CR-840 puzzler
- Thread starter RayN
- Start date
If I've been following along correctly, you now have both predriver and power pack out of the circuit. It would be very unlikely that anything else in the power amplifier circuitry would produce an output offset - all that's left are "passive" parts and the PCB traces. Still, there's no harm in verifying. Predriver pins 6 and 15 are power supply connections - those voltages are normal. That all other pins are at 0V is also normal under the circumstances.
Cheers,
chazix
RayN
Active Member
Yep, I've got one.
RayN
Active Member
Waiting for the predriver to get here, I put the right channel output driver back in and I get sound out of the speaker, halfway there. But I have two more questions while I wait: 1) the tuner works but the volume is extremely low, and I get nothing if I engage the FM mute switch. Just curious if anyone knows what would cause this. I have to push the volume all the way up just to barely hear a station. 2) for some reason my voltages around the power supply area are now running about 10V high. This happened before I put the output driver back in. Where I'm supposed to see ~50V I get 60 and the ~41V areas are now showing 50V. Is this perhaps related to the fact that one output driver is out and just emulated with resistors?
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No. Power supply voltages should not vary by much depending on whether the power amps are intact. (Unless the power amps are not only intact, but being run into a load at substantial volume. In that circumstance, the bulk supplies will sag to some extent.)
When you talk about ~50V, are you referring to the rails that power the predriver (pins 6 and 15)? Those rails should be regulated. If they are varying, there is a power supply problem.
(Or maybe a measurement problem. What are you using as a ground connection for your meter?)
It would be a good idea to not install any more power amplifier components until and unless you are confident that the power supply is healthy.
I don't have any insight about the tuner question. What about AM? (Note that external antennas are required to get any appreciable reception. On the CR-x40 series, this is true for both FM and AM. That is, you need two antennas, typically a "T" for FM and a loop for AM.)
RayN
Active Member
Yes, I'm getting ~ +-60V at pins 6 & 15 as well as at various legs of T801 - 805. I'm using a DMM with an alligator clip on black attached to the chassis ground. I finally got the SM with the correct schematic so I'm trying to figure out where the bump in voltage is coming from.
I guess it's too late now, but it is very risky for any predriver that's installed to be exposed to those voltages. I don't suppose you have a variac, so you could power the receiver at maybe 90VAC until you get the regulated power supplies straightened out? (A variac can be used in conjunction with a DBT. The variac limits voltage, and the DBT primarily limits current.)
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RayN
Active Member
Ok, digging back into this thing, although it's quickly starting to be not worth my time. However, I'm learning. Looking at the yellow outline in the image I've included, the voltage I'm seeing at the collector of TR801 is ~64V, so 10V too high. If I lift the cathode of D801 I measure ~22V coming out of the diode. So I figured some active component must be making this extra voltage. I removed TR801 and TR803 and still saw 64V at the collector pad of TR801. Now I'm thinking there must be a shorted component somewhere. Thoughts?
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I'm sorry for how this sounds, but I don't think your reasoning has any basis in the fundamentals of how this circuitry is supposed to work.
The DC voltage at the collector of TR801 should be the result of the AC coming from the power transformer, half-wave-rectified by D801, and smoothed by C803. You might web surf for "half wave rectifier" to learn about this, in particular to learn what DC voltage to expect, given the 39VAC input.
I can think of only a couple of explanations for the excessive DC voltage reading at TR801's collector:
- You have a grossly high voltage on your AC power line, like 140V. If that were so, you'd probably be changing light bulbs in the house every week or so.
- The power transformer has a quite unusual defect, such that more than 39VAC is on the orange wire. I don't think I've ever encountered this, and I can't think of a defect that could cause it without manifesting other symptoms.
- Your DC voltage readings are not really with respect to zero. That is, the point you are using as ground for your meter connections is not really at 0V. I think we already established that you are using metal parts of the chassis, which should be fine. I think this explanation is, let's say, the least implausible. It would mean that there is a bad connection somewhere in the ground network, perhaps a cracked trace on the circuit board.
RayN
Active Member
@chazix You are not wrong when you say my reasoning needs work, I am certainly just a learning novice and this did have me confused...primarily because this is the first I've heard of a half wave rectifier. Now I've read about it and it helps quite a bit. But why did I see a different voltage at the diode cathode when lifted as opposed to when it's soldered down? I'm reading the correct A/C voltage at the fuse which means my transformer is fine. I have two DMMs, I'll try measuring 801 and 802 collectors at the same time. I'll also see what I can find out about the ground.
Thank you! I appreciate the information.
Thank you! I appreciate the information.
When soldered down, C803 comes into play, tending to maintain the DC voltage near the + peak that is reached by the AC voltage. The peak voltage from 39VAC is sqrt(2)*39, about 55V. There is some loss compared to the AC peak due to the voltage drop across D801 and other factors.
When lifted, the DC voltage reading will depend on how your meter responds to an unfiltered half-wave rectified AC voltage, which I don't know; but it will read much less than the peak, unless you have a very, very unusual meter.