Rotel RB-871 Mark II Channel Imbalance

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Hi:

Just picked up a Rotel RB-871 MK II Power Amp.

I noticed that one channel is 15-20dB lower than the other.

The fuses on the bad side measure good. The sound from the lower output

side is clean and not distorting when you bring the volume up to match the other normal working channel.

Any ideas of why the channel imbalance.

I will be checking transistor by transistor when I get a chance.


It is dual mono from the Bridge rectifiers and on. It shares power from a single large toroidal transformer.

Any ideas would be helpful to get me started.

Note I have tried it with two preamps and I also swapped the inputs around to confirm it is indeed the power amp at fault.

Thanks in advance,
 
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Hi Dan,
Don't mess around with transistors. If there was a problem here, the sound wouldn't be clear and you would likely have a DC problem on the output also. Power supply details are not going to affect the diagnosis with this.

You most probably have some bad coupling capacitors, so much for the highest probable cause. The gain of an amplifier circuit are set by the ratio of feedback resistors. The two most probable problems are the coupling capacitor, and the negative feedback to ground capacitor (normally a value of 100 uF or larger). If that cap goes open (broken solder connection?), the gain will drop to 1, or zero dB gain. That pretty much sounds like what you are seeing there. That would be C605 in the RB-870 schematic (I haven't got the diagram for yours). It should be pretty straight forward to find if it were on my bench though. So not impossible for you to find.

If you are capable of doing very close matching with transistors, you could take the opportunity to match the differential pair (long tail pair) in each channel. Thermally bond them together of course! Try to keep the temperature of each the same. If you are not experienced in doing this, do not touch the transistors. It doesn't take much to overheat or zap (static) transistors if you aren't very familiar with working with them. This is when a good technician is worth their weight in gold!

-Chris
 
All fixed and working.... Negative Feedback resistor....Thanks Chris!

Now I am trying to fix the tuner section of a Rotel RTC-940AX.
No hiss or static ... dead silence from the tuner output to preamp.........

No AM or FM...All other inputs are working.... else works...

Any ideas?

Thanks in advance,
 
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Hi Dan,
It's great you have your amplifier problem solved. Do you clean the flux off your connections after soldering?

For the tuner, there is normally a regulated supply running at about +12 VDC to supply both the AM and FM tuner sections. Look for an overheated voltage regulator. Check it's connections. Of course, when something has run that hot for a while, I like to replace the hot part with new.

Other issues you may find are a leaky ceramic capacitor on the osc input pin of the controller for tuning, or a bad osc output buffer from the FM front end. That will stick the tuning voltage completely up or down no matter what frequency you are set to receive. I have seen one shorted capacitor on the output of the MPX chip that killed everything. Uncommonly, that one ran the AM audio through the MPX chip as well. This is not common, but it has happened. The capacitor was a polystyrene type that a previous tech had shorted by applying too much heat while resoldering the connection. Use a clip on heat sink to help control the heat coming up the lead, polystyrene really doesn't deal with heat too well. Otherwise, lovely capacitors.

-Chris
 
Troubleshooting the RTC-940AX tuner

For the tuner.... it looks like the ouput (emitter) of the Q911 (Medium Power NPN 2SD313 -heatsinked) that feeds the circuit in tuner reads about +2.6V instead of the +13.7 volts on the schematic.
I will check the decoupling caps (in case they are shorting to ground & loading this PS output?) on the output of this PS (13.7V) as well the TO-92 (2SC536) that feeds the base of Q911 as well.

Stay tuned.

Thanks,
 
Success! RTC-940AX Preamp Tuner is Working Now!

Hi Chris:

Got the RTC-940AX working... the resistor feeding the base of the TO-92 that fed the Q911 was opened up. Now the Preamp-Tuner works totally and as a side benefit the motorized volume control work as a result of the 12Volt feeds the motor circuit as well.

Now my final troubleshooting deals with Rotel RTC-850 Preamp-Tuner that has a 10-15 dB imbalance between the channels. The tuner has only static I believe (I have to check again tomorrow as it getting late) so it may not be related. All the other inputs work....

This may be a tougher one as I believe there is no really gain stage other than for the phono input.

Let me know what you think.

Thanks in advance,
 
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Joined 2004
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Hi Dan,
Great! Another problem solved.

Do you happen to have a schematic for this thing by chance? This would really help right about now. As for any gain stage, the same things apply as they would to a power amplifier. Coupling caps and resistor ratios set the gain. The additional wrinkle is that you have a muting circuit for the outputs to the amplifier. These can easily go bad and cause similar problems that reduces your signal levels. So here we go ...

Output RCA connections to the PCB at the rear panel. Audiophile cables can also expand the connectors inside the jack so that normal RCA plugs make very poor contact. I test for the bad connections by moving the RCA plug around to see if it makes and breaks contact - but very gently! You don't want to excite bad connections at the PCB interface. I will very often resolder the RCA jack connections unless they apear to be great under strong light and magnification.

Muting problems involving transistors. If you see a BJT type transistor, you need to use the same type. These are not normal transistors and other types will break down right away usually. A common transistor number is 2SC2878. These are specially designed and have a much higher emitter-base reverse breakdown voltage. These may test great, so the only test will be removal. DC voltage on the signal output is also a good indicator that your muting transistor has a defect. Other faults can also cause this to happen, so disconnect the muting transistor, power back up and retest.

J-Fets may also be used for muting. Again, disconnection is about the only way to confirm this. Be aware that when these parts are disconnected, you have no muting so you will have to reduce the amplifier volume, or switch the speakers off when turning on or off. Some preamps may put a nice spike on the audio when changing functions as well. You have been warned.

Signal relays in series with the signal can suffer oxidized contacts. Replace the relay if you find this. You can test by shorting across the N.O. contacts to the moving contacts. Again, you have no muting action once you have done this.

You may find muting transistors in various places throughout a circuit. Keep your eyes open.

Bad solder connections on front panel controls (volume, balance, tone ...). Resolder these if there is any hint they may be bad. The same things happen with interconnecting wire / cables. Cracked PCB traces near the sides and corners may be damaged possibly.

As for any further gain stages beyond the phono EQ amp, I sure hope there are. You will probably have a tone control circuit. It needs 15 ~ 20 dB worth of gain to compensate for tone control network losses. Don't forget about any other things like muting and loudness circuits. After the volume control, there ought to be a buffer at the very least! Otherwise, you will not be able to drive the cables connecting to your amplifier.

An oscilloscope is great to follow signals with. That will save you a ton of time. When soldering, use a real station if possible. The Solomon stations with the digital display sell commonly for $120 CDN or less. These are great stations, so no excuse on this. Also, use the proper liquid flux. Sayal carries it, as should your normal parts jobbers. I think it is MG Chemicals #835-100ml for a 100 ml bottle. To remove solder from a connection, it's often helpful to add a little first. Then, use the large solder sucker (the one with recoil folks) to clear out the bulk of solder. Add a touch of flux, then some solder. Don't use so much solder that it forms a ball. It should have concave sides and wet the lead and PCB well. You can also scrape the component lead before applying the flux if they are oxidized. Your power transistor in the regulator should have had it's leads cleaned first for sure, then re-installed without huge amounts of solder. Fresh thermal compound as well. Clean up all your flux with lacquer thinner and a toothbrush after your soldering is done. Inspect for solder joint quality, "solder bridges" and solder balls or strings. Do not apply power until you have inspected your own work. I do all of this every single time, so do as I do!

-Chris
 
Hi Dan,
Great! Another problem solved.

Do you happen to have a schematic for this thing by chance? This would really help right about now. As for any gain stage, the same things apply as they would to a power amplifier. Coupling caps and resistor ratios set the gain. The additional wrinkle is that you have a muting circuit for the outputs to the amplifier. These can easily go bad and cause similar problems that reduces your signal levels. So here we go ...

Output RCA connections to the PCB at the rear panel. Audiophile cables can also expand the connectors inside the jack so that normal RCA plugs make very poor contact. I test for the bad connections by moving the RCA plug around to see if it makes and breaks contact - but very gently! You don't want to excite bad connections at the PCB interface. I will very often resolder the RCA jack connections unless they apear to be great under strong light and magnification.

Muting problems involving transistors. If you see a BJT type transistor, you need to use the same type. These are not normal transistors and other types will break down right away usually. A common transistor number is 2SC2878. These are specially designed and have a much higher emitter-base reverse breakdown voltage. These may test great, so the only test will be removal. DC voltage on the signal output is also a good indicator that your muting transistor has a defect. Other faults can also cause this to happen, so disconnect the muting transistor, power back up and retest.

J-Fets may also be used for muting. Again, disconnection is about the only way to confirm this. Be aware that when these parts are disconnected, you have no muting so you will have to reduce the amplifier volume, or switch the speakers off when turning on or off. Some preamps may put a nice spike on the audio when changing functions as well. You have been warned.

Signal relays in series with the signal can suffer oxidized contacts. Replace the relay if you find this. You can test by shorting across the N.O. contacts to the moving contacts. Again, you have no muting action once you have done this.

You may find muting transistors in various places throughout a circuit. Keep your eyes open.

Bad solder connections on front panel controls (volume, balance, tone ...). Resolder these if there is any hint they may be bad. The same things happen with interconnecting wire / cables. Cracked PCB traces near the sides and corners may be damaged possibly.

As for any further gain stages beyond the phono EQ amp, I sure hope there are. You will probably have a tone control circuit. It needs 15 ~ 20 dB worth of gain to compensate for tone control network losses. Don't forget about any other things like muting and loudness circuits. After the volume control, there ought to be a buffer at the very least! Otherwise, you will not be able to drive the cables connecting to your amplifier.

An oscilloscope is great to follow signals with. That will save you a ton of time. When soldering, use a real station if possible. The Solomon stations with the digital display sell commonly for $120 CDN or less. These are great stations, so no excuse on this. Also, use the proper liquid flux. Sayal carries it, as should your normal parts jobbers. I think it is MG Chemicals #835-100ml for a 100 ml bottle. To remove solder from a connection, it's often helpful to add a little first. Then, use the large solder sucker (the one with recoil folks) to clear out the bulk of solder. Add a touch of flux, then some solder. Don't use so much solder that it forms a ball. It should have concave sides and wet the lead and PCB well. You can also scrape the component lead before applying the flux if they are oxidized. Your power transistor in the regulator should have had it's leads cleaned first for sure, then re-installed without huge amounts of solder. Fresh thermal compound as well. Clean up all your flux with lacquer thinner and a toothbrush after your soldering is done. Inspect for solder joint quality, "solder bridges" and solder balls or strings. Do not apply power until you have inspected your own work. I do all of this every single time, so do as I do!

-Chris

Hi Chris:

I will take your advice wholw-heartedly and apply to this repair as well.

I will keep you updated on my progress.

Thanks again
 
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Hi Steve,
Thank you very much! I'll grab these two.

It's so much better and easier when I can give advice focused on the exact model. These circuits are pretty similar in the way they work, so the check points are about the same no matter what the make and model are for the common stuff. Manuals are critical once the design becomes interesting though.

Hi Dan,
it was actually the Neg Feedback Decoupling Cap 100uF 25V Black Gate Cap.
Yup.

The rest of the information is simply a list of the more common things I would look at. It's pretty comprehensive as you can see. The intent is to focus your attention in directions that are probably causes instead of going off on a tangent.

-Chris
 
Further Details on the Rotel Preamp-Tuner

Hi Chris:

The symptoms for the Rotel RTC-850 are specifically:

The one channel is 15-20dB down in level and the sound is distorted as well.

The tuner (FM & AM) works fine BTW....

I will take your advice in regards to the muting as well as any preamp gain stages and go from there.....

So if you or any others have any specific ideas on the repair..... please let me know.

Since the PS is shared between both channels, then this is not a factor.

I will concentrate on the coupling & localized PS decoupling for the preamp
and I will see if there are any resistors/transistors that have failed or drifted off next.

I am printing out the schematics and I will use this as the starting point.

Thanks in advance,
 
The 2 sets of coupling (3 on each channel) caps measure correctly.

Next is to check the output of the 2 buffering Opamps (5532's) that are located for the volume, balance & tone controls circuits.

I am going to determine where the muting circuit is on the schematic and trace that out as well.

I will keep you posted.... any ideas please let me know....

Thanks,
 
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Joined 2004
Paid Member
Hi Dan,
Okay, we know a couple things off the bat then. Only one channel affected means your power supplies are fine. Did you check the tape output jacks and headphone output by chance? Just other easy to get to test points. Because this is a preamplifier, the headphones must be driven by a headphone amp. In this case, it's an NJM4556D (8 pin DIP case style) that lives on the headphone jack PCB itself. This is a very common part to be used here as the NJM4556 was designed for this job in particular. You may also find them used for signal output buffers in various devices.

The muting is accomplished by using a relay to short the signals to common, so that's about the best way to do this. Bad relay contacts should cause possibly a failure to mute on one channel, or distortion only during muting. Look at RY801 on page 17 just below and to the right of the centre of that page. Q812 and Q813 energize the coil.

IC501 and IC502 on the tone / balance PCB are the buffer - amplifiers you wanted to check. Op amps seldom fail unless they are directly connected to the outside world and bad things happened to them. If you look back in the circuit, you will find the selector IC (TC9152P). They use one for "listen" and switches for "tape send" duty. It isn't unusual to find one of these ICs damaged when they are connected directly to the input jacks - as these are. You can always probe the various pins of that IC with a 'scope probe to check signals. Measuring the input jacks for DC bias is another check that can sometimes indicate a bad chip. You can also insert some series resistance in line with your test signal and probe that input, select various inputs and see if your signal is distorted once that input is selected. Then desolder the signal outputs from the TC9152P and repeat to see if the signal is still distorted. Doing this avoids having to remove what might be a good chip.

Straight forward troubleshooting and signal chasing. You'll figure this out.

-Chris
 
New Observsations Part #2

Signal Voltage from Left & Right outputs are unbalanced with Right output being about 2-2.5X greater than the left, before and after Relay RY801.
SO this eliminates RY801 (the Muting relay as the culprit).

The left channel is not distorted like I thought previously....

I am following the signal backwards towards the buffers...


It is strange that the Headphone circuit outputs equal volume (via headphones) but the preamp out does not.......

Stay tuned....


Chris any thoughts???

Thanks in advance,
 
Success! - Cheaps ALPS Volume Potentiometer

I narrowed it down to a mistracking volume pot.

The signal voltages were correct up to the volume pot divider network, but once it left the board and on to the balance control, the voltages diverted by 3-4X delta between the 2 channels.

So I measured the impedances for the left & right side of pots and I am getting a full 25% diversion between them .

Sometimes I would hear distortion and sometimes I didn't (???) so I figured that the wiper is not making good contact with the slider at times..

I am going to add FaderLube to the pot to see if I can get it close to spec.

Stay tuned.

Thanks,
 
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