Board A test results; 00.7Ω I could not get it any lower 
Have moved pre-amp, cd player to bench and set all up ready, with board C at eye level (good to see and work on) propped up against the cabinet all ready for tomorrow
Very pleased about having to read e (emitter) values only
I had a paid job this morning; customer let the oil tank run empty, hence a lot of airlocks.
Have moved pre-amp, cd player to bench and set all up ready, with board C at eye level (good to see and work on) propped up against the cabinet all ready for tomorrow
Very pleased about having to read e (emitter) values only
I had a paid job this morning; customer let the oil tank run empty, hence a lot of airlocks.
The airlocks sound like fun 
OK... Board A. Slightly confused. We should be measuring millivolts (AC) here when the tone is playing.
OK... Board A. Slightly confused. We should be measuring millivolts (AC) here when the tone is playing.
No problem
Now this looks interesting. 0.095 vac is 95 millivolts. That's in the right ballpark for a line level signal. 0.007 vac is just 7 millivolts, so where has the signal gone ?
As I mentioned earlier, the level will be a bit lower, but not by that amount. Just a few percentage points lower, thats all.
This needs investigating. At this point I'm going to suggest you double check the result and maybe confirm with your other meter. Lets prove beyond doubt that these initial readings are good. The only doubt would be that the meter is upset by DC present on the emitters (mine wasn't... I would expect yours not to be either). We have to be 100% certain on this though.
Assuming the readings are correct we are back to looking at the area of the control.
Next steps would be to check the level at the other end of resistor 3404. You should be seeing around 80 millivolts or more signal there. Then check on the wiper of the pot. It should be the same as you get on 3404.
I'll be out for much of the morning but this looks promising
Now this looks interesting. 0.095 vac is 95 millivolts. That's in the right ballpark for a line level signal. 0.007 vac is just 7 millivolts, so where has the signal gone ?
As I mentioned earlier, the level will be a bit lower, but not by that amount. Just a few percentage points lower, thats all.
This needs investigating. At this point I'm going to suggest you double check the result and maybe confirm with your other meter. Lets prove beyond doubt that these initial readings are good. The only doubt would be that the meter is upset by DC present on the emitters (mine wasn't... I would expect yours not to be either). We have to be 100% certain on this though.
Assuming the readings are correct we are back to looking at the area of the control.
Next steps would be to check the level at the other end of resistor 3404. You should be seeing around 80 millivolts or more signal there. Then check on the wiper of the pot. It should be the same as you get on 3404.
I'll be out for much of the morning but this looks promising
Retest using the Fluke again gives a different set of readings; C402 .052 vac, 6400 emitter .004 vac, 6401 emitter .006 vac (using the Mastech meter gives these readings C402 .092 vac, 6400 emitter .015 vac, 6401 emitter .015 vac).
Have set-aside the Mastech meter and will continue to use the Fluke.
440 Hz test tone result other side of 3404 leading to 2401 gives .050vac
Have set-aside the Mastech meter and will continue to use the Fluke.
440 Hz test tone result other side of 3404 leading to 2401 gives .050vac
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This is definitely worthy of investigation because something isn't right. The signal is way to low.
I think the Fluke is the better meter for this, its certainly the one I would trust given the choice. I'm sure we are onto something now
This is just for information so you can see what we should be registering on the meter. The diagram (its actually a working simulation) shows the input stage of the Philips and I've marked the parts up with the same numbers for easy reference.
The pot is on full which means 3406 connects directly to cap 2401.
At the right is a cap connecting to 'your meter'. That's just to remove the DC from the simulation so that the traces line up and can be compared... your meter should be doing this already.
You can see the input voltage (Green trace C402) and the voltage on the transistor (Blue trace marked 6400_Emitter_as_read_on_meter) should be virtually identical signal wise and yours are not.
This limits our search at this stage to the pot (remember I said it should really be removed to preclude the possibility of some weird internal short), and to capacitor 2400 (it could be leaky and pulling the signal down) or something odd around the transistor.
Easiest ways to find out:
Isolate cap 2400. If its the cause the level will come up on the transistors.
If that is no good then if you isolate 3406 then you should read the full signal voltage on pin 2 of the pot (its middle leg). If you do then the problem is around the first transistor. If you don't then the problem is most likely the pot and it needs to be removed to prove it.
(why 3401 and 3403 are not in the simulation... because they can only interact with the preamp side of things and we know by measurement the signal is present on C402)
I think the Fluke is the better meter for this, its certainly the one I would trust given the choice. I'm sure we are onto something now
This is just for information so you can see what we should be registering on the meter. The diagram (its actually a working simulation) shows the input stage of the Philips and I've marked the parts up with the same numbers for easy reference.
The pot is on full which means 3406 connects directly to cap 2401.
At the right is a cap connecting to 'your meter'. That's just to remove the DC from the simulation so that the traces line up and can be compared... your meter should be doing this already.
You can see the input voltage (Green trace C402) and the voltage on the transistor (Blue trace marked 6400_Emitter_as_read_on_meter) should be virtually identical signal wise and yours are not.
This limits our search at this stage to the pot (remember I said it should really be removed to preclude the possibility of some weird internal short), and to capacitor 2400 (it could be leaky and pulling the signal down) or something odd around the transistor.
Easiest ways to find out:
Isolate cap 2400. If its the cause the level will come up on the transistors.
If that is no good then if you isolate 3406 then you should read the full signal voltage on pin 2 of the pot (its middle leg). If you do then the problem is around the first transistor. If you don't then the problem is most likely the pot and it needs to be removed to prove it.
(why 3401 and 3403 are not in the simulation... because they can only interact with the preamp side of things and we know by measurement the signal is present on C402)
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Lol, just take it slowly. I'm hoping we might pin this down very shortly.
I'll be back in a little while.
I'm guessing this reading is OK. I want you to try something now you are getting familiar with measuring signal levels. We have around 36 millivolts on C402. I want you to switch the speaker OFF and unplug the signal lead from it.
Now play the tone and measure the signal level at the 'plug end' of that lead. Just connect the meter across the outer shield and inner pin (black lead on shield) and see how it compares to the 36 millivolts you had before.
I'm wondering if something right at the front end of the speaker is loading the preamp excessively.
I'll be back in a little while.
I'm guessing this reading is OK. I want you to try something now you are getting familiar with measuring signal levels. We have around 36 millivolts on C402. I want you to switch the speaker OFF and unplug the signal lead from it.
Now play the tone and measure the signal level at the 'plug end' of that lead. Just connect the meter across the outer shield and inner pin (black lead on shield) and see how it compares to the 36 millivolts you had before.
I'm wondering if something right at the front end of the speaker is loading the preamp excessively.
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