Rotel RB-871 Mark II Channel Imbalance - diyAudio
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Old 22nd July 2011, 08:55 PM   #1
dtm1962 is offline dtm1962  Canada
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Default Rotel RB-871 Mark II Channel Imbalance

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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Old 22nd July 2011, 09:22 PM   #2
anatech is offline anatech  Canada
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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
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Old 22nd July 2011, 09:43 PM   #3
dtm1962 is offline dtm1962  Canada
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Thanks Chris...

I will check the caps out to see which one of the Blackgates are at fault. The coupling I believe is the 10uF & the NF Ground I believe is a 100uF. I will check the solder connections 1st.

I will let you know what I find.

Thanks for your response!
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Old 22nd July 2011, 10:48 PM   #4
dtm1962 is offline dtm1962  Canada
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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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Old 22nd July 2011, 11:07 PM   #5
anatech is offline anatech  Canada
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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
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Old 22nd July 2011, 11:35 PM   #6
dtm1962 is offline dtm1962  Canada
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Default 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,
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Old 23rd July 2011, 03:13 AM   #7
dtm1962 is offline dtm1962  Canada
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Default 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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Old 23rd July 2011, 05:28 AM   #8
anatech is offline anatech  Canada
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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
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Old 23rd July 2011, 05:52 AM   #9
dtm1962 is offline dtm1962  Canada
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Quote:
Originally Posted by dtm1962 View Post
All fixed and working.... Negative Feedback resistor....Thanks Chris!
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Old 23rd July 2011, 05:54 AM   #10
dtm1962 is offline dtm1962  Canada
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Hi Chris:

For the RB-971 Power Amp repair.... it was actually the Neg Feedback Decoupling Cap 100uF 25V Black Gate Cap.

Sorry for the confusion....
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