So voltages as follows
At test points marked on schematic
R111 good channel 8vdc bad 5.6vdc
R104 good channel 8.7vdc bad 6.2vdc
R130 good channel 7.4vdc bad 5.1vdc
R113 good channel 8.1vdc then C106 side 32.7vdc bad channel 5.6vdc then C106 side 24.3vdc
At test points marked on schematic
R111 good channel 8vdc bad 5.6vdc
R104 good channel 8.7vdc bad 6.2vdc
R130 good channel 7.4vdc bad 5.1vdc
R113 good channel 8.1vdc then C106 side 32.7vdc bad channel 5.6vdc then C106 side 24.3vdc
Can you just clarify that one please 🙂
In post #1 you say the midpoint will not go under 40 volts. C106 is the bootstrap capacitor and does connect to the midpoint. The 'top' end of C106 would be around +45 volts on a good channel, the bottom end the midpoint..
So I've been measuring this between pins 1 and 5 as per dada manual as the mods on that are what I intend to do, if I measure between pins 5 and 9 the voltage won't adjust up ( but will go down) from 24.5vdc, am I looking at this the wrong way around?
Hi Chris,
I would think you would measure between pins 8-9 (common point) and whatever test point. Note that pin 1 is listed as 67V, that has to be with respect to pins 8-9. Convention also supports this, I have always measured it this way working on almost everything including a 303.
I would think you would measure between pins 8-9 (common point) and whatever test point. Note that pin 1 is listed as 67V, that has to be with respect to pins 8-9. Convention also supports this, I have always measured it this way working on almost everything including a 303.
OK, so measuring between incorrect pins,
Measuring between pin 9 ( ground ) and pin 5 ( output ) I get 25.5vdc this will not adjust up to 33.5vdc but will down.
Measuring between pin 9 ( ground ) and pin 5 ( output ) I get 25.5vdc this will not adjust up to 33.5vdc but will down.
Hi Chris,
Okay, let's reset. Would you please measure the test points between the channels again (sorry). We'll get everyone on the same page. We all have to start from zero again.
Okay, let's reset. Would you please measure the test points between the channels again (sorry). We'll get everyone on the same page. We all have to start from zero again.
Hi Anatech, thanks for all the help and measure again, no problem
R104
Good channel 9vdc
Bad channel 6.4vdc
R111
Good channel 8.5vdc
Bad channel 5.9vdc
R104
Good channel 9vdc
Bad channel 6.4vdc
R111
Good channel 8.5vdc
Bad channel 5.9vdc
Okay, looks like TR100 is drawing too much current. That causes TR101 to conduct heavier and starves TR102 which should lead to a higher midpoint voltage. Why?
If TR102 were leaky, that could do it. A high resistance with R116 or R117 (4K7) may also cause this. If C101 was leaky, TR100 would be starved and appear to be drawing too much, allowing TR101 to conduct more heavily and starve TR102.
It's difficult to troubleshoot because things are still working in the active range. Nothing is open or shorted. I would pull C100 and C101 to see what happens to the DC conditions.
If TR102 were leaky, that could do it. A high resistance with R116 or R117 (4K7) may also cause this. If C101 was leaky, TR100 would be starved and appear to be drawing too much, allowing TR101 to conduct more heavily and starve TR102.
It's difficult to troubleshoot because things are still working in the active range. Nothing is open or shorted. I would pull C100 and C101 to see what happens to the DC conditions.
Ok both off and voltage test points,
R104 Bad channel 6.5vdc
R111 bad channel 5.9vdc
So same as before
R104 Bad channel 6.5vdc
R111 bad channel 5.9vdc
So same as before
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One should eliminate the possibility of a fault in the output triples where there is a cute shortcircuit protection scheme with two 1S 920 diodes around each driver transistor base. If it is not possible to set the voltage at the output ahead of the output capacitor, one triplet could be passing too much current dragging this dc point down or if else the IQ bias is out of adjustment. This amplifier does not have a great mean level of standing current as reckoned by Linsley-Hood said to be about 4 m.a. He noted that there is a drawback in that this did not allow a significant margin of Class A operation to act as a cushion if there were unexpected variations in operating conditions or device characteristics.
It might be easier to start investigations from the output backwards to the input.
It might be easier to start investigations from the output backwards to the input.
Hi mjona,
Best not to flip back and forth. Offset (center voltage) is normally determined by the voltage amplifier, however in these odd designs other faults can affect these things as well. I am not a fan of this design at all. Repaired many. They work okay but the design can give rise to really odd problems.
Anyway, best to follow one train of troubleshooting. Jumping to other trains of thought lead to confusion and wasted time for all.
Now if you look at what was found so far, the voltage amps section is not operating normally. That much we know. Could a driver or output affect it? Sure.
Best not to flip back and forth. Offset (center voltage) is normally determined by the voltage amplifier, however in these odd designs other faults can affect these things as well. I am not a fan of this design at all. Repaired many. They work okay but the design can give rise to really odd problems.
Anyway, best to follow one train of troubleshooting. Jumping to other trains of thought lead to confusion and wasted time for all.
Now if you look at what was found so far, the voltage amps section is not operating normally. That much we know. Could a driver or output affect it? Sure.
The difference is the same as one good diode junction which could be by coincidence To decide whether or not most multimeters include a diode test setting. You can use that in circuit with power off or or you could switch to the voltage range to measure Vbe of individual transistors and use intuition if the measurements are significantly out of range off +/-0.6 Volts. for the particular polarity of the transistor.
So at the minute tested good are,
Tr100, Tr101 Tr102
C100, C101
R116, R117, R130, R113
All tested out of circuit
Any other parts in the voltage amp to check check?
The low voltage at R111-R108 should shut down TR102 and raise the voltage. Therefore something else is drawing the collector of TR102 down. Is bias low or normal? A leaky TR104 or low gain / open TR103 could do that.
At this point I would normally place the negative meter probe on the output (L100). Then you can measure the output transistors to see bias voltages accurately. Check drops across the emitter resistors (R124,R125 - collector I know). Check current flows through each output stage transistor by measuring voltage drops in their emitter resistors. Compare to the other channel if you find something odd, or want to confirm.
At this point I would normally place the negative meter probe on the output (L100). Then you can measure the output transistors to see bias voltages accurately. Check drops across the emitter resistors (R124,R125 - collector I know). Check current flows through each output stage transistor by measuring voltage drops in their emitter resistors. Compare to the other channel if you find something odd, or want to confirm.
We’ve already established that all the transistors are working correctly.
I don’t know where JLH got 4mA from. Not the service manual, certainly. It says 5 to 10mA, and that is certainly enough.
I got this information from his book "Valve and Transistor Audio Amplifiers" his quote was in the context of the symmetry of Vin/I out curves in each output triplet half. He described these triplet halves as virtually identical with complimentary symmetry. I don't see any need to split hairs over his comment and what the service manual recommends. One could try both standing current settings and decide which level works best.