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Old 30th Nov 2021, 12:01 pm   #1
captainsemtex
Diode
 
Join Date: May 2020
Location: Vancouver, British Columbia, Canada.
Posts: 5
Question Mission Cyrus 2 hum/buzz repair advice please

I am the original owner of a first generation Mission Cyrus 2 + PSX. After 30+ years I recently started to get a loud hum/buzz across every input. It doesn't matter if any inputs are connected or not.

From reading other posts with a similar topic, the problem is likely failing capacitors around the regulated voltage supply for the pre-amp section.

However, I have used my mini-oscilloscope and I am getting unexpected readings... so I am wondering if this is the only problem?

I have attached the relevant part of the Cyrus 2 PSU circuit diagram from the service manual.

The output from the transformer T1 is +/- 31VAC which goes direct to the two voltage regulators VR1 and VR2 via some rectifying diodes and smoothing capacitors.

Attached image "VR2 In" shows +30VDC at the input pin of VR2 with a little ripple, but image "VR1 In" shows -28VDC at the input pin of VR1 with a lot more ripple. So I believe that suggests a problem with C64.

However, the output from VR2/VR1 should be +/- 17.8VDC.

Image "VR2 Out", measured at the positive terminal of C68, only shows +6V... but the signal is "clean".
Image "VR1 Out", measured at the negative terminal of C67, shows -28VDC with a lot of ripple!! In fact, VR1 In and VR1 Out are essentially the same!?!

VR1 and VR2 are both new devices, as are C67 and C68.

I appreciate any ideas what may be going on here.

Thanks in advance for your help.

Ian
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Old 30th Nov 2021, 12:48 pm   #2
Analogue man
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Default Re: Mission Cyrus 2 hum/buzz repair advice please

From your description it rather suggests that VR1 has failed.
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Old 30th Nov 2021, 12:59 pm   #3
TonyDuell
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Default Re: Mission Cyrus 2 hum/buzz repair advice please

I assume the downward pointing triangle is a common ground, as is the midpoint of the transformer secondary. They are probably linked at one point only to reduce noise. But check they _are_ linked. An open-circuit common connection causes all sorts of odd faults!

Check the resistors associated with the regulators (I assume the latter are LM317/LM337 or similar) and that they are connected to the right points, no open-circuit tracks.
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Old 2nd Dec 2021, 6:51 am   #4
captainsemtex
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Location: Vancouver, British Columbia, Canada.
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Talking Re: Mission Cyrus 2 hum/buzz repair advice please

Thank you for the replies. Checking that the centre tap of the transformer was tied to ground was something I hadn't considered. I checked... and it is.

From the results I got certainly some capacitors were gone and VR1 looked like it could be dead too.

Anyway, I replaced C63, C64, C65 and C66 and... voila!! No buzz/hum!! Everything is working again I also checked the output voltages for VR2 and VR1 and they are now +/- 18V.

Inspired by this success I intend to replace every capacitor. None of the electrolytic capacitors look bad, but when I measured C65 and C66 after removing them, their values were nowhere near what they should have been. Some of the polypropylene capacitors in the MM/MC area also look questionable.

Thank you for your help.

Best wishes
Ian
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Old 2nd Dec 2021, 10:52 am   #5
Ted Kendall
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Default Re: Mission Cyrus 2 hum/buzz repair advice please

Don't get carried away here - electrolytics do age, but go about replacement carefully. Some of the tracks on the small signal boards may be delicate. As long as the polypropylenes are sound, I'd let them be.
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