I measured left and right terminals without the speakers in.
On power on/off the left side spikes when the relay clicks and shows OL for a moment before falling back down. It shows OL on both mV and V scales so it must be a pretty big spike.
The right channel only peaks in the tens of mVs so nowhere near the left channel.
On power on/off the left side spikes when the relay clicks and shows OL for a moment before falling back down. It shows OL on both mV and V scales so it must be a pretty big spike.
The right channel only peaks in the tens of mVs so nowhere near the left channel.
Hmm... this could be very tricky to diagnose but lets investigate a couple of things first.
How many seconds (about) does the relay need before it clicks in?
How long after it clicks in does it take for the voltage to fall back to near zero?
Remind me where we are with the bias settings... are we on minimum bias with the preset at 500 ohms for both channels and neither bias transistor linked out?
If so then do you want to try giving the left channel a little bit of bias current and see if it changes the problem at all.
How many seconds (about) does the relay need before it clicks in?
How long after it clicks in does it take for the voltage to fall back to near zero?
Remind me where we are with the bias settings... are we on minimum bias with the preset at 500 ohms for both channels and neither bias transistor linked out?
If so then do you want to try giving the left channel a little bit of bias current and see if it changes the problem at all.
It takes about 2/3 seconds after powering on.
About the same time to fall down again, 2/3 seconds.
left channel is 500 ohms.
right channel we didn't play with.
Yes, I will give it a try
Brought it up to 5.3mV stable over 8mins or so.
No change in the voltage spiking at the terminals though.
Right then... I'll have to have a think on that one (tomorrow probably )
First thoughts are that we need to quickly isolate the power amp inputs to be sure nothing is going on there. That would be done by linking out either Q329 or Q330 from collector to emitter.
That would apply a short at the input of the amplifier and prove if the problem was with the power amp itself or whether it was something going on in front of it.
Beyond that I would try and look more closely at what the original mishap might have done... those two reversed connectors you mentioned but that needs a bit of figuring out.
One point though, once the offset at the speaker is low (near zero) then it shouldn't really make a noise when you power off if the relay drops out instantly... although instant is open to interpretation I know.
To be continued then...
)First thoughts are that we need to quickly isolate the power amp inputs to be sure nothing is going on there. That would be done by linking out either Q329 or Q330 from collector to emitter.
That would apply a short at the input of the amplifier and prove if the problem was with the power amp itself or whether it was something going on in front of it.
Beyond that I would try and look more closely at what the original mishap might have done... those two reversed connectors you mentioned but that needs a bit of figuring out.
One point though, once the offset at the speaker is low (near zero) then it shouldn't really make a noise when you power off if the relay drops out instantly... although instant is open to interpretation I know.
To be continued then...
Right then... I'll have to have a think on that one (tomorrow probably
First thoughts are that we need to quickly isolate the power amp inputs to be sure nothing is going on there. That would be done by linking out either Q329 or Q330 from collector to emitter.
That would apply a short at the input of the amplifier and prove if the problem was with the power amp itself or whether it was something going on in front of it.
Beyond that I would try and look more closely at what the original mishap might have done... those two reversed connectors you mentioned but that needs a bit of figuring out.
One point though, once the offset at the speaker is low (near zero) then it shouldn't really make a noise when you power off if the relay drops out instantly... although instant is open to interpretation I know.
To be continued then...
Alright, Thanks Mooly
About the noise when powering off - despite the offset being low - when powering off the voltage spikes to OL just like it does when powering on.
Could you try one more quick test.
On the good channel, do you see any similar kind of spike in voltage if you measure before the relay? Same for when you power up.
You can easily measure on that on either end of R20 on the relay board.
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When this is powered off the relay should drop out 'instantly' and it can do that because the relay driver board (that chip) looks for the AC going missing.
So it seems that instant isn't quite instant (and that's normal, there will be a few tens of milliseconds delay) and during that time as the rails fall the offset voltage changes.
So is it a power amp problem or a sudden change in input voltage. Try linking out those mute transistors and see if it alters the fault.
On the good channel, do you see any similar kind of spike in voltage if you measure before the relay? Same for when you power up.
You can easily measure on that on either end of R20 on the relay board.
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When this is powered off the relay should drop out 'instantly' and it can do that because the relay driver board (that chip) looks for the AC going missing.
So it seems that instant isn't quite instant (and that's normal, there will be a few tens of milliseconds delay) and during that time as the rails fall the offset voltage changes.
So is it a power amp problem or a sudden change in input voltage. Try linking out those mute transistors and see if it alters the fault.
Attachments
Well, measuring across R20 it does seem to spike but at a different time. It shows OL once during the first time protection/bulb lights cycle stage then on the subsequent bulb lights (usually 2 or 3 times) it shows a lower voltage and finally when the relay closes it does not spike. This is when the left side spikes - when the relay closes.
I linked Q330...
Before hooking up the speaker I measured at the terminals.
Now the protection/bulb lighting cycle takes much longer (6 or 7 times) before the relays close.
When they close it looked like a smaller spike unless i'm mistaken. Before the short i'm fairly sure the DM showed OL in the V range as well as mV range. With the short OL did not appear with V but only with mV.
After trying with the speaker it still causes the pop.
I used the min/max feature on the DM to check:
power on...
Bad side: Max => 1.3V ... Min => -3.3V
Good side: Max => 1.16V ... Min => -1.17v
power off...
Bad side: Max => OL ... Min => -0.02V
Good side: Max => 0.04V ... Min => -0.01V
So the powering off spike is clearly different - but powering on is not such a huge difference but there only a pop on the bad side.
Thanks for all those readings... and I'm struggling to think where we should begin with this tbh.
You haven't got it accidently set to bridged mode (little switch at the back) ? Nope, didn't think so...
I think we also have to try this on full mains before getting to heavy delving any further. You should turn the bias back down first for safety though.
This may sound a strange thing to do but when the amp is on and the offset is near zero do you still hear a pop if you select/deselect the speakers using the front panel switches?
I'm just wondering if there is any kind of instability present which wouldn't show on a DVM (that needs a scope). That could cause a DC shift in the output but because it is an AC quantity you could think you are seeing zero volts DC on a meter when in fact there is an asymmetric high frequency signal present. That's a bit of a far out theory however
You haven't got it accidently set to bridged mode (little switch at the back) ? Nope, didn't think so...
I think we also have to try this on full mains before getting to heavy delving any further. You should turn the bias back down first for safety though.
This may sound a strange thing to do but when the amp is on and the offset is near zero do you still hear a pop if you select/deselect the speakers using the front panel switches?
I'm just wondering if there is any kind of instability present which wouldn't show on a DVM (that needs a scope). That could cause a DC shift in the output but because it is an AC quantity you could think you are seeing zero volts DC on a meter when in fact there is an asymmetric high frequency signal present. That's a bit of a far out theory however
hmm, that short in the left channel really does seem to affect the right channel too!
with short..
Bad side: Max => 3.3 ... Min => -2.03
Good side: Max => 1.13 ... Min => -1.17
without:
Bad side: Max => 1.14V ... Min => -3.23V
Good side: Max => 0.008V ... Min => -0.072V
The above are just for powering on.
with short..
Bad side: Max => 3.3 ... Min => -2.03
Good side: Max => 1.13 ... Min => -1.17
without:
Bad side: Max => 1.14V ... Min => -3.23V
Good side: Max => 0.008V ... Min => -0.072V
The above are just for powering on.
Does the right channel audibly pop with the short in place?
The output side of the relay is effectively floating until the contacts close so you have to measure from ground and to R20 or R21 to get a true result. It doesn't matter which end of the resistor you measure to as they are across a coil (which appears as a short at DC).
The output side of the relay is effectively floating until the contacts close so you have to measure from ground and to R20 or R21 to get a true result. It doesn't matter which end of the resistor you measure to as they are across a coil (which appears as a short at DC).
It remains silent with or without - which baffled me after seeing the measurements.
Good, that's how I was measuring.