Stage Line 500W amp repair, help please

Thread Starter

Rookieme

Joined Jan 26, 2021
308
Having +15 volts on IC3 pin 3 indicates that either the base emitter junction of Q16 or it's emitter is not connected to zero volts. This will mean that over temperature will not be detected on the left hand channel. (You had a similar problem with the emitter connection of Q42 at around post #396.)
What was causing the loss of -15 volts on pin 4 of IC3 ? When you say "all other readings are correct" do you mean that the fan now works when shorting the base to emitter on either Q16 or Q42 ? ( When you are shorting the base to emitter of Q16 you may also be making contact between the emitter on the 0 volt track.)

Les.
i think the bad reading on IC3 PIN3 was operator error, after this the fan still did not work, and i mean i went back over all the pins and all the other readings were the same
 

Thread Starter

Rookieme

Joined Jan 26, 2021
308
ok so with that done i am going to run the left channel at low power and monitor from 0v to T16 and watch for a voltage drop in the hope Q16 gets hot and starts to conduct and send a low voltage to pin 3 on ic3 to operate the op anp and switch on the fan, please correct me if this is wrong,

pete
 

LesJones

Joined Jan 8, 2017
3,701
I don't think the bad reading on IC3 pin3 was operator error. In the first set of readings in post #515 I think you got pins 2 & 3 mixed up. You listed IC3 pin 2 as having about +14 volts on it but I think that was pin 3. You then give the reading of IC4 pin 2 as about + 14 volts. This is the same point as IC3 pin 3. When I added in the test to IC4 pin 2 it was to see if there was break in continuity between these two points.

Les.
 

Thread Starter

Rookieme

Joined Jan 26, 2021
308
(You had a similar problem with the emitter connection of Q42 at around post #396.)
( When you are shorting the base to emitter of Q16 you may also be making contact between the emitter on the 0 volt track.)

Les.
i think you may be right Les, T16 i am getting 14vdc and T11 i get around 0.644vdc the sort of voltage you get across a base/emitter bridge, it would appear there is a connection problem between Q16 base or emitter i think
 

Thread Starter

Rookieme

Joined Jan 26, 2021
308
T16 is for the right hand channel. you need to monitor T15.
Les.
yes i know, i have switched to the right channel to compare, as of now the voltage is slowly dropping on T16, i am looking to see if the fan comes on at all on the right channel as this seems almost in full working order, but i may have to use the resistor rather than the speaker as the right channel hardly gets warm compared to the left i think the left still has a couple of gremlins, after this i will remove the board and inspect for connection issues with Q16 then return and monitor this channel again
 
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Thread Starter

Rookieme

Joined Jan 26, 2021
308
ok fantastic news, on the right channel the voltage atT16 gets to about 0.562vdc and the fun cuts in and back off at 0.565vdc, so i would say that side is good
 
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LesJones

Joined Jan 8, 2017
3,701
T11 is the reference voltage for the LEFT hand channel. (I calculated it to be 0.569 volts) This voltage should be stable. The voltage on IC3 pin 3 is compared with this voltage to decide if the fan should be on or off. The reference voltage on T11 will change very slightly when the output of Q3 pin 1 changes state due to positive feedback via R128. This makes Q3 behave like a Schmitt trigger. This makes the on/off switching more positive.
T15 voltage changes as the base emitter voltage of Q16 changes due to temperature change.

Les.
 

Thread Starter

Rookieme

Joined Jan 26, 2021
308
on inspection Q16 did indeed have a bad connection, but when i fix this i lose all output from both channels and the bridge function does not work, i think i am going to replace Q16 and Q42, does anyone have any suggestions for a good replacement,

pete
 

LesJones

Joined Jan 8, 2017
3,701
I suggest NOT trying the bridge function until both channels are working correctly. (It is possible the faulty channel could damage the good channel.)
I do not agree with replacing Q16 or Q42 until you understand how they are causing the channels to stop working. The first step is to measure the voltages on T15 and T16 again.

Les.
 

Thread Starter

Rookieme

Joined Jan 26, 2021
308
it seems every time i take the board out of this thing i end up with a new problem or different readings, Q16 fault is now fixed but left channel audio is distorted, T15 reads 0.660 vdc and T16 reads 0.652 vdc, this is with the amp cold
 

LesJones

Joined Jan 8, 2017
3,701
The readings on T15 and T16 are about what I would expect.
I suspect that the distortion and the heating problem are related to each other.
I suggest making a table of the following voltages with respect to 0 volts.
Q1 base, Q10 base, Q12 base, Q18 base, Q18 emitter, T5.
A22 base, Q11 base, Q12 base, Q20 base Q20 emitter.

Les.
 

Thread Starter

Rookieme

Joined Jan 26, 2021
308
this is what i get

positive side output tails

Q1 base = - 0.506 vdc
Q11 base = 0.00 vdc
Q12 base = 0.00 vdc
Q18 emitter = 0.00 vdc
T5 = -0.005 vdc

negative side output tails

Q22 base = - 4.589 vdc
Q11 base = -4.17 vdc
Q13 base = 0.001 vdc
Q20 emitter = - 0.006 vdc

from the readings it looks like there is a connection issue from Q7 base to Q1,
Q7 base reads - 3.927 vdc

and if i remember correctly from the last time i chased the output tails the last two transistors are not active until the amp is driven with some input
 

LesJones

Joined Jan 8, 2017
3,701
I don't see a problem around Q7.
Q22 base -4.589
Q7 base -3.927
----------------------
-0.662 (Q7 vbe) This is about what I would expect from the forward biased drop across the Q7 base emitter junction.
Edit. I have realised that I answered you comment about what I though you would have been looking at rather than what you said.
In these calculations I am making these assumptions. Q7 Vbe = 0.6 volts. The base current of Q7 is low enough not to have a significant loading effect on the potential divider R22 R23. And the current through R21 is 2.6 mA based on the calculations done for the right hand channel.
The voltage between the base and collector = 0.6 x R22/R23 = 0.6 x 15/3.1 = 2.9 volts We need to add the voltage across R21 to this value.
This will be 68 x 0.0026 = 0.177 volts so the calculated difference between Q7 base voltage and Q1 base voltage will be 2.9 + 0.177 volts = 3.077 volts. As Q7 base is at - 3.927 volts Q1 base would be at -3.927 + 3.077 = -0.85 volts. As I have only estimated Q7 Vbe I think this is near enough. If you want you can measure Q7 Vbe and re do the calculation.

I do see a problem around Q11 Q13 There seems to be 4.17 volts between Q11's base and emitter. Measure the voltage DIRECTLY on Q11's emitter. This is to check for continuity between Q11 emitter and Q13 base. (I think I remember having a broken track in this area on the right hand channel.) If there is no break here remove Q11 and test it.

Les.
 
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Thread Starter

Rookieme

Joined Jan 26, 2021
308
i found the fault on the left channel, it was Q13 base had a faulty connection, i think this is from moving and bending the board when its removed and re fitted, now have good clear audio on both channels,

I think I may inject a sine wave and to drive the left channel with my resistor fitted and monitor T15 for voltage drop and the fan coming on,

what's your thoughts?

pete
 
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Thread Starter

Rookieme

Joined Jan 26, 2021
308
I don't see a problem around Q7.
Q22 base -4.589
Q7 base -3.927
----------------------
-0.662 (Q7 vbe) This is about what I would expect from the forward biased drop across the Q7 base emitter junction.
Edit. I have realised that I answered you comment about what I though you would have been looking at rather than what you said.
In these calculations I am making these assumptions. Q7 Vbe = 0.6 volts. The base current of Q7 is low enough not to have a significant loading effect on the potential divider R22 R23. And the current through R21 is 2.6 mA based on the calculations done for the right hand channel.
The voltage between the base and collector = 0.6 x R22/R23 = 0.6 x 15/3.1 = 2.9 volts We need to add the voltage across R21 to this value.
This will be 68 x 0.0026 = 0.177 volts so the calculated difference between Q7 base voltage and Q1 base voltage will be 2.9 + 0.177 volts = 3.077 volts. As Q7 base is at - 3.927 volts Q1 base would be at -3.927 + 3.077 = -0.85 volts. As I have only estimated Q7 Vbe I think this is near enough. If you want you can measure Q7 Vbe and re do the calculation.

I do see a problem around Q11 Q13 There seems to be 4.17 volts between Q11's base and emitter. Measure the voltage DIRECTLY on Q11's emitter. This is to check for continuity between Q11 emitter and Q13 base. (I think I remember having a broken track in this area on the right hand channel.) If there is no break here remove Q11 and test it.

Les.
you were absolutely spot on again Les
 

Thread Starter

Rookieme

Joined Jan 26, 2021
308
ok used my sig gen to input 1000 Hz sine wave and got the left channel up to temperature, the fan now activates on the left channel, monitored T15 with the DMM it went down to about 0.560 vdc before the fan spun up, however i think this is a success, also the left channel heat sync is not getting anywhere near as hot as is it was,
is there any more testing to be done or are we calling this done, i do still need to remove the heat sync and clean and thermal paste everything properly, and try and find a way of keeping Q7 and Q29 pressed against the heat sync,

pete
 
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