Hi,
For a wile I wasn't satisfy with the sound quality of my system.
Each week I blamed different component, the speaks, the pre amp or the power amps, maybe I was just looking for a reason to replace it all ;-)
I decided before buying new equipment to give it a try and refresh some of the components of the rotel rb-960bx power amp.
I own two units therefore I will be able to compare between them.
After searching the web for guidance I found that I need to:
1.change the electrolytic capacitors in the power supply
2.change the input coupling capacitors
The power supply capacitors are 50v 1000uf N.C neg.black towa
any recommendation for a replacement?
Also, could you advise on the input coupling capacitors,I am a bit confuse regarding the type and the value.
According to the bom one of them is p.p 100v101h and the second is m.p 100v 104k, I understand that those are Japanese measurement units.
On the board I found 100 100u wima and k100 1818 c7 ero capacitors.
Also do I miss any other components that needed to be replaced?
And no you have any tips for good quality cleaning?
For a wile I wasn't satisfy with the sound quality of my system.
Each week I blamed different component, the speaks, the pre amp or the power amps, maybe I was just looking for a reason to replace it all ;-)
I decided before buying new equipment to give it a try and refresh some of the components of the rotel rb-960bx power amp.
I own two units therefore I will be able to compare between them.
After searching the web for guidance I found that I need to:
1.change the electrolytic capacitors in the power supply
2.change the input coupling capacitors
The power supply capacitors are 50v 1000uf N.C neg.black towa
any recommendation for a replacement?
Also, could you advise on the input coupling capacitors,I am a bit confuse regarding the type and the value.
According to the bom one of them is p.p 100v101h and the second is m.p 100v 104k, I understand that those are Japanese measurement units.
On the board I found 100 100u wima and k100 1818 c7 ero capacitors.
Also do I miss any other components that needed to be replaced?
And no you have any tips for good quality cleaning?
104 and 101 are values in pico farads, a standard way of marking where the last digit is the multiplier. 104 is 100nF or 0.1uF and 101 is 100pF. K and H are tolerances, K is -/+10% and H is -/+3%
To be honest (my opinion), component changes like these will be bring little to no actual audible difference. The unit sounds as it does because of the basic circuit topology.
To be honest (my opinion), component changes like these will be bring little to no actual audible difference. The unit sounds as it does because of the basic circuit topology.
Well the amp is about 27 years old, electrolytics caps may well be failing, depends on the
amount of use its had, how hot it runs, etc, so the best way is to measure a few before deciding. The ones in the proection circuitry and the decoupling caps on the amp board(s) are the first ones to pay attention to as they lead to definitely failure modes (protection kicking in spuriously, amp oscillation, respectively). Failing reservoir caps will show up as reduced power handling ability, fairly easy to spot.
If you can lift a few caps and measure capacitance, leakage and ESR and all seems within specs, you probably don't need to worry for another decade.
amount of use its had, how hot it runs, etc, so the best way is to measure a few before deciding. The ones in the proection circuitry and the decoupling caps on the amp board(s) are the first ones to pay attention to as they lead to definitely failure modes (protection kicking in spuriously, amp oscillation, respectively). Failing reservoir caps will show up as reduced power handling ability, fairly easy to spot.
If you can lift a few caps and measure capacitance, leakage and ESR and all seems within specs, you probably don't need to worry for another decade.
My choice for electronic equipment (populated PCB's) would be hot water and detergent.
Post #10:
Washing an amplifier board
Post #10:
Washing an amplifier board
I found the service manual on line. There is at least one error in the schematic and the power supply caps are 10,000uF, not 1,000uF.
Input coupling caps are 10uF@50V: C601 and C602.
More likely to fail are the feedback decoupling caps 100uF@25V: C607b and C608.
Smaller caps like the RF filters C603 and C604 (100pF) are unlikely to fail.
There are a couple minor improvements I might try but otherwise this is relatively modern circuit.
You could check the idle bias as per the service manual, although I notice they recommend ~18mA per transistor (4mV/0.22Ohm) which is a bit low if you are fussy about crossover distortion.
Input coupling caps are 10uF@50V: C601 and C602.
More likely to fail are the feedback decoupling caps 100uF@25V: C607b and C608.
Smaller caps like the RF filters C603 and C604 (100pF) are unlikely to fail.
There are a couple minor improvements I might try but otherwise this is relatively modern circuit.
You could check the idle bias as per the service manual, although I notice they recommend ~18mA per transistor (4mV/0.22Ohm) which is a bit low if you are fussy about crossover distortion.
Im not sure what’s in your 960bx but if remember correctly a lot of Rotel equipment from that time period used quality BHC Aerovox power supply capacitors. They may still be in good shape.
Hi,
The RB-960BX circuit topology is the same as found in the power amp sections of RA-820AX and RA-931, etc.
So, you could do the quantum leap in Rotel sound quality by upgrading your 960-BX as per my thread "improving a Rotel amp THD by 20dB!"
I now sell the special CM1 and VAS3 modules for £10 a piece. That covers technical assistance from me when/if you need it.
People all around the world have by now successfully done the upgrades.
Your call.
Cheers,
Per
By coincidence I have just inherited a Rotel RA840BX to be repaired.
The fix was easy - a new Zener in the RIAA stage reg. Strange because a "professional" repairer had told the owner it was uneconomical to repair!
I have measured distortion of the power amp stage (no preamp as such, it's passive) and it's quite low distortion: 0.0051% at 1W into 10R load. It's easy to get less, by dialing up the quiescent current, but the heatsinks are not big enough for this to be viable.
The circuit uses unusual compensation, no local decoupling (like Naim. To keep the 0V rails clean?) and has a 0.22R ressistor instead of the more common LC output "buffer".
This circuit was designed/tweaked by Stan Curtis. IME Stan has a knack of making things sound way better than they ought to. Stan states in his HIfi Critic articles that there were lots of iterations of this ciruit and that the layout is one of the most important things to get right, for good sound.
People have complained about the 0V (star) grounding being a complete mess in the RA820. In my 840BX,, although the star point looks untidy, it's technically correct.
I found that lots of noise comes from the phono stage 0V being connected to the chassis (which of course is connected to safety earth). This is fine, until you connect it to something else with an earthed ground e.g. an oscilloscope.
I will be very interested to read about the OP's findings,, since he has an original to compare to.
I'm not planning any mods of my 840BX. The MC phono stage is excellent. The power amp is at least as good as my NAP200 clone (which was constructed with very carefuly matched BC239s etc). The HD profile and amount is almost identical to the NAP FWIW.
Funilly enough, one of my first amps was an RA820. I remember fitting some big smoothing caps to it and being shocked at how crap it sounded afterwards. That was when I first heard about Stan.
The fix was easy - a new Zener in the RIAA stage reg. Strange because a "professional" repairer had told the owner it was uneconomical to repair!
I have measured distortion of the power amp stage (no preamp as such, it's passive) and it's quite low distortion: 0.0051% at 1W into 10R load. It's easy to get less, by dialing up the quiescent current, but the heatsinks are not big enough for this to be viable.
The circuit uses unusual compensation, no local decoupling (like Naim. To keep the 0V rails clean?) and has a 0.22R ressistor instead of the more common LC output "buffer".
This circuit was designed/tweaked by Stan Curtis. IME Stan has a knack of making things sound way better than they ought to. Stan states in his HIfi Critic articles that there were lots of iterations of this ciruit and that the layout is one of the most important things to get right, for good sound.
People have complained about the 0V (star) grounding being a complete mess in the RA820. In my 840BX,, although the star point looks untidy, it's technically correct.
I found that lots of noise comes from the phono stage 0V being connected to the chassis (which of course is connected to safety earth). This is fine, until you connect it to something else with an earthed ground e.g. an oscilloscope.
I will be very interested to read about the OP's findings,, since he has an original to compare to.
I'm not planning any mods of my 840BX. The MC phono stage is excellent. The power amp is at least as good as my NAP200 clone (which was constructed with very carefuly matched BC239s etc). The HD profile and amount is almost identical to the NAP FWIW.
Funilly enough, one of my first amps was an RA820. I remember fitting some big smoothing caps to it and being shocked at how crap it sounded afterwards. That was when I first heard about Stan.
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Hello @LevPri, did you end up getting this project done? I noticed the 10000 50v caps for the power supply have three prongs? Having a hard time finding the same capacitor and wanted to see what you went with. Plenty of two prong snap ins but not three.
If anyone could help me out I have the other caps ready to order. Thanks in advance.
If anyone could help me out I have the other caps ready to order. Thanks in advance.
kalidupakid, your post is now here:I am posting here to build up enough rep with the mods to converse with AngelP to purchase modules to upgrade my Rotel RB-960BX. Please help me out if you are able to. Thanks.