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Bose amplus 100 amplifier weird noises

refmatt

Sep 23, 2015
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Hi

I have this Bose amplifier that had a number of problems when i got it like no power which i have eventualy sorted, however i now get good sound out of it for about 10 seconds then it sounds terrible like in the video attached, this is at a low volume and with a number of different sources etc.

If anyone has a clue on where to start that would be great ? Unfortunatly i dont have a scope :(

Circuit diagram is attached also





Thanks in advance
Matt
 

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dorke

Jun 20, 2015
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How does the AMP sound in low output volume?
What was the "no power" problem you found?
 

refmatt

Sep 23, 2015
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How does the AMP sound in low output volume?
What was the "no power" problem you found?
Thanks for the responce the c
Volume doesnt seem to make a difference to the sound, it sounds distorted at all volumes. And the no power issue was mainly due to the mains switch essentially falling apart i think due to over use ! So just soldered a new one in :) thanks
 

refmatt

Sep 23, 2015
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Yeah, good catch, I was busy at work, lots of things on my mind and typing on here at the same time ;)

yes, tho 1000uF is probably way too big I personally wouldn't use anything more than around 10 uF




I did state that this was for the preamp area stages, not the power amp ;)
Thanks for the responce, i will order a 10uf at 50v cpaacitor and give this ago :) it all sounds so easy when people who know what there doing explain things :) thanks
 

refmatt

Sep 23, 2015
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Thanks for the lack of acknowledgement
As a result, I have deleted my helpful posts
Sorry davenn that second comment was ment to be a reply to your first post apologies i thought i clicked on yours.
 

dorke

Jun 20, 2015
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What is feeding the input of the AMP?
It should be a differential signal with shild(+,-,GND).

Can you try and see if there is distortion on the line-outputs(differential as well)?
 

refmatt

Sep 23, 2015
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The input to the amp is a differential signal.

I have used the speaker scope to test around everywhere i test seems to be distorted the same as the output, when i test tp1 which is just after the resistor it is doing it even though the input when tesed sounds fine through the scope. Does this mean them resistor could be to blame or is this reading false as it is coming from else where ?
Thank you agian
 

dorke

Jun 20, 2015
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It looks like some replies are missing from the discussion...
Please remind me ,what are you finally using for the "speaker scope"?
In any case, it will be loading the circuit under test .
If you use it for testing low impedance circuits it may be o.k,
but for hiZ circuits it isn't.

If I understand correctly ,
the signal to the left of R100 is not distorted but the one to the right is.

No,the resistor is not the cause(but check that it's resistance value is o.k).
When using the "speaker scope" what was your reference point?

A general note:
If the input stage causes distortion ,you will get it in every point along the path till the output.

The first stage is a differential to single ended buffer.

Let's check the first stage:
disconnect the input cable to ch
1.without power connected to the unit,
check resistance between G1 and Analog GND(marked by a triangle) for High impedance.
2.with power on and input cable to ch1 disconnected .
check U100 pin 1 for zero Vdc (TS1 point)
check U100 pin 2,3 for zero Vdc (TP3,TP4)
check U100 pins 4,8 for +/- 12Vdc

If all the above are good ,use the "speaker probe" but in a differential mode !
probe between M89-M90 and TP1-TP2,what do you get?
 

dorke

Jun 20, 2015
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Forgot to say :all Vdc measurements to Analog GND.
and,Here is the CircuitStage1.jpg
 

refmatt

Sep 23, 2015
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It looks like some replies are missing from the discussion...
Please remind me ,what are you finally using for the "speaker scope"?
In any case, it will be loading the circuit under test .
If you use it for testing low impedance circuits it may be o.k,
but for hiZ circuits it isn't.

If I understand correctly ,
the signal to the left of R100 is not distorted but the one to the right is.

No,the resistor is not the cause(but check that it's resistance value is o.k).
When using the "speaker scope" what was your reference point?

A general note:
If the input stage causes distortion ,you will get it in every point along the path till the output.

The first stage is a differential to single ended buffer.

Let's check the first stage:
disconnect the input cable to ch
1.without power connected to the unit,
check resistance between G1 and Analog GND(marked by a triangle) for High impedance.
2.with power on and input cable to ch1 disconnected .
check U100 pin 1 for zero Vdc (TS1 point)
check U100 pin 2,3 for zero Vdc (TP3,TP4)
check U100 pins 4,8 for +/- 12Vdc

If all the above are good ,use the "speaker probe" but in a differential mode !
probe between M89-M90 and TP1-TP2,what do you get?
Hi

Thank you so much for your time i really appreciate it.

I have firstly measured the resistance between c103 and r104 which i believe are the points you meant, when i first did this it was reading 19.21 kohm then the second few times i did it i was getting roughly 9kohms ?

As for the u100 on ts1 and tp 2 and 3 there was 0v

On pins 4 and 8 which i measure on c108 and c106 i was also getting next to 0v it was 0.1 ish and when on mv it was all over the shop.

I am not really sure what this means ?

Thanks matt
 

dorke

Jun 20, 2015
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Pin 4 and Pin 8 should measure +/- 12V DC (with power on).
If it is zero, you found a meaningful problem.
Trace those lines they should get to a regulator(s).
 

Mapromonyc

Feb 14, 2019
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Feb 14, 2019
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Hi I k ow thinks an old post but I picked up a used amplus 100. After plugging in and turning on the switch I dont get any light to indicate power. Am I missing something?
 

Ylli

Jun 19, 2018
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Schematic is available here: https://elektrotanya.com/bose_amplus-100.pdf/download.html
Start by checking the fuses, F600, F601, and F602. If the fuses are good, then take out your voltmeter and check the voltage on (either end) of both R602 and R603. Should find about +20 volts on R602 and -20 volts on R603. Now check the voltages on the outputs of U600 and U601. Should be +/- 12 volts respectively. If good so far, does it sound like the relay K600 is pulling in?
 
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