Leestereo's Restoration & Upgrade of a Rotel RA-970BX
- Thread starter Leestereo
- Start date
eradoncic
Active Member
Well, I doubt it is oscillation- 970BX is well decoupled, and in other similar model to my expirience both mentioned op amps does not oscillate. Even a LM6170 stands good at that place and it is 100MHz/2000V/uS unit.
Something else is made noise there, check the socket, use quality one.
Having Rotel RA1070, the only tried op amp oscillating was AD828, and it is 130MHz/450V/us. At the end, left OPA2132 as standard and/or AD712, both ful, ritch sound
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theDG
Member
I should have said makes an oscillating sound sort of a bzz bzz
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eradoncic
Active Member
Sure. Either for 980/985 and 1070- the schematics is very similar to RA970.
Just these days, I’m refreshing one of the mentioned models (RA-1070).
Replaced all signal DC blocking caps as mentioned using Nichicon ES, 10uF/50V, since to my ear these sounds much better than Wima MKS (somewhat sterile).
Also big sound improvement is replacing decoupling C511/512 caps of the line preamp (IC501/502) with some fresh caps. I’ve used Nichicon KZ here, 220/35V, but want to try also FG 470uF.
The results/sound are unexpectadly good with KZ. Better than say- Pana FC or Rubycon ZA/ZL, which were previously on the board.
Other caps in such position are optional to replace since these are decoupling caps for the tone control ICs, and such- you may or may not replace, depending how much you use the tone contols.
You may consider also replacing the AD712 IC with some of your more liking (good and modern cadidate are LM4562/49720/ OPA1642/2134/2132/OP275…name it. I finaly opted for MUSES8920- very clean and good sound balance).
AD712 is somewhat harsh, but dynamic and sparkling sound typical for AD, so you may left this as-is.
My 5c.
Edits: typos
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I realize that seeing a thread with the same component in question anyone might be having an issue with is like seeing a shiny object for a magpie, or squirrel to a dog, HOWEVER, it is ALWAYS BEST PRACTICE to start your own thread instead of adding onto someone else's thread. Can cause confusion in the long run for others, and detracts from the excellent thread Leestereo originally intended this to be.
If you want specific members to see your thread because of the work they did, use @____________ with whatever usernam you want to get pinged.
Sorry, rant off......
If you want specific members to see your thread because of the work they did, use @____________ with whatever usernam you want to get pinged.
Sorry, rant off......
@Leestereo,
Embarking on an RA970BX resto, similar to yours.
Started with replacing busted fuses, now there is sound. Though horrendous.
My testing equipment includes a multimeter, my speakers, my ears and eyes.
Would you agree that these power filter caps are likely to be shot?
I read that yours tested well, but did you consider what you would replace them with if they did not?
Do you have any recommendations regarding brand and model and / or upsizing both capacitance and voltage.
Rotel slit foil electrolytics are 10000uf and 50V, I was considering 12000uf 63V if they fit? Brand not yet decided, but Mundorf is out of the budget.
I measured them as 55mm high and 35mm diameter, they are tight up against each other and near the top of the cabinet, dont think anything physically larger would fit.
Keen to hear your ideas.
Embarking on an RA970BX resto, similar to yours.
Started with replacing busted fuses, now there is sound. Though horrendous.
My testing equipment includes a multimeter, my speakers, my ears and eyes.
Would you agree that these power filter caps are likely to be shot?
I read that yours tested well, but did you consider what you would replace them with if they did not?
Do you have any recommendations regarding brand and model and / or upsizing both capacitance and voltage.
Rotel slit foil electrolytics are 10000uf and 50V, I was considering 12000uf 63V if they fit? Brand not yet decided, but Mundorf is out of the budget.
I measured them as 55mm high and 35mm diameter, they are tight up against each other and near the top of the cabinet, dont think anything physically larger would fit.
Keen to hear your ideas.
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mrehorst
Super Member
I just recapped an RA-840BX3 and RT830.
Those caps are definitely dead. I wouldn't power the thing up with caps that look like that. They and all the other electrolytics in the unit need to be replaced. Those caps may have taken out the rectifiers when they blew.
If it's like my 840BX3 and has a phono stage, there may be a couple tantalum caps used for interstage coupling. In my unit those caps were noisy and caused motorboating in one channel of the phono preamp. I'd replace those, too.
Those caps are definitely dead. I wouldn't power the thing up with caps that look like that. They and all the other electrolytics in the unit need to be replaced. Those caps may have taken out the rectifiers when they blew.
If it's like my 840BX3 and has a phono stage, there may be a couple tantalum caps used for interstage coupling. In my unit those caps were noisy and caused motorboating in one channel of the phono preamp. I'd replace those, too.
p4spooky
Active Member
I used Nichicon - LKS1J123MESC. These are exact fit (see my restoration thread HERE using @Leestereo recommendations here)
Spooky has a recommendation for new rectifiers with heat sinks.
Will likely go for those.
No phono on the RA970BX, unfortunately.
@Leestereo
Some restorers are critical of the metalized polyester film caps when i showed your project.
They recommended polypropylene film caps of same spec, admitting that they are large and more expensive and may need to be squeezed in or have long leads to fit.
Would you mind sharing your thoughts around film cap selection for this project?
Thanks!
Some restorers are critical of the metalized polyester film caps when i showed your project.
They recommended polypropylene film caps of same spec, admitting that they are large and more expensive and may need to be squeezed in or have long leads to fit.
Would you mind sharing your thoughts around film cap selection for this project?
Thanks!
Leestereo
Super Member
Polypropylene film capacitors are preferred to polyester film types however since they are typically 10X the physical size of the polyester types, and 20X-30X larger than aluminum electrolytic capacitors, there is simply insufficient space to install them on a board as replacements for stock components. Further, the installation of such oversized film capacitors is very likely to introduce noise into the circuit, and or even possibly induce oscillation see https://sound-au.com/articles/capacitors.htm for further details.
mrehorst
Super Member
When I recapped my RA_840BX3 I ordered WIMA PP film caps to replace the tantalum caps used for interstage coupling in the phono preamp section. I did not realize how big they were until they arrived. I don't think there's any way I could have shoe-horned them into the amp. I ended up using some audio grade electrolytics to replace the tantalum caps.
Marcel27
Active Member
I had a crackling sound in the right channel, only when I used the Tone control. It was the NJM2114 that was defect after so many years. I never replace old opamps with new opamps. I replaced the NJM2114 with a NJM2068 and often by the same time-bound types. It is an older type, smaller slew rate, a little more THD and more noise. The best part is I cannot hear the difference.
The result sounds good.
[edit] I do never recapping. I only exchange defect components. I like the "old" sound.
