amp not working


2008-10-18 11:59 am
hi folks i came home today to find one of my trusty old Rotel RB03 power amps is no longer working.
i know this site for mainly for DIY and modding but can any one find the time to help a fellow HIFI enthusiast out?
There is no sound no power light and no bridged mono light.
i have limited knowledge with electronics but did manage to mod up a marantz cd67 chaning out most caps op amps etc so i can use a soldering iron.
i simply don't know my way round the internals of a power amp. View attachment rotel_rb_03.pdf

hopefully the pdf manual has attached.

Thanks in advance for any replies.

Hi Payload thanks for the reply i did check the trigger switch position.
On the schematic there is 240v at inputs 1 & 2 (CON 2) there is also 240 at 1,2,3 & 4 (CON 3) all on pcb X-1240E02 these all go to the toroidal transformer also 1 feed to the trigger board X-1291.
returning back to pin 3 (CON 1)from the trigger board there is 24v
From toroidal a red connects to 1 black to E1 and orange to 2 on pcbs X1240E01 assuming black as GND the red and orange show 1.210v when the amp is switched on and 0.4 when switched off.
There is power at 126 and 127 near the big caps i will print a blow up of the schematic tomorrow to identify the parts.


2008-03-12 9:59 pm
you wrote in your 1st post, that there is no power light;
the power indicator LED gets its voltage from the auxiliary transformer T101 which also powers the relay RY101 which turns on the main power toroids; if the power LED does not light up this obviously means that there is no output from T101 and hence the relay is not activated;
there are 2 fuses F101 and F903 associated with that auxiliary psu, did you check them ?
b.t.w. pull the power cord before you go inside ... !
Thanks payloadde i checked fuses visually straight away, the fuse you spotted f903 i thought was a resistor!
I pulled the board and checked a few times getting OL so looks like the culprit.
It never ceases to amaze me just how good at this you guys are!
I still have questions if its ok with you?
When i search for axial fuses they seem to be mostly fast blow is that correct?
The parts list lists it as Lead type fuse R251-250 is that simply axial fuse rated at 250v?
Thanks for you help i would never have worked out the probable cause as what looks like a little green resistor lol
The breaking capacity tells you the maximum current that the fuse can interupt and allow the arc to extinguish.

Fortunately in the UK you have BS1362 fuse in your plug top. It gives the high rupture capacity (HRC) that you may require in the event of a catastrophic failure.
Then you have the back up of the main distribution board, where your MCBs are probably rated to >=5kA

But that reinforces why you must only fit BS1362 fuses into your plug top. Glass fuses must not be used where HRC fuses are required.


2008-03-12 9:59 pm
The fuse in question here (F903) is a quick acting (labelled F, Fast) 250mA on the low-voltage secondary side of the auxiliary power transformer; so voltage is not the issue here, nominal current is; the max. breaking capability 30A is O.K. here, the secondary of that little transformer will not even come close to 30 amps.
The fuse you picked as a replacement is labelled FF - meaning super-fast acting; this might not be the best choice because you already lost the F-type;
now the question is, was there a reason why the original fuse blew ? before fitting a new one, I would check components downstream of this fuse;
primary suspects could be the rectifier bridge D101 and the smoothing cap C101; I am not saying they are necessarily broken - fuses may deteriorate with age and with each turn-on there is a surge current into the cap and maybe it is a little under-rated, too - but I'd check them anyway at least with an Ohm-meter; now that the fuse is out you may just probe across both AC pins (labelled "~") of D101 which is a bridge rectifier and you should get an OL reading after a couple of seconds; the cap will need some time to charge, so be patient; test with probe cables reversed, too.
Since you are about to buy a couple of fuses anyway you might consider to order a new rectifier and cap as well - they are cheap enough ...
By the way the part list shows D101 as "power diode 1.5A" but it is in fact a small 1.5 amp bridge rectifier - again voltage is not the issue here, and 1.5A is about the smallest you can buy these days; C101 is a 470 micro-farad (uF) electrolytic rated 25 volts.
When you replace them, remember polarity matters (or take a photo before you start de-soldering things).
good luck ... and don't forget to pull the plug ...