Sure. I took all the measurements without the 150R resistors and everything is ok. Output transistors are all within 10mV ± 1mV.
You did a great job so far, taking things apart and reassembling is not without risks, that's why small steps are needed to prevent later disappointments.
Now it's time to test the slow start circuit.
1) With the 4 blue caps discharged, measure the bias current at switch on.
An number of seconds the bias should be practically zero before the full bias will be activated. Output voltage should be around zero volt.
2) When switching off, measure when bias becomes zero again and then immediately measure the +/- Vcc unreg, it should be around +/- 40 Volt.
4) Let the 4 blue caps discharge
3) Connect a 180K resistor in parallel to the external 6K8 resistors that's connected to + Vcc Pre Reg
Now switch on and measure the LS output voltage, it should start at ca zero volt and after a few seconds go to ca 3 volt.
5) discharge the 4 caps again and change the 180k resistor to the other 6K8 resistor.
Repeat measuring the LS output voltage. It should go from zero voltage to -3V after a few seconds.
When this all works, your amp is so far 100% and you can start mounting the top board.
P.S. did you change the four large caps on the upper corners of the baseboard and the upper board. The can be trouble makers.
Hans
Now it's time to test the slow start circuit.
1) With the 4 blue caps discharged, measure the bias current at switch on.
An number of seconds the bias should be practically zero before the full bias will be activated. Output voltage should be around zero volt.
2) When switching off, measure when bias becomes zero again and then immediately measure the +/- Vcc unreg, it should be around +/- 40 Volt.
4) Let the 4 blue caps discharge
3) Connect a 180K resistor in parallel to the external 6K8 resistors that's connected to + Vcc Pre Reg
Now switch on and measure the LS output voltage, it should start at ca zero volt and after a few seconds go to ca 3 volt.
5) discharge the 4 caps again and change the 180k resistor to the other 6K8 resistor.
Repeat measuring the LS output voltage. It should go from zero voltage to -3V after a few seconds.
When this all works, your amp is so far 100% and you can start mounting the top board.
P.S. did you change the four large caps on the upper corners of the baseboard and the upper board. The can be trouble makers.
Hans
Thank you Hans. Actually, the channel I'm currently testing I still haven't fully disassembled. I thought of first testing it so if I find problems with the switch on circuit I can more easily diagnose it and repair.
Yes I'm going to replace the big silver caps. I was thinking of Panasonic FC 100V, what do you think? Alternatively I have Rubycon MXG 160V.
Does the 180K resistor need to be a specific wattage or a 1/2 watt one will do?
After switch on, it takes 3-4 seconds before bias quickly rises to about 30mV (it will then slowly settle to 27,5mV).
When switching of, bias quickly falls to zero volts in less then 1 second, Vcc unreg voltages are still about 70s Volts when bias goes to zero....
Yes I'm going to replace the big silver caps. I was thinking of Panasonic FC 100V, what do you think? Alternatively I have Rubycon MXG 160V.
Does the 180K resistor need to be a specific wattage or a 1/2 watt one will do?
After switch on, it takes 3-4 seconds before bias quickly rises to about 30mV (it will then slowly settle to 27,5mV).
When switching of, bias quickly falls to zero volts in less then 1 second, Vcc unreg voltages are still about 70s Volts when bias goes to zero....
Last edited:
Ok now I looked at the LS output and I'm worried.
First of all, this channel used to have 0,1 mV offset and all I did is remove the top board.
Now the DC offset is ca -800 mV when switched on and ca -200mV when switched off. I guess it will go towards zero as the caps discharge but never reach zero volts.
Then I looked at the LS output during turn on and turn off.
I did a video with the scope so it is easier than explaining. It is 0,5V per divisions so the spike gets to 4 volts it seems. Turn off video is a different time scale so the ripple isn't shown.
I think that this is how it used to be in both channels before I disassembled the amp, except for the DC offset which used to be 0 V on this channel (right), and 40mV slowly stabilizing at 0V on the other channel.
Seems that the slow start circuit is working since for a few seconds the output stays at zero, but then the amp still isn't stable when the output shows.
It could be that the two 6k8 resistors not being matched since when I switched them around I got +800mV offset
First of all, this channel used to have 0,1 mV offset and all I did is remove the top board.
Now the DC offset is ca -800 mV when switched on and ca -200mV when switched off. I guess it will go towards zero as the caps discharge but never reach zero volts.
Then I looked at the LS output during turn on and turn off.
I did a video with the scope so it is easier than explaining. It is 0,5V per divisions so the spike gets to 4 volts it seems. Turn off video is a different time scale so the ripple isn't shown.
I think that this is how it used to be in both channels before I disassembled the amp, except for the DC offset which used to be 0 V on this channel (right), and 40mV slowly stabilizing at 0V on the other channel.
Seems that the slow start circuit is working since for a few seconds the output stays at zero, but then the amp still isn't stable when the output shows.
It could be that the two 6k8 resistors not being matched since when I switched them around I got +800mV offset
Last edited:
What's the difference between Mov342 and Mov343
Don't worry about what you see on the LS output because the feedback is loop is open.
Output voltage depends on how equal the 6k8 resistors are ,the ripple on Vcc Pre Reg and a bit on the Hfe of the output transistors.
What you see is o.k.
The 180K resistor can be any resistor, 0.25Watt is o.k.
Hans
Don't worry about what you see on the LS output because the feedback is loop is open.
Output voltage depends on how equal the 6k8 resistors are ,the ripple on Vcc Pre Reg and a bit on the Hfe of the output transistors.
What you see is o.k.
The 180K resistor can be any resistor, 0.25Watt is o.k.
Hans
Connecting the 180k resistor in parallel to the 6k8 one gives me +2,4V and -1,4V of DC offset, repsectively. Again, switching around the two 6k8 resistor gives me +1,2V and -3V, respectively.
If all this is ok, I will then just replace old electrolytic and do general restoration on this board. I will reassemble the other channel board in a few days and will do the same tests, since I'm waiting for a few parts still.
Any specific tests I should do after mounting the top board, other then checking voltages and LS output?
Thank you again for your assistance.
Stefano
If all this is ok, I will then just replace old electrolytic and do general restoration on this board. I will reassemble the other channel board in a few days and will do the same tests, since I'm waiting for a few parts still.
Any specific tests I should do after mounting the top board, other then checking voltages and LS output?
Thank you again for your assistance.
Stefano
Sorry I forgot about that. So I repeated the test. After initial switch on output would stay still (approximately 100-200mV, same as it is with the amp switched off). After 2-3 seconds I see the ~ -2 V spike (not 4V as I previously said by mistake), after a fraction of a second the voltage stabilizes at +1,9V or -2,4V, with the 180k resistor on the positive or negative side, respectively (I also changed 6k8 resistors to a better matched pair to get closer voltages between pos and negative).
This pot is meant to adjust the offset of the inverting input amplifier.
But since this amp is ac coupled it makes no difference on the LS output offset. Left from P1008 you see a 14k7 resistor sitting between two 10uF blue caps.
On the left side of this resistor you can measure the offset and if needed adjust it with the pot.
Hans
But since this amp is ac coupled it makes no difference on the LS output offset. Left from P1008 you see a 14k7 resistor sitting between two 10uF blue caps.
On the left side of this resistor you can measure the offset and if needed adjust it with the pot.
Hans
