Protection Circuit Issues Carver C-2

[DreadPirate]

Hi folks. I was setting up my Carver C-2 Pre Amp and M-200t Amp to do some LP to CD transfers. The Main 1 Output from the C-2 goes to the M-200t. The Main 2 Output I had setup with a phono to jack to come into my Line Input sound card. In a moment of brilliance, I unplugged the jack from the soundcard while everything was on. I now get an intermittent thump, at about once a second out of the speaks. The protection circuit on the M-200t kicks in and the lights flicker at the thump. I am guessing this is not good :]. I pulled off the M-200t, hooked up a tapedeck to its input and it appears to work fine, so I believe the problem is with the C-2s output. I do get fine sound out of the headphone jack, though. I have a multi-meter and an esr-meter and a desire to learn by screwing up if necessary.

Can I take a VDC reading at the Main 1 Output of the C-2 (it is an RCA plug) to see if the spike is there? If so, where do I go from there?

[anatech]

Hi DreadPirate,
Interesting fault. Check for DC on the output of the C-2. It may be possible you've damaged it. I wish I had the schematic for this.

Look to see if there are muting transistors across the output signal to ground. They may leak DC if damaged. You can not replace with any other type if they are 2SC2878.

Your amp should have input coupling caps.

-Chris

[DreadPirate]

Do the input coupling caps offer protection to the amp? Can I rule out any problems with the amp given the tape deck test I did? I am guessing I grounded the signal when I unplugged the jack from the sound card. How would this backfeed into the amp?

I am not convinced the fault is in C-2 (pre-amp). I monitored Main 1 for a while and readings were in the millivolt range, though I think the meter did go to -1 for an instant (using a DMM). How else can I confirm C-2 is at fault without damaging other equipment?

After looking under the hood of C-2, there are perhaps 2 or 3 ICs, 3 or 4 transistors, lots of resistors. All the devices look undamaged. Would a shotgun replacement of the transistors be a reasonable approach?

I can take a pic if you would like to see it. The C-2 service manual seems hard to find, but I will pick up and share the 200mt service manual if that is the unit needing work.

[anatech]

Hi DreadPirate,
Your test with the amp would seem to clear it for faults.

The negative spike from the preamp could cause this fault. We really need a schematic before embarking on component changes. A look and a couple tests normally would pinpoint the fault.

Pick up the service manuals anyway. Since you own this equipment, it's in your best interest to have this information. I believe I saw a couple sites for download. If they are available from Sunfire - so much the better.

Disconnecting the cable from the computer may have injected a large signal from the difference in ground potentials. I would not expect parts cracked and / or smoked, but rather a softer failure. So observing the PCB with your eyes will not show a fault.

-Chris

[DreadPirate]

Thanks to Mike at www.vintageaudiomanuals.com, I now have the schematic for the C-2. It is here:

http://depositfiles.com/en/files/307...ematic.jp.html

Let me know if it comes through fine. Also, if there is a place to post it for the use of others. I can also print it out in any size you want and mail it if you want an enlarged hard copy for yourself.

I would apprecate any suggestions on how to evalaute the fault. Thanks.

[DreadPirate]

I think its worth repeating that I am getting good sound out of the headphones, the thumping only happens when the amp is connected through the Main Output.

[anatech]

Hi DreadPirate,
Replace both Q104 and Q204, muting transistors. They are J Fets, type 2N5458.

You can measure the gate voltage, then the drain voltage. If you see a DC voltage on the drain the same polarity as the gate the Fet is damaged. Confirm by removing the Fets and turning the preamp on. Wait 5~10 sec and turn the amp on. The amp should not cycle and it should play music. Turn the amp off first then and wait 30 sec before turning the C-2 off to avoid a big thump.

Thank you very much for the schematic! Consider picking up the amplifier one while you can.

-Chris

[DreadPirate]

A few final questions:

How do I measure these voltages? I can see from the spec sheets which is the D, S, and G, do I just set the meter to VDC, place my black lead on a suitable ground and probe D, S, G to read their voltages? If so, what is a good ground to use?

What is a good source for these transistors that would not have too excessive shipping? I am going to check RS, but suspect they will not have these.

Thanks for your help.

[anatech]

Hi DreadPirate,

do I just set the meter to VDC, place my black lead on a suitable ground and probe D, S, G to read their voltages?

Yup. That's about it. Should be around -11~12 V unmuted as I recall. I hope you are using a DVM and not a cheap analog meter. The analog meter may load down any leakage and give you a false measurement.

What is a good source for these transistors that would not have too excessive shipping?

RS may have it. They are very common J Fets. There must be a place close by. The cost of shipping will exceed the cost of these parts for sure.

Do you experiment, or build anything? If so, just include with your next parts order. They are not expensive. Garden variety J Fet with many uses. Buy extra and experiment.

-Chris

[DreadPirate]

Just wanted to let you know I replaced the muting transistors and all is well. The headphone output might not be as high as I would like, not sure if this is related, as I don't remember what it was like before. Thanks again.