Lafayette LR-9090 right channel out

[aurora01]

Working on this lr-9090 no right Channel I posted earlier last week but can find original thread,

The protection relay engages I have the same voltages on the output transistors on the two amp boards, but looking at the voltage coming off the rectifier diodes pin 11 is showing positive voltage when the diagram shows should be negative , i checked the filter caps and there ok, guess my question is am I looking at a rectifier issue and would this cause an outage on the right channel

 

[Robisme]

The schematic looks to be wrong. If you look at the rectifier diodes, the positive side of the sine wave will conduct through the diodes that connect to pin 11. Pin 11 should be a positive voltage.

Rob

 

[peteCWA]

Rob is correct. And, if it was supposed to be negative, C7 polarity would be in error.
Drawing is incorrect. I can't remember if we issued a service addendum and/or later schematics were corrected.

 

[peteCWA]

What are you using for inputs to test audio output?
Engage the STEREO/MONO to MONO.
Exercise several times the Audio Mute Button and the Tape Monitor switch.
Over time, switches with little use, can develop oxidation on the internal contacts and become intermittent or non functional. Exercising the switches can wipe away the oxidation and return the switch to good use.

 

[aurora01]

What are you using for inputs to test audio output?
Engage the STEREO/MONO to MONO.
Exercise several times the Audio Mute Button and the Tape Monitor switch.
Over time, switches with little use, can develop oxidation on the internal contacts and become intermittent or non functional. Exercising the switches can wipe away the oxidation and return the switch to good use.

I’ve tested the voltage readings on Aux, I have tried deoxit on all the switches several times, it’s possible that the protection relay contacts could be dirty for right channel but would I not. Have the same voltage on the outputs if that were the case, I’m wondering if the problem is on the tone board,

 

[aurora01]

I’ve tested the voltage readings on Aux, I have tried deoxit on all the switches several times, it’s possible that the protection relay contacts could be dirty for right channel but would I not. Have the same voltage on the outputs if that were the case, I’m wondering if the problem is on the tone board,

I’ve tried Both tuner and Aux for audio testing

 

[peteCWA]

I would be very careful using Deoxit on switches especially if the internal structure holding the contacts is made of paper phenolic material. Sometimes just using WD-40 will work fine to clean the internal push button switches without causing long-term problems.
What are you using for test equipment?
Have you checked the obvious; both speakers are working and connected properly?
Do you have the same problem with stereo headphones?
What's your trouble shooting experience level?

With stereo circuitry, you have two identical circuits. With an oscilloscope, it would be very easy to see where signals are passing and where they are not.
Also, with careful viewing of the schematic, you could switch the L and R input connections to the audio output board make sure both audio output channels there are working. If they are, the problem is further back in the chain.
I would also not provide any signal to the working channel when doing trouble shooting.

 

[aurora01]

I would be very careful using Deoxit on switches especially if the internal structure holding the contacts is made of paper phenolic material. Sometimes just using WD-40 will work fine to clean the internal push button switches without causing long-term problems.
What are you using for test equipment?
Have you checked the obvious; both speakers are working and connected properly?
Do you have the same problem with stereo headphones?
What's your trouble shooting experience level?

With stereo circuitry, you have two identical circuits. With an oscilloscope, it would be very easy to see where signals are passing and where they are not.
Also, with careful viewing of the schematic, you could switch the L and R input connections to the audio output board make sure both audio output channels there are working. If they are, the problem is further back in the chain.
I would also not provide any signal to the working channel when doing trouble shooting.

This unit has 8 emitter resistors I’ve checked and compared the left and right channels and they are pretty close to one another, I’ve gone through all the resistor on both amp boards and found one defective and replaced, resoldered to resistors on the power supply board that were cracked, my knowledge of schematics is limited on tuners but I do have a background in automotive module schematics so I’m not unfamiliar with circuitry.

I’ve done many cleanings on stereo equipment using different kinds of Deoxit.

This tuner is a bit more complicated because it can handle 3 sets of speakers,. I’ve tried the aux side with a cd player and still nothing from right channel so it’s not limited to the tuner, I don’t have headphones at home but will check that when I get to my audio store.

I may pull the left channel amp boards out and remove the relay and clean but I doubt it’s going to prove anything

 

[peteCWA]

You didn't answer some of my questions but I'll let that go.
The RECEIVER is not that complicated in regards to the number of speakers that can be connected;
You can use Speaker A or Speaker B or Speaker C or Speakers A and B or Speakers A and C.
It's all in the switching and not a lot circuit mystery.

In reviewing my old repair history for the LR-9090 back in the 20th century, it looks like I took in a total of 72 of them for repair over a span of years. One complaint that was somewhat common was low sound or no sound in one channel.

The attached drawing (I did very quick but it should readable) identifies where al these controls mentioned below are on the front panel:
In 12 instances, BALANCE control was turned to far to the left or right.
Cleaned and exercised DUAL TAPE MONITOR switch – 2 times
Cleaned and exercised AUDIO MUTE switch – 2 time
Cleaned and exercised MODE switch – 3 times
Cleaned and exercised TREBLE TURNOVER switch – 2 times
Cleaned and exercised BASE TURNOVER switch – 1 time
Cleaned and exercised TONE switch – 3 times

The rest of the repairs had to do with various semiconductors, caps, resistors on the audio output boards failing, coupling caps between stages going defective, transistors or parts falling in various parts of the receiver circuitry.

If you have the right consumer grade test equipment, finding the problem goes a lot quicker and with a lot less diddling in the dark.

 

[Robisme]

Working on this lr-9090 no right Channel I posted earlier last week but can find original thread,

Here is the original thread. I had noticed that is was mistakenly labeled a Hitachi. Didn’t mention it in my reply because it didn’t really matter.

Maybe you could have them merged.

Rob

 

[aurora01]

You didn't answer some of my questions but I'll let that go.
The RECEIVER is not that complicated in regards to the number of speakers that can be connected;
You can use Speaker A or Speaker B or Speaker C or Speakers A and B or Speakers A and C.
It's all in the switching and not a lot circuit mystery.

In reviewing my old repair history for the LR-9090 back in the 20th century, it looks like I took in a total of 72 of them for repair over a span of years. One complaint that was somewhat common was low sound or no sound in one channel.

The attached drawing (I did very quick but it should readable) identifies where al these controls mentioned below are on the front panel:
In 12 instances, BALANCE control was turned to far to the left or right.
Cleaned and exercised DUAL TAPE MONITOR switch – 2 times
Cleaned and exercised AUDIO MUTE switch – 2 time
Cleaned and exercised MODE switch – 3 times
Cleaned and exercised TREBLE TURNOVER switch – 2 times
Cleaned and exercised BASE TURNOVER switch – 1 time
Cleaned and exercised TONE switch – 3 times

The rest of the repairs had to do with various semiconductors, caps, resistors on the audio output boards failing, coupling caps between stages going defective, transistors or parts falling in various parts of the receiver circuitry.

If you have the right consumer grade test equipment, finding the problem goes a lot quicker and with a lot less diddling in the dark.

Thank you for the reply, I’ve gone through all the switches a number of times cleaning and cycling them on and off, I checked dc offset and left is 4.5 mv positive and right is 4.5 mv negative, I don’t have a scope just a fluke meter and cap tester currently , I will probably remove the amp boards and recap and dig further,

My last question is I’ve traced the amp wires and looking at the schematic the board closest to the transformer would handle the right channel correct?

 

[Robisme]

Can you see the inputs to the amp boards? Or the outputs of the preamp? Play music, no speakers attached, and check for AC voltage. May have to turn it up about half way.

Play music that is does not have a lot of quiet passages. The voltage will vary with he music.

This will tell you if you are getting signal from the preamp.

Rob

 

[aurora01]

Can you see the inputs to the amp boards? Or the outputs of the preamp? Play music, no speakers attached, and check for AC voltage. May have to turn it up about half way.

Play music that is does not have a lot of quiet passages. The voltage will vary with he music.

This will tell you if you are getting signal from the preamp.

Rob

, I will look into your suggestion but the amp boards are tight on this unit

 

[aurora01]

Update: removed the right amp board and found this burnt trace may have been repaired before, I removed and cleaned the damaged trace and soldered in a jumper wire, I tested all the resistors around the area and all were good so not sure the cause, I used a dim bulb tester and it dimmed down but the bulb was still glowing I’ve read these huge transformers can cause this, I did go round the amp boards with a infrared red camera and looked ok

My dc offset is now 10 mv both left and right before the bad Channel was -9mv

I now have audio

 

[aurora01]

Update: removed the right amp board and found this burnt trace may have been repaired before, I removed and cleaned the damaged trace and soldered in a jumper wire, I tested all the resistors around the area and all were good so not sure the cause, I used a dim bulb tester and it dimmed down but the bulb was still glowing I’ve read these huge transformers can cause this, I did go round the amp boards with a infrared red camera and looked ok

My dc offset is now 10 mv both left and right before the bad Channel was -9mv

I now have audio

 

[Robisme]

Glad you got it sorted out.

Rob

 

[aurora01]

You didn't answer some of my questions but I'll let that go.
The RECEIVER is not that complicated in regards to the number of speakers that can be connected;
You can use Speaker A or Speaker B or Speaker C or Speakers A and B or Speakers A and C.
It's all in the switching and not a lot circuit mystery.

In reviewing my old repair history for the LR-9090 back in the 20th century, it looks like I took in a total of 72 of them for repair over a span of years. One complaint that was somewhat common was low sound or no sound in one channel.

The attached drawing (I did very quick but it should readable) identifies where al these controls mentioned below are on the front panel:
In 12 instances, BALANCE control was turned to far to the left or right.
Cleaned and exercised DUAL TAPE MONITOR switch – 2 times
Cleaned and exercised AUDIO MUTE switch – 2 time
Cleaned and exercised MODE switch – 3 times
Cleaned and exercised TREBLE TURNOVER switch – 2 times
Cleaned and exercised BASE TURNOVER switch – 1 time
Cleaned and exercised TONE switch – 3 times

The rest of the repairs had to do with various semiconductors, caps, resistors on the audio output boards failing, coupling caps between stages going defective, transistors or parts falling in various parts of the receiver circuitry.

If you have the right consumer grade test equipment, finding the problem goes a lot quicker and with a lot less diddling in the dark.

Update: has been fixed, found a burnt trace on amp board, replace defective resistor as well, thanks for the help.

 

[aurora01]

Glad you got it sorted out.

Rob

Yes I fixed it, found bad trace on amp board and defective resistor