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Sanity check on connecting signal generator to receiver aux-in

B

bdondo

Active Member
I'm troubleshooting a Kenwood KR-5600 (see also: this thread) where I recapped and replaced some transistors on the power amp board. Now I have very weak audio at the left speaker but seemingly good audio volume in the right channel.

To troubleshoot, I first traced the signal in the working channel to have something to compare to.* To do this, I:
  • Connected the positive end of the signal generator cable (alligator clip) to the tip of an RCA cable, and wrapped electricians tape around the sleeve to prevent an accidental short (I should probably cut off the jack on one end and connect directly to the wire).
  • I connected the black signal generator clip to chassis ground on the receiver.
  • I disconnected the ground clips from the scope probes.
  • Set the signal generator to 1KHz at 1Vpp (I also tried lower voltages but this seemed to produce a better signal to trace).
  • Connected one scope probe to the positive end of an 8-ohm dummy load and observed the amplified signal.
  • Plugged the receiver into a dim-bulb tester and turned it on.
  • Connected the other probe to various spots in the power amp signal path and was able to trace the signal everywhere I thought I should see it (I think...).
So then I decided to check the left channel:
  • Powered down.
  • Moved RCA cable from right to left aux input.
  • Moved first scope probe to left dummy load
  • Powered on with DBT
  • DBT went bright and relay shut off power to the amp.
So now I'm wondering if I just did something really dumb here, like maybe I'm not supposed to connect the signal generator black cable to chassis ground? I don't see how that would cause a short thought. I'm hoping to at least remove operator error from the equation here. I since noticed that the specs for the amp show 150mV input for aux, but I know that real-world aux-in devices are higher than that.

I searched all over this site and the whole web and didn't find anything that really explained exactly how to connect the signal generator to the input. I just see more general statements like "connect the signal generator to an aux input" without much detail. Do people cut off the ends of RCA cables and connect to the bare wires? If I want to test both channels at the same time, do I feed the same signal generator signal to each RCA channel? I figure I can make a little jig of some kind to mount the RCA cables on to keep them organized and away from each other.


* I'm not totally convinced that the right channel is working properly (I don't measure any gain after the first stage), but at least the signal is going all the way through.
 
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Depending on what type connector you have on your sig gen, get a T adaptor for RCA cables. Then you can just plug the RCA ends into the Aux. inputs.

My sig gen has BNC jacks so I got BNC to female RCA adaptors.
 
Yeah, I actually have a double but was going to pull it apart. The one male to two female adapter should work well and take up less space.
 
My signal generator for troubleshooting is an old phone with a signal generator app. I use a headphone to L/R RCA cable to feed both channels at the same time. This makes it easy to test both channels at the same time for comparison between channels.

One advantage of the phone as signal generator is I do not have to worry about ground connections because the phone is isolated. I like battery operated test equipment because it easily avoids improper grounding issues between the device under test and the test equipment.

Another advantage is that the phone volume level easily controls the signal generator output to both channels to a level suitable for equipment.

As for why your DBT went bright when testing the left channel, that is a head scratcher based on your described hookup. I would double check the wiring, particularly at the speaker outputs. Maybe hook everything back up and with power off, measure resistance across the left speaker output. Hopefully it is 8 ohms.
 
RTally said:
My signal generator for troubleshooting is an old phone with a signal generator app. I use a headphone to L/R RCA cable to feed both channels at the same time. This makes it easy to test both channels at the same time for comparison between channels.

One advantage of the phone as signal generator is I do not have to worry about ground connections because the phone is isolated. I like battery operated test equipment because it easily avoids improper grounding issues between the device under test and the test equipment.

Another advantage is that the phone volume level easily controls the signal generator output to both channels to a level suitable for equipment.

As for why your DBT went bright when testing the left channel, that is a head scratcher based on your described hookup. I would double check the wiring, particularly at the speaker outputs. Maybe hook everything back up and with power off, measure resistance across the left speaker output. Hopefully it is 8 ohms.
Click to expand...

The phone signal does work nice, but I have found it can be a bit weak. Is there a way to boost the signal to make it closer to what service manual request for testing? Even if you turn the volume up all the way the phone's signal is still low. Perhaps are some phones better than others?
 
RTally said:
My signal generator for troubleshooting is an old phone with a signal generator app. I use a headphone to L/R RCA cable to feed both channels at the same time. This makes it easy to test both channels at the same time for comparison between channels.

One advantage of the phone as signal generator is I do not have to worry about ground connections because the phone is isolated. I like battery operated test equipment because it easily avoids improper grounding issues between the device under test and the test equipment.

Another advantage is that the phone volume level easily controls the signal generator output to both channels to a level suitable for equipment.

As for why your DBT went bright when testing the left channel, that is a head scratcher based on your described hookup. I would double check the wiring, particularly at the speaker outputs. Maybe hook everything back up and with power off, measure resistance across the left speaker output. Hopefully it is 8 ohms.
Click to expand...
I've been tempted to try a phone for a signal generator but haven't yet. The battery-operated aspect would be nice.

I ended up getting a BNC->RCA adapter and an RCA splitter, and it works well.

As for the issues with the circuit, I took power amp board out and double-checked everything. Turns out I had a capacitor in backwards (C14, which is in the right channel circuit. I'm not sure if that was the cause though. I had also swapped out the 200K single-gang balance control, which had four legs, with a 100K dual-gang three-legged pot. So I swapped the old one back in as well (the old one has a broken shaft and I can't find an exact replacement). So one of those two things was likely the problem, but not sure which. I'd normally fix one problem at a time and then test, but this receiver is a real pain to access the power amp board.

Screenshot 2023-10-13 at 7.16.12 AM.jpg
 
Warren-Oates said:
The phone signal does work nice, but I have found it can be a bit weak. Is there a way to boost the signal to make it closer to what service manual request for testing? Even if you turn the volume up all the way the phone's signal is still low. Perhaps are some phones better than others?
Click to expand...

A cable from headphone plug to RCA is a common way to connect phones and portable devices to sound systems. Before I got my streamers, I used a cable connection from my phones & iPad to various sound systems to play music. Generally, I set phone volume to max (or just below) and use the receiver/preamp volume control.

I had a portable mp3 player that had a weak output. I used a CMOY headphone amp to boost the output. A side benefit is that it also made the SQ much better.
 
RTally said:
A cable from headphone plug to RCA is a common way to connect phones and portable devices to sound systems. Before I got my streamers, I used a cable connection from my phones & iPad to various sound systems to play music. Generally, I set phone volume to max (or just below) and use the receiver/preamp volume control.

I had a portable mp3 player that had a weak output. I used a CMOY headphone amp to boost the output. A side benefit is that it also made the SQ much better.
Click to expand...
The classic CMOY. I built one a long time ago, but it seems like I never got it working right. I ended up building an O2 headphone amp, which works great. Good idea using a headphone amp to boost the signal. I suppose that would work if an iPhone, etc. couldn't put out 150mV or whatever is needed.
 
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