Stage Line 500W amp repair, help please

Thread Starter

Rookieme

Joined Jan 26, 2021
279
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.
Will do this tonight, and no the - symbol has not changed, i just double it up so it is clearer, i am doing this all by mobile phone,

Pete
 

Thread Starter

Rookieme

Joined Jan 26, 2021
279
Here the results you requested with power off and the fan removed,
Resistance across Q50 C and E,
This result slowly climbs or declines depending on which way round i have the probes but its almost the same, 0.790MΩ and 0.702MΩ,

Fan in and power on,
IC3 PIN 2=0.565 IC3 PIN 6=0.559
IC3 PIN 3=0.662 IC3 PIN 5=0.659
IC3 PIN 1=14.817 IC3 PIN 7= -13.952
IC3 PIN 4 = -15.047

R122 one end = 14.801
R122 other end = 0.558

R127 one end = 0.558
R127 other end= 0.002

Pete
 

LesJones

Joined Jan 8, 2017
3,571
It looks like IC3b is faulty as the conditions on it's inputs are correct but the state of it's output is wrong. OR there is a short to pin 7. Check the track from IC3 pin 7 for a short to something else. Is the part number of IC3 just 4560 or are there some letters in front of the 4560 ? Is there any indication of the manufacturer of IC3 ? Is IC3 a DIL package device or surface mount ? Searching for datasheets on just 4560 brings up a number of different dual op amps with different specifications so you may have trouble finding the correct spare part.

Les.
 

Thread Starter

Rookieme

Joined Jan 26, 2021
279
When this thing is finished, which now i believe it soon will be,
Would there be any benefit to fitting some of the componants that were not fitted from factory,
as in the missing outputs and associated componants,
Could this be a possible upgrade and make the unit stronger and more reliable
 

Thread Starter

Rookieme

Joined Jan 26, 2021
279
@LesJones
If we assume its IC3B at fault here,
Could i remove the connection from D40 cathode end to disconnect it from the circuit just leaving IC3A to control the fan, if it does it will confirm IC3B is faulty ?
 

LesJones

Joined Jan 8, 2017
3,571
IF YOU HAVE CONFIRMED THAT THERE IS NOTHING SHORTING TO THE ETCH FROM IC3 PIN 7 THAT COULD BE PULLING IT NEGATIVE then we know IC3b is faulty because the non inverting input is more positive than the inverting input which should make the output pin positive. (From voltage readings in post #482) But the voltage readings show that it is negative.
Also from the voltage readings in posts #473 and #482 we know D40 is forward biased with about 0.6 volts drop. So it's anode end (IC3b output end.) must be pulling it negative.
If you do not believe this reasoning you can disconnect one end of D40.

Les.
 

Thread Starter

Rookieme

Joined Jan 26, 2021
279
I will pull the PCB out again and follow the etch from IC3B PIN 7 and see if there is a short to somewhere it shouldnt be,
I think i may have a heatsink for my resistor to mount to, see photos20211022_143329.jpg20211022_143407.jpg
 

Thread Starter

Rookieme

Joined Jan 26, 2021
279
Here is a photo of the underside of IC3, all looks ok, Pin 1 is top right,
I do see dry joints on T13 and T14 which i will sort when the new BA 4560,s turn up20211022_160548.jpg
 

Thread Starter

Rookieme

Joined Jan 26, 2021
279
Well the resistor works a treat, dosent even get warm, however the outputs do, moreso Q13, this part of the heatsink and that particular transistor gets quite hot, i did notice when just probing around Q20 base voltage is about 0.3v higher than the rest,
20211022_175337.jpg
 

LesJones

Joined Jan 8, 2017
3,571
You need to be more clear about what you are displaying. I am guessing that the scope trace is the output of the left hand channel (As you mention Q13 and Q20) with the 8 ohm load connected and using a divide by 10 probe. As your scope is displaying 81 mV RMS and I am assuming a divide by 10 probe the actual voltage on the output will be 810 mV RMS.
As the load is 8 ohms the power output is V^2/R = 0.81*2/8 = 0.082 watts. I would not expect your load resistor to get warm.
You say " Q20 base voltage is about 0.3v higher than the rest " I assume by higher you mean more positive. Can you clarify EXACTLY what you mean by "the rest"
As MrChips points out using DC coupling would give us a better indication if the DC conditions in the output stages are correct.
It would be a good idea to voltage tests that you did on the output stages of the right hand channel on this channel to see the DC conditions are about correct.

Les.
 

Thread Starter

Rookieme

Joined Jan 26, 2021
279
I am going to wait untill the new IC3 arrives, then install the IC socket and new chip, then i will start some testing if all is well
 
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