4230 No sound on right main channels, Output voltage off. Need Help!

[Davron94]

Hey All! I've been working on this unit for a while now getting both amp boards back into shape. The receiver was never taken care of very well and had previously been worked on. I'm currently working on the Main Channel board after replacing all of the output wires (due to corrosion), The trim pots (due to instability with readings), transistors, quite a few resistors and all the caps on both boards.

I disconnected the Rear board (which currently gives off a very bright DBT) when isolated. Not sure why yet, but the Main board is a low DBT, however, I have no sound at all from the right channels. Its completely dead, like its disconnected. Here's the interesting part- Both left and right channels bias and DC balance are set properly, and the left channels are outputting sound fine. Its baffling to me that it is completely dead on the right channel speaker outputs. The TIP41 outputs on H001 and H002 though are reading weird voltage with no voltage on base and emitter, and collector is about 23VDC. Left channel H003 and H004 are normal at approximately 24VDC-base, 48-collector, 24-emitter. Emitter resistors have been changed out also, and are at ~.7 ohms. I cannot find any issues with this board. All transistors have been checked and rechecked.

Any Ideas on what would be causing the voltage issue on the outputs and no sound output?

 

[Davron94]

No one around to give advice or support on this issue?

Would anyone know what to check for if the right main channels section of the board is completely out, but the left channels output is perfectly fine? There was a wiring issue and that was corrected, and now both output transistors for the right channels (front and rear) on the board are showing proper voltage of B-23, C-47, E23. Both bias and DC balance are set properly also. But it's like the right channels are completely disconnected when trying to listen through speakers. Is there a way to check the preamp circuitry on these quad receivers? Thanks All!

 

[mrk229]

Have you checked and cleaned the remote/local switch on the rear panel? ALL audio goes through it.

 

[Davron94]

Have you checked and cleaned the remote/local switch on the rear panel? ALL audio goes through it.

Yes, all switches and pots have been thoroughly cleaned. It's not like a static, distortion, or low volume issue. There is nothing coming through at all, dead silent. It seems like a disconnected wire somewhere, but I cannot find anything apparent visually and checking through the schematic I have continuity from the board to the power switch and then to the right speaker terminals.

 

[mrk229]

If you follow the schematic To & From that remote local switch it might be a good test point for a scope to see if audio is making it that far.

And the legs on the volume control itself are also good test points to look at audio with the scope.

If you don't have a scope look into making a simple audio signal tracer that you can use.

 

[dankik]

I disconnected the Rear board (which currently gives off a very bright DBT) when isolated.

I have no sound at all from the right channels. Its completely dead, like its disconnected.

I tried tracing the signals from the speaker terminals to the P700 boards.
If you are referring to the "Rear Channel board", there isn't any. Looking at the schematic there are two boards as follows:

• One P700 is for the Main LF and Main LR.
• The other P700 is for the Main RF and Main RR.

If you removed one of the P700 boards, you would remove either the Left Front/Rear or Right Front/Rear.
When you say "I have no sound at all from the right channels", it is probably because you removed the P700 for the Main RF and Main RR.

Its baffling to me that it is completely dead on the right channel speaker outputs.

It's because you removed the Right channel P700.

 

[mrk229]

I tried tracing the signals from the speaker terminals to the P700 boards.
If you are referring to the "Rear Channel board", there isn't any. Looking at the schematic there are two boards as follows:

• One P700 is for the Main LF and Main LR.
• The other P700 is for the Main RF and Main RR.

If you removed one of the P700 boards, you would remove either the Left Front/Rear or Right Front/Rear.
When you say "I have no sound at all from the right channels", it is probably because you removed the P700 for the Main RF and Main RR.

It's because you removed the Right channel P700.

View attachment 3718773

I was wondering about that. The schematic is so dense I could not follow it that well on my screen. Have to print out as large as possible and use a straightedge to follow things.

 

[dankik]

@mrk229, I just copy and paste parts of the schematic drawing into a drawing program (Visio) then draw and overlay colored lines to follow the paths. It's a little hard to see above because it's a large portion of the schematic.

 

[dankik]

To determine what is shorted on your remove right channel (RF, RR) board, remove the four output transistors H001-H004 then plug the board back in and see if you still have a short (DBT lamp glowing brightly). Note which transistor went into each position so you can reinstall them later.
Do you have a transistor tester or DMM that has a diode/transistor test function?

 

[Davron94]

Thanks for the input, guys! I did receive incorrect information from someone somewhere and went with it regarding the amp boards, and I was chasing a ghost! While inspecting the lines running from the boards to the speaker switches, and power switch to the terminals and the rear channel selector switch, I just realized myself this afternoon that the boards are separated by left and right channels, not Main and Remote channels, which clears this up quite a bit and gets me back on the right track. Thank you for confirming this!

To MRK229's point, the schematic is quite busy! Although I have become quite familiar with the amp board layout! So, also thank you dankik for drawing those trace lines! That is a huge help for understanding the circuitry pathways.

But yes, I have done quite a bit of component replacement on these boards and am skilled at testing the outputs with a DMM. Now I just need to troubleshoot the right channel board to see where the failed component(s) is.

 

[Davron94]

Ok, found some strangeness while trying to troubleshoot. I removed the output leads and heatsink, and ran on the DBT and got a low light. I tested all the outputs forward and reverse bias and all good. The weird thing is that most of the leads to the outputs have continuity to ground. I disconnected the +power rail from the board J721 and still get most pins grounding out. The only 2 grounds going to the board are the J707 and J708, on both boards. but then looking at the schematic, there should only be one external lead to ground on each board, on this one its J707. should the additional ground be removed. Could that be the issue?

*Edit* Nevermind on the ground removal. I went ahead and disconnected it from the filter cap, and it did not make a difference. Still have everything going to ground.

 

[Davron94]

To determine what is shorted on your remove right channel (RF, RR) board, remove the four output transistors H001-H004 then plug the board back in and see if you still have a short (DBT lamp glowing brightly). Note which transistor went into each position so you can reinstall them later.
Do you have a transistor tester or DMM that has a diode/transistor test function?
View attachment 3718792

Thanks for the direction on the testing. I did remove the heat sink and outputs and tested them. The H002 and H004 were bad. I replaced them and all tested good. I tested the P700 board by itself and got a very low DBT. It almost seems too low. But then I reconnected the outputs and got an extremely bright DBT. So it seems like its something in the Main RR section of the board causing the issue. I've checked all resistors, and transistors. The only anomaly that I can find is that botn R739 and R740 both read OL in circuit, but then when lifting a leg they read the 51k. On the working board, in circuit, they both read ~5.8k. So it seems that maybe there is something else bad in the circuit? EDIT* I replaced the 51k resistors and they read the same as the left channel board, but still have a very bright DBT.

 

[rBuckner]

Did you mess with adjusting the bias pot? If it's end stopped to whichever side is full tilt bias, it may do that. If so, adjust both bias pots to about where the left side pots are turned.

 

[Davron94]

Did you mess with adjusting the bias pot? If it's end stopped to whichever side is full tilt bias, it may do that. If so, adjust both bias pots to about where the left side pots are turned.

I did replace them and turned them down to about 60 ohms. the other board's pots read about 120 ohms when set to 15mV. I thought about that also and rechecked them. I'm thinking its something simple that I'm missing! I don't know if its wise to run power to the board on the DBT to check voltages, but I don't want to have to keep pulling components to check them. Something is blowing H002 and H004 immediately and then the mains fuse.

 

[rBuckner]

I did replace them and turned them down to about 60 ohms. the other board's pots read about 120 ohms when set to 15mV. I thought about that also and rechecked them. I'm thinking its something simple that I'm missing! I don't know if its wise to run power to the board on the DBT to check voltages, but I don't want to have to keep pulling components to check them. Something is blowing H002 and H004 immediately and then the mains fuse.

I'm not sure whether or not a lower bias pot resistance is lowering the bias on a working amp so you may be going the wrong way! I bet you don't have a variac to be able to adjust the line voltage coming in but if you did, I'd suggest having a meter on the bias points and bring up the variac until it had say 10mV, then adjust the pot in the direction that would bring it down, and go nearly to the end. Both front and rear sides. A variac with a line current meter is always best. A DBT is still kind of dangerous as you know!

My amp repair mantra... If a bad amp channel has had power and is still bad, every single transistor and diode must be checked and all power resistors before any attempt at repowering!!.

 

[Davron94]

I'm not sure whether or not a lower bias pot resistance is lowering the bias on a working amp so you may be going the wrong way! I bet you don't have a variac to be able to adjust the line voltage coming in but if you did, I'd suggest having a meter on the bias points and bring up the variac until it had say 10mV, then adjust the pot in the direction that would bring it down, and go nearly to the end. Both front and rear sides. A variac with a line current meter is always best. A DBT is still kind of dangerous as you know!

My amp repair mantra... If a bad amp channel has had power and is still bad, every single transistor and diode must be checked and all power resistors before any attempt at repowering!!.

I was able to find the short that was creating the bright DBT on the right amp board. It was a bad break in the trace coming from the R737 trim pot down to the diode. it was hidden under the solder. I ran a jumper and the short is gone. But now, no bright DBT, but there is no voltage at the emitter resistors and the DC nor balance can be set. Both are showing no DCv. Any thoughts on where to go when no voltage at the emitter resistors?

 

[rBuckner]

Well, that was an open, not a short, and bias was way too high, an overcurrent situation. So back to basics. Do the other channels (left front and left rear) still work? If not check the voltage on the main filter cap on the chassis. Other than that, did the power wire at J721/722 get busted off? Ground still making it to J703/4/8?

 

[Davron94]

Well, that was an open, not a short, and bias was way too high, an overcurrent situation. So back to basics. Do the other channels (left front and left rear) still work? If not check the voltage on the main filter cap on the chassis. Other than that, did the power wire at J721/722 get busted off? Ground still making it to J703/4/8?

Yes, sorry an open. but left front and rear are good with ~48 VDC coming from the cap. Ground is good on J703/4/7/8. Main RR side of board is good on both 24 VDC and 15mV Bias. both boards have 47.5 VDC at J721 and J722.

 

[Davron94]

Still on this one. Got the right front channel working but now cannot set the R737 above 10mV. DC balance is good at 24V.

Anyone know what could be causing this?

 

[Davron94]

Hi All, I finally got the amp boards working correctly and have really good sound quality and power. This unit is louder than I expected! Does not get hot at all under heavy load.

One issue that the unit is still having is with the balance in the front channels. The right channel is lower in volume than the left when the slider is centered. It has to be moved almost halfway to the right end from center to get a perfect balance. Anyone know what could be causing this and how to fix it?