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New to Site with MXR-2000 issue


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Greetings from Raleigh-Durham area. I repair vintage HiFi as a hobby. Few years ago I repaired a MXR-130 and loved the innovative technology used in the PS and FM stereo noise reduction design. Recently bought a Carver MXR-2000 that has loud hum in power transformer with -69vdc and +69vdc rails measuring at -/+99 volts on both. TRIAC was replaced with no change. All Zener diodes voltages ZD801 through ZD805 are normal.  Appears that voltage monitoring circuit may be driving TRIAC to 100% duty cycle. I see design uses optocoupler to provide signal to control drive to the Q801 through Q804 to determine switch on/off time to the TRIAC to maintain the +/-69VDC. Is it safe to remove the TRIAC so I can power up rig to T/S the regulator circuitry? Thanks in advance for any suggestions.

 

Mark

 

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Hi Mark, and welcome to thecarversite.com . We're glad you're hear. ;) 

 

The Carver receivers are quite popular here, and many have them and have worked on them. Hopefully, one of them will chime in with answers to your questions.

 

You might consider starting a thread in the Receiver area of the forum with your questions.

 

https://thecarversite.com/forum/82-receivers/

 

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Welcome @markdd, glad you found us.

 

I'm not a tech, but there are some here - just haven't seen your question yet.  

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Welcome Mark!!!!!

Hope that you get your MXR2000 sorted soon. I bought mine new about 35 years ago and outside of recap and relay work, it is has been just awesome. Congratulations, hope that you enjoy yours as much as I have.

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On 5/9/2024 at 10:52 AM, markdd said:

Greetings from Raleigh-Durham area. I repair vintage HiFi as a hobby. Few years ago I repaired a MXR-130 and loved the innovative technology used in the PS and FM stereo noise reduction design. Recently bought a Carver MXR-2000 that has loud hum in power transformer with -69vdc and +69vdc rails measuring at -/+99 volts on both. TRIAC was replaced with no change. All Zener diodes voltages ZD801 through ZD805 are normal.  Appears that voltage monitoring circuit may be driving TRIAC to 100% duty cycle. I see design uses optocoupler to provide signal to control drive to the Q801 through Q804 to determine switch on/off time to the TRIAC to maintain the +/-69VDC. Is it safe to remove the TRIAC so I can power up rig to T/S the regulator circuitry? Thanks in advance for any suggestions.

 

Mark

 

Wow.  If you have got +/- 99volts where +/- 69 volts are supposed to be, do NOT turn it on again.  Other rail voltages in the amp will be high.  The probability of venting one of the big power supply caps is very high.

 

The Tirac is behaving as if it is shorted.  You said you replaced it.  Double check that it's wired correctly.  

 

Troubleshooting will required a Variac.  With a Variac, you can vary the AC line voltage down to something that yields WAY lower than +/- 99 volts.  From there you may be able to diagnose the problem.

 

If you don't have a Variac consider sending it to a professional.  Further testing with voltages that high will not end well.

 

 

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Posted (edited)

As @wrf has suggested, you need to use a Variac. I use one on all equipment. That way you have control of the power and watch what is going on. Have you tried to adjust the SVR801 to see if High rail voltage will drop?  By reading your post, you have left a lot of unanswered questions. 

Edited by Rob
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