Not yes, I suspect...
there may be other components gone bad.
I made some voltage measurements shown in the attached jpeg.
The 24 V measures ok but the neg 18 Voltage reads as negative 0.5 V
I assume that something has failed a wee bit back along the line and I assume I should check the circuit that should produce the neg. 18 Volts.
there may be other components gone bad.
I made some voltage measurements shown in the attached jpeg.
The 24 V measures ok but the neg 18 Voltage reads as negative 0.5 V
I assume that something has failed a wee bit back along the line and I assume I should check the circuit that should produce the neg. 18 Volts.
Attachments
-0.5v is the voltage you will get if the 18v zener diode (B805 it think) was short cct.
First check if you have -27 volts on the collector of Q802. Then check the voltage on the its base. I'll bet its 0v.
Finally with the amp turned off put your meter on the diode check scale and measure across the zener. A good one will measure open cct one way and 0.6v the other. Replace the zener before you turn the amp on again, ie don't turn it on with the zener out.
Cheers
First check if you have -27 volts on the collector of Q802. Then check the voltage on the its base. I'll bet its 0v.
Finally with the amp turned off put your meter on the diode check scale and measure across the zener. A good one will measure open cct one way and 0.6v the other. Replace the zener before you turn the amp on again, ie don't turn it on with the zener out.
Cheers
Hi Quasi,
Alan, If the area around a capacitor is running warm (discoloured PCB), it might be better to just replace it part. The capacitor might be good, but they are not generally an expensive part. For you, it might be a lot of trouble to get.
-Chris
I second that! It's very easy to forget to put the diode back in, especially if you take so much as a phone call in between.
Alan, If the area around a capacitor is running warm (discoloured PCB), it might be better to just replace it part. The capacitor might be good, but they are not generally an expensive part. For you, it might be a lot of trouble to get.
-Chris
Are you able to isolate the power supply outputs from the rest of the works?
We need to prove that the power supply is not being dragged down by a short elswhere. So lets isolate an entire module first and prove the fault in or out from that.
Unfortunately -0.5 could also be the voltage a shorted IC drags a power supply down to.
Good luck.
We need to prove that the power supply is not being dragged down by a short elswhere. So lets isolate an entire module first and prove the fault in or out from that.
Unfortunately -0.5 could also be the voltage a shorted IC drags a power supply down to.
Good luck.
I figured it out..the 18 V is for operating a tape connection..
Anyway...if you read back at the start you'll see this is all new to me...electronics..but for the past week I've been getting a grounding in practical application. I know I have a lot to learn but it is fascinating.
So....according to your approach how do I proceed?
Anyway...if you read back at the start you'll see this is all new to me...electronics..but for the past week I've been getting a grounding in practical application. I know I have a lot to learn but it is fascinating.
So....according to your approach how do I proceed?
Re: Isolating the power supply..
Hi,
You posted this while I was typing and so my posted startin with "I don't know" was in response to the previous post.
If the DC power supply components are on the same board then isolating this from the rest is not as easy.
But you seem to have enough information to proceed anyway.
What we need are the voltages between components. The reason for this is you may have a good component but a faulty connection.
Can you please connect your voltmeter to the + of C802 or the transformers center-tap and measure along R803, R804 and R810. We need to know the voltages at each junction between these components.
Cheers
hotscot said:Do you mean the power lead coming from the transformer and going to the board? It's a three pin connection that can be easily disconnected from the pcb. It's certainly providing the correct voltages,
(If that's what you mean
Hi,
You posted this while I was typing and so my posted startin with "I don't know" was in response to the previous post.
If the DC power supply components are on the same board then isolating this from the rest is not as easy.
But you seem to have enough information to proceed anyway.
What we need are the voltages between components. The reason for this is you may have a good component but a faulty connection.
Can you please connect your voltmeter to the + of C802 or the transformers center-tap and measure along R803, R804 and R810. We need to know the voltages at each junction between these components.
Cheers
Progress of sorts. I found a broken trace and managed a repair.
I now get the 18 volt output successfully
However....a resistor began heating up..and I mean a nice rosy red colorsort of heat.
If any of you guys are curious it was R771 on the schematic.
R772 on the other hand was fine.
If this behaviour indicates catastrophic damage I'm willing to stop.
I now get the 18 volt output successfully
However....a resistor began heating up..and I mean a nice rosy red colorsort of heat.
If any of you guys are curious it was R771 on the schematic.
R772 on the other hand was fine.
If this behaviour indicates catastrophic damage I'm willing to stop.
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