C4000 Hologram Board Bleedthrough Distortion

My C4000 preamp has been with me since Feb 1982. A couple years later, I had the "B board" hologram revision done. Ever since that revision, the preamp has had audible hiss and hum when the hologram was engaged. But that's not the main topic here, just a background context.

Some time in the late 1980s, my C4000 began producing what I would describe as "bleedthrough distortion" when the hologram was engaged. This bleedthrough was constant, regardless of master volume setting. With the volume at zero, the bleedthrough was the only signal audible, when input signal was present. Normally, this bleedthrough would cease after a short 5-10-minute warmup and it was fine. I lived with that for many years.

This past year, however, I've been noticing sporadic and occasional 'crackling' that would be audible on louder peaks of input signal, but then would clear up after a while. For some time, I thought the problem was dirty pots on an EQ in the ext proc loop, and other times, it seemed the switches on the C4000 were the culprit. Nearly ALL of the pushbuttons are noisy and will impart crackling noise in the output if touched in the slightest manner while playing.

But this past week, I think I narrowed it down to the Sonic Hologram hardware itself, as I was observing a particularly nasty kind of distortion on louder musical peaks and with the input levels at higher values. If the input level from signal source is low, there is no audible distortion--this only happens when louder portions of program appear at the C4000's inputs. I was able to RELIABLY repeat the injection and removal of distortion by engaging and disengaging the Sonic Hologram during a particular piece of music where the distortion was particularly dominant.

Now I think the problem is specific to the Hologram card itself. Has anyone confronted this particular gremlin in the C4000 and if so, have you been able to locate the component that was at-fault? My intuition tells me it's probably a semiconductor flaw, possibly ultrasonic oscillation triggered by signal levels above a certain threshold and not a capacitor issue, but I'd like to get a better idea as to what it might be before I tear the unit apart and start working on it.

I'm guessing here (and it is truly that), but it could be that you are clipping some of the opamps. Do you have noise reduction, or any other processing in other than SH? Remember, you can't get more out of an opamp than you put into it, and I believe the C-4000 only has +12v rails for opamps. More modern equipment usually sues at least +15v.

I'm certain that it's not a clipping problem. It used to clear up with time, or temperature. Now it's constant. Besides, I have another device in the ext proc loop that will clip badly if overdriven, so that limits levels into the preamp greatly.
The problem has gone from temperature-related, to 'always there' now. I hear it particularly with piano, as there is little harmonic energy to mask it. Particularly 2 octaves above middle C, when those notes are played fortissimo, I get the occasional crackling. Sometimes if I raise the input level a lot, and then back it off and then play the track again, the crackling doesn't reoccur. It's sporadic. Almost seems like a dirty potentiometer, but it's not that entirely.

My Model 4000t is also producing hiss and distortion when CD signal is feed in (can hear leaking sound from tweeter), whereby the master volume is fully shut, even though hologram switch is disengaged (out). Expertise advice is required. Thank you.

Remark: The problem still exists after changing all EXAR 4136CPs' to NTE997s' in Hologram board.

My Model 4000t is also producing hiss and distortion when CD signal is feed in (can hear leaking sound from tweeter), whereby the master volume is fully shut, even though hologram switch is disengaged (out). Expertise advice is required. Thank you.

Remark: The problem still exists after changing all EXAR 4136CPs' to NTE997s' in Hologram board.

You may have a related kind of component failure, but in a different section of the preamp.

Unfortunately, I don't have a schematic or even a block diagram of the C4000, so it is impossible to know exactly what is going on in there.

I suspect that the 'distortion' we are hearing is parasitic oscillation in some section of the circuitry.

Back in the late '70s and early '80s, I used to work for a the repair department of a datacomm company and we often dealt with a mode of failure that came to be known as "TI crud". The Texas Instruments ICs that had silver plating would tarnish their pins over time and under the right conditions, the tarnish would grow dendrites between pins, some of which were somewhat conductive paths. The result was erratic operation, parasitic oscillations and ultimate destruction of the IC. In some cases, we could remedy the problems with the application of a typewriter eraser to abrade the corrosion off of the pins. Then we'd apply a clear epoxy coating to seal it.

The problems with my Carver C4000 are remeniscent of the TI crud problems I dealt with in the service shop more than thirty years ago. It's definately not clipping. It's most likely parasitic oscillation, which makes a clicking, crackling kind of sound when it happens.

Yeah, one of the nice things about silver ICs (and connectors made with a silver pin) is that Silver Oxide is a good conductor.

Do you hear the static with Tone, Peak Unlimit, and Correlator disengaged?
I looked on eBay, searched web, and there are no C-4000 manuals for sale.
Try this site:
http://www.hifiengine.com/manuals/carver/c-4000.shtml

I put together my C-4000 service manual from a few different versions.
I reorganized and bound it with glossy cover.
When Bob Carver and I were discussing C-4000 circuitry, he exclaimed
"Wow, where did you get this service manual? I want one."

I don't have much spare time.
If I did, I would scan mine and offer it to the databases.
You won't solve your problem without the schematics.

Would it be a kind of "induction" of signal take place between input/output, as they share the same board?

I don't know how loud you turn up the volume.
Bleedthroughs happen when you listen to an unconnected input
while another source is going.
Some people use shorting plugs on unused inputs.

I don't know how loud you turn up the volume.
Bleedthroughs happen when you listen to an unconnected input
while another source is going.
Some people use shorting plugs on unused inputs.

While CD player is playing and connected to preamp, selector switch in CD position and volume switch is fully shut, I still can hear weak and distorted music out of tweeter. Not hear when track stop or CD player is paused.

I think I traced the bleedthrough problem... it's a poorly designed ground reference on the Bass and Treble controls.. the ground for the pots is on a different PCB than the rest of the tone amp circuit. That is just asking for trouble.. eddy currents, slight resistance and voila... noisy ground on the pots, causing bleed into the rest of the preamp.. Very severe with the Tone engaged. Almost nonexistent with Tone disengaged.

See my new post about the modifications and improvements I'm making to my C4000. That unit is a disaster in terms of power supply lead routes, lack of proper DC bypassing, and just overall very poor PCB layout practice. I've been working to correct many of these things in the past week.