Mod Cyrus II power train for PSX? - diyAudio
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Old 29th May 2004, 08:56 AM   #1
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Join Date: May 2004
Location: Bath
Default Mod Cyrus II power train for PSX?

I am hoping to obtain a PSX to go with my Cyrus II (Serial C227881 ca. 02/1990) but understand that there is some internal 'dealer mod' needed to the amp.

Does someone know precisely what this is?

I expect to have to remove a pair of links (maybe on the + and - rails) or maybe fuses to avoid the PSX and the internal toroid 'competing' to power the output stage.

However I don't confidently know exactly what to alter.

PS another problem (any suggestions appreciated) is that the input selector on some channels is poor. You have to work and twist the input selector knob and position it maybe off-notch (off detent position) to get the stereo signals, at least on some sources. This is a shame.

Any suggestions/info most welcome!
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Old 29th May 2004, 01:46 PM   #2
sreten is offline sreten  United Kingdom
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Replace the input selector if you can.

The wiring of the PSX should be self evident.

I believe the output stage is powered by the PSX
so you have to remove links to the internal supply.

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Old 29th May 2004, 03:16 PM   #3
Geoff is offline Geoff  United Kingdom
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There are two fuses in the Cyrus 2 (labelled F1 and F2) located on the pcb between the toroid and the heatsink/output transistors. Remove these before connecting the PSX.

Noisy/intermittent selector switches are a common problem on the Cyrus 1/2 in my experience. I have been unable to find a suitable replacement so you may well have to do as I did. That is the judicious use of contact cleaner spray whilst rotating the switch from end-to-end. This should provide a (temporary) cure, but I found it necessary to repeat the treatment every six months or so (depending on how often the switch is rotated during normal use). I also found it beneficial to connect sources to switch positions that had otherwise been unused (eg connecting the CD player to 'Video' or 'Tape').
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Old 29th May 2004, 07:26 PM   #4
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Thanks for the advice.

Geoff, I have the old brochure wherein a picture of the Cyrus I PCB is shown. Maybe the board is common between One and Two variants, only some components omitted on the lowly-er One.

Indeed I see FS1 and FS2 - ??FB 15A - taking the current to the power transistors. That's good - no links to cut thru, just fuses to detach when running off PSX?


I emailed Mission (because I could) asking for a selector SX component but no reply. In any case it looks a bit like desoldering a DIL component - a bit fiddly. Off to buy some cleaner for when I take the lid off, non-invasive surgery preferred. 'Working' the knob brings some relief, consistent with the proposal that inactivity is bad (possible tarnishing or something). I have a new Humax freeview box hence a new source for Radio and TV to serve....

my first post and a quick helpful reply, thanks

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Old 30th May 2004, 07:16 AM   #5
Geoff is offline Geoff  United Kingdom
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The boards for the Cyrus 1 and 2 were the same. The only differences between the two, so far as I am aware, is that the 2 had an improved phono stage, higher supply rail voltage for the power amp section and larger output transistors (parallel pairs of the same output transistor were used in earlier models).
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Old 27th September 2004, 02:57 PM   #6
jimboy is offline jimboy  United Kingdom
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Default Cyrus II selector switch, Amp I->II upgrade?

Call Jo Moody at Cyrus with your credit card number handy and she will despatch a new selector switch. Cost c.GBP10 +VAT. Not too hard to replace if you can desolder and solder okay. Try cleaning first though as that often results in a huge improvement!

Phone: + (0) 1480 435577 ext 203
Fax: + (0) 1480 437715

I have a question re. the similarity between the Cyrus I & II. What would be required to upgrade a Cyrus I to a II. New output tannies, caps, and what else? Has anyone tried this?

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Old 27th September 2004, 03:51 PM   #7
Did it Himself
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Best phone her I think; I've had nil response to my emails and have heard stories of the same from others.

The upgrade from I to II would require new transformer (maybe PSU caps as well) and depending on the revision, additional output transistors. This is of course neglecting the phono stage.

If you can get hold of a PSX then that will give you by far the biggest upgrade and will also give you (power amp wise) a II.
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Old 26th July 2006, 05:32 PM   #8
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Default cyrus 3 amp u/s

Dear Kind audio people,

I have a bit of a problem, my beloved Cyrus 3 has gone "pop!.

It started when it was clicking and poping in one channels speakers, then it just blew fuses (the one on the back (not the intrernal ones for each chanel which are fine)

I took out the teansformer and found it was "bad, it showed a blue spark when power was applied and blew the fuse.

The PCB where the diodes lived also showed a blue spark and blew fuses when i applied a PSXa power supply (after modding the input to accept the outboard PSU as you do!) So the txformer and rectfiers stage is shorting up i assume.

So this is much to good an amp to give up on, its fab and had a good phone stage , so

can any one supply (no pun intedned) a circuit of said amp , OR HOW ABOUT CONVERTING IT TO 12V SLA BATTERYS, that would be fantastic....... i think.... please discuss............Ta!

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Old 26th July 2006, 05:42 PM   #9
AndrewT is online now AndrewT  Scotland
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I suspect a short downstream of the rectifier has caused the transformer to overheat and finally short out internally.

Your PSXr supplies only part of the circuit and the original transformer is retained for less demading duty.

So you still have to repair the Cyrus 3 and then decide on whether to add a PSXr.
regards Andrew T.
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Old 26th July 2006, 06:35 PM   #10
Did it Himself
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I'm not familiar with the 3, but assuming it's basially the same as the 2 you might be able to replace the recitifier diodes and buy a new smaller toroid from Farnell to power the pre-amp.

I would do some further investigations first though like on what may have taken the PSU out.
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