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Old 14th April 2012, 01:40 PM   #11
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wasnt checking the correct place or bias. the big white resisters.
they are reading 2.5mv so according to schematic im going with 220 resisters over to metal pillors under ribbon cable to increasd bias. See if this works!!!

Ill be back.!
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Old 14th April 2012, 08:41 PM   #12
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Well I used a 220r resistor across solder pillars and then turned on amp. Checked bias and it was rocketing up going above 100mv and rising so I turned amp of!! Could fell the heat coming from transistors!! And still the bong noise on start up. What to do next ??? I'm stumped again!!!
I will not be defeated !!! Help!!!!'
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Old 14th April 2012, 10:54 PM   #13
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You shouldn't have soldered anything there, it is a test point to measure with a volt meter. I still think checking the bias current is barking up the wrong tree. Your previous reading of approx 130mV indicates approx 300mA bias, which is too high. However, both channels are consistent.

Have you checked R51 and R71 yet? Also, any comment on what may have caused the problem?
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Last edited by richie00boy; 14th April 2012 at 10:59 PM.
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Old 15th April 2012, 08:25 AM   #14
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Hi richie i have now checked resistors (on the board) and the results are as followed
position r51 55k
position r71 74.5
having looked at the schematic the said resistors should be 75k but r51 is not reading this,but is this because its still in circuit???

Go easy on me im trying my hardest here, im a builder by trade so all this electrical jiggery is quite new to me but im finding it very interesting and i hope very rewarding when i have fixed this amplifier!! I HOPE!!!:
THANKS FOR PATIENCE
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Old 15th April 2012, 11:21 AM   #15
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R51 has a bigger capacitor across it which will give a reduced reading, so I think what you have is fine.

Now I see that you measured 2.5mV across the big white resistors R107, well this shows the bias current to be approx 57mA which is perfect.

Please can you confirm if the amp just started behaving this way or if something was unplugged etc and it started doing it.
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Old 15th April 2012, 11:53 AM   #16
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This was my dads Amplifier so I don't know what happened to it. Not been used for around 5 yrs. this is why I am determined to fix it . So sorry no history to help!!!
What's next to check
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Old 15th April 2012, 03:48 PM   #17
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Is there any noise or cone movement when you turn it off? I seem to recall even a good working Cyrus 2 Issue 7 amp has some turn on noise/thump - the question is how bad is it and is yours worse than normal.
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Old 15th April 2012, 04:37 PM   #18
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Since this is it an old power amplifier you should check the power supplies voltages for ripples. Read all voltages with the voltmeter selector switch in AC. All voltages should read less than .5 volts. Hold the probes until you can see the reading it is stable. It is time when reading DC in AC.
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Old 15th April 2012, 06:35 PM   #19
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The turn on noise is quite bad and defiantly not correct. No noise when turn of,
But after doing more probing with my multi meter I have found something!!!!! I think.
Position vr1 transistor 8930 (lm337t) is reading 18v from out pin right side
Vr2 8948 (lm317t) is reading 7.2 on the out pin middle. This should also be 18v but is not!!!
Have I found something here???????? When reading the transistors I'm grounding the neg to chassis and pos to pin I'm assuming this is correct.
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Old 15th April 2012, 06:51 PM   #20
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Vr1 and Vr2 should read +/-18 volts.
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