Hi Harry,
I forgot something. There are two thermal sensors that complete the circuit for the speaker relay. If any of the connections are open - no relay.
So check for a short across terminals 1 - 5 and from terminals 38 - 7 (?? I can't read these). Just look for the same device that will mount on the other heat sink. You can also make sure you have a positive voltage on one or both pins of D203 (across the relay coil). No B+ means there is an interruption in that circuit (with the thermal breakers).
Let me know please.
-Chris
I forgot something. There are two thermal sensors that complete the circuit for the speaker relay. If any of the connections are open - no relay.
So check for a short across terminals 1 - 5 and from terminals 38 - 7 (?? I can't read these). Just look for the same device that will mount on the other heat sink. You can also make sure you have a positive voltage on one or both pins of D203 (across the relay coil). No B+ means there is an interruption in that circuit (with the thermal breakers).
Let me know please.
-Chris
Chris,
Diode reads 40.8 volts on one side 20.5 on the other this is with the solid state timer still in the circuit which engages the relay. The relay is funtional with the timer in place,when I restore the original circuit and remove the timer the relay will not energize. Should I be overly concerned with the 76.9 volts I have on B+? It should be 74volts, would it be better to concentrate on the power supply first? Harry
Diode reads 40.8 volts on one side 20.5 on the other this is with the solid state timer still in the circuit which engages the relay. The relay is funtional with the timer in place,when I restore the original circuit and remove the timer the relay will not energize. Should I be overly concerned with the 76.9 volts I have on B+? It should be 74volts, would it be better to concentrate on the power supply first? Harry
Hi Harry,
Don't worry about that small difference in B+ at all.
I guess some technician couldn't troubleshoot the original fault so they bypassed it. Let's concentrate on faults in the original relay circuit then. Return the circuit to it's original format. Test individual parts as you get the opportunity.
Check R212 (270R), Q207, ZD601 and ZD602. Look for overheated parts and low hFE for Q212. You may find that C605 and C606 (4u7 10V) have been cooked to death. Another problem may be leaky transistor Q201 and dried up capacitors C201, C202 and C203. If you don't have a good capacitor tester, simply replace the lot. It's faster anyway.
-Chris
Don't worry about that small difference in B+ at all.
I guess some technician couldn't troubleshoot the original fault so they bypassed it. Let's concentrate on faults in the original relay circuit then. Return the circuit to it's original format. Test individual parts as you get the opportunity.
Check R212 (270R), Q207, ZD601 and ZD602. Look for overheated parts and low hFE for Q212. You may find that C605 and C606 (4u7 10V) have been cooked to death. Another problem may be leaky transistor Q201 and dried up capacitors C201, C202 and C203. If you don't have a good capacitor tester, simply replace the lot. It's faster anyway.
-Chris
Hi Harry,
I am pretty sure I know what's wrong then. Locate D508, D509 and C517 (1 uF 50 VDC). If you turn the unit on, you should read a negative voltage between the anodes of D508, D509 and the negative of C517 with respect to ground. I will bet you measure possibly a small negative voltage. If you had a 'scope it would show a very high ripple waveform, or full wave rectified wave form if the cap is completely open.
Replace this capacitor. When you turn the amp on, make sure you are measuring the voltage test point and be ready to correct it.
Let me know how this works out please.
-Chris
Okay, this is what we will concentrate on. Normally I can troubleshoot with the unit in front of me instead of just replacing parts.
Normal when the above is true, this diode is across the relay coil.
I am pretty sure I know what's wrong then. Locate D508, D509 and C517 (1 uF 50 VDC). If you turn the unit on, you should read a negative voltage between the anodes of D508, D509 and the negative of C517 with respect to ground. I will bet you measure possibly a small negative voltage. If you had a 'scope it would show a very high ripple waveform, or full wave rectified wave form if the cap is completely open.
Replace this capacitor. When you turn the amp on, make sure you are measuring the voltage test point and be ready to correct it.
Let me know how this works out please.
-Chris
Chris,
Changed C517 and C522 which is not indicated on the schematic you have both are 1uf 50 volt caps. Still no relay and now B+ has jumped from 76.9 to 81.6 volts and will not adjust any lower. FYI the service manual for the M500 has 3 different versions of a schematic I am going to send you the one that I believe is relevant to the unit in question. I will try to do this tomorrow rather today as it is now about 1:30 A.M. Harry
Changed C517 and C522 which is not indicated on the schematic you have both are 1uf 50 volt caps. Still no relay and now B+ has jumped from 76.9 to 81.6 volts and will not adjust any lower. FYI the service manual for the M500 has 3 different versions of a schematic I am going to send you the one that I believe is relevant to the unit in question. I will try to do this tomorrow rather today as it is now about 1:30 A.M. Harry
Hi Harry,
Okay, so we now know that cap was bad for sure. It also affects the voltage regulation. Now you have to measure a few things again.
Please measure the terminal voltages around the following transistors.
Q202, 203, 204, 205, 206, 207. This will allow a better diagnoses. I am aware you are measuring some again, but the voltages should have changed.
I will stand by for the proper manual, thank you. Yes, it's now 1:40 here. I'm heading to bed now.
-Chris
Okay, so we now know that cap was bad for sure. It also affects the voltage regulation. Now you have to measure a few things again.
Please measure the terminal voltages around the following transistors.
Q202, 203, 204, 205, 206, 207. This will allow a better diagnoses. I am aware you are measuring some again, but the voltages should have changed.
I will stand by for the proper manual, thank you. Yes, it's now 1:40 here. I'm heading to bed now.
-Chris
Hi Harry,
The list has some errors. You must have been tired. These following relationships must be true according to the schematic :
Q202 C = Q203 E should be close to Q205 B through 1 K (?)
Q202 B = Q204 E
Q205 B = Q206 C
So, what I can tell from your measurements is that Q205 B should be at a higher voltage unless Q202, Q203 or Q206 are conducting. A short on C202 or short failure (C-E) in any of these transistors would also cause the same thing.
Please also try to test for a C-E short on these transistors, the diodes and capacitors. Then please confirm your readings. Also please check to confirm our negative voltage across the 1 uF caps you changed earlier. It may be in backwards possibly.
Since another technician has been into this unit, carefully clean the foil side of the board and check for cracked traces. Remember, the guy was in panic mode.
-Chris
The list has some errors. You must have been tired. These following relationships must be true according to the schematic :
Q202 C = Q203 E should be close to Q205 B through 1 K (?)
Q202 B = Q204 E
Q205 B = Q206 C
So, what I can tell from your measurements is that Q205 B should be at a higher voltage unless Q202, Q203 or Q206 are conducting. A short on C202 or short failure (C-E) in any of these transistors would also cause the same thing.
Please also try to test for a C-E short on these transistors, the diodes and capacitors. Then please confirm your readings. Also please check to confirm our negative voltage across the 1 uF caps you changed earlier. It may be in backwards possibly.
Since another technician has been into this unit, carefully clean the foil side of the board and check for cracked traces. Remember, the guy was in panic mode.
-Chris
Chris,
With respect to the power supply 9.71 volts on the Cathode side of D508 & 509, Anode side measures -16.5 volt. Now on to the protection circuit C202=236 uf, Q202,203,206 no emitter to collector short all of them oscillate using my Micronta Dynamic Transistor Tester (dont laugh I have had this thing a long time). I thought it was time to interject some humor into this process. Took readings again
Q201 E)45.5 C)-.332 B)45.3
Q202 E) 0 C) .047 B)-.005
Q203 E).047 C) 0 B).828
Q204 E)-.005 C).195 B) 0
Q205 E) 0 C) 45.2 B).048
Q206 E) 0 C).049 B)-.347
Q207 E) 0 C) 45.2 B) 0 Thanks, Harry
With respect to the power supply 9.71 volts on the Cathode side of D508 & 509, Anode side measures -16.5 volt. Now on to the protection circuit C202=236 uf, Q202,203,206 no emitter to collector short all of them oscillate using my Micronta Dynamic Transistor Tester (dont laugh I have had this thing a long time). I thought it was time to interject some humor into this process. Took readings again
Q201 E)45.5 C)-.332 B)45.3
Q202 E) 0 C) .047 B)-.005
Q203 E).047 C) 0 B).828
Q204 E)-.005 C).195 B) 0
Q205 E) 0 C) 45.2 B).048
Q206 E) 0 C).049 B)-.347
Q207 E) 0 C) 45.2 B) 0 Thanks, Harry
Hi Harry,
Your Micronta checker is fine to use. Your transistors appear to be okay. I am not very familiar with it, so I don't know how sensitive it is to leakage.
Looking at your numbers, I see problems with the following :
Q203 E).047 C) 0 B).828
Q204 E)-.005 C).195 B) 0 The bold terminals are connected to each other. Look for a break in the foil here.
The base of Q205 should be high, bringing the emitter terminal positive as well. This point is at 0V basically and this area is where your remaining fault is. Either the resistor between Q201 E and Q205B is open (or the trace open), or you have a low impedance around C202 anode, Q206 C or Q205 B (same point), or near Q202 C or Q203 E. This would drop your voltage through the resistor connecting this point to Q205 B. Q206 opens the relay when you turn the power off. It is working properly now.
It should be interesting to find out what your problem was since there where two faults here in the same circuit. The second fault may have been caused by the first technician (in panic mode, remember?).
-Chris
Your Micronta checker is fine to use. Your transistors appear to be okay. I am not very familiar with it, so I don't know how sensitive it is to leakage.
Looking at your numbers, I see problems with the following :
Q203 E).047 C) 0 B).828
Q204 E)-.005 C).195 B) 0 The bold terminals are connected to each other. Look for a break in the foil here.
The base of Q205 should be high, bringing the emitter terminal positive as well. This point is at 0V basically and this area is where your remaining fault is. Either the resistor between Q201 E and Q205B is open (or the trace open), or you have a low impedance around C202 anode, Q206 C or Q205 B (same point), or near Q202 C or Q203 E. This would drop your voltage through the resistor connecting this point to Q205 B. Q206 opens the relay when you turn the power off. It is working properly now.
It should be interesting to find out what your problem was since there where two faults here in the same circuit. The second fault may have been caused by the first technician (in panic mode, remember?).
-Chris
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