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9090DB Right Channel only works in Mono

If both channels work when in Mono, the amplifier, protection relay and speaker selector switch are all working for both channels. Forget the driver board. The dropout is occurring in the preamp in the signal path prior to the mono switch. In a 9090DB most of the signal path through the preamp is prior to the mono switch.

- Pete
 
LBPete said:
If both channels work when in Mono, the amplifier, protection relay and speaker selector switch are all working for both channels. Forget the driver board. The dropout is occurring in the preamp in the signal path prior to the mono switch. In a 9090DB most of the signal path through the preamp is prior to the mono switch.

- Pete
Click to expand...

Hi Pete,
Thank you, I really appreciate the reply.
I am leaning towards have a qualified person resolve this.
Any recommendations?
Thanks again
Harry
 
I don't have a specific recommendation in that area but google is your friend if you want to find something close by. Search for Stereo repair. Here's one that came up in North Myrtle Beach, Tommy's TV and electronics. They have a lot of good google reviews and have been in business since 1967. No harm talking to them.

- Pete
 
LBPete said:
I don't have a specific recommendation in that area but google is your friend if you want to find something close by. Search for Stereo repair. Here's one that came up in North Myrtle Beach, Tommy's TV and electronics. They have a lot of good google reviews and have been in business since 1967. No harm talking to them.

- Pete
Click to expand...
Thanks, Pete

I reached out to Tommy's and he doesn't work on stereos. He was a nice gentleman and spoke with me for several minutes. He thought that there might be someone in the Charlotte, Charleston, or Wilmington areas. I did a Google search for vintage audio and didn't turn up anything within a reasonable driving distance.

I am sure your prior post on the problem is in with the Dolby board is correct.

I reassembled the unit and started playing with the switches in the front end. Here are my observations
  • In Stereo mode and the Dolby NR is switched to Dolby FM both Power Meters register a strong signal. Even though the Power Meters are both showing a strong signal still very little volume out of the right channel.
  • In Mono mode the volume is consistent for both speakers, the Power Meters register a lower output signal.
  • Significantly better sound quality when Dolby NR is in Dolby FM switch setting.
  • The unit seems to get very hot, starting in the back center near the transistors. I don't ever remember it running this hot and is concerning.
I may consider removing the Dolby board again but hesitate, it was a lot of effort with no apparent results.

Any thoughts?

Thanks!

Harry
 
FWIW I just fixed a Yamaha R-500 receiver that was only putting sound out through both channels in mono. Fortunately in that case the only thing between the inputs and the mono switch was the input selector. The switch was the type that converts rotary input to linear movement of contacts along a rail. One of the sliding contacts was especially dirty, so I cleaned it with a piece of paper soaked in Deoxit, and cleaned the contact rails with a piece of white drafting eraser. Now it works perfectly.

The problem is it required fully desoldering the switch, disassembly of the front panel to remove the switch, and disassembly of the switch itself. Not a simple job.

The upside was in this case it was fairly easy to diagnose without desoldering anything. Continuity checking the switch inputs to output for each channel revealed the problem.
 
crv00 said:
FWIW I just fixed a Yamaha R-500 receiver that was only putting sound out through both channels in mono. Fortunately in that case the only thing between the inputs and the mono switch was the input selector. The switch was the type that converts rotary input to linear movement of contacts along a rail. One of the sliding contacts was especially dirty, so I cleaned it with a piece of paper soaked in Deoxit, and cleaned the contact rails with a piece of white drafting eraser. Now it works perfectly.

The problem is it required fully desoldering the switch, disassembly of the front panel to remove the switch, and disassembly of the switch itself. Not a simple job.

The upside was in this case it was fairly easy to diagnose without desoldering anything. Continuity checking the switch inputs to output for each channel revealed the problem.
Click to expand...

Thank you AK for the insights.

I am thinking it is in the switches as well. In addition to the behaviors I noticed in my last post to Pete, I don't think some of them are working as expected. For example, the Mid Tones dial switch is working fine, there is a clear difference in sound as I move it from side-to-side but the Bass and Treble switches don't seem to impart any changes in tone when tested end-to-end. Likewise with a few of the push-button switches don't seem to affect any changes in the sound.

In lieu of shipping the entire unit I was wondering if it would be possible to find a technician and pull the Dolby and Equalizer Boards for rebuilding. Do you think that is a viable idea?

Thanks

Harry G
 
The dropout is occurring at a point prior to the mono switch. That switch is on the same board as the Dolby push switches. That pretty much means the dropout is related to the Dolby and it's push switch board. Any of the related switches as well as the Dolby board pass throughs could be responsible. Before you tear anything apart, "exercise" all those switches and note if there are any changes in the sound when you move them.

If you do open it up to clean the switches, look at the wires that connect the switch board to the Dolby board. Early units with have a green flexible circuit board like connector. Later production uses black ribbon wire. If you have the green flex pc board, be very very careful with it. It breaks easily and that will ruin you day.

- Pete
 
LBPete said:
The dropout is occurring at a point prior to the mono switch. That switch is on the same board as the Dolby push switches. That pretty much means the dropout is related to the Dolby and it's push switch board. Any of the related switches as well as the Dolby board pass throughs could be responsible. Before you tear anything apart, "exercise" all those switches and note if there are any changes in the sound when you move them.

If you do open it up to clean the switches, look at the wires that connect the switch board to the Dolby board. Early units with have a green flexible circuit board like connector. Later production uses black ribbon wire. If you have the green flex pc board, be very very careful with it. It breaks easily and that will ruin you day.

- Pete
Click to expand...

Hi Pete,
Thanks for the reply.
I had the older ribbon cables, they were white, not black. I already replaced them. I am not looking forward to removing the Dolby board and Push Switchboard but will. I've already tried cleaning the switches several times with no effect but will try again.
Thanks again
Harry
 
If they were white, they are probably not original. There is no guarantee that more cleaning will fix it. The only way to verify where the dropout is occurring is by tracing the signal with an oscilloscope. Did "exercising" the switches change anything?

- Pete
 
Any chance you ever got this figured out? I've been having the exact same issue with mine for years. This will be my third time wasting many days troubleshooting lol...
 
burrmanchu said:
Any chance you ever got this figured out? I've been having the exact same issue with mine for years. This will be my third time wasting many days troubleshooting lol...
Click to expand...
Flip the Dolby selector switch a few times. If the channel comes and goes the Dolby board needs to be rebuilt.
 
LBPete said:
If they were white, they are probably not original. There is no guarantee that more cleaning will fix it. The only way to verify where the dropout is occurring is by tracing the signal with an oscilloscope. Did "exercising" the switches change anything?

- Pete
Click to expand...
Hello, I am helping Harry out with his issue. The first thing was that under a DBT the bulb was super bright so turned it off. Removed driver board and no issues with DBT. I then suspected some issues with the rebuilt driver board with some wrong value resistors. I had a spare working 9090DB driver board that I plugged in this time no issue with DBT. Checked bias / dc offset and it was good.

Issue with right channel only working on Mono is still there. I did a signal trace and right channel is dropping once it reaches the volume pot. Signal going into dolby board is good. I suspect there is issues with the dolby switch and thinking about pulling the dolby board out. Anyone happen to know some areas on the bottom side of Dolby board to check signal path ? I wasn’t able to find that info from schematic.
 
I had to take my selector switch apart to really clean it. deoxit spray only helped temporarily. then coated with F5
 
If the Dolby board has not had the feedthrough rivets reworked, it is almost certainly bad feedthrough(s) causing the problem. They all need to be reworked to have a reliable receiver. It is very involved. Read posts on this forum that describe the process.

P.S. I do not pin the rivets, I just resolder with a good thick solder fillet. I have done many, and have never had an issue.
 
AE Bob said:
If the Dolby board has not had the feedthrough rivets reworked, it is almost certainly bad feedthrough(s) causing the problem. They all need to be reworked to have a reliable receiver. It is very involved. Read posts on this forum that describe the process.

P.S. I do not pin the rivets, I just resolder with a good thick solder fillet. I have done many, and have never had an issue.
Click to expand...
That has already been done on this unit.
 
plyzo said:
That has already been done on this unit.
Click to expand...
With all due respect to AEBob, just resoldering the feedthrus does not work long term. You have to insert a bus wire though the feedthru, bend it over on both sides of the board and solder it on both sides. This is the only way to make the unit reliable. there are about 60 feedthrus. You have to remove the wafer switch and do every feedthru. If the unit has the green flex cable, it must be replaced with ribbon cable. the green cables become intermittent.
 
plyzo said:
I did not perform the work on the dolby board but it looks as if it was done properly with the rivets.
Click to expand...
Use a scope and see if both channels are going into the Dolby board. They enter (from the selector switch if memory serves) and then go out to the tape outputs and eventually go out again. I bet the channel is lost in the Dolby board.
 
Signal is coming into the dolby board.
So with the scope and checking the mono switch, I am only getting signal on the right side of the switch circled in RED. I assume that the green side should be getting a signal as well but it's not. When I move to mono then Red circle and the one above it have a signal.

F2654.png
 
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One other interesting thing is that only the right side meter is working which is the signal that is being lost.
 
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