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Old 23rd July 2011, 06:56 AM   #11
dtm1962 is offline dtm1962  Canada
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Quote:
Originally Posted by anatech View Post
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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Old 23rd July 2011, 06:59 AM   #12
dtm1962 is offline dtm1962  Canada
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Hi Chris:

I have the service manual for the RTC-850..... I will review your ideas and apply it to the repair with the aid of the schematic.

Thanks again,
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Old 23rd July 2011, 01:37 PM   #13
sregor is offline sregor  United States
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Many Rotel manuals (including RTC-850) can be found at http://bwgroup-support.com/rotelservicemanuals.html
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Old 24th July 2011, 01:54 AM   #14
anatech is offline anatech  Canada
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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,
Quote:
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
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Old 24th July 2011, 07:01 PM   #15
dtm1962 is offline dtm1962  Canada
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Default 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,
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Old 24th July 2011, 08:04 PM   #16
dtm1962 is offline dtm1962  Canada
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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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Old 24th July 2011, 08:35 PM   #17
anatech is offline anatech  Canada
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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
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Old 24th July 2011, 10:39 PM   #18
dtm1962 is offline dtm1962  Canada
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New Observations:

The headphone output is perfectly balanced but the preamp out channel imbalance is still off by 10-15dB.

The muting circuit (full attenuation) functions correctly.......

It narrows it down to the R801 relay and ????
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Old 24th July 2011, 11:28 PM   #19
dtm1962 is offline dtm1962  Canada
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Default 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,
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Old 25th July 2011, 12:13 AM   #20
dtm1962 is offline dtm1962  Canada
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Default 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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