# Stage Line 500W amp repair, help please

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,090
Let's use the space between our ears a bit. Vbe is Voltage, Base to Emitter. Maybe better expressed as V(be)

First a little research: https://aetechron.com/Diagnosing-overtemperature-7548-7796.shtml say heat sink temperature below 100C, Using the setpoint number in the 500 amplifier, room temp of 25C and the slope, you get 96.8 degrees as a setpoint. Makes sense.

You need a transistor (a p-n junction) with a Vbe of around 420 mV at room temperature.

You need a transistor (a p-n junction) with a Vbe of around 463+600mV = 1.063 V at room temperature.
Anyway, it has to be bigger than 463 mV.

Usually transistors made in the same batch will have the same characteristics. You might have to buy 20 and then pick 2 for Q16 and Q42. You might get lucky and have two already. Remember close, not the same. Every 10 mV is a degree C.
The lower the Vbe at room temperature, the circuit will "read" a hotter temperature.

"selected" components are not unusual. manufacturer's might say "Q42 selected" and that's it.

We are doing a bit of reverse engineering here.

R121/R126 and R122/R127 are voltage dividers. I won't do the math. They are dependent on the supply voltage. This kinda shows you that ripple and the value of the +15V supply matter. Other critical designs like a voltmeter will use a voltage reference. here we don't care if it's between 90C and 110C that the fan comes on or one comes on at 95 and the other 110C.
"Too hot" doesn't mean > 98.6F like it does in a human.

The entire point of using that diode is to NOT get 15V and to prove an open. It was NOT designed to turn the overtemperature LED off.

We want to as few measuremnts as possible to identify the fault(s).

Last edited:

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,090
WRONG: Shorting V(be) of Q42 should make the lover temp light go out and the fan to go off. There is some time constant, A short across a diode or a V(be) junction is still a short.

RIGHT: Shorting V(be) of Q42 should make the lover temp light go out and the fan to go off. There is some time constant, A short across a diode or a V(be) junction is still a short.

EDIT: Wrong answer thanks to Les. A short will make the overtemp light come on.

Last edited:

#### bertus

Joined Apr 5, 2008
21,851
Hello,

The tester in post #419 seems to show a forward voltage.
Perhaps you can try to find a pair with some close forward voltages.

Bertus

#### LesJones

Joined Jan 8, 2017
3,701
Re post #442 Shorting B-E on Q42 should make the right hand channel over temp LED come on.
I was thinking of suggesting doing this when the open circuit fault was fixed as a test that the overtemp circuit was working.
NOTE. Vbe drops as the temperature increases.
If these tests are OK I think we can start on the fault that is causing the left channel overtemp LED to be lit.

Les.

Last edited:

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,090
Look like I got confused a bit:

T15=0.433vdc and LED3 is associated with T15
T16 14.754vdc This was the supposedly good overtemperature channel.

So, there were/are two problems. I mistakenly thought that T16 was causing the overtemp "LED" to turn-on.
In fact, T16 @ 15V would permanently turn it off.

OOps. Thanks @LesJones for paying more attention.

Q16/Q42 are obsolete and it's difficult to select a transistor based on Vbe. I only saw a few datasheets where Vbe vs temperature at some collector current was shown.

My 420mV might be wrong too. @bertus got a setpoint of 493 mV. Lets; try this again:

95C (The heat sink bad temperature is) 95-25 is 60 degrees hotter which is 60 dec C * 10 mV/ deg C. or 60 mV.

so 493+600 mV

You need a transistor (a p-n junction) with a Vbe of around 463+600mV = 1.063 V at room temperature.
Anyway, it has to be bigger than 463 mV.
I'm suggesting that if the setpoint is 95C, then the room temperature value
should be 25C which is 70C (I used 60, big deal). Add 10 mV/deg C.

I hope I got this right.

This is closer to the 0.6 V that we normally use for Vbe.

Sorry.

Last edited:

#### Rookieme

Joined Jan 26, 2021
308
Re post #442 Shorting B-E on Q42 should make the right hand channel over temp LED come on.
I was thinking of suggesting doing this when the open circuit fault was fixed as a test that the overtemp circuit was working.
NOTE. Vbe drops as the temperature increases.
If these tests are OK I think we can start on the fault that is causing the left channel overemphasised LED to be lit.

Les.

I will do ths soon,
Also the parts to put the left chanel togother have atrived but will get to that once this problem is fixed

Last edited:

#### Rookieme

Joined Jan 26, 2021
308
Re post #442 Shorting B-E on Q42 should make the right hand channel over temp LED come on.
I was thinking of suggesting doing this when the open circuit fault was fixed as a test that the overtemp circuit was working.
NOTE. Vbe drops as the temperature increases.
If these tests are OK I think we can start on the fault that is causing the left channel overtemp LED to be lit.

Les.
@LesJones
Hi
Yes shorting the base and emitter of Q42 does indeed bring the thermal right LED 4 ON

Now that the break on Q42 emitter is fixed relative to T26 i get 0.60vdc at T16

I also think Q16 is good , however it is not currently fitted, the temp LED 3 is still on

This is with all the diode bridges removed from Q42 and Q16

Last edited:

#### Rookieme

Joined Jan 26, 2021
308
Here is Q16 in the tester

Last edited by a moderator:

#### LesJones

Joined Jan 8, 2017
3,701
With Q16 NOT fitted.
With the black lead from your meter connected to T26 measure the voltages on T13, T15, IC4 pins 1, 2, 3. Measure the voltage between the ends of R151. Report these results.

Les.

#### Rookieme

Joined Jan 26, 2021
308
Here are the measurements you requested,
Relative to T26
Q16 NOT FITTED

T13 = 0.508 vdc
T15 = 0.435 vdc
IC4-A
Pin 1 = 6.815 vdc
Pin 2 = 0.006 vdc
Pin 3 =0.508 vdc
Measured across R151 = 0.000 vdc

#### Rookieme

Joined Jan 26, 2021
308
Bit of a development, sat with the amp on tring to compair IC4-A and IC4-B against each other and IC4 has decided to let go and burn up,
Today is not a good day,
I do have a couple IC TLO320 op amps if they would work as a replacement ?

Last edited:

#### LesJones

Joined Jan 8, 2017
3,701
Re post 450. The readings indicate almost a dead short between IC4 pin 2 ans the 0 volt rail.
This could be caused by C52, an internal fault in IC4 or IC 3.
With the power off measure the resistance between IC4 pin 2 and the 0 volt rail. Note the reading.
As IC4 has been destroyed cut pin 2. Do the resistance test between the track that went to pin 2 and the 0 volt rail again If the short has cleared IC4 was the cause. If not try removing C52 to see if that is the cause of the short circuit. If the capacitor is not the cause then it must be IC3.
As zophas has pointed out
IC4 is NOT an op amp. Use the correct parts.

Les.

#### Rookieme

Joined Jan 26, 2021
308
Ok from IC4 pin 2 to the 0v rail i get 34.092kΩ,
With pin 2 cut from IC4 i get roughly the same 34.089kΩ,
Do you still want me to remove C52 and recheck ?

#### LesJones

Joined Jan 8, 2017
3,701
It looks like the short was internal to IC4. Measure the resistance between the remains of pin 2 on IC4 and the 0 volt rail. If you get a low reading IC4 was the cause. If not then something else was causing the short that must have been disturbed thar was causing the short.

Les.

#### Rookieme

Joined Jan 26, 2021
308
With pin 2 cut from IC4 i get roughly the same 34.089kΩ,
This was from the remains of the pin2, to 0v rail

#### Rookieme

Joined Jan 26, 2021
308
Found a heatsink for my resistor for testing the left chanel ,also waithing on my new ICs turning up, may be a while before they turn up

Last edited:

#### LesJones

Joined Jan 8, 2017
3,701
You will need MUCH BIGGER heat sink than that to dissipate 150 watts.

Les.