Sorry Pete. I haven't been supporting you as I should have been.
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First some notes on using your multimeter. I should have explained this earlier, and I think you've figured it out already, but it doesn't hurt to be clear.
Always have the probes in the COM socket (black probe) and VΩmA socket (red probe) unless we ask you to measure a high current. (We probably won't ask you to do that.)
To measure voltage, use the V
DC ranges on the top left of the range switch.
The amplifier needs to be powered up for voltage measurements, and you need to be very careful not to let the probe slip and short things together, because that can damage components, even if the short is only very brief.
If possible, clip one probe (usually the black one) onto a terminal so you can forget about it and concentrate on the red probe only.
This amplifier does have dangerous voltages in it, and not just in the mains circuitry. So make sure your hands and arms don't make contact with anything in there. I find that a large sheet of corrugated cardboard under your forearm and wrist is good for keeping your skin from touching anything, while still giving your hand some solid support so you can control the probe firmly.
Since this amplifier doesn't have any voltages over about 130V in it, you can start with the 200 range, which measures voltages up to 200V. Connect the probes to the two points in the circuit where you want to measure the "voltage between", or across the component where you want to measure "voltage across".
The meter will display the voltage in volts. If the reading is less than 20, change to the 20 range, because this will give you another digit of resolution. The displayed value is still in volts.
If the reading is less than 2.00, change to the 2000m range. The meter now displays the voltage in mV (millivolts), so for example if it displays 1325, the voltage is "1325 mV", which is the same as 1.325V. The maximum indication on this range is 1999 mV (just a fraction under 2000 mV, which is 2V).
If the reading is less than 200, you can change to the 200m range. The reading is still in mV but there is an extra digit of resolution. This isn't important in an audio amplifier so there's no need to bother with it.
For measuring resistance, use the Ω ranges at the bottom left of the range switch.
The process is the same as with voltage, but you normally measure with the unit powered down. You still need to avoid touching things because your skin and body resistance could affect your measurements.
Resistance is measured in ohms (Ω). 1000Ω equals one kilohm (kΩ) and 1,000,000Ω equals one megohm (MΩ). Your meter displays values in Ω and kΩ only.
If you know what resistance you expect, choose the lowest range that's higher than that value. If the resistance is higher than the maximum that the range can display, the meter shows an overload indication (0L or 1. or something like that). So choose the next range up, as necessary.
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Back to the amplifier.
We need to figure out what's going on with the voltages around R108 back in post #169.
The first photo in post #239 shows us that near that part of the circuit, there are two power resistors (which get hot and can damage the PCB) and a large capacitor stuck to the board with brown goop which can sometimes become corrosive with age.
We also have some very mysterious measurements around R108. Bob's suggestion in post #254 would explain them, but it seems somewhat unlikely to me. Whatever the problem is, we need to fix it.
The problem circuit is actually very simple - just two resistors and a zener diode in series:
Q46 should be removed from the board, so we can ignore it. I've indicated that with the red cross on the wire that goes to Q46's base.
So we have three components in series - R108, R117 and D38 - connected between two voltage rails - +45VL (which is running at about +66V because the output transistors are disconnected so there's no load on the power supply), and -45VL (which is running at about -66V for the same reason).
So these three components have about 132V total connected across them. This causes current to flow through the circuit. The same current flows through all three components, and the total voltage is split into three voltages that add up to the total.
The current causes a roughly constant voltage of 51V to appear across D38, because that's what a 51V zener diode
does. The remaining voltage, around 80V, is split between R108 and R117 in proportion to their resistance.
Their total resistance is 28 kΩ so 1/28th of 80V, which is about 2.8V, should appear across R108, and 27/28 of 80V, which is about 77V, should appear across R117.
BUT when you measure those voltages, there is no voltage across R108. That is very mysterious, because according to the diagram, there are no other components in the circuit, and you measured R108 as having the correct resistance!
Current must be flowing through D38 and R117 because there is plenty of voltage across them, but it doesn't seem to be flowing through R108 because there is no voltage drop across it, despite it having the right resistance.
This is why Bob suggested that perhaps the bottom end of R108 is shorted to +45VL (that would provide a path for the current through R117 and D38) but the top end is disconnected (that would explain why R108 measures the right resistance).
So we need to concentrate on that part of the circuit - everything physically close to R108. I would like to see a photo the same as the first photo in post #239 but rotated so the writing is the right way round and, if possible, showing Q46 and R108 without interference from the large white resistors marked 100RJ, and a photo of the underside of the board in exactly the same area.
If you have refitted Q46, can you please remove it again, and before you take the photos, clean the underside of the board with solvent (isopropyl alcohol is best) and a toothbrush and/or cotton swab, and then power up and re-measure the voltages from post #169, then power off and re-measure resistance across R108.
