Stage Line 500W amp repair, help please

Thread Starter

Rookieme

Joined Jan 26, 2021
304
Ý
You will need MUCH BIGGER heat sink than that to dissipate 150 watts.

Les.
Seriously?

The resistor does not fit in the centre anyway, i could have pilfered a bigger one from the charger i stripped out but thought this would be adequate,
Removed IC4 tonight as it smoked everytime i powered the amp up, the new IC3 and IC4 are in the post, do i need to do any testing before replacing these ? ,

I plan to install some IC sockets and replace only IC4 to start with,
Also now i have both thermal LEDs on, these were on before i removed IC4
 
Last edited:

Jon Hoover

Joined Oct 10, 2019
34
I have followed this thread for the last few months. It's addicting like a soap opera! I can't wait for this item to be repaired so we can find out what was wrong with it. Keep plugging away and don't give up!
 

Thread Starter

Rookieme

Joined Jan 26, 2021
304
I have followed this thread for the last few months. It's addicting like a soap opera! I can't wait for this item to be repaired so we can find out what was wrong with it. Keep plugging away and don't give up!
Hahaha, try being on my end, the guys here are fantastic, i to just want to finish this thing and move on to my next project, (which i already have, coming soon), i have learnt so much, admittadly probably really frustrating for the guy's helping me , hope to have this boxed off and working propperly soon

Pete
 

Thread Starter

Rookieme

Joined Jan 26, 2021
304
Ok so,
Refitted Q16,
Replaced D4, D5, D6, D7,
Replaced Q7
And now i have power on both chanels,
As in both left and right chanels play clear audio,
I still think there is a problem with the fan speed, possibly IC3A, i get 14VDC at the positive side of C52 but only 0.6VDC at the posative side of C53, also
R123 and R131 are verry hot
 

bertus

Joined Apr 5, 2008
21,764
Hello,

Have a look at Q50. The PCB underneath is discoulored. Maybe it has been to hot.
That is the transistor driving the fan.

Bertus
 

Thread Starter

Rookieme

Joined Jan 26, 2021
304
Hi bertus,
i did look at Q50,
It only seems to have -- voltage at all points, C, B and E,

Or it that the idea, as its PNP and only forwads -- voltage
 

LesJones

Joined Jan 8, 2017
3,655
With the power off and the fan connector (Con5) unplugged measure the resistance between the collector and emitter of Q50. Do this with bot polarities of the test leads.

Re connect the fan connector and switch the power back on.
With the black meter lead connected to T26 measure the voltage on the following points.

IC3 pin 2 IC3 pin 6
IC3 pin 3 IC3 pin 5
IC3 pin 1 IC3 pin 7
Q50 emitter
Q50 base
Q50 collector
Report the results.

It looks like the fault around IC4a was removing power to the first long tail pair (Q4 and Q5) of the left hand power amplifier as it is now working.

Les.
 

LesJones

Joined Jan 8, 2017
3,655
The results from post #473 do not make sense. With respect to 0 volts are the readings all positive, all negative or some positive and some negative ?

Les.
 

LesJones

Joined Jan 8, 2017
3,655
I did not know what yo meant with two negative signs in front of the numbers. the readings do make sense if they are all negative.

Les.
 

Thread Starter

Rookieme

Joined Jan 26, 2021
304
With the power off and measuring the resistance across connection 5 reads
0.75kΩ slowley rising or falling depending on which way round i ha e the probes, probably due to C47 charging and discharging,
With respect to T26

IC3 pin 2= 0.563 IC3 pin 6= --13.943
IC3 pin 2= 0.652 IC3 pin 5= 14.801
IC3 pin 1=14.798 IC3 pin 7= 0.558
 

LesJones

Joined Jan 8, 2017
3,655
Pete, Can you clarify if the way of representing negative numbers is taught in schools has changed since I went to school. (Which was a long time ago.) I was taught to represent a negative number by preceding it with a single dash. So minus 5 would be -5.
You seem to use two dashes preceding the number. So you write minus five as --5. This is to save confusion in the future when I am helping younger people.

I would like you to add a few more tests to those I requested in post #474

IC3 pin 2 IC3 pin 6
IC3 pin 3 IC3 pin 5
IC3 pin 1 IC3 pin 7
Also test the following points.
IC3 pin 4
both ends of R122 (You don't need to try to identify the ends. That should be clear from the readings.)
both ends of R127 (You don't need to try to identify the ends. That should be clear from the readings.)

Les.
 
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