Teac C-3RX - Noise With dbx Engaged

[David_NC]

Background: a friend gave me this deck about a decade ago. Recently, I decided it was time to bring it out of retirement. It's in good condition overall, but it needed a capstan belt, which I've replaced. After lubing a few parts, the mechanical portion seems to work just fine. Playback is now good after cleaning controls and switches.
The only issue it seems to have is really loud, distorted sound and a pegged VU meter in the right channel when a tape is being played with dbx switched on. This does not occur with no NR or with Dolby or ever in the left channel. With the monitor set to source, and a source connected to the input jacks, the dbx behaves normally. This leads me to believe that the issue is with the right encoder section of the dbx board. I'm really not sure where to go next with this and would appreciate any thoughts, experience, or ideas! Thanks.

edit: This leads me to believe that the issue is with the right decoder section of the dbx board, not the encoder.

 

[Blue Shadow]

Playback with dbx adn the meter pegs monitoring tape
Set to source, no problem

This indicates the problem is in the decode function. Dbx is encoded onto the tape and decoded from the tape.

Then the issue is just following a signal through the left and right dbx decode functions to find where a mono signal differs.

 

[David_NC]

I may need to repost this question in a different forum; the Tape forum seems to be pretty quiet.
It's time to get back on this one. I'd like to get this deck working properly, as it's a nice deck with one real issue. I tested a bunch of points previously and found a few with the noise. Below are a couple pics - one of the deck's innards and one of the dbx board. I circled the area where the signal on the right channel was dramatically different from the one on the left; the difference being the high level distortion.


I will get a much better pic of the components in question so they can be identified. If anyone has had any experience with an issue like this, I'd love to hear it. Thanks.

 

[dankik]

First, I have a couple of questions:

1. With dBx OFF (on both record and playback), you can record and playback normally, without any problems... correct?
2. If you record with dBx ON and playback with dBx OFF what do you hear (it may sound a little compressed)?
3. If you record with dBx OFF and playback with dBx ON what do you hear (it may sound a little compressed)?
4. With dBx ON during recording and ON during playback (if I understand you correctly) you hear a loud distorted sound that pegs the meter (only on the right channel), correct?
5. With the monitor set to source you are not monitoring the tape, but the input signal and I would guess the dBx is not applied to this source signal that's why you don't hear the noise.

Do you have the schematics for the C-3RX. If not, you can download the SM and schematic from hifiengine.com.
https://www.hifiengine.com/manual_library/teac/c-3rx.shtml
Sorry for the following questions, just trying to establish the extent of what we can do:

1. Do you have an oscilloscope? If you do, then we can trace the wiring from the input of the dBx board and compare the bad channel audio signal with the good channel.
2. Do you have a good soldering iron and do you know how to unsolder/solder components on a PCB?

 

[David_NC]

First, I have a couple of questions:

1. With dBx OFF (on both record and playback), you can record and playback normally, without any problems... correct?
2. If you record with dBx ON and playback with dBx OFF what do you hear (it may sound a little compressed)?
3. If you record with dBx OFF and playback with dBx ON what do you hear (it may sound a little compressed)?
4. With dBx ON during recording and ON during playback (if I understand you correctly) you hear a loud distorted sound that pegs the meter (only on the right channel), correct?
5. With the monitor set to source you are not monitoring the tape, but the input signal and I would guess the dBx is not applied to this source signal that's why you don't hear the noise.

Do you have the schematics for the C-3RX. If not, you can download the SM and schematic from hifiengine.com.
https://www.hifiengine.com/manual_library/teac/c-3rx.shtml
Sorry for the following questions, just trying to establish the extent of what we can do:

1. Do you have an oscilloscope? If you do, then we can trace the wiring from the input of the dBx board and compare the bad channel audio signal with the good channel.
2. Do you have a good soldering iron and do you know how to unsolder/solder components on a PCB?

Hi,
1 correct
2 sounds bad / compressed
3 dbx on when playing back gives very high level distortion that pegs the meter, all on the right channel. Left channel is fine
4 3 above happens when dbx switch is on during playback
5 correct. Source is unaffected
I have the schematics and scope and have traced the issue to the area circled in blue at the top of the second picture. The opamps were fine; signal in yielded a similar output but at a higher level. There are two black components at the far left of the blue circle; I meant to include those inside the circle. I believe the noise starts there, based on what my scope showed. It's been a while, and I can't recall every detail, but those two stand out. I'll get a better pic of them showing ID's and details.

 

[dankik]

So, here's something interesting. If you look at the red circled variable resistor. Its rotation does not look like the other 3 below it (which look all about the same). Also, if you look at any column (2nd, 3rd, 4th) of other VRs the rotations look about the same for each channel.

Did you move the red circled VR?

• Take a marker and put a reference mark on both the rotating part of the VR and the board (so you can return it back to this position) then try to turn it to match the rotation position similar to the other 3 in that column and see if that helps.
• I can't read the board ref numbers but if you get a sharper photo we can look to see what that VR controls. Looks like it might be the input gain control.

 

[Tinman]

Before you get all carried away, did you deoxit the switches? I have had a few of these decks and they tend to get cranky if they sit a long time. I had to always clean the switches and exercise them a bit. I now have a C-3X, but same issues with the toggle switches on the right. Make sure those are working correctly BEFORE assuming there is an electronic problem.

BTW, we are now seeing not only capacitor failures on these decks, but also the film caps are going bad. (The red and green one's) This is bad news, but it's happening. More work....

Lastly, good spot on the VR. I'd think it odd that it could cause anything other than a level difference, but you never know. I have yet to see everything.

If you own an ESR meter, I'd poke the electrolytic caps just to see what shape they are in. I'd ABSOLUTELY check the big one's in the power supply in these decks!

 

[oldvinyldude]

Ditto on cleaning contacts first.

Similarly, I would clean the VR in question, but before doing so, it would be interesting to see what voltage is present on the outbound leg of it. In addition to marking it for returning it to it's 'as delivered' position, it would be good to have this value, as cleaning might change it's resistance, even though it is returned to the right marked spot.

Let us know if cleaning\changing the VR position has any effect on the distortion and level (after cleaning\testing of the items that Tinman suggested.