Cyrus 2 crackling sound help.

Steven Z

Member
2015-05-11 10:26 am
Hi Guys,

I really need some help here. I own a Cyrus 2 amp ( CEN C 85 33) printed on the board and behind is printed issue 04. But when I checked my serial number (202856) and it's within the range for issue 06 according to the service manual.. unless the serial number on my case is messed up..

Upon checking on some of the components it's doesn't match the Issue 06 components but matches some remarks next to it, stating its was previously etc etc component.

I'm kinda lost here, I've googled everywhere for service manual but I can only find those with Issue 06/07... have anyone come across Cyrus 2 Issue 04 service manual ?

My problem started when I replaced my caps and I didnt adjust the bias.. after a while i heard some crackling sound.. turned it off and check all components.. luckily everything is working fine but now i'm afraid to turn on the amp again without adjusting the correct bias.

I tried to get reading from the emitter resistor base on service manual 06 and 07.. it doesn't work..

Anyone have any idea how to find the emitter resistor?

Any help is very much appreciated.. thank you.
 
When an amplifier is returned to Cyrus for service an updated board is fitted as a repair. Setting the bias should be a doddle. the emitter resistor is connected to the output stage emitter. A bit obvious but there it is.
R93/4 is the 0R47 adjustment is made with RV2/3 and should be between 8 and 20mV. Measure the voltage across. Don't forget, No signal and No speaker load for this measurement!
There is no off set adjustment to be done.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
We seem to have had a run on questions along the lines of 'I recapped my amp and now......'

Replacing caps will not alter the bias at all. If the amp was working perfectly before and now seems to have an intermittent fault (the crackling), then you need to recheck very carefully for any problems with your soldering/inadvertent solder splashes/incorrectly fitted parts.
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
How about a schematic for a bit of light entertainment? :rolleyes: This is original issue 6 but dated 1999, long after production ceased in 1987 - not that there should be any significant difference to the output stage as with issue 7 and TOG (single pair of output transistors). cyrus 2 power amplifier service manual Download page :: Schematics Unlimited

It's an awful drafting and numbering system but there are 8 such emitter resistors, one for each output device, R87-94. Note the number R47 also shown at each resistor is a lame-brained conflict of the alphanumeric resistance value system used. It means 0.47 ohms, not part number R47 as do R87,88 etc. The instructions for setting though, specify a test point that is not marked other than by part number.

Pick which ever of 4 emitter resistors you like in the channel corresponding to RV3 or 4, but manual calls for R93(L) or R94(R). This assumes that the output transistors are all still functional after a DIY "recapping" but as Mooly pointed out, there should be no need to fiddle with bias if you only replaced caps (and it is tricky with the slow response to the heatsink temp.) anyway.

Now to the problem. Your OP says "crackling". What does this refer to, occasional crackling noises or full-on distorted sound, like notes that cut off as soon as they begin to rise in volume?
 
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