There are two current limiting diodes, one next to each driver transistor. The inboard ends of these are connected to pins 4 and 6 respectively, and they are a convenient place to attach multimeter probes in mV mode to measure the bias voltage.
I think it’s the only difference, or maybe also the 6k8/8k2 resistor mentioned above.
They didn’t make 115,000 of these.
They didn’t make 115,000 of these.
Really only one way to find out if everything is normal... hook it up to your DBT and then send it.
Going back to your original problem, did you measure RV200 while turning it? Sometimes intermittent connections can only be seen when you vary the value. It should read a fairly smooth change when you vary it... if it jumps around then you might want to dig into it a bit more, either by cleaning it or replacing it outright.
Yes, it seems fine.
Here is my DBT with 60w bulb:
Results:
I would say the light stays steadily on with about half intensity.
From pin 1 to 9 I measure 39V that cannot be changed with RV200.
No smoke though 😀
Your DBT looks good 👍 So an half lit bulb suggests a moderate current flow. What wattage bulb is that?
Can you measure the voltage between pins 4 and 6 as that will give an idea of current flow in the output stage. Compare with the good channel too. You are measuring between and not to each pin. The expected voltage would be very small, around 5 to 10 millivolts.
Can you measure the voltage between pins 4 and 6 as that will give an idea of current flow in the output stage. Compare with the good channel too. You are measuring between and not to each pin. The expected voltage would be very small, around 5 to 10 millivolts.
The 67 volts might be low because of excess current draw. The bulb is preventing that current going to high and as a consequence you see the voltages fall.
There are two ways of working on this... if you want to look at the regulator first which is a sensible idea then I suggest you disconnect the supply to the bad channel (pin 1) and leave just the good channel as a load for the regulator.
It would still be good to know the voltage between those two points I mentioned though because it gives useful info on a possible fault condition. Have we excess current draw or not? Its all about gathering evidence at this point.
There are two ways of working on this... if you want to look at the regulator first which is a sensible idea then I suggest you disconnect the supply to the bad channel (pin 1) and leave just the good channel as a load for the regulator.
It would still be good to know the voltage between those two points I mentioned though because it gives useful info on a possible fault condition. Have we excess current draw or not? Its all about gathering evidence at this point.
So once the regulator board is fixed I expect so see 67V across even with the DBT?
In the good channel I measured 4mV between 4-6 and 0.1mV in the bad channel.
I will disconnect pin1 from the bad channel to work on the regulator board.
Regulator board:
- Haven't tested the capacitors but they look good.
- I know the resistors and diodes are good.
- Both transistors show signs of life (although I think the smoke came out of the Tr201).
Not necessarily, if you use the bulb then the 67 volts will only be correct provided the current draw from the output of the regulator is low enough not to pull the voltage down. As more current is drawn the bulb lights more and the resistance of the bulb increases (this is why they work so well protecting things). As the resistance rises, the voltage across the mains transformer falls and so the input to the regulator also falls. If there is insufficient voltage then the output from the regulator will be low.
That is good at this point as it shows little current flow.
I would still do that despite the low current flow. Keep the good channel connected.
When working on the regulator you need to check that the input voltage is correct. This is measured between pins 12 and 15 on the board. I think its around 95 volts going into the board so be careful not to short anything when you measure.
To understand how the bulb tester actually works you might find it instructive to measure the resistance of your bulb when it is cold. Take it out of the holder and measure its resistance across the pins. It will be very low ohms.
If your mains voltage is 230 volts and the wattage of the bulb is 60 watt then it normally draws around 0.26 amps. I=W/V = 60/230 = 0.26A The resistance when lit normally would R=V/I = 230/0.26 = 885 ohms.
So measure your bulb when cold 😉 Do you see... the resistance of the filament increases rapidly when its hot. That is why it is so good as a protection device. The more current your amp draws, the hotter the filament becomes and its resistance increases which limits the power that can ultimately be supplied to the amp. If the amp draws little current (no fault) the bulb stays cold and the filament stays at a low ohms value and so lets nearly full mains voltage get through to the transformer.
I measured 70ohms so about 10x resistance once hot 😉
With the DBT I measure 40V across 12-15 and 39V across 1-9 😕
I checked both 4-6 again and the "bad" one reads 0.1mV and the good one now reads "1" on my multimeter. I should point out the the 4mV reading I got yesterday from the good board was not stable. At first it also gave me "1" but then it kind of stabilize on 4mV, not today.
👍 quite a difference hot vs cold.
The supply is being dragged down. Disconnect the bad channel and see what the voltages come up to. If by '1' you mean 1 volt then that means a high current is flowing. Lets see if we can get the correct voltages on the regulator with only the good channel connected.
If you are wondering why I don't suggest disconnecting both channels and working entirely on the regulator alone it is because of the way the regulator is configured which is very unusual.
The +95 volts across the regulator input has the plus 95 connected directly to the amp and now labelled as 67 volts. It is the negative or 'zero volt' side that is actually regulated. The regulated output is now called 'ground' and that is the ground point of the main amplifier. This means if you meausure from amplifier ground to the big reservoir cap negative terminal you would actually see around negative 28 volts. That makes it a bit non intuitive to work on. The regulator also likely needs some loading to operate correctly and would appear not to work if run with no load.
So that is why I say to keep the good channel in place as a load.
The +95 volts across the regulator input has the plus 95 connected directly to the amp and now labelled as 67 volts. It is the negative or 'zero volt' side that is actually regulated. The regulated output is now called 'ground' and that is the ground point of the main amplifier. This means if you meausure from amplifier ground to the big reservoir cap negative terminal you would actually see around negative 28 volts. That makes it a bit non intuitive to work on. The regulator also likely needs some loading to operate correctly and would appear not to work if run with no load.
So that is why I say to keep the good channel in place as a load.
Quoting google: "indicates that the measured value is outside the meter's range or is an open circuit"
That kind of "1".
But as I said, it gave me 4mV yesterday after some time.
Will disconnect one channel and see what happens between 12-15.
Sorry for being a noob. Just put my multimeter in the next mode and I'm clearly reading 235mV on the good channel, of course it was out of bounds.
Edit2: I disconnected pin 1 from the bad channel and nothing changed between 12-15, still 40V
Edit2: I disconnected pin 1 from the bad channel and nothing changed between 12-15, still 40V
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That is very high.
Lets start again and see where we get trying a different approach. Despite all I said above let us see where this gets us......
1/ Disconnect both channels from the regulator. You need only disconnect pin 1 on each amp board at this stage to do that.
2/ With the power on and your meter on DC volts put the black lead on pin 13 of the regulator and the red lead on pin 12. What voltage do you get? Do you see approximately 67 volts?
3/ If the voltage is low or the bulb is still lit then we seem to have a problem in the power supply as well as issues with the amp boards.
With both channels disconnected we get no light from the bulb.
Measuring 13-12 we get 65V to 80V depending on RV200.
Good news right? Does this mean the regulator board is in good condition, as in able to provide 67V?
Measuring 13-12 we get 65V to 80V depending on RV200.
Good news right? Does this mean the regulator board is in good condition, as in able to provide 67V?
That does sound promising, yes. We don't know how it holds up under load of course but yes, its good 👍
So set it for the correct 67 volts and move on to working on one channel at a time. I would begin with what you think is the good channel. Set the bias current preset to maximum resistance so that it appears as a 2.2k in circuit and not as a zero ohm resistor. Maximum resistance will give minimum bias current.
If the bulb lights and the voltage falls then check again carefully what the small voltage across pins 4 and 6 is. It should be near zero volts.
Turn this preset so it appears as a 2.2k
So set it for the correct 67 volts and move on to working on one channel at a time. I would begin with what you think is the good channel. Set the bias current preset to maximum resistance so that it appears as a 2.2k in circuit and not as a zero ohm resistor. Maximum resistance will give minimum bias current.
If the bulb lights and the voltage falls then check again carefully what the small voltage across pins 4 and 6 is. It should be near zero volts.
Turn this preset so it appears as a 2.2k
I reconnected the "good" channel and set RV101 to max, in my case 2.7k
Bulb lights up and the same 235mV across 4-6 😕
Edit: Maybe switch up the channels and try with the other one?
Bulb lights up and the same 235mV across 4-6 😕
Edit: Maybe switch up the channels and try with the other one?
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Yes, do that. Let us see if one channel really is still good or not.
The 235mv across 4 and 6 tells us that a current of 0.39 amps is flowing (being drawn from the power supply). I=V/R = 0.23/0.6 which is 0.39 amps. 0.6 ohm is the combined series resistance of the two 0.3 ohm resistors.
The 235mv across 4 and 6 tells us that a current of 0.39 amps is flowing (being drawn from the power supply). I=V/R = 0.23/0.6 which is 0.39 amps. 0.6 ohm is the combined series resistance of the two 0.3 ohm resistors.