Mooly I suspected my small fluctuations might be due to the amp not being fully warmed up. In fact the manual says turn VR3 and VR4 all the way to the left and let amp warm up for 10 minutes. But I've noticed when those trimmers are at that position full current is flowing thru the board and the heat sinks get really hot. Too hot to touch. Saw this with my amp as well as this one. So I'm afraid to leave it in that state for that long. Once voltage is dialed in the heat sinks temp drops considerably.
I get nervous when I read 'turn CCW or turn CW, or turn to left, turn to right' because it depends which way round you look at them.
The bias presets should be initially set to minimum resistance which will give minimum current in the output stage. What you do then is turn them up slowly bringing the bias up in a couple of stages to the correct value leaving time between each adjustment for the amp to equalise its temperature.
You are aiming to get the bias at the stated value when the amp is thoroughly warm (say 1 hour with its cover in place). As soon as you take the top off the bias is going to change again so be quick measuring... its only ever a compromise I'm afraid.
We know far more about designing amps that can maintain a stable bias current today than back when these were designed. You just have to get it as near as practicable. You will find if you play music and then recheck the bias that it will be different yet again... you can't do anything about that with this design I'm afraid.
The bias presets should be initially set to minimum resistance which will give minimum current in the output stage. What you do then is turn them up slowly bringing the bias up in a couple of stages to the correct value leaving time between each adjustment for the amp to equalise its temperature.
You are aiming to get the bias at the stated value when the amp is thoroughly warm (say 1 hour with its cover in place). As soon as you take the top off the bias is going to change again so be quick measuring... its only ever a compromise I'm afraid.
We know far more about designing amps that can maintain a stable bias current today than back when these were designed. You just have to get it as near as practicable. You will find if you play music and then recheck the bias that it will be different yet again... you can't do anything about that with this design I'm afraid.
Amplifiers are designed to reject noise and ripple on the rails (the PSRR or power supply rejection ratio) and so on a good amplifier, any ripple present on the rails is highly rejected and does not appear at the output.
And there is no correlation between AC rail ripple and DC offset voltage. Some meters can react a little unpredictably when trying to measure small a small AC component riding on a much larger DC voltage.
I hear you Mooly. This time around I'm going to leave trimmer where it is. Near right and let it warm up. Then I'll check it and adjust. Then I'll check those .5 ohms and L1/L2 again.
You'll never stop the bias varying, in fact it might realistically only stay within 50% of a nominal value as the amplifier heats and cools. The DC offset will also vary a little with temperature and rail voltage changes... not by much but enough to notice in measurement.
Mooly, to be clear on my last measurements, I am seeing the same voltage at L1/L2 as I am at the .5 ohm resistors (and was able to trace the common path on the board). Left channel is fluctuating only a few mV either side of -3mV. Right channel has a bit more fluctuation -4mV to -15mV after the amp has warmed up.
But as you said, and having read a few posts online about fluctuating voltage being common with the older gear, this may be normal. My fear is that I feel I may have introduced this fluctuation as I did not notice it before replacing resistors, caps and trim pots.
After warming things up, I am still seeing steady +36.9v at points 17 and 45, and -44v at points 13 and 16. I also checked the Collectors on Q5-10 and they appear to match closely with those listed on the circuit board diagram on page 47 (+44 and -44v where appropriate).
But as you said, and having read a few posts online about fluctuating voltage being common with the older gear, this may be normal. My fear is that I feel I may have introduced this fluctuation as I did not notice it before replacing resistors, caps and trim pots.
After warming things up, I am still seeing steady +36.9v at points 17 and 45, and -44v at points 13 and 16. I also checked the Collectors on Q5-10 and they appear to match closely with those listed on the circuit board diagram on page 47 (+44 and -44v where appropriate).
I don't think you will have done anything to cause a 'fluctuation'. Honestly 🙂
The reason I kept mentioning L1/L2 is because a little way back you mentioned seeing 0.6 volts or so on these resistors. One end of each 0.5 ohm goes to L1/L2 and so the voltage at this common point has to be the same as at the speaker terminals. The other end of each 0.5 ohm resistor will be either above or below the voltage on L1/L2 by an amount that depends on the bias current flowing.
The reason I kept mentioning L1/L2 is because a little way back you mentioned seeing 0.6 volts or so on these resistors. One end of each 0.5 ohm goes to L1/L2 and so the voltage at this common point has to be the same as at the speaker terminals. The other end of each 0.5 ohm resistor will be either above or below the voltage on L1/L2 by an amount that depends on the bias current flowing.
OK, I have to apologize for the confusing numbers. While I originally saw the .6V at the .5 ohms, that must have been my measurement before replacing things. Now I see the fluctuating voltage at L1/L2, .5 ohms and speaker outputs (-1 to -5mV left, -4 to -15mV right).
So the question is: Are these numbers acceptable or should I be looking at other components on the amp board? As I said earlier, after replacing things, I played the amp for a few hours and it sounded just fine. No static or crackling from either channel.
So the question is: Are these numbers acceptable or should I be looking at other components on the amp board? As I said earlier, after replacing things, I played the amp for a few hours and it sounded just fine. No static or crackling from either channel.
No problem 🙂
Those voltages are absolutely fine for this type of amplifier. You'll not improve the DC stability any more.
Those voltages are absolutely fine for this type of amplifier. You'll not improve the DC stability any more.
Thanks Mooly for all your help (again)! I'll do some more burn-in testing at higher volumes to see if things are stable and the click-off issue the owner was seeing is no longer a problem. I'm also waiting for a new power lamp to arrive as his is burnt out...
Hi Mooly, so I've got a new pilot lamp for power-on and was checking the circuit voltage that is supplied to the bulb and I'm seeing a fluctuating -30mV to -160mV. Does that sound right? I temporarily connected the new bulb and it lights, but its fairly dim as you might expect for an 8V bulb.
I traced things on the Switch Circuit Assembly board (page 51) and see that Point 10 that goes to bulb is separated from Point 13 coming from Power Transformer (secondary) by a 1A fuse and R8 750 Ohm (3W) resistor. Current along all these paths is fairly consistent at -30mV to -160mV.
I traced things on the Switch Circuit Assembly board (page 51) and see that Point 10 that goes to bulb is separated from Point 13 coming from Power Transformer (secondary) by a 1A fuse and R8 750 Ohm (3W) resistor. Current along all these paths is fairly consistent at -30mV to -160mV.
Hi Tom,
I'm not quite following you on that one 🙂
So the new bulb lights and is OK ? but you are wondering why the voltage is only a few millivolts and fluctuating. I haven't got the circuit in front of me but from what you are describing, the bulb is AC fed from the transformer (a common arrangement) and so you would need your meter on AC volts to read the voltage.
I'm not quite following you on that one 🙂
So the new bulb lights and is OK ? but you are wondering why the voltage is only a few millivolts and fluctuating. I haven't got the circuit in front of me but from what you are describing, the bulb is AC fed from the transformer (a common arrangement) and so you would need your meter on AC volts to read the voltage.
Oops, that might explain it. So I'm seeing 44V A/C to the bulb. It does light, I just thought it didn't seem all that bright. I'll go ahead and install it. Thanks Mooly.
So you know, the bulb should be an 8V 55mA bulb according to manual and the closest match I could find is 8V 50mA. This should be ok, right?
The bulb match sounds fine.
You will read the full AC voltage from the tranny with no bulb fitted, fit the bulb and it becomes a series circuit with the bulb and 750 ohm in series to limit the current.
You will read the full AC voltage from the tranny with no bulb fitted, fit the bulb and it becomes a series circuit with the bulb and 750 ohm in series to limit the current.
Bulb installed but not happy with how dim it is. Unfortunately, after a couple of hours playing the amp yesterday, it stopped playing. After letting it cool down an hour, the speaker relay still wasn't clicking on during power up. I haven't checked it out yet this morning.
So it sounds like you still have problems. Go back to the basics of checking for excess offset and excess bias currents. Those are the only two 'inputs' to the protection circuit. If they are OK then you are looking at a problem in the protection circuit/relay driver itself.
I reflowed my solder work and rechecked offset and bias. Seems a bit more stable but still seeing about 6mV at the .5 ohms. I'm playing it now to see if it happens again.
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