the plus + in front of a positive voltage. I'm not surprised it has crossed you up as well.

Other than Q2 collector, these readings seem reasonable.

Nope.....

As it is the Q1 collector voltages are very wrong if they truly are + 46.8v and -46.8v

Q1 - (a double transistor)
Both collectors - + and - 46.8 (polarity correct)
B - .58
Both emitters - .001

they should both be negative

and the q2, q3 emitter voltages

Q2
C - 7.08 (a touch high)
B - 46.9
E - 47.5

Q3
C - 1.16
B - -46.9
E - -47.5

are impossible, the emitters are connected!!! one is at +47.5v and one is at -47.5 volts??

So all three come out and all circuits and resistances tested. An open resistor could destroy transistors.

By testing circuits I mean to check not at the component under test's leads but rather the leads of components it is supposed to be connected to. You are verifying foils and connections as well as parts values.

 

[KingBubba]

Mark, I understand your comment about the listing of voltages and the dashes. This has been cleaned up and the polarities have been corrected, as they were simply clerical errors in transposition to the AK page.

As far as this:

You missed or didn't understand my last suggestion,
Quote:
It may help to run the voltages on all the transistors a/b so to speak.

instead of a/b I should have said left / right. as in compare the left and right channel voltages on each transistor.

You missed that I said this in post 3:

It will be tomorrow before I disassemble the left side. Any thoughts for working on the right side any more?

As for the Q2 and Q3 polarity, according to this:

the negative reading on the emitters is correct, unless the diagram is wrong. I did recheck the voltages and they are all negative on base and emitter of Q2 and Q3 as shown on the foil side diagram.

As previously stated, today I will disassemble the left side and get readings from there.

 

[moermusic2]

I also thought I had a soft right channel. But with 2 different amps? Now believe my softness is due to a weak mid driver in the right spkr tower. Good luck 'Bubba.

 

[markthefixer]

Mark, I understand your comment about the listing of voltages and the dashes. This has been cleaned up and the polarities have been corrected, as they were simply clerical errors in transposition to the AK page.

As far as this:



You missed that I said this in post 3:



As for the Q2 and Q3 polarity, according to this:

the negative reading on the emitters is correct, unless the diagram is wrong. I did recheck the voltages and they are all negative on base and emitter of Q2 and Q3 as shown on the foil side diagram.

As previously stated, today I will disassemble the left side and get readings from there.

I misunderstood "disassembling the right side"....

And the revised voltages posted are all reasonable, and with correct 0.6v base to emitter voltage differentials. Right side voltage measurements aren't needed - but there's a slight chance they would turn something up.

As for the initial symptoms, I have to take time later to look closer.

 

[KingBubba]

I have now disassembled the left side and I must say I am puzzled because the readings on this board are not as close as the right side. I have double checked that it is the right amp-in input and right speaker output that are low in volume and they are. The inputs and outputs trace directly to the right board and yet it is the left board that shows discrepancies. The voltages are as follows:

AWH-052 L

Q6
C -52.75
B -1.15
E -.6

Q5
C +52.6
B +1.09
E +.55

Q4
C +1
B +46.81
E +47.5

Q2
C +13.5
B -47.37
E -47.99

Q3
C -1.13
B -47.7
E -47.99

Q1
C -47.2 top and -47.1 bottom
B +.033 top and +.033 bottom
E +.65

I don't know what to make of this side, but there is no doubt that the bias balanced easily and it sounds good when a source is played through it. :scratch2:

 

[KingBubba]

The wire wrap on pin 18 on right broke today and I soldered it. The left side pin 18 looked weak too; so I touched it with solder to beef it up. With this amp disassembled, it would take an octopus to shift it around without stressing the wire wraps. In hopes that that was the problem with the right side, I tested for audio after doing this and, unfortunately, there was no change. The right is still 1/10th the volume of the left. I also retested the power input voltages on both cards and got a solid + an - 53 volts on both boards; so power is definitely not the issue. I also tried the headphones and came up with the same results, the right side sucks the big one.

 

[goat67]

I have now disassembled the left side and I must say I am puzzled because the readings on this board are not as close as the right side. I have double checked that it is the right amp-in input and right speaker output that are low in volume and they are. The inputs and outputs trace directly to the right board and yet it is the left board that shows discrepancies. The voltages are as follows:
I don't know what to make of this side, but there is no doubt that the bias balanced easily and it sounds good when a source is played through it. :scratch2:

AWH-052 L               AWH-052 R  
Q6                      Q6
[B]C  -52.75 [/B]              C -51.56  (-51.4)
B  -1.15                B -1.16  (-1.2)
E  -.6                  E -.60  (-0.6)

Q5                      Q5
[B]C  +52.6  [/B]              C +51.4  (51.4)
B  +1.09                B +1.09  (1.2)
E  +.55                 E +.53  (0.6)

Q4                      Q4
C  +1.0                 C +1.1  (1.2)
[B]B  +47.51 [/B]              B +46.4  (46.6)
[B]E  +46.89[/B]               E +47.05  (47.2)

Q2                      Q2
[B]C  +13.5 [/B]               C +7.08  (5.0)
[B]B  -47.8 [/B]               B -46.9  (-46.4)
E  -47.2                E -47.5  (-47.0)

Q3                      Q3
C  -1.13                C -1.16  (-1.2)
[B]B  -47.7  [/B]              B -46.9  (-46.4)
E  -47.19               E -47.5  (-47.0)

Q1                      Q1
C -46.8                 C  -47.2 and -47.1  (-46.4)
[B]B - .58 [/B]                B  +.033 and +.031  (0.0)
[B]E - .001[/B]                E  +.65  (0.6)

I thought I would make it easier for people to see the boards side by side
you have issues with the board identified as the left board.

Just an FYI just because a board is mounted on the left side does not mean it drives the left channel

I need to read your other thread to see what you have done.

 

[markthefixer]

Take Q4 out of circuit and test it, take R13 out of circuit and test it.

Q4 is a current source, and R13 is 160 ohms.

R13's supposed to have (48 - 47.2) 0.8v across it's 160 ohms for 5 mA of current.

Instead it's (48 - 1.09 ) / 160 = 290 mA which is absolute B.S.

Then considering Q4:
C +1.0
B +47.51
E +1.09

collector to emitter is only 90 millivolts...
Too low for a Vce(sat)...
it's shorted collector to emitter....

base to emitter is (47.51 - 1.09 ) = 46.42
Vbe max is about 5 volts, after which the transistor is toast.
the base junction is probably blown right off the chip.

R13's the culprit, it went high on resistance, and as the base to emitter voltages climbed, and the transistor turned on harder and harder to get that base to emitter voltage back to 0.6v. It never made it, the base to emitter voltage exceeded 5 volts and blooie...

If you didn't remove R13 from the circuit to test it, you could have been reading another current path.

All this, of course, assumes that the q4 emitter voltage reading is correct.

As for this right/left board / right/left channel stuff, please detail exactly how you determined this.

I call left sides those to the left when the front panel knobs are facing me with the panel lettering correct side up.

 

[KingBubba]

The Q4 emitter reading, on AWH-052 L,was a misread on my part, off my notes. It has been corrected at +46.89. My apologies to all. That reading is on the left side, which is not where the problem lies.

Goat67, thanks for the paperwork cleanup you did, sorry for my error. Your note abut left vs. right was one of the first things I checked. The right side board does indeed feed the right speaker out. I even double checked that yesterday and noted it in my post. Thanks for any help on this.

Mark, I apologize for wasting your time. I, unfortunately have never been good at paper work. In High school, I failed chemistry due to lousy paperwork mistakes and yet my chemistry score in my college boards was the highest of all my scores. I'll try harder to be more accurate.

 

[markthefixer]

Have you tried subbing in a pair of a992's for q1?

Is Q1 original? Is the original Q1 available??

Which transistors were replaced, with what?

 

[goat67]

Kingbubba
Can you provide a summary of what has been done on the left channel
From reviewing your posts I believe you have replaced the output transistors due to a mishap on setting the bias
and caps also 2 resistors it would help to get all of this here in this thread.

Q4 is still not right.
You differently have a problem with the board that you are calling left

I will update my post with your new value.

Still means issues on the left board

 

[KingBubba]

Q1 on both boards is original. The one on right is very close to spec:

Q1 AWH-052 R
Actual____________________Spec.
B +.001___________________0v
B and +.001_______________0v
C -46.8___________________-47
C -46.8___________________-46.4
E +.58____________________+.6

I would consider replacing it with a double A992, or a new 2SA798, but the readings are so close to spec. I can't understand why a replacement would be necessary. I had already started a new thread yesterday looking for the answers to where to find a 2SA798 replacement and whether creating your own double transistor is a possibility. Mark, you have just pointed out that you can create your own with 2 A992s so that question is now answered. There are a number of places I can order them from so I could get a new one if need be. I just am not so sure, at least on the right board, that the need be.

Goat67, I am not trying to be impolite by bumping you off to a thread, but all information you might require is in the original thread:

http://www.audiokarma.org/forums/showthread.php?p=3391348#post3391348

I don't have time to rewrite the story of this amp today; as I am in the middle of a 26 hour workday and things are about to start hopping here at the theater in a short while. Any assistance is greatly appreciated but the stumper is still the right board's insistence to only be 1/10th the volume of the left. Any info about the left side will be appreciated, but it's the right side that has my full attention at this time.

Thanks to all for your contributions, all of which will be considered and again sorry for my errors. I hope to keep them down in the future by double checking.

 

[goat67]

Q1 on both boards is original. The one on right is very close to spec:

Q1 AWH-052 R
Actual____________________Spec.
B +.001___________________0v
B and +.001_______________0v
C -46.8___________________-47
C -46.8___________________-46.4
E +.58____________________+.6

I would consider replacing it with a double A992, or a new 2SA798, but the readings are so close to spec. I can't understand why a replacement would be necessary. I had already started a new thread yesterday looking for the answers to where to find a 2SA798 replacement and whether creating your own double transistor is a possibility. Mark, you have just pointed out that you can create your own with 2 A992s so that question is now answered. There are a number of places I can order them from so I could get a new one if need be. I just am not so sure, at least on the right board, that the need be.

Goat67, I am not trying to be impolite by bumping you off to a thread, but all information you might require is in the original thread:

http://www.audiokarma.org/forums/showthread.php?p=3391348#post3391348

I don't have time to rewrite the story of this amp today; as I am in the middle of a 26 hour workday and things are about to start hopping here at the theater in a short while. Any assistance is greatly appreciated but the stumper is still the right board's insistence to only be 1/10th the volume of the left. Any info about the left side will be appreciated, but it's the right side that has my full attention at this time.

Thanks to all for your contributions, all of which will be considered and again sorry for my errors. I hope to keep them down in the future by double checking.

Well you need to quit looking at the board you consider the RIGHT channel board the problem is in the board you have marked as the LEFT channel board good luck

And have a nice day

 

[KingBubba]

Goat67 Please Come Back

PM sent to Goat67 in hopes that will please rejoin this thread. Writing what you mean to say in these threads is difficult and I am trying to do my best. If I offended, it was completely unintentional. I apologize for any misunderstanding due to my poor wordsmanship. I am trying to do the best I can with this project and am trying to concentrate on one thing at a time to avoid personal confusion and mistakes. This was the intent of my comments, not to be offensive. I am a very slow hunt and peck typer and I sometimes take shortcuts in what I write, just to get it done. I gladly welcome assistance from any member who wants to help. I'm just trying to keep the info coming in where I am working so I don't get confused and f**k something up......again. :yes: Please forgive.

Gary

 

[goat67]

I responded to your PM. No problem all is well.

We need to be clear on what we are talking about on these boards.

From MarkTheFixer:
This advice was for the board you have labeled as left.

Take Q4 out of circuit and test it, take R13 out of circuit and test it.

Q4 is a current source, and R13 is 160 ohms.

R13's supposed to have (48 - 47.2) 0.8v across it's 160 ohms for 5 mA of current.

Instead it's (48 - 1.09 ) / 160 = 290 mA which is absolute B.S.

Then considering Q4:
C +1.0
B +47.51
E +1.09

collector to emitter is only 90 millivolts...
Too low for a Vce(sat)...
it's shorted collector to emitter....

base to emitter is (47.51 - 1.09 ) = 46.42
Vbe max is about 5 volts, after which the transistor is toast.
the base junction is probably blown right off the chip.

R13's the culprit, it went high on resistance, and as the base to emitter voltages climbed, and the transistor turned on harder and harder to get that base to emitter voltage back to 0.6v. It never made it, the base to emitter voltage exceeded 5 volts and blooie...

If you didn't remove R13 from the circuit to test it, you could have been reading another current path.

All this, of course, assumes that the q4 emitter voltage reading is correct.

As for this right/left board / right/left channel stuff, please detail exactly how you determined this.

I call left sides those to the left when the front panel knobs are facing me with the panel lettering correct side up.

Since you updated the Q4 emitter reading some of this does not apply.
Are you sure on the readings on the board you have labeled left?

Verify them to what I posted.

Question did you have to replace the outputs that are connected to the left board due to a bias setting mishap?

Did you replace any other transistors on this board?

I seem to recall you had issues soldering on the left board. Verify you do not have any solder bridges.

Check connections and joints around Q1

For the other board have you replaced any transistors?

Bummer on the long work day(s)

When you get a chance check this out so we can try and help you along the path to a Jamming Amp again.

 

[markthefixer]

If I remember everything correctly, supposedly everything on the right board has been checked, and it's the amp board that has the low volume.

Since the voltages are reasonable, you need to check (or recheck) R9 & R8 out of circuit (lift one lead) for the correct values, and C5 as well.

We need to do a real time reading of the input voltage (right at R2 and the base of q1) at the amp input, and output, for both channels. No guessing, verifying...

We're assuming that the input is getting from the RCA plug to the base...

The gain on both channels could be the same, and just that the signal is getting cut down on one channel.

Why were R1 & R3 replaced again?


And..... take your time...a slow thread gives me time to think.... we need thinking now.....

My quick responses are not because I want to speed things up in the thread, but rather to get something helpful down that can be acted upon whenever the questioner comes back, as well as not allowing the thread to fall through the cracks so to speak.

Making a mental note - "I'll get to that later" just doesn't work....