Hi guys, I've got this immaculate Rotel ra-840 that looks very well cared for. There is not a single piece of fluff anywere inside! With no load I've got 11mv on one channel (regardless of source selected) and 2.4 (ish) volts on the other channel. One fuse on the combined pre/power board is blown. I'm seeing 21 volts on that (see pic) and 15mv on it's neighbour. There's 76volts coming from the supply board, does that sound right?
I built a chip amp some years ago but I've forgotten most of what I learnt. I have noticed a lot of the joints are dark grey as opposed to shiny but no components seem loose/wobbly. Any thoughts for a willing learner?
I built a chip amp some years ago but I've forgotten most of what I learnt. I have noticed a lot of the joints are dark grey as opposed to shiny but no components seem loose/wobbly. Any thoughts for a willing learner?
21VDC at the output is obviously not good. The usual suspect when the output stage has gone DC, is that the output semis themselves are shorted, C to E. These are 2 parallel, complementary pairs of 2SD1047/B817 but being parallel components, only one pair has to have a short to show the fault. Check and compare the power transistor and their driver transistor B-E and C-E voltages carefully, preferably using hook probes or at least sharp pointed meter probes for security (hard points are best) that won't easily slip and make any existing damage even worse, involving more than just the power transistors (that's all too easy to do, incidentally). When unpowered, you can also check the resistance between the +/-33V supply rails and the output rails to confirm a short before needing to pull any leads or remove the transistors to check/compare.
BTW, these transistors are long obsolete and there are plenty of fakes out there. Last I checked, only genuine KTD1047/KTA817 complements could be considered equivalent to the original Sanyo semis. Unfortunately, ST micro only supply the NPN type.
BTW, these transistors are long obsolete and there are plenty of fakes out there. Last I checked, only genuine KTD1047/KTA817 complements could be considered equivalent to the original Sanyo semis. Unfortunately, ST micro only supply the NPN type.
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Hi vu meter,
76 V from the supply board sounds spooky given only +/-33V rails on the power amp rails. Between which two points did you find 76V? What voltage do you observe on the power amp rails? The TB-130 board schematic implies +/-33V, or about 66V rail-to-rail. So 76V seems a bit disturbing.
Are you able to post a service manual rather than a screen shot? Some of the component values are hard to decipher.
To the forum community:
I confess the Rotel supply scheme confuses me. There seems to be no center tap from the transformer secondary to circuit ground. What keeps the amp supplies symmetrically balanced re ground? Am I missing something? I'm attaching a simplified sketch of the supply and one power amp as I interpret them. Is there a center-tap not shown? Else why have two 5A fuses in series?
Back to the ra-840: On the problem channel, would you report the rail supply voltages, the output voltage before the blown Fuse (F601), the input base (Q603), and the feedback base (Q605), all with respect to ground? Please report negative voltages with a minus sign.
Thanks!
Quad also used DC balancing circuits for dual rails from single wound transformers in a few of their amplifiers - at least their 306 model amps. in my experience. A Full Rotel 840 manual is available for viewing etc. here: https://www.audioservicemanuals.com/r/rotel/rotel-ra/rotel-ra-840/322108-rotel-ra-840-service-manual
Ian, thank you for the manual link. Page 7 has a pictorial showing a center-tap missing in the schematic. I feel much better. 😀
Thanks for your replies gents. I believe I may be in error stating 70 odd volts from the supply. I was measuring across what I now believe to be the pos and neg output wires marked in the pic, which I'm guessing combined to make 76v?. I've since measured each one to earth and they are 39v apiece. I noticed where the red wires from the transformer enter the board were loose so pulled the board out and resoldered everything in sight. Upon powering up nothing exploded so I view that as a small victory!
No changes though. Btw, I'm sure it's me but shouldn't there be a voltage reading when measuring across earth and the red/green transformer wires? I'm seeing nothing at all, well a momentary flicker on the meter then nowt.
Sorry about the scrawl, I was writing with the mouse!
No changes though. Btw, I'm sure it's me but shouldn't there be a voltage reading when measuring across earth and the red/green transformer wires? I'm seeing nothing at all, well a momentary flicker on the meter then nowt.
Sorry about the scrawl, I was writing with the mouse!
You're seeing +/- 39V rather than the nominal 33 indicated on the schematic. That seems a bit high--- about 18% re nominal. To be prudent, would you compare your actual mains voltage with what is probably specified on the amp's rear panel? If the power transformer isn't being over driven, then we can assume 39V supplies are as intended.
You have to dig through the manual's pictorials of the transformer to discover that there are two secondary windings, both having a black lead tied to ground. If you have your DVM set to AC volts, you should find AC present. If you forget and leave the meter set to DC, you'll probably see average of the sine wave which is 0VDC.
Thanks again for the replies. Ok, my ac outlet is 241v. (You were quite correct about my meter's setting Ian).
I've taken some more readings; So the red transformer wires where they meet the two fuses (f901 + f902) are 28v across earth.
The green transformer wire at fuse f903 is 9v. The other green wire is 0v. I checked the resistance between each of the output rails? that I marked on yesterday's pic and the red transformer wires (supply rails?). With the meter set to 200k one side is 30 and the other is 10. With the meter set to 20k one side is 3 and the other is 1 which seems consistent but have to admit I don't really understand what I'm seeing. If you need a steam loco fixed I'm your man but electrickery can lean me feeling like I'm back in junior school, so thanks for bearing with me.
I've taken some more readings; So the red transformer wires where they meet the two fuses (f901 + f902) are 28v across earth.
The green transformer wire at fuse f903 is 9v. The other green wire is 0v. I checked the resistance between each of the output rails? that I marked on yesterday's pic and the red transformer wires (supply rails?). With the meter set to 200k one side is 30 and the other is 10. With the meter set to 20k one side is 3 and the other is 1 which seems consistent but have to admit I don't really understand what I'm seeing. If you need a steam loco fixed I'm your man but electrickery can lean me feeling like I'm back in junior school, so thanks for bearing with me.
Can you determine if the unit has been worked on before?
Are the output transistors that are bolted to the heatsink the correct type (compare with the other channel) and that also applies to the driver transistors feeding them.
Can you see anything that shows it has been looked at previously. Any clues with the soldering?
The voltage you are seeing on the blown fuse (F601) doesn't immediately suggest a shorted output stage. If it were shorted you would be seeing the full rail voltage on the fuse. Its either a less common fault or I was wondering if it was oscillating (unstable) due to previous work.
You have to start somewhere though 🙂
Are the output transistors that are bolted to the heatsink the correct type (compare with the other channel) and that also applies to the driver transistors feeding them.
Can you see anything that shows it has been looked at previously. Any clues with the soldering?
The voltage you are seeing on the blown fuse (F601) doesn't immediately suggest a shorted output stage. If it were shorted you would be seeing the full rail voltage on the fuse. Its either a less common fault or I was wondering if it was oscillating (unstable) due to previous work.
You have to start somewhere though 🙂
Hi Mooly. I did wonder that myself. Couldn't see anything that looked out of place but just as I was about to assume it was a virgin I noticed a solitary fingerprint on the inside of the meter glass. Further investigation revealed one missing screw from the facia plate. All eight of the output transistors look identical. None look like they are any newer than the others. Mind you, that thermal paste looks very abundant. Would it leave the factory like that? Also, am I correct in thinking there's just two driver transistors?
Sadly, that refurb. work has either mr DIY's or a very inexperienced repair tech's signature (and thermal paste) all over it.
Yeah, looking at the pic without the alchohol filter, it certainly looks like there's paste residue/imprints from previous larger transistors. Why on earth wouldn't you wipe it all off?!
It could well have lots of paste, its there to do an important job. Yep, two driver transistors, one upper NPN and one lower PNP that drive the parallel pairs of output transistors.
Your probably wondering what's next...
The offset voltage that you measured on the fuseholder, the '21 volts'. Can you confirm it was Plus 21 volts and not Negative 21 volts.
For diagnosing faults at a distance careful voltage measurement can go a long way but you need to be very careful not to accidently short any pins. Seeing all the voltage on all the transistor nodes would help pinpoint any issue.
For example
Q601
E =
B =
C =
Q603
E =
B =
C =
and so on.
The offset voltage that you measured on the fuseholder, the '21 volts'. Can you confirm it was Plus 21 volts and not Negative 21 volts.
For diagnosing faults at a distance careful voltage measurement can go a long way but you need to be very careful not to accidently short any pins. Seeing all the voltage on all the transistor nodes would help pinpoint any issue.
For example
Q601
E =
B =
C =
Q603
E =
B =
C =
and so on.
Thanks Mooly. My probe just broke, which is suprising as it's chinese! Is this measuring with the transistors in place?
In place and the amp switched on. Providing the fault is a 'DC' fault and the amp is not oscillating or unstable (in which case it is a scope job because the results on a DVM then become meaningless) then voltage checks should reveal where there is an issue.
Got it. Might have to go dark for a day or two while I source some decent probes. Postal strike in the UK at the moment and maplin electronics stores are long gone. The Internet is a double edged sword.
Update. Armed with my trusty lack of knowledge I've measured the three pin voltages of all eight main output transistors. (emitter/base/collector?)
I measured from left to right (looking from the front of the unit). T1 e -33v b +35v c -34v q618?
T2 e-34v b +35v c -34v q622?
T3 e -34v b -35v c-35v q620?
T4 e -34v b -34v c -35v q624?
T5 e +21.5v b +54.1v c +21.4 q617?
T6 e +21.5v b +39.8v c +22v q621?
T7 e +21 b -34v c +21.5v q619?
T8 e +21.4 b -39.5v c +22v q623?
F601 is still +21v, however, things have changed on it's neighbour F602 which is now -35v! I'm sure this has resulted from me shorting one of the pins while measuring. I've attached pic of which one. I spose it had to happen. As soon as it did the heatsink got very warm indeed! No smoke though!
Speaker dc has changed: l/h -35.9v (prev 11mv) r/h +17v (prev +2v)
I measured from left to right (looking from the front of the unit). T1 e -33v b +35v c -34v q618?
T2 e-34v b +35v c -34v q622?
T3 e -34v b -35v c-35v q620?
T4 e -34v b -34v c -35v q624?
T5 e +21.5v b +54.1v c +21.4 q617?
T6 e +21.5v b +39.8v c +22v q621?
T7 e +21 b -34v c +21.5v q619?
T8 e +21.4 b -39.5v c +22v q623?
F601 is still +21v, however, things have changed on it's neighbour F602 which is now -35v! I'm sure this has resulted from me shorting one of the pins while measuring. I've attached pic of which one. I spose it had to happen. As soon as it did the heatsink got very warm indeed! No smoke though!
Speaker dc has changed: l/h -35.9v (prev 11mv) r/h +17v (prev +2v)
Thanks for taking the measurements.
We have issues, big issues. I really think this might be outside your comfort zone... 🙂
The power transistors as looked at from the front will have the pins in the order of B-C-E. The PNP ones (the 2SB's) are the same pattern. If you look at the circuit you will see there are two pairs per channel. Providing the 0.22 ohm resistors that go to the emitters are OK (not open circuit) then the collector, the base and the emitter of each 2SD and each 2SB in that channel should have the same voltage.
The voltage measured between base and emitter (so measured between the pins) should be in in the 0.65 volt region.
The middle pin of each goes to the appropriate supply rail.
The mishap with the other channel has likely zapped at least one 'upper and perhaps also one 'lower' output transistor in the parallel pairs. That in itself may not be a huge issue. The other channel with the 21 volt offset was the interesting one but it would have needed the voltages on all the transistors in that channel recording accurately so that we can see what is really going on.
We have issues, big issues. I really think this might be outside your comfort zone... 🙂
The power transistors as looked at from the front will have the pins in the order of B-C-E. The PNP ones (the 2SB's) are the same pattern. If you look at the circuit you will see there are two pairs per channel. Providing the 0.22 ohm resistors that go to the emitters are OK (not open circuit) then the collector, the base and the emitter of each 2SD and each 2SB in that channel should have the same voltage.
The voltage measured between base and emitter (so measured between the pins) should be in in the 0.65 volt region.
The middle pin of each goes to the appropriate supply rail.
The mishap with the other channel has likely zapped at least one 'upper and perhaps also one 'lower' output transistor in the parallel pairs. That in itself may not be a huge issue. The other channel with the 21 volt offset was the interesting one but it would have needed the voltages on all the transistors in that channel recording accurately so that we can see what is really going on.
