Stage Line 500W amp repair, help please

LesJones

Joined Jan 8, 2017
4,174
To save confusion we will work on one channel at a time. let's start with the left channel. I think this channel should pass an audio signal now. Connect a speaker to the output of the left channel via a resistor of about 100 ohms (This is to reduce the risk of destroying the speaker.) It may need to dissipate up to about 25 watts under fault conditions. Inject an audio signal from the headphone connector of a battery powered radio, cassette player, MP3 player etc via a non polorised capacitor (About 0.47 uF) into the junction of R11 and R12. (This is where you injected the DC voltage.) Don't forget to connect the headphone common to the 0vP line on the amplifier. You should hear the audio from the speaker but don't expect it to be very loud due to the resistor in series with the speaker and the fact that the voltage gain of this part of the amplifier is only 6 (This is 6 times not 6 dB.). If this works the next step is to inject the signal into earlier stages.

Les.
 

Thread Starter

Rookieme

Joined Jan 26, 2021
308
Ok, here is something interesting, i have connect an input audio signal to the phono jack on the back of the unit, then connect one end of the speaker connections to 0vp and the other to the -inL, then swapped to the right chanel, both siges give clear audio, (although not very loud) also the gain on the front seems to function correctly, however when i inject audio here the power amp has no output at all either chanel
 

LesJones

Joined Jan 8, 2017
4,174
At least we now know that the pre-amp section is working. I think it would be a good idea to repeat the tests injecting the + and - DC to the input but with some load on the output. Your 100 ohm high power resistor that you used in series with the speaker should do to start with. Before doing that check the voltage (With respect to 0vP) at the anode end of D3. I would expect it to be about +34 volts. Do you understand how the feedback system in the power amp works ? It subtracts one sixth of the output voltage from the input voltage and compares it with zero in the longtailed pair formed by Q4 and Q5. Any difference from zero is amplified to drive the positive or negative power stages to adjust the output in the direction to balance the inputs to the longtailed pair. Think of it like a servo system.

Les
 

Thread Starter

Rookieme

Joined Jan 26, 2021
308
At least we now know that the pre-amp section is working. I think it would be a good idea to repeat the tests injecting the + and - DC to the input but with some load on the output. Your 100 ohm high power resistor that you used in series with the speaker should do to start with. Before doing that check the voltage (With respect to 0vP) at the anode end of D3. I would expect it to be about +34 volts. Do you understand how the feedback system in the power amp works ? It subtracts one sixth of the output voltage from the input voltage and compares it with zero in the longtailed pair formed by Q4 and Q5. Any difference from zero is amplified to drive the positive or negative power stages to adjust the output in the direction to balance the inputs to the longtailed pair. Think of it like a servo system.

Les
No, i am completely lost at this point (feedback system) lol,
I understand where to connect the battery and where on D3 to take the reading, this is what i am struggling with, i understand transistors, NPN and PNP, i understand how they need base voltage to be forward bias, but think i need to do some more reading, it looks a simple circuit but its not haha

Also i think there may be an issue with D4 on the left chanel, with the diode setting on my meter i get continuity both was round, anyway, i will connect the battery and post my results shortly oh and 68Ω is the closest i have
 

Thread Starter

Rookieme

Joined Jan 26, 2021
308
Ok with the battery + connected to the 0v and -- connected to the IN--L via a 3k resistor, i connected T4 and T5 with a 68Ω resistor, black probe of the meter on T26 and red probe on the anode of D3 i get, 27.46vdc,
Would you like me to do this again with polarity reversed?
 

LesJones

Joined Jan 8, 2017
4,174
The voltage test at D3 anode was to make sure that the signal "control-L" (Which originates at the over temp detection circuit.) was at the correct state for the amplifier to work. Although 27 volts is a bit lower than I expected I don't think it is low enough to cause a problem. I think you mentioned that you worked with single phase and three phase power so I imagined you worked in a factory and you looked after CNC machines and so would be familiar with servo systems. This is a rather complex project to tackle with your limited knowledge. It has a lot of protection circuitry to protect the amplifier from over temperature, over current and clipping on the output.
When I said repeat yesterdays test but with a load on the output it was to see if you still got the + and - 3 volt readings on the output with it driving a load.

Les.
 

Thread Starter

Rookieme

Joined Jan 26, 2021
308
The voltage test at D3 anode was to make sure that the signal "control-L" (Which originates at the over temp detection circuit.) was at the correct state for the amplifier to work. Although 27 volts is a bit lower than I expected I don't think it is low enough to cause a problem. I think you mentioned that you worked with single phase and three phase power so I imagined you worked in a factory and you looked after CNC machines and so would be familiar with servo systems. This is a rather complex project to tackle with your limited knowledge. It has a lot of protection circuitry to protect the amplifier from over temperature, over current and clipping on the output.
When I said repeat yesterdays test but with a load on the output it was to see if you still got the + and - 3 volt readings on the output with it driving a load.

Les.
No i install UPS systems and battery backups for work,
And yes i agree, this amp is a learning project for me
 

Thread Starter

Rookieme

Joined Jan 26, 2021
308
I havent changed anything on the preamp side, its completely 100% original , so surely the fault has to be in the output power amps
 

LesJones

Joined Jan 8, 2017
4,174
Yes I agree so I need the result of the test I asked you to carry out with the 100 ohm (Or so) load on the output to see if it still manages to give close to the calculated output voltages at the output for both + and - 1.5 volts input. (The same as the tests you did before but with this small load on the output.) The result of this test will give an idea where the fault MIGHT lie. But further tests will be required to get to the actual fault. As you don't understand how the amplifier works you will have to wait for me or other members of the forum to direct you to the next tests to make. This will be a very slow process compared to what would happen if we here working directly on the amplifier.

Les
 

Thread Starter

Rookieme

Joined Jan 26, 2021
308
LEFT CHANEL:
battery -- to --IN L
68Ω resister across T4 and T5,
T4 measures 0.341vdc
T5 measures 0.000vdc
Polarity reversed
T4 measures --0.387vdc
T5 measures --0.000vdc

Hope this helps
 

LesJones

Joined Jan 8, 2017
4,174
I think you have mixed up T4 and T5 in post #71. T4 is the 0vP point which is the same point electrically that you have (Or should have.) the negative meter lead connected so that should read zero.
Measure the voltage at the following points (If I don't say with respect to in future it will be with the negative meter lead connected to 0vP.) first with nothing connected to the input then +1.5 connected then -1.5 connected.
Junction of R14 and R15
Junction of R16 and C3
junction of C5 and R19
D14 cathode (Cathode is the end with the band on.)
D1 anode.
(This should be 15 readings.)

Les.
 

Thread Starter

Rookieme

Joined Jan 26, 2021
308
Ok before i continue, i have to say that this project is also running on another forum where i am also taking advice and guidance, everyone has there own way of doing things and some are easier for me to follow than others, is this an issue between forum users?,
 

LesJones

Joined Jan 8, 2017
4,174
It's not a problem that different people have different approaches. I suggest sticking to one groups suggestions and follow it through to the end. In normal circumstances I would recommend finding someone that lived close enough to help you but that might mean waiting a long time.

Les.
 

MrChips

Joined Oct 2, 2009
30,704
This is not a simple repair. It is difficult enough with one helper. You need to be focused on one task at a time. Having many cooks in the pot is going to be very confusing and make your life very difficult. My recommendation would be to stick with one forum. I have intentionally stayed on the sideline and let Les take the lead.

If you want to see a similar repair with similar situation as yours you ought to see this:

https://forum.allaboutcircuits.com/threads/technics-su-vx800-amplifier.143789/

This is running on over 3 years.
 

Thread Starter

Rookieme

Joined Jan 26, 2021
308
We have not left.
Show us photos of your oscilloscope and signal generator.
Have you decided which forum you are going to follow?
I appreciate everyone's help so far on both forums, however i think the direction on here is easier to follow, i will upload photos of the scope and sig gen soon, there only cheap ones but should enable me to give better information (once i learn how to use them propperly)
Cheers
 
Top