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Leestereo's Restoration & Upgrade of a Rotel RA-970BX

Unfortunately, since this 970BX restoration work is now ~4 years old (1 computer and 1 dead hard drive ago), I no longer have that specific BOM.
 
Thanks for the excellent write up @Leestereo. i have owned an RA970bx since new and sadly its now distorting in the left channel - visually only one cap (c524) has a slightly raised top and everything else looks in order. Would love to do a recap as per your instructions. I have two questions for you

1. What series of Wima caps did u use?? Im having trouble identifying the ones you have used because none of the current once have 'F3" stamped on them
2. I never, never use the tone controls - can i pull one of the good 220uf caps in the tone circuit ie C527/528 and use that to replace the bad cap in C524 as they are identical.

Sorry for the newbie questions im a novice but i do know how to solder and remove components and so i thought i would have a crack at this myself. I have actually ordered Nichicon Muse cap for C524/C525 already but i was concerned because i couldnt find any with 105 degree rating only 85 degree. Hence the idea to just take a cap from the tone circuit....
 
patrick s said:
1. What series of Wima caps did u use?? Im having trouble identifying the ones you have used because none of the current once have 'F3" stamped on them
Click to expand...

The replacement WIMA capacitors are MKS2 types. The "F3" designation is a manufacturing code.

patrick s said:
2. I never, never use the tone controls - can i pull one of the good 220uf caps in the tone circuit ie C527/528 and use that to replace the bad cap in C524 as they are identical.
Click to expand...

The C524 capacitor is a local filter for the regulated -16VDC rail as is C528 (which is in parallel with C524). If there was a problem with C524, there would be distortion in both channels. Hence, swapping these is not likely to resolve the distortion problem in the left channel. If you use only headphones is the distortion still apparent?

patrick s said:
...I have actually ordered Nichicon Muse cap for C524/C525 already but i was concerned because i couldnt find any with 105 degree rating only 85 degree...
Click to expand...
The Nichicon Muse capacitors are not available with 105°C temperature rating.
 
Just to stick my nose in here. I run a Rotel RB976Mk2 6 channel power amp. The Mk1 and Mk 2, although looking similar are completely different internally and sound different too. The Mk2 is a modular design with each amplifier pair on a separate board and heatsink whereas the Mk1 is a single board unit.. I increased the size of the PSU smoothing caps and replaced the bridge rectifiers with high power types bolted to the casing for improved heat sinking. I replaced a few caps with better types and tidied the internal wiring. The power transformer has 3 sets of secondary windings and is rated at 700VA - It's huge.
These units sound superb and I think they are very underrated sonically. The specs given are very conservative as the amp is capable of producing far more than it's quoted 60 watts per channel. Personally I like the 8 and 9 series Rotel amps. I have had a number of them over the years and I've never been disappointed.

Incidently, the later 10 series RMB1066 (The one with false heatsinks on the front with input level controls hidden in between the fins) is internally identical to the RB976Mk2. There is no service manual available for the 1066 so you use the 976Mk2 one.
 
Thats great thanx ill get those WIMA mks2 caps on order then. Im so glad i mentioned which cap i thought looked suspect @Leestereo so i will provide further clues. The picture attached is my unit and I've circled in red the caps that are not flat on top. The large 10,000UF cap feels slightly rippled on top. I recall having a bad interconnect which blew fuses in the pre and power sections from memory which have since been replaced. Channels would drop in and out and this confused me greatly - i rectified this by cleaning the switches with contact spray. The distortion originally was on/off but these days its more on than off although and is not really noticeable until the volume is at 3 oclock and usually only manifests in the woofer not the tweeter. Yes the distortion is present with phones too but again only in the left channel. Which components do u suspect are bad given that info??
RA970BX.jpg

Would also like to draw ur attention to this article in DIY Audio which recommends replacing the opamp IC501 with a texas instruments LME4890. Would u be in agreement with that?? https://www.diyaudio.com/forums/solid-state/271483-rotel-ra-970-bx.html.

Thanks for your input @Powertech. Yes fine amps for the dollars. Really have to spend double the money to hear significantly better. Im not quite sure the point u are trying to make however. Are u suggesting that the ra970bx which i have may be different internally to a Ra970bx mk2 and hence the potentially one bad cap c524 might only affect one channel rather than two??
 
@Leestereo @Powertech Can anybody assist with this PLEEAASSSSSSSSSEEEEEEEEE???? Think i have found whats causing the distortion in one channel, Resistor R901 and Diode D903 readings are all over the place and eventually goes to OL on a multimetre. Whereas corresponding resistor R902 and diode R904 read 3.2kilo ohms and 1.2m milli ohms on the metre conistently. Can anybody point in the right direction as to replacement parts and it would be particularly helpful if i could get part numbers. These are near the large power supply caps.fault.jpg
 
Another example of good budget gear were some HK from the late 80's/90's that used poor quality caps. Found some green sleeve 85°C TK brand electrolytics that went low in value due mainly because of heat. Replaced them with some 105°C Nichicon or Panasonic.

@patrick s , maybe you have a faulty Zener D903, but I also see that cap on the left with a strange black spot on its top (+) side? Best thing to do is to measure the 3 components (Zener, resistor and electrolytic) off board.
 
patrick s said:
...Think i have found whats causing the distortion in one channel, Resistor R901 and Diode D903 readings are all over the place and eventually goes to OL on a multimetre...
Click to expand...

Its unlikely that the resistor is faulty; the D903 zener can be replaced with a BZX55B16.
 
Cheers thanx for the info @Leestereo . I wasnt sure of the specs for the zener but a google search indicates BZX55B16. is a vishay zener diode with a 500mW 16v spec yes???. Ive measured and re measured the value of that resistor R901and its always OL on the meter???? So dunno, but at such a small cost cant hurt to replace it. @Karl vd Berg the black spot is glue that they use to glue the cap to the board. The cap is not leaking. An interesting bit of trivia CAPS c526/527 are labelled as 220uF but they are BOTH reading approx 550uf on a meter!!! Im measuring from the underside of the circuit board so im confused whats happening i would have thought it wouldnt make any difference if components are on off the board to test values. Whilst i was at it i measured the two large power supply caps rated at 10,000uF but reading at approx 12,000uF for both :)
 
Hi all, I'm doing one of these up as per the thread except there is no sound. Just the merest whisper and some quiet hiss. the only thing that seems off to me is that there is a short across the two ~ of the bridge rectifier positions on the circuit board even after it has been removed, the bridge rectifier itself is ok everything else seems to measure ok?
 
oops :oops: I found some dry joints! now there is sound on the right but not a lot of volume, but the left is still just a bit of hiss will retest tomorrow
 
Hello, I am new to this site, which I stumbled upon when researching the Rotel RA 970 BX. The Lee's Stereo rebuild article is stunning, and I wonder if anyone know's if they are still in business, and whether such a refurbishment would still be possible. I am writing from Montreal, Canada, and would be running this amp into Vecteur Premiere speakers to start, with the intent of upgrading the speakers if necessary. Many thanks for any help you can provide, and thanks for the community - John Mac Master
 
patrick s said:
Cheers thanx for the info @Leestereo . I wasnt sure of the specs for the zener but a google search indicates BZX55B16. is a vishay zener diode with a 500mW 16v spec yes???. Ive measured and re measured the value of that resistor R901and its always OL on the meter???? So dunno, but at such a small cost cant hurt to replace it. @Karl vd Berg the black spot is glue that they use to glue the cap to the board. The cap is not leaking. An interesting bit of trivia CAPS c526/527 are labelled as 220uF but they are BOTH reading approx 550uf on a meter!!! Im measuring from the underside of the circuit board so im confused whats happening i would have thought it wouldnt make any difference if components are on off the board to test values. Whilst i was at it i measured the two large power supply caps rated at 10,000uF but reading at approx 12,000uF for both :)
Click to expand...

Hi Patrick, I am a new member here and was interested in hearing how your RA-930BX modifications went? (if you are still on here 2 years later haha). I am doing my RA-930AX up in a similar way and from what i recall about multimeters and capacitance is that without removing the caps from the board you can get some very strange readings as the current the multimeter uses to test capacitance will flow out of the cap through to the rest of the circuit. This was the culprit for my strange readings and resulted in me needing to remove the caps from the board for testing. There is a way to test capacitors that are sill soldered in accurately but honestly, just dee-soldering them for testing will likely be easier than having to buy more equipment and learn different testing techniques. Since the amps are now close to 27 years old, I would say most would need replacing and I have not even removed them yet for testing. I am definitely a victim of the speaker popping on power up and down and this has steadily increased to quite worrisome levels. For probably the last 10 years I was able to avoid dealing with this by connecting my speakers up to the "Remote" connectors and playing the role of a protection relay manually by waiting 5-10secs before pushing the Remote button in. Apart from this however the amp works wonderfully. 27 years is an impressive innings for a amp (let alone one that is considered budget) and really shows that the old Rotels were incredibly hardy amps. During that 27 years I did not maintain the amp at all apart from replacing a fuse at one point. This is even more astounding in context of me or my dad (who originally bought it) using it near daily for those 27 years. The purist in me is cringing at the idea of changing parts out for newer better ones and somehow I have not noticed the gradual decline in SQ over the years. I guess my ears have been adjusting to the incremental decline or maybe my ears themselves have been declining hahaha. Regardless, I am excited to bring this amp back to life!

If anyone else is doing up an 930AX or has in the past please let me know! I would be very interested to hear how you went and what parts you chose!
 
MMM, my RA 970BX works well with the selector on aux1 or aux2 but with CD or tuner selected there is considerable hum over the music through the speakers?
 
Update!
Someone else has fiddled with this one before me :biggrin:. Originally Q902 had blown and peculiarly seems to have damaged the tape in/out phono sockets which have burn marks on them. The screening contacts on the dual CD/Tuner phono inputs were not making good contact because they seemed to have been shortened so were not going through the PCB properly. Maybe the Tape - CD/Tuner quad blocks were swapped by the previous owner? Am awaiting some replacement quad pcb phono block for the tape in/out but will probably have to make them fit as they probably won't line up correctly.
 

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