Anyone seen this Sine Wave before?

[EJB14]

Hi all;

Hoping that someone has seen this before and can tell me what to check out.

NAD 1700 Preamp; has two sets of preamp outputs - "Normal" and "High" - The first pic is of the problem using a1Khz signal as an input to the CD inputs - notice the small dip at the downturn of the positive peaks? This only occurs using the 'Normal' output. The "High" Outputs produce a nice sine wave that matches the wave I play into the input. The Tape Out's produce the same nice wave. The preamp sounds great though headphones (which makes sense since the headphones are attached to the high output).

The RightMark Audio Test software is reporting significant distortion on the normal outputs (> 3%) while reporting a much more acceptable 0.02% on the high outputs.

Just replaced all the capacitors in the preamp section with Nichicon FG's hoping that was the problem. Given the high outputs are operating per spec, I am wondering if the outputs are amplified separately or if there is something in line with the normal output only that is not operating per spec.

Thanks!
-Ed


"Normal" Pre-amp output wave:


"High" Pre-amp output wave:

 

[awillia6]

Do you have a square wave input signal available? Also, what are the p-p voltage levels of the input and output signals?

Also, while I don't see a schematic for your 1700 anywhere, one for the 1300 is generally available.

 

[EJB14]

I can generate a 1Khz Square wave, yes and use that as input. I can get you the peak to peak of the in and out - I have tried varying the input level to lower levels, and even at low levels, it still produces a similar imperfect sine at the normal outputs. What value should I be using for an input peak to peak? Line Level (1.4 V RMS, near 4 peak to peak?) or something less?

Right now the unit is completely apart, so would have to put it all back together to get that info. I am trying to work out what is different between the high and normal outputs. Obviously there must be an amplifier in there somewhere..
Thanks!
-Ed

 

[awillia6]

Don't bother putting the unit back together just to run another waveform, especially since you've determined the problem to be both consistent and independent of input level (within reason). You likely have enough to go on at present to move forward with power-off in-circuit component checking. Do you have the 1700 schematic or are you working off the 1300 circuit?

 

[petehall347]

positive side is where to look . sorry if i stated the obvious .

 

[EJB14]

Yeah, I guess the 1300 schematic is all I can work with here - I cannot seem to find a schematic of the 1700. Nice that it is very similar! Has both the High and Normal outputs. Keep in mind that I am a newbie to most of this, but learning fast -

yes, that does look obvious - positive side - have to work out how to do that - hey - that brings up a point - the problem is exactly the same in both channels (left and right) of the normal output. If it was a single transistor that was the problem, wouldn't that only affect one channel? So it is some part of the normal output only (which puts the whole problem near the very end of the circuit I would think) that affects left and right.

Hmm... Thanks again for everyone's help!

 

[petehall347]

power supply my best guess .be a good start .

 

[awillia6]

Keep in mind that I am a newbie to most of this, but learning fast -

Hmm... Thanks again for everyone's help!

You came to the right place to get tossed into the deep end of the pool. Best way to learn how to swim they told me. Glug, glug, glug...

Be careful dorking around in there with the power on—REAL careful.

 

[EJB14]

I should read my own posts - "a single transistor" was the problem!!

From the pcb right next to the balance control, a three wire bundle (red/white/black) goes all the way across the PCB and is attached right in front of the normal outputs. Right next to this wire, is a single transistor. There is another three wire set going to what looks like a differential amp nearby, and then another wire set from the output of that amp back to the high output. And of course, another single transistor right next to high output ( same type as the normal outputs - SC2878A 8F Toshibas).

I pulled both these transistors, high output one tested as a NPN, hFE of 600 or so. The normal output one my tester claimed it was PNP or NPN depending on which PIN was in the '1' slot of the tester, hFE of 150 - confusing the tester, and must be bad. I have a set of BC550CTA NPNs on hand, and found one with a similar hFE. Had to twist the pinouts to match the Toshiba, and installed it and put it back together and got a perfect sine when I tested. Rightmark reporting THD at 0.0025% - awesome!

I think I will need to replace the BCA550CTA though - while the BCA's specs are close in most areas, the Toshiba's Vebo (Emitter-base voltage) max is 25V. The BCA550CTA is 5 V. I cannot seem to find a suitable replacement on mouser, so may have to resort to buying some originals off ebay or perhaps I'll get lucky and there is one in the AM circuit someplace I can steal.

Thanks again for all the help!

 

[EJB14]

Now this begs a question - why would the design have a single transistor right in front of the outputs? What is the purpose of that? It obviously affects both channels..Hmm...

 

[leesonic]

This doesn't make sense to me. From the 1300 schematic, it appears the high output takes it's feed from the normal input, then amplifies it. You would expect both waveforms to look weird. It also looks like plugging in headphones mutes the output, not that I suspect the headphone jack. I would check the waveform before the FETs Q519 and Q520 and see what that looks like. Maybe there is a bad transistor or resistor downstream of these (you've recapped it already right?), but even then it doesn't make sense that the normal waveform is distorted and the high isn't.

Lee.

 

[EJB14]

Hey Lee - I agree - I stared at the schematic of the 1300 for about an hour and was coming to the same conclusion as you. But the 1300 schematic does not match the 1700 implementation exactly. There is a single transistor on each output as I stated. One of them was good (high output), one was bad (normal output). Replaced the one on the normal output and all is good again.

 

[gadget73]

Single transistor affecting both channels? Can't be used as an amplifier or buffer. Perhaps a voltage regulation device for the amplifier that actually fed the output jack? Thats about the best guess I can offer without seeing a schematic of the specific device in question.

These don't have any sort of peak limiter or VU indicator that could be fed from that transistor do they?

 

[EJB14]

Single transistor affecting both channels? Can't be used as an amplifier or buffer. Perhaps a voltage regulation device for the amplifier that actually fed the output jack? Thats about the best guess I can offer without seeing a schematic of the specific device in question.

These don't have any sort of peak limiter or VU indicator that could be fed from that transistor do they?

There is no VU indicator on this unit, and I don't think it has a peak limiter...

 

[EJB14]

FYI - I finally found the service manual on manual-buddy and coughed up the $5. I uploaded it to hifiengine.com so others can have at it. While waiting for them to post it, here's a link to it on my google drive if anyone is interested. I would sure like to know what that single transistor might be for.
Thanks again everyone - this site is great!

NAD 1700 Service Manual
https://drive.google.com/open?id=1QjHDqTlV3bN3sSeKZMEsMxA-be_DOtmB

 

[BinaryMike]

The 2SC2878s are used as muting devices in this design. They're connected backwards and driven into conduction by current from Q807. The backwards connection yields low gain and minimizes muting thumps. The high Vebo spec is probably necessary to prevent audio signal distortion at high output levels, so you're right to seek out exact replacements in this case.

 

[awillia6]

Sorry, didn't mean to screw you up with the Model 1300 detour, read that the units were basically clones. Not so, obviously.

 

[EJB14]

Awesome! Thanks BinaryMike! And no worries Awilla6 - its all good and has been very helpful. This thing sounds very good with my replacement transistor, which I have been keeping at lower volumes mostly anyway.

 

[restorer-john]

Muting transistors are notorious little troublemakers. I've run into plenty of dead/dying/shorted ones in CD players over the years.

 

[EJB14]

Was searching for replacements today and found this:

Great article on the 'muting' transistor, and the 2SC2878 in particular:
http://www.electroschematics.com/9660/muting-transistor-attenuator-circuits-2sc2878/

Note how they talk about it being very susceptible to ESD, due to the fact that the positive pin of your RCA gets there first, and if you haven't discharged the static - guess who gets the brunt of it - the muting transistor.

Trying to figure out the codes. Mine as a 2SC2878 A 8F. What is the '8F' - a production run? The A is in regards to the hFE, so I will match that, but the finding all kinds of other values where the 8F is.