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Old 6th April 2005, 02:10 PM   #1
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Unhappy having trouble setting quiesent current?

hi there, i have built two identical amplifier boards based on MJL21193/94 ouput drivers, one works, the other is being impossible!

as it currently stands, i have checked everything i think i possibly can!!

the transistors all seem OK (i even replaced all 4 output drivers) and all parts have correct polarity.

the resistors are all in the right spots (checked with ohm-meter) and all the diodes are right way around. no soldering errors at all.

when i apply power, with VR1 set to full anti-clockwsie, there is no voltage at all across the 'safety resistors' and VR1 does not change this when adjusted.

voltage at the output = ~0.45V (as is documented it needs to be)

now the reason i am posting is becasue i have FINALLY found something different between the two boards:

the working board has a 300 ohm resistance across the input

the non-working board has over 20 mega ohms!!!

i guess this is not good?

can anyone help me troubleshoot this? i assume there is meant to be a low resistance across input like on the working board?

attached is a circut diagram and schematic if it helps.

i note that the diagram says there should be voltage of 0.62V across the 47ohm resistor next to Q6 and a voltage of 0.9V across the 68ohm resistor next to Q7

i measure 0.71V next to Q6 and only 0.1V next to Q7 (my mains voltage is a bit higher than 240V BTW)

thanks for any direction you might have to offer
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Old 6th April 2005, 02:11 PM   #2
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this is the circut layout:
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Old 6th April 2005, 02:29 PM   #3
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When you measure impedance/resistance at the input where exactly are you measuring? If it is right at the start of the amp then because the input is AC coupled through the cap you will never get a stable reading here, so you can ignore both your readings mentioned above.

Sorry I can't help more, but if you have no voltage across your safety resistors that means a lot of current is trying to be consumed by the amp, so the output must be short. Try removing the very last output pair base connection from the board and see if they pass any current by measuring the voltage across their emitter resistors.

Which SC design is that? It looks nice.
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Old 6th April 2005, 02:41 PM   #4
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Quote:
Originally posted by richie00boy
When you measure impedance/resistance at the input where exactly are you measuring? If it is right at the start of the amp then because the input is AC coupled through the cap you will never get a stable reading here, so you can ignore both your readings mentioned above.


yeah, at the start next to the 2.2uF electro, i just thought they should measure the same (or similar),

oh well... i thought i was onto something

Quote:
Sorry I can't help more, but if you have no voltage across your safety resistors that means a lot of current is trying to be consumed by the amp, so the output must be short.
but there is no heat anywhere?

Quote:
Try removing the very last output pair base connection from the board and see if they pass any current by measuring the voltage across their emitter resistors.
OK, will try tomorrow, it is getting late here..

Quote:

Which SC design is that? It looks nice.
it is the 175W into 4ohm design from April 1996

thanks for your advice...!
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Old 6th April 2005, 03:35 PM   #5
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If there really is no heat ANYWHERE and really not even millivolts across the safety resistors then you cannot be getting any power onto your board.
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Old 6th April 2005, 06:10 PM   #6
sam9 is offline sam9  United States
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When you can't set the bias, in my limited experienc, it's because for some reason the VAS transistors are not turning on. The reason is probably not ibn the VAS itself but in the input section.

If I had this problem I would recheck the value of each resistor, then do a systematic point to point continuity check touching, whenever possible, the component leads rather than the pcb traces or solder joints. It might be helpful to remove the caps temporarily to measure resistance.

With the caps in place but the amp not connected to a power supply, an AC test signal applied to the inputs should appear at the base input transistor in the LTP but no further along the signal path. It should also appear at various other locations in the input section before it reached the differential pair. See if you get the same AC reading for both good and bad amps.

By this point you will have eliminated some possabilities and may even be closing in. You might also try temporaritly removing the diff pair transistors when you do the above inorder to isolate the input components from the rest of the amp. If youget the same readings for both amps that way you at least know the problem is not located in the pre-LTP input components.

If you are still looking after this, start comparing quiesent voltages around the diff pair, the current mirror and the CCS. Don't for get to look for bad or backwards diodes, PNP vs NPN, etc.

This may have been of no help at all but it is where I would look.

One more technique: I have uncovered some major "gotchas" simply by stepping aaway for a day or two. The answer sometimes just pops out of thin air while at the gym, fishing or knocking back a beer at the local.
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Old 7th April 2005, 03:38 AM   #7
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Quote:
Originally posted by richie00boy
If there really is no heat ANYWHERE and really not even millivolts across the safety resistors then you cannot be getting any power onto your board.
must be a new definition of "cannot" that i am un-aware of?

there is full supply +/- rail voltage at the collectors of each output transistor. (i assume that this is correct?)

and, as i stated, there is voltage at the two points as specified by the circut diagram,

Quote:
i measure 0.71V next to Q6 and only 0.1V next to Q7 (my mains voltage is a bit higher than 240V BTW)
sam9 - thanks heaps for those steps, i think i understand most of them, and will give it a go...
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Old 7th April 2005, 08:47 AM   #8
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OK, let me rephrase that. If there really is no heat ANYWHERE and really not even millivolts across the safety resistors then either you cannot be getting any power onto your board OR your amplifier cannot be drawing any current.

As you can measure voltage on the board then I suspect the latter. This would point to the input stage LTP constant current source not working. If this doesn't work then the VAS will be open circuit and no drive on the output stage.
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Old 7th April 2005, 11:29 AM   #9
boholm is offline boholm  Denmark
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What are the voltages across the two 220 ohms next to Q4 and Q5? - They should be the same.

And as a check-up: What is the voltage across the two 150 ohms next to Q1 and Q2? - They also should be the same.

Compare all these measurements to the other (working) board. That should give some hints.

And maybe a picture of both sides of the fawlty board itself, please. Just to see, if we can spot anything, that you have missed - you know; fresh eyes
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Old 7th April 2005, 11:47 AM   #10
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Quote:
Originally posted by boholm
What are the voltages across the two 220 ohms next to Q4 and Q5? - They should be the same.


both are 0.47V +/- 0.01V

Quote:
And as a check-up: What is the voltage across the two 150 ohms next to Q1 and Q2? - They also should be the same.
both are 0.26V +/- 0.01V

Quote:
And maybe a picture of both sides of the fawlty board itself, please. Just to see, if we can spot anything, that you have missed - you know; fresh eyes
very embarased to show the underside, as my desoldering skills on the output transistors leave alot to be desire but they all connect to their relative points correctly with continuity tests.

there is lots of shiny flux, but i have held the board up to light and the tracks are all OK, and there are no obvious bridges...

BTW - i have just bought a new set of input stage transistors to install tonight...
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