Stage Line 500W amp repair, help please

Thread Starter

Rookieme

Joined Jan 26, 2021
308
Ok heres what i did,
I looked at the connection points of Q42 B and E, as you suggested the E legg where the copper was showig was bad,
I removed Q42 and put it in my tester (photo in post #419)

I have now re-fitted Q42 and confirmed continuity from the B to the temp +R connection and E to the 0v connection as shown in post #409,connection point 2,
I still have the laft temp LED on also protect and overheat on the front panel, and i have removed the 1n4148 diode bridge

Pete
 

Thread Starter

Rookieme

Joined Jan 26, 2021
308
Is this circuit attached to the protection circuit?
I have been trying to follow it and it brings me to Q2 on the left chanel
 

bertus

Joined Apr 5, 2008
21,851
Hello,

The Q42 is used as temperature sensor on the right channel.
The Q16 is used as temperature sensor on the left channel.

sta160-12 temp sensors.png
It is clear now why the collector pins are cut off.

You could compare the voltages between the left and right sensor.

Bertus
 

Thread Starter

Rookieme

Joined Jan 26, 2021
308
Hello,

The Q42 is used as temperature sensor on the right channel.
The Q16 is used as temperature sensor on the left channel.

View attachment 248704
It is clear now why the collector pins are cut off.

You could compare the voltages between the left and right sensor.

Bertus
Hello bertus,
Yes i did notice that, i thought Q16 and Q42 were actualy part of the amplifier circuit,
There actualy not connected at all apart from being bolted to the heatsink to read the temperature,
@KeepItSimpleStupid said that IC4A and IC4B is an OR gate, meaning pins 2 OR 3 on IC4A must be active to produce current,

Would this be correct ?

Also Q16 and the resistors R136 and R151 must make up a voltage devider to give 0v at T15 ??

Pete
 

bertus

Joined Apr 5, 2008
21,851
Hello,

The temperature sensors are the B-E diodes of the transistors Q16 and Q42.
Here I pasted the transistors in the temperature control schematic:
sta160-12 temp schematic.png
IC3 controls the fan.
IC4 makes the temperature failure indicator.

Bertus
 

Thread Starter

Rookieme

Joined Jan 26, 2021
308
From that, it looks like Q16 could be shorted, if that were the case would T15 be -- voltage ?,

Ah but then no, because Q16 emitter is connected to 0v
 

Thread Starter

Rookieme

Joined Jan 26, 2021
308
Hello,

You could compare the voltages on T15 and T16.
Those are indicators for the temperature.

Bertus
I did that and the results were
T15=0.433vdc
T1614.754vdc
However we did find a break in the connection between Q42 Emitter and the track after this,
 

bertus

Joined Apr 5, 2008
21,851
Hello,

There are only two parts, R152 and Q42, between T16 and 0V.
Is R152 measuring the given 120 Ohms?
Is the connection between the bottom of R152 and the base of Q42 OK?

Bertus
 

Thread Starter

Rookieme

Joined Jan 26, 2021
308
Seems i am having some soldering issues, looking at Q42 again, now the B legg is not attached propperly, i have been using some solder paste that was reccomended to me (see photo)
This seems to just make a mess,
going to fix thia connection and bridge Q42 and Q61 with a diode and see what happens20210923_204550.jpg20210923_204623.jpg
 

Thread Starter

Rookieme

Joined Jan 26, 2021
308
Thank you for that, i do have solder, some with flux and some without, this board is extremely fragile, 1 second to long and the tracks start to lift, i also have some solder wick and a sucker but its hard to ger a clean joint,

Ok i have bridged Q16 and Q42 with a 1n4148 diode, now relative to T26 i am getting,
T15 = 0.420 vdc
T16 = 0.597 vdc,
I still have LED 3 on and overheat and protect on the front on
 

bertus

Joined Apr 5, 2008
21,851
Hello,

T15 is below the reference voltage of 0.493 Volts.
T16 is above the reference voltage of 0.493 Volts.

It is clear why LED 3 is still on.

Bertus
 

bertus

Joined Apr 5, 2008
21,851
Hello,

T13 and T14 are the reference voltages.
They are around 0.493 Volts.
There is a hysteresis build in the comparator.

Bertus
 
Your solder paste isn't optimum. It's not bad either.

https://www.tme.com/Document/523a674f6ad16a9d7ed0db3f6da41f7b/Pasta lutownicza EN.pdf

I must be missing something here,
This is not clear to me :-(,

How did you calculate the referance voltage ?
This is an application where the transistors could be selected based on Vbe. The diode substitution was to get something below 15V which you did finally manage to do.

Now, replace those transistors where Vbe is about the same at room temperature.

T16 is 597 mV at room temperature I presume. As it warms up (597-493)/10 or about 10 degrees C hotter it will switch.

@bertus has calulated that the comparitor will switch below 493 mV +- some hysteresis

The diode was just to get a value below 15V. Now, you need the right Vbe value.

The temperature coefficient slope is a material property. What Vbe is at a given temperature is a fabrication property.
 

Thread Starter

Rookieme

Joined Jan 26, 2021
308
This is an application where the transistors could be selected based on Vbe
is this the voltage across the base and emitter?

Now, replace those transistors where Vbe is about the same at room temperature.
are you asking me to replace Q16 and Q42 for ones that have the same base and emitter voltage at room temperature?

if so how would i work that out?
 
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