For your own curiosity yes, then you know what to expect. It is as adason says, your meter is on wrong range.
Yes I see makes sense, would it unreasonable to expect it might just max out and say -20v instead of -1? Presumably it would show just '1' if I tried to measure more than +20 on 20v scale. I guess these are nuances of the DMM I will learn...
The 4 volt AC is measured between those two points rather than to ground. It is the filament (heater) supply for the display tube and can also be measured across the end pins of the display. The filament supply is not ground referenced but is instead referenced to the -30 rail via R3 and R4 and the Zener D4. That allows the filament supply to float at a voltage set by the Zener which is typically around 5.6 volt. So the filaments will be around -24 to to -25 volts as measured from ground but still reading 4 volts AC between the two points.
If the display is working all these supplies will be OK and these supplies will not stop the player working.
If the display is working all these supplies will be OK and these supplies will not stop the player working.
Thanks - that's very interesting.
The displays illuminates for a fraction of a second when I press the stand-by button - then cycles back to stand-by. Only once did the display remain on, that was when I first powered up after swapping out the main board and psu form the donor machine. the display remain illuminated until I tried to eject and since then I get the cycle as described above with display only on for a fraction of a second.
I'm trying to rule out the psu board before diving deeper into the main board. Unfortunately I can't swap out just the PSU because of that plastic connector which broke when I tried to remove it from the original player so it is coupled with the main board for now.
The displays illuminates for a fraction of a second when I press the stand-by button - then cycles back to stand-by. Only once did the display remain on, that was when I first powered up after swapping out the main board and psu form the donor machine. the display remain illuminated until I tried to eject and since then I get the cycle as described above with display only on for a fraction of a second.
I'm trying to rule out the psu board before diving deeper into the main board. Unfortunately I can't swap out just the PSU because of that plastic connector which broke when I tried to remove it from the original player so it is coupled with the main board for now.
Se we do that by checking all voltages from the power are present and correct even when the player is in a faulty state. That means all the voltages I highlighted in post #2.
I would doubt there is any problem with the display power supply voltages but you should for completeness just check that the 4 volt AC is still present. Easy to measure between R3 and R4 (look at the diagram)
On the original psu which I have removed and which is not hooked up to a board I get exactly 4.5v AC across the two pins (4 and 5).
On the unit I am testing which is hooked up only 3.7v on pins 4 and 5. I traced to what I think is R3 and R4 - 3.6v, 2.2v between the two and 0 v on the other side (see photo).
Is the drop to 3.6v expected when hooked up?
On the unit I am testing which is hooked up only 3.7v on pins 4 and 5. I traced to what I think is R3 and R4 - 3.6v, 2.2v between the two and 0 v on the other side (see photo).
Is the drop to 3.6v expected when hooked up?
It sounds reasonable as the voltage will fall under load. Any problem with the -30 volt rail and/or the 4 volt AC will only affect the glass filament display tube, it will not alter any other functionality on the player.
Is this significant: - I am testing Q2 which is a transistor with the heatsink: 2SB1370.
On the removed board form original player, DMM in diode mode black probe on base, red on collector 715, move red to emitter 715. Same test with problematic board: 605 and 226.
If I switch polarity - so red probe on base, I get 1 and 1 on the removed board, On the problematic board it starts at about 1800 and changes to 1, then on the emitter I am getting 255.
From what I've read, including here
https://www.diyaudio.com/community/threads/testing-transistors-with-dmm-or-vom.18151/
These readings suggest an issue?
On the removed board form original player, DMM in diode mode black probe on base, red on collector 715, move red to emitter 715. Same test with problematic board: 605 and 226.
If I switch polarity - so red probe on base, I get 1 and 1 on the removed board, On the problematic board it starts at about 1800 and changes to 1, then on the emitter I am getting 255.
From what I've read, including here
https://www.diyaudio.com/community/threads/testing-transistors-with-dmm-or-vom.18151/
These readings suggest an issue?
Q2 is the 7 volt regulator and we confirmed by measurement that the correct voltage (+7 volts) was present on the collector. No further testing is really needed as long as that 7 volts is stable and constant but 
for your own curiosity...
You can not reliably test transistors in circuit because of interaction with other components. They must be removed or electrically isolated from the rest of the circuit.
The removed board will read differently compared to the board that is fitted in the player. For one thing it is connected to the big reservoir cap and bridge rectifier which will confuse readings. Also any residual voltage from charged caps can also skew readings.
Q2 has a 220 ohm between base and emitter and that in itself means the transistor should read 'leaky' between base and emitter if tested in circuit.
When your meter is on the diode test range it is actually supplying a small test current from the meter leads and the reading you see is actually volt drop between the probes. So 715, 605 and 226 are actually 715mv, 605mv and 229mv. 600 to 700mv is the normal volt drop across a silicon diode or transistor junction.
With your player, once you have measured all those voltage rails on the power supply and found them OK you have to say that the power supply is OK and the fault is elsewhere.
for your own curiosity...You can not reliably test transistors in circuit because of interaction with other components. They must be removed or electrically isolated from the rest of the circuit.
The removed board will read differently compared to the board that is fitted in the player. For one thing it is connected to the big reservoir cap and bridge rectifier which will confuse readings. Also any residual voltage from charged caps can also skew readings.
Q2 has a 220 ohm between base and emitter and that in itself means the transistor should read 'leaky' between base and emitter if tested in circuit.
When your meter is on the diode test range it is actually supplying a small test current from the meter leads and the reading you see is actually volt drop between the probes. So 715, 605 and 226 are actually 715mv, 605mv and 229mv. 600 to 700mv is the normal volt drop across a silicon diode or transistor junction.
With your player, once you have measured all those voltage rails on the power supply and found them OK you have to say that the power supply is OK and the fault is elsewhere.
I'm kind of out of ideas - at least as far as my capabilities will allow.
I've traced the 7v and and 5v to the ribbon connectors which look like where they terminate (to me) - I get 5v and 7v and only a slight drop when I power-up.
There are rails over the board, some so fine I can barely see them, I read that faulty capacitors might be the cause, there are a number of larger round capacitors but since they have to be removed to be tested I don't see how I can realistically test these as it is not practical to remove each individual cap.
The only other tactic i can think of is to swap over every cap, transistor and resistor that is removable in a scattergun approach but that really is my last resort as next stop is a hammer and a lot plastic and metal in the re-cycling bin.
I've traced the 7v and and 5v to the ribbon connectors which look like where they terminate (to me) - I get 5v and 7v and only a slight drop when I power-up.
There are rails over the board, some so fine I can barely see them, I read that faulty capacitors might be the cause, there are a number of larger round capacitors but since they have to be removed to be tested I don't see how I can realistically test these as it is not practical to remove each individual cap.
The only other tactic i can think of is to swap over every cap, transistor and resistor that is removable in a scattergun approach but that really is my last resort as next stop is a hammer and a lot plastic and metal in the re-cycling bin.