Papa Hemi Posted July 15, 2015 Share Posted July 15, 2015 This amp has returned home after being out on loan, but it is noisey and drops a channel intermittently. I have traced the issue to power supply in the CyrusII, the toroidal tx only having output on one side. In pic, 26v ac on yellow, 0v on gray. Not having much experience with tx - is this a usual fault? Does anyone fix these? Link to comment Share on other sites More sharing options...
Owen Y Posted July 16, 2015 Share Posted July 16, 2015 Yo Papa! So you are not getting any (AC) voltage one side of the TX, referenced to the centre tap? Should be ~26vac either side. If you think that is the case, then you could desolder the 3 output wires (check zero volts on the big PSU caps first), then measure DC ohms of the TX sec winding, ea side, to see if one side is 'open', or differs. That might tell you what to do next. Link to comment Share on other sites More sharing options...
Papa Hemi Posted July 16, 2015 Author Share Posted July 16, 2015 OK - desoldered the TX - checked dc Ω - inconclusive. Checked AC - 28v both sides. Initially thought that I might have a crook diode so I will be checking those next. Link to comment Share on other sites More sharing options...
Papa Hemi Posted July 16, 2015 Author Share Posted July 16, 2015 And they check out ok - with about .535v drop across them all. I am puzzled as in my initial trouble-shooting I had ac from only one side of tx, now seems ok out of circuit. Might just put it all back together and see... Link to comment Share on other sites More sharing options...
Owen Y Posted July 16, 2015 Share Posted July 16, 2015 Firstly, it looks like you are looking at the Cyrus II - the problem is not with the PSX then? These wee things apparently have dual symmetrical +/- PSUs, so you can next check the outputs of the regulated supply ccts. The Service Manual is here - http://www.hifiengine.com/manual_library/cyrus/2.shtml Of course, these ancient amps, likely could do with some nice new PSU caps. Link to comment Share on other sites More sharing options...
Papa Hemi Posted July 17, 2015 Author Share Posted July 17, 2015 I do have the PSX but this issue is in<span style="line-height: 1.5;"> the power supply of the cyrusII. Yes I have the manual, and it was the test for the regulated supply output that led me to the tx as the issue.</span> Link to comment Share on other sites More sharing options...
Owen Y Posted July 17, 2015 Share Posted July 17, 2015 OK... a one-ch problem suggests that it is NOT in the PSU (which would usually affect both chs). A headache if it is intermittent. The noise... what sort? If hum, then of course suspect the filter caps or Gnding. Link to comment Share on other sites More sharing options...
Papa Hemi Posted July 19, 2015 Author Share Posted July 19, 2015 I have been thinking... The issue is a buzzing that is independent of volume on one channel. It is there regardless of the PSX being in use, when the PSX is used it provides power to the power amplifier circuit, the PSU in the cyrus then only powers the pre-amp of the cyrus, so the large PSU caps are redundant (see attached circuit), instead there is a secondary PSU for the preamp. Looking at the 470uF (50v) caps in that circuit I see that there is some visible signs of deterioration in the form of blistering on the side of one cap. I will replace these caps, resolder the tx, and go from there. Link to comment Share on other sites More sharing options...
Papa Hemi Posted July 19, 2015 Author Share Posted July 19, 2015 OOps, no circuit diagram, wrong format - was a tiff, now converted. Link to comment Share on other sites More sharing options...
Owen Y Posted July 19, 2015 Share Posted July 19, 2015 Well spotted - that is definitely a sign of a cap about to haemorrhage itself (usually happens at the top). Take note how the caps are connected +/-/+/- (for the +/- dual symmetrical supply rails) - ie. not both -ves Gnded. I wonder if there is some heat from the diode in close proximity? Maintain some air gap if you can. Link to comment Share on other sites More sharing options...
Papa Hemi Posted July 26, 2015 Author Share Posted July 26, 2015 Thanks for that Owen, I have ordered new caps and they are sightly smaller in diameter, so that may mitigate any issues from the heat from the diode. Will update upon installation of the new caps. Link to comment Share on other sites More sharing options...
Owen Y Posted July 28, 2015 Share Posted July 28, 2015 Keep us posted, pics if it happens ;-) Link to comment Share on other sites More sharing options...
Papa Hemi Posted August 1, 2015 Author Share Posted August 1, 2015 I have now replaced the preamp psu caps, setting them back a bit from the diode (thanks Owen). Re-assembled now and sounding fine (cross fingers) with the PSX driving the power amp section. I have reinstated the PSX as there still seems to be an issue with the psu on the Cyrus - ac voltage at rectifier is double on one side to what it should be, negligible on the other (D8 and D5 in the attached circuit). I have checked the diodes as ok on my dmm, perhaps there is another issue. Link to comment Share on other sites More sharing options...
Owen Y Posted August 1, 2015 Share Posted August 1, 2015 Hi Papa - good work - correct me if I'm wrong but... The new caps are on the preamp supply side (+/-18v) - but the 80v/0v measured is the power amp supply side? Incidentally, that should be VDC measurement at the diode output-to-cap connection - not VAC. And 80vdc is not so good if the 10,000uF C62 cap is only 50v rated? If the correct +/-18v is generated on the preamp side, then that would suggest that the PTX is fine - but the problem is on the power amp supply side - ie the diode bridge etc on the +/-40v side. (Maybe check again those diodes, lifting one end of ea to DMM check them individually.) But I'd suspect C61/62 -replace them too, with nice modern ones. Could be something amiss/damaged downstream (ie regulators) but I'm guessing unlikely 0V without something being obvious. Cheers. Link to comment Share on other sites More sharing options...
Papa Hemi Posted August 1, 2015 Author Share Posted August 1, 2015 Thanks the lot for that Owen, always nice to get your input. Yes I need to investigate further. Played the line stage for quite a while, it sounds good. No joy however on the phono input side -something truly awry there. A switching problem looks to be the first issue... starting to look like a can of worms. Link to comment Share on other sites More sharing options...
Owen Y Posted August 2, 2015 Share Posted August 2, 2015 Re. C61, you could just temporarily replace it with a big uF cap you might have on hand (at least 100v rated) and/or test it with a DMM - if you don't have capacitance, ohms test should be way up in the MΩ. Link to comment Share on other sites More sharing options...
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