Phew. The trouble a little resistor can cause, huh?YEAH!
It turned out that R696 was faulty. After I exchanged it, the relay works
If you look at the thread again, you'll see how we systematically went through the relay driver circuit (though obviously starting at the input would have saved a good amount of time, but you never know that before).
Those don't fail all too often unless operated very close to their maximum working voltage spec, which isn't the case here. If they do, there usually is leakage or bulging (note: old fixing glue around them is normal), commonly along with audible hum in the speakers.
Now, onto the power amp. Proceed with great caution here, for an accidental short or somesuch could make it go poof.
First of all, check the speaker fuses (F601). They must be well under an ohm. Also inspect the fuseholders for corrosion and clean them if necessary.
Also test all the resistors involved in feedback and thereabouts - R601, R653, R657, R659, plus R615 and R671.
Now let's start at the input stage.
Find the junction of R611, R613 and R685. Verify about -18.6 V. (The other ends of these resistors should be at -16.1, -16.7 and -37 V, respectively.)
Locate the junction of R605 and Q601 emitter. Verify about +34.8 V.
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They have 0,1 Ohm
Also test all the resistors involved in feedback and thereabouts - R601, R653, R657, R659, plus R615 and R671.
measure with resistors soldered the all have the right values
Now let's start at the input stage.
Find the junction of R611, R613 and R685. Verify about -18.6 V. (The other ends of these resistors should be at -16.1, -16.7 and -37 V, respectively.)
R611 -11,4 V, -9,7 V
R613 -11,4 V, -9,4 V
R685 -33,4 V, -11,8 V
Locate the junction of R605 and Q601 emitter. Verify about +34.8 V.
33,8 V
Your negative supply seems a little low. That's where one end of R685 should be connected. Do you also get -33.4 V on R625 or R627? With the amplifier off, try determining resistance between the R692/R698 junction (@ relay driver) and R685. They should be more or less directly connected (ohms, maximum).
Do you have any means of measuring small AC voltages (like a 2V~ range)? Then you could probe for ripple on the supplies using a suitable coupling capacitor (not necessary for a fancy multimeter, but some cheap ones tend to deliver entirely messed up AC reading with DC present).
Anyway, even with -B1 being a bit low the amp should still work fine.
Another resistor to check at the input: R603, 6k8.
On to the next differential amplifier stage. Since you have already located R625 and R627, determine the voltages across them in operation. One end should be on negative supply, the other about 2.3 V higher. If so, verify that both are about the right value with the unit off (doing this in circuit is usually not a problem, since bad resistors tend to drift high and can only ever read lower when installed, so if you find one that's high it's probably defective).
This is the stage turned on by the relay, which connects it to its current source around Q607. Not only is there some gain here, level shifting is also performed.
In the next stage (we're getting into medium power territory), check 100 ohm resistors R629, R635, R673 and R675, plus 560 ohm R631. Funny things could happen if any of them were out of spec. Also check 2k2 R681 and R683 and make sure they do in fact connect to ground.
In operation, all of the 100 ohmers should drop about 1.6 volts from the respective supplies.
Incidentally, the +/-0.53 V readings indicated for the non-ground ends of R681 and R683 (Q623/625 bases) in the schematic are nonsense. They should be about +/- 1.1 V from output level, as we have two base-emitter voltage drops from there to the output. (Bias pot adjustment range should cover about +/- 0.9 to +/- 1.2 V.)
Also check quiescent current by measuring voltage across TP1/TP3.
It is certainly not impossible that the amp is oscillating for some reason. I suppose you don't have a scope?
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Another resistor to check at the input: R603, 6k8.
Its ok
On to the next differential amplifier stage. Since you have already located R625 and R627, determine the voltages across them in operation. One end should be on negative supply, the other about 2.3 V higher. If so, verify that both are about the right value with the unit off (doing this in circuit is usually not a problem, since bad resistors tend to drift high and can only ever read lower when installed, so if you find one that's high it's probably defective). This is the stage turned on by the relay, which connects it to its current source around Q607. Not only is there some gain here, level shifting is also performed.
They are both ok and dropping voltage about 2 volts
In the next stage (we're getting into medium power territory), check 100 ohm resistors R629, R635, R673 and R675, plus 560 ohm R631. They are all ok
Funny things could happen if any of them were out of spec. Also check 2k2 R681 and R683 and make sure they do in fact connect to ground. They do
In operation, all of the 100 ohmers should drop about 1.6 volts from the respective supplies.
They do drop 1,6 volts
Also check quiescent current by measuring voltage across TP1/TP3.
I did this before, but didnt let the amp heat up for some time. Now after the amp is running for 10 minutes, its still drifting! But its possible to adjust quiescent current to around 2,5 mV after about 20 minutes
Its ok
On to the next differential amplifier stage. Since you have already located R625 and R627, determine the voltages across them in operation. One end should be on negative supply, the other about 2.3 V higher. If so, verify that both are about the right value with the unit off (doing this in circuit is usually not a problem, since bad resistors tend to drift high and can only ever read lower when installed, so if you find one that's high it's probably defective). This is the stage turned on by the relay, which connects it to its current source around Q607. Not only is there some gain here, level shifting is also performed.
They are both ok and dropping voltage about 2 volts
In the next stage (we're getting into medium power territory), check 100 ohm resistors R629, R635, R673 and R675, plus 560 ohm R631. They are all ok
Funny things could happen if any of them were out of spec. Also check 2k2 R681 and R683 and make sure they do in fact connect to ground. They do
In operation, all of the 100 ohmers should drop about 1.6 volts from the respective supplies.
They do drop 1,6 volts
Also check quiescent current by measuring voltage across TP1/TP3.
I did this before, but didnt let the amp heat up for some time. Now after the amp is running for 10 minutes, its still drifting! But its possible to adjust quiescent current to around 2,5 mV after about 20 minutes
Not particularly useful - you'd need something with a few MHz of bandwidth to spot oscillation (which I suspect might be happening). Any old Hameg would do fine.I have a software scope
UT-71C here.
I think you could give the '61B a shot probing for ripple on +/-B1. Hmm, it it also has a frequency counter up to 10 MHz. Not a scope, but might show whether there is anything fishy going on (a low-level sinewave output from the amp may be needed to keep it from locking onto noise and what have you).Your measurements look entirely OK to me, so I'm kinda running out of ideas here. We may need to take a look at the input stage again. Maybe the input Qs are leaky or something.
What are the DC voltages across R615 (22k) and R671 (270k)? R671 hopefully does have a low-resistance connection to ground?
On the input side, what are the voltages across R601 (15k) and R603 (6k8)?
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Eek!
Well, we haven't looked at the 0.22R emitter resistors yet (R645/47/49/51). Or the solder joints on the finals (output drivers), for that matter. I suggest you check both.
Loop gain seems to be very low.
An unrelated question:
Could it be that C625 is a 2.2 µF type and not, as I initially suspected, 2.2 nF? That would mean about a 34 dB closed-loop gain in the audible range, which seems more in line with what I'd expect and what you find in the power amps of other Rotels.
This is supposed to ensure that bias current is not excessive even when the amp is nice and toasty.
I suggest you decrease bias again until DC offset is where it was before.
What's DC offset like if there is a load connected (you can enable the "remote" speakers and measure there, headphone output would be fine, too)?
I suggest you decrease bias again until DC offset is where it was before.
What's DC offset like if there is a load connected (you can enable the "remote" speakers and measure there, headphone output would be fine, too)?
Since I adjusted bias, my DC offset went higher
Also I can't adjust the offset anymore with its trimpot DC offset is not changing with load either...So, hmm shall I continue with the measurements you suggested before? I get a bit nervous now running the amp now for longer time...Still my connected headphone sounds ok when I play music...
Eek!
Well, we haven't looked at the 0.22R emitter resistors yet (R645/47/49/51). Or the solder joints on the finals (output drivers), for that matter. I suggest you check both.
Loop gain seems to be very low.
my multimeter reads 0,3 Ohm, I guess thats ok...
It could be you do need a coupling cap. Do you happen to have a small film cap (>= 15 nF or so) or bipolar electrolytic with at least a 50..63 V rating floating around?My Multimeter reads freaky values between 13-160 volt AC
Also in any case check whether R671 is OK (270k) and connected to ground, and the DC voltages on both ends in operation. Same for R603. Any kind of offset found here will be seen on the output. Can you measure on the ends of C605 (otherwise use R607 and R609)? A bad connection on Q603's base alone could be enough to cause substantial offset, since base biasing current must flow through R601 and R603 to ground. Between R601 and R607 (ends connecting to Q603) there must be no more than the usual 0.6 V B-E voltage drop.
It's certainly not impossible that one of the drivers (Q623/Q625) is toast. How warm do they get in operation? Any suspicious-looking solder joints on them? Also check their emitter reistors R641/R643, 910R 2W.
Which voltages do you get when (carefully) measuring on the non-grounded end of R683 and the non-supply end of R641? Same for R681 and R643. For reference, we also need the output DC offset.
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http://dl.dropbox.com/u/163999/Web Bilder Foren/"rotel_ra-820bx3_sm_[ET]".pdf
Hey, this is the only commercial japanese amp I know which is inverting !
I did not read all the file but this may help. I had recently to repair a Rotel amp not working and presenting almost full DC power voltage at one of the outputs. The output fuse protection was not burnt but happily, the connected speaker survived.
It took me some time to find the faulty component, to my experience, it is a very rare failure : unexpectedly, it was the main feedback resistor which was open-circuit despite having no slightest sign of distress at all (overheat for example).
Hey, this is the only commercial japanese amp I know which is inverting !
I did not read all the file but this may help. I had recently to repair a Rotel amp not working and presenting almost full DC power voltage at one of the outputs. The output fuse protection was not burnt but happily, the connected speaker survived.
It took me some time to find the faulty component, to my experience, it is a very rare failure : unexpectedly, it was the main feedback resistor which was open-circuit despite having no slightest sign of distress at all (overheat for example).
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