Those readings are looking more realistic.
Yes, you do need to turn the bias to minimum and if that doesn't work then I'm going to suggest you either unsolder or just snip out (cut one side) of R754 which was 4k7 and I think we changed to 3k9.
If you cut it out you force a zero bias condition. The amp should run in that state.
If it does then go back to the original 4k7.
I did wonder about those BD's being fitted incorrectly. Hopefully they are OK though.
Yes, you do need to turn the bias to minimum and if that doesn't work then I'm going to suggest you either unsolder or just snip out (cut one side) of R754 which was 4k7 and I think we changed to 3k9.
If you cut it out you force a zero bias condition. The amp should run in that state.
If it does then go back to the original 4k7.
I did wonder about those BD's being fitted incorrectly. Hopefully they are OK though.
Minimum bias = Max resistance on the trimmer (R756), correct?
If that's correct, then the trimmer is already set to minimum bias.
I'll remove/cut R754, retest and report back.
R754 replaced with brand new 4K7 resistor (as I lost the original one).
Results of voltage tests from R768 centre to base of:
Upper transistors:
Q762 = 739mV
Q758 = 1.403V
Q754 = 2.004V
Lower transistors:
Q764 = -692mV
Q760 = -1.321V
Q756 = -1.947V
Serial lamp limiter bulb is on and bright, but does not flicker. Power LED is on and does not flicker.
Those readings look to be in the right ball park. Good 
Assuming the bias preset is set for max resistance (minimum current) then it looks like we need to tweak the value of the 4k7.
Three things to do because information is everything.
1/ Measure the voltage across the two end pins of R768 while the bulb is bright. That will let us calculate the value of current that is flowing now. Useful to know.
2/ Do a quick measure of the supply rails. How much lower than normal are they with the bulb lit. The fact the power LED is on is a good sign... it suggests the rails are fairly high.
3/ You can either snip or lift one end of R754 (the 4k7) or 'risk' increasing its value to say 5k6 or 6k8 (risk on the basis its all now OK, not that anything will go pop).
Snipping is the easiest. If all is good then the current should fall and the bulb dim. If that happens then the resistor has to be altered anyway. If it doesn't dim then there is a further issue. So up to you on that one
I would just snip it and see if the bulb dims.
Also remember that the good channel is drawing current and so that will add to the bulb brightness.
Assuming the bias preset is set for max resistance (minimum current) then it looks like we need to tweak the value of the 4k7.
Three things to do because information is everything.
1/ Measure the voltage across the two end pins of R768 while the bulb is bright. That will let us calculate the value of current that is flowing now. Useful to know.
2/ Do a quick measure of the supply rails. How much lower than normal are they with the bulb lit. The fact the power LED is on is a good sign... it suggests the rails are fairly high.
3/ You can either snip or lift one end of R754 (the 4k7) or 'risk' increasing its value to say 5k6 or 6k8 (risk on the basis its all now OK, not that anything will go pop).
Snipping is the easiest. If all is good then the current should fall and the bulb dim. If that happens then the resistor has to be altered anyway. If it doesn't dim then there is a further issue. So up to you on that one
I would just snip it and see if the bulb dims.Also remember that the good channel is drawing current and so that will add to the bulb brightness.
Those readings look to be in the right ball park. Good
Assuming the bias preset is set for max resistance (minimum current) then it looks like we need to tweak the value of the 4k7.
Three things to do because information is everything.
1/ Measure the voltage across the two end pins of R768 while the bulb is bright. That will let us calculate the value of current that is flowing now. Useful to know.
The voltage across R768 = 100mV
Where should I measure the supply rails? Should I be measuring J801 or collector of Q702 and Q704?
3/ You can either snip or lift one end of R754 (the 4k7) or 'risk' increasing its value to say 5k6 or 6k8 (risk on the basis its all now OK, not that anything will go pop).
Before I replaced R754, I powered on the amp and the bulb did dim; so no need to remove or snip it again.
Snipping is the easiest. If all is good then the current should fall and the bulb dim. If that happens then the resistor has to be altered anyway. If it doesn't dim then there is a further issue. So up to you on that one
Also remember that the good channel is drawing current and so that will add to the bulb brightness.
So 100mv across 0.2 ohms gives 0.5 amp. This is where the bulb tester saves tears at bedtime.
The supply can be measured on any of the collectors of those six transistors in the output stage... but be careful, no slips with the probe.
Why not just snip that 4k7 and see if the bulb dims. That's the end result we want.
The supply can be measured on any of the collectors of those six transistors in the output stage... but be careful, no slips with the probe.
Why not just snip that 4k7 and see if the bulb dims. That's the end result we want.
The voltage on the collector (w/ ref to ground):
Q762 = 13.14V
Q764 = -13.15V
Before I replaced R754, I powered on the amp and the bulb did dim; so no need to remove or snip it again.
Thanks. So they are fairly low and that is because of the current draw in the output transistors.
So at this point all still looks OK given those earlier readings on the transistors, but we must get that current down to essentially zero before applying full mains. Otherwise it will just self destruct.
Quickest way to prove all is good is to snip the resistor. Its got to be done and the result of that will leave us with two choices.
1/ The current falls away and that essentially means its all going to work OK once we tweak that resistor value.
2/ The current doesn't fall away... that would mean more tests as something would still be amiss.
I'm hoping its the first option that its basically fixed.
So at this point all still looks OK given those earlier readings on the transistors, but we must get that current down to essentially zero before applying full mains. Otherwise it will just self destruct.
Quickest way to prove all is good is to snip the resistor. Its got to be done and the result of that will leave us with two choices.
1/ The current falls away and that essentially means its all going to work OK once we tweak that resistor value.
2/ The current doesn't fall away... that would mean more tests as something would still be amiss.
I'm hoping its the first option
that its basically fixed.Thanks. So they are fairly low and that is because of the current draw in the output transistors.
So at this point all still looks OK given those earlier readings on the transistors, but we must get that current down to essentially zero before applying full mains. Otherwise it will just self destruct.
Quickest way to prove all is good is to snip the resistor. Its got to be done and the result of that will leave us with two choices.
1/ The current falls away and that essentially means its all going to work OK once we tweak that resistor value.
2/ The current doesn't fall away... that would mean more tests as something would still be amiss.
I'm hoping its the first option
I have snipped R754 as per your request and now the voltage across R768 = 0V
That's the result we wanted.
You should find that the bulb is now very dim and that the rails have come up to near normal values.
There should be no significant DC voltage present (as measured from ground) to the middle pin of R768.
All that means its basically all fixed and working correctly.
All that remains to be done is select a suitable value to replace that 4k7 with, and I'm afraid its a little bit trial and error for that.
A 6k8 might be a good first 'guess' and allow the trimmer to set the bias current correctly.
You need a value that allows the voltage across R768 to be turned to zero (as you now have) and yet allows the trimmer to set the required voltage across that resistor.
Does that make sense?
You should find that the bulb is now very dim and that the rails have come up to near normal values.
There should be no significant DC voltage present (as measured from ground) to the middle pin of R768.
All that means its basically all fixed and working correctly.
All that remains to be done is select a suitable value to replace that 4k7 with, and I'm afraid its a little bit trial and error for that.
A 6k8 might be a good first 'guess' and allow the trimmer to set the bias current correctly.
You need a value that allows the voltage across R768 to be turned to zero (as you now have) and yet allows the trimmer to set the required voltage across that resistor.
Does that make sense?
And also... keep the bulb in place at this stage.
When you are happy the bias trimmer is working OK and with a suitable range we then turn the bias back down before finally applying full mains and resetting it for real.
Ideally you want a value for the resistor that allows the bias to be correct when the trimmer is turned to around 50% rotation.
I think we said the correct bias is when you have around 8 millivolts between the end pins of R768.
Slow and steady I'll look in tomorrow.
When you are happy the bias trimmer is working OK and with a suitable range we then turn the bias back down before finally applying full mains and resetting it for real.
Ideally you want a value for the resistor that allows the bias to be correct when the trimmer is turned to around 50% rotation.
I think we said the correct bias is when you have around 8 millivolts between the end pins of R768.
Slow and steady
I'll look in tomorrow.And also... keep the bulb in place at this stage.
When you are happy the bias trimmer is working OK and with a suitable range we then turn the bias back down before finally applying full mains and resetting it for real.
Ideally you want a value for the resistor that allows the bias to be correct when the trimmer is turned to around 50% rotation.
I think we said the correct bias is when you have around 8 millivolts between the end pins of R768.
Slow and steady
I have purchased a number of common values of resistors. I bought 5k6, 6k2, 6k8 & 7k5; done some tests then decided to buy some 5k1 resistors.
Here are the results:
R754 = 5k6 ohm
Adjusting R764 = 0 to 13mv across R768
R754 = 5k1 ohm
Adjusting R764 = 0 to 45mv across R768
R754 = 4k7 ohm
Adjusting R764 = 0 to 100mv across R768
I think 5k1 ohm would best to use, what do you think Mooly?
I currently have a 5k1 ohm resistor on R754 and have adjusted R764 so that there is 8mV across R768. Also the resistance on R764 = 503.7 ohm.
That all sounds very good and it would appear its all going to work 
and yes, use the 5k1
Here is what you do next.
1/ Turn the bias back to zero.
2/ Remove the bulb and apply full mains.
3/ Reset the bias back to give the 8 millivolts. Leave the amp on and allow the heatsink to stabilise in temperature for 10 minutes. Make sure the 8 millivolts doesn't rise to high. It will vary a bit which is fine, what we don't want is it going up and up on its own.
Remember there is no safety net now (the bulb) and so be sure you have the meter connected correctly and securely at all times as you carefully turn the preset.
4/ Final check that there is no DC voltage at the speaker terminals.
Now connect speakers and test the amp.
and yes, use the 5k1Here is what you do next.
1/ Turn the bias back to zero.
2/ Remove the bulb and apply full mains.
3/ Reset the bias back to give the 8 millivolts. Leave the amp on and allow the heatsink to stabilise in temperature for 10 minutes. Make sure the 8 millivolts doesn't rise to high. It will vary a bit which is fine, what we don't want is it going up and up on its own.
Remember there is no safety net now (the bulb) and so be sure you have the meter connected correctly and securely at all times as you carefully turn the preset.
4/ Final check that there is no DC voltage at the speaker terminals.
Now connect speakers and test the amp.
That all sounds very good and it would appear its all going to work
Here is what you do next.
1/ Turn the bias back to zero.
2/ Remove the bulb and apply full mains.
3/ Reset the bias back to give the 8 millivolts. Leave the amp on and allow the heatsink to stabilise in temperature for 10 minutes. Make sure the 8 millivolts doesn't rise to high. It will vary a bit which is fine, what we don't want is it going up and up on its own.
Remember there is no safety net now (the bulb) and so be sure you have the meter connected correctly and securely at all times as you carefully turn the preset.
4/ Final check that there is no DC voltage at the speaker terminals.
Now connect speakers and test the amp.
5k1 not soldered into R754.
Here's the results with full mains:
1) reset bias to zero.
2) powered on amp with full mains.
3) Adjusted R764 to get 8mV across R768.
4) set timer for 10 mins.
5) after 4-5 mins voltage across R768 drops to 6mV.
6) after 10 mins, checked for voltage on the speaker terminals, and there isn't any.
Question:
Should i adjust the bias so it starts at 10mV, so it can then settle at 8mV?
That's a pretty good result actually and the fall in current is simply due to the sensing transistor warming on the heatsink. Exactly what should happen on a design like this.
The difference between 8mv and 6mv won't give any audible difference and so I would suggest leaving it as it is.
Time to give it a listen methinks
The difference between 8mv and 6mv won't give any audible difference and so I would suggest leaving it as it is.
Time to give it a listen methinks
That's a pretty good result actually and the fall in current is simply due to the sensing transistor warming on the heatsink. Exactly what should happen on a design like this.
The difference between 8mv and 6mv won't give any audible difference and so I would suggest leaving it as it is.
Time to give it a listen methinks
Mooly
The AMP is now working properly.
Two nights ago I had the amp running for several hours straight during the evening with no issues.
I downloaded the tracks from:
10 of the best songs to test your speakers | What Hi-Fi?
and played them using a raspberry pi (w/ DAC hat) and moode audio player. It sounded great.
The voltage across R768 fluctuated a little, ranging from 6mV to 11mV, usually depending on the volume.
My client now whats me to do all the mods from this post:
Marantz PM66 KI tweeks
And finally:
I just want to say a massive heart-filled thank you for last 5 months (on and off) of help you have (certainly) gave me; and hopefully others too.
You are are a huge credit to this community!
I have posted on other forums and on one particular forum the first response was just a moan about not following one or more particular petty rules.
Thanks for the kind words... its really appreciated and great to hear that your amp is all up and running now.
I'm not sure what to think of that modification list, you would need to look very very carefully at what is involved. Black Gate caps have been out of production for well over a decade... and there is a lot of scope for things to go wrong when undertaking something like that.
and great to hear that your amp is all up and running now.I'm not sure what to think of that modification list, you would need to look very very carefully at what is involved. Black Gate caps have been out of production for well over a decade... and there is a lot of scope for things to go wrong when undertaking something like that.
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