Have you done cold checks on the transistors to see if there leaky?
Also changing parts with full checking will waste time and money only change parts that are faulty plus to much solding and desolding will weaken them tracks.
Check all voltages from ht to the +/- rails that feed the op-amps.
Also changing parts with full checking will waste time and money only change parts that are faulty plus to much solding and desolding will weaken them tracks.
Check all voltages from ht to the +/- rails that feed the op-amps.
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Ok, getting there! I would suspect that if you measure at the end of the lifted resistors you will get 7v....
I think that the DC offset that is higher on channel 1 will be due to the low voltage across Z3 it may be that you will need to get a new Zener if replacing the capacitor doesn't bring back up the voltage to near enough 6.2v make sure you get a good tight 1% tolerance Zener.
The protection circuit is probably Q20-23 or D9-10 something in that area at fault. Measure voltages. Around components. There is an 18v veneer is there 18v across it?
I think that the DC offset that is higher on channel 1 will be due to the low voltage across Z3 it may be that you will need to get a new Zener if replacing the capacitor doesn't bring back up the voltage to near enough 6.2v make sure you get a good tight 1% tolerance Zener.
The protection circuit is probably Q20-23 or D9-10 something in that area at fault. Measure voltages. Around components. There is an 18v veneer is there 18v across it?
Evening guys,
I tracked down a scope and a signal generator. I stuck a 1V 200hz square wave in, traced it to the TL074, to the gain pots and then to the chokes. To my non experienced/skilled self the wave on the scope looked great. I also stuck a sine wave in and the looked good to.
I've not done any cold checks on the transistors and the rail/supply voltages look good.
Chris, I thought that I'd find that 7v but it didn't turn up, I'm guessing it might be coming from the display board??
I'll see what happens when the display board goes back on, that connector 5 is just a ribbon cable soldered to the board.
My theory is that something failed on the display board and has taken out the protect circuit in the process, but that's all guess work.
I've got the zeners on order to but they're are 2%, bugger
It looks like RS don't do them in 1%, These are the ones I ordered Link
Those Q20-Q23 (BF422 and BF423) are no longer available any idea on what alternative I could use if there is a fault with them?
I'll have a measure of that 18v zener in the morning
I tracked down a scope and a signal generator. I stuck a 1V 200hz square wave in, traced it to the TL074, to the gain pots and then to the chokes. To my non experienced/skilled self the wave on the scope looked great. I also stuck a sine wave in and the looked good to.
I've not done any cold checks on the transistors and the rail/supply voltages look good.
Chris, I thought that I'd find that 7v but it didn't turn up, I'm guessing it might be coming from the display board??
I'll see what happens when the display board goes back on, that connector 5 is just a ribbon cable soldered to the board.
My theory is that something failed on the display board and has taken out the protect circuit in the process, but that's all guess work.
I've got the zeners on order to but they're are 2%, bugger
It looks like RS don't do them in 1%, These are the ones I ordered Link
Those Q20-Q23 (BF422 and BF423) are no longer available any idea on what alternative I could use if there is a fault with them?
I'll have a measure of that 18v zener in the morning
Just to follow up on the as I may not be about.
If with Connector 5 removed, the resistors lifted and the protection circuit says it is going into protect then the fault is purely in the protection circuit. For the circuit types and the voltages involved I don't think the fault would have been caused by anything other than a part failing and I should think it will be located purely within the protection circuit.
The offsets you are measuring on the PCB for the amp circuit is back to a reasonable level.
The input opamp is not happy and needs replacing for sure.
I think that because you have lifted the end of the resistors closest to the protection circuit that when you measure on the resistor you get no 7v, this is why I suggest checking the anode of D9.
I wouldn't spend any time worrying about the rest of the amp (though a recap of the small value capacitors and Zener diode mentioned before would be sensible.
If with Connector 5 removed, the resistors lifted and the protection circuit says it is going into protect then the fault is purely in the protection circuit. For the circuit types and the voltages involved I don't think the fault would have been caused by anything other than a part failing and I should think it will be located purely within the protection circuit.
The offsets you are measuring on the PCB for the amp circuit is back to a reasonable level.
The input opamp is not happy and needs replacing for sure.
I think that because you have lifted the end of the resistors closest to the protection circuit that when you measure on the resistor you get no 7v, this is why I suggest checking the anode of D9.
I wouldn't spend any time worrying about the rest of the amp (though a recap of the small value capacitors and Zener diode mentioned before would be sensible.
Thank you Chris for your help and taking the time to write your thoughts on this.
Unfortunately I didn't get chance to do anything today.
I'll report back here on my findings.
I'm in no rush to get the amp back up and running and I don't mind on waiting for your guidance if I get stuck.
Hopefully the parts will arrive tomorrow, I think there are enough caps to replace most if not all as RS only sold them in large quantities.
Unfortunately I didn't get chance to do anything today.
I'll report back here on my findings.
I'm in no rush to get the amp back up and running and I don't mind on waiting for your guidance if I get stuck.
Hopefully the parts will arrive tomorrow, I think there are enough caps to replace most if not all as RS only sold them in large quantities.
With the display board soldered back on and with R63 and R64 leg off still, I'm now getting 5v on both speaker terminals.
It seems the 5v is coming from pins 2 and 4 on that connector 5 from the display board.
There is no 7v on D7 before or after the connector back on.
R70 is shown as a diode on the diagram but it's a resistor (180k)
There is no voltage across z7, does that sound right?
Is it sounding like the fault is on the display board seeing there is 5v coming from it and ending up at the speaker posts?
It seems the 5v is coming from pins 2 and 4 on that connector 5 from the display board.
There is no 7v on D7 before or after the connector back on.
R70 is shown as a diode on the diagram but it's a resistor (180k)
There is no voltage across z7, does that sound right?
Is it sounding like the fault is on the display board seeing there is 5v coming from it and ending up at the speaker posts?
Ok, this is a little odd. There should be nothing on the speaker terminals at all, if the protection circuit is engaged. I wonder if you are getting erroneous readings from induced voltage in the probes...
It is the voltage at D9 that needs looking at with the resistors still disconnected.
Can you confirm that the protection circuit is still kicking in even with the resistors lifted.
With connector five now connected and the resistors still lifted, what voltages do you see at the tail end of the inductors.
It is the voltage at D9 that needs looking at with the resistors still disconnected.
Can you confirm that the protection circuit is still kicking in even with the resistors lifted.
With connector five now connected and the resistors still lifted, what voltages do you see at the tail end of the inductors.
Ok but on connector 5 pins 2 and 4 you get 5v?
If you have the amp off you should be able to measure continuity from inductor 1 to pin 2 off connector 5 and the same with inductor 2 to pin 4.
If you are measuring 5v correctly on pins 2 and 4 then in theory you should be reading that at the inductor...
I think something like C40 is responsible for delaying the speakers being connected.
Check the voltage across C40
What is the voltage at the base of Q25?
If you have the amp off you should be able to measure continuity from inductor 1 to pin 2 off connector 5 and the same with inductor 2 to pin 4.
If you are measuring 5v correctly on pins 2 and 4 then in theory you should be reading that at the inductor...
I think something like C40 is responsible for delaying the speakers being connected.
Check the voltage across C40
What is the voltage at the base of Q25?
Connector 5 pins 2 and 4 you get 5v? Yes
If you have the amp off you should be able to measure continuity from inductor 1 to pin 2 off connector 5 and the same with inductor 2 to pin 4. I see I should on the diagram but I don't when I try.
If you are measuring 5v correctly on pins 2 and 4 then in theory you should be reading that at the inductor... Strangely I don't see the 5V at the cokes. I've been measuring with negative probe on the negative speaker post but I tried the negative probe on the ground of the display board and got the same.
Voltage across C40, 18V
Voltage at the base of Q25, base 17.32V, collector 62.5V (does that sound right?) Emitter 16.75V. As the collector voltage seemed high to me I thought I'd report it.
If you have the amp off you should be able to measure continuity from inductor 1 to pin 2 off connector 5 and the same with inductor 2 to pin 4. I see I should on the diagram but I don't when I try.
If you are measuring 5v correctly on pins 2 and 4 then in theory you should be reading that at the inductor... Strangely I don't see the 5V at the cokes. I've been measuring with negative probe on the negative speaker post but I tried the negative probe on the ground of the display board and got the same.
Voltage across C40, 18V
Voltage at the base of Q25, base 17.32V, collector 62.5V (does that sound right?) Emitter 16.75V. As the collector voltage seemed high to me I thought I'd report it.
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