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

[Goldie99]

Superb - very well detailed, explained & reasoned, with excellent photos & diagrams

I appreciate it's probably original, but could I just ask what the purpose of the transistor 'encapsulation' / 'potting' is ? e.g., the 2 / 4 white resin (?) filled objects in the last 2 photos.

I came across similar recently in a Sony TA-F444ES, with one particular transistor 'potted', but it's analogue in the opposite channel left normal / 'unpotted' - I never did work out why ??

 

[NAD80]

The company I work for used to cover an IC with hard potting to hide the actual part #. Wondering if the potting is to act as a heat sink?

 

[Leestereo]

Another fine thread. Very well thought out approach (as usual). A couple questions:

I understand why you'd replace electrolytics with film capacitors, but why replace them with bipolar electrolytics if films aren't viable?

What does the replacement of the zeners do?

Thanks for the kind words.

Bipolar electrolytic capacitors are preferred to polarized types for signal path duty due to their lower distortion (see the "Capacitor Sound" articles by Cyril Bateman published in Electonics World, 2002 for further details). Film capacitors are still preferred over the bipolar electrolytic types, but due to their physical size, their use as replacements/upgrades for electrolytic capacitors on circuit boards is typically limited to film capacitors that are <10µF. The precision zener diodes offer increased stability, lower radiated noise and improved reverse current ( http://www.vishay.com/docs/85604/bzx55-se.pdf ); the rationale for their use is similar to using soft recovery UF400x diodes to replace 1N4004 types.

 

[Leestereo]

Superb - very well detailed, explained & reasoned, with excellent photos & diagrams

I appreciate it's probably original, but could I just ask what the purpose of the transistor 'encapsulation' / 'potting' is ? e.g., the 2 / 4 white resin (?) filled objects in the last 2 photos.

I came across similar recently in a Sony TA-F444ES, with one particular transistor 'potted', but it's analogue in the opposite channel left normal / 'unpotted' - I never did work out why ??

Thanks for the compliment.

Yes, the white sleeves and the gray glue (resembles silicone RTV) are original; this arrangement is to thermally couple the differential transistor pairs (for improved DC stability).

 

[Leestereo]

Rotel RA-970BX Power Amplifier Stage

Signal Path Capacitors:
The stock capacitors in the input high-pass filters (C601, C602) are 4.7µF Black Gate electrolytic capacitor and these were upgraded to a WIMA 2.2µF stacked film types. The original capacitors for C607 and C608 (which set the low end cut-off of the negative feedback loop for the left and right channels, respectively) were 100µF Black Gate polarized electrolytic types. These were upgraded to 100µF Nichicon MUSE ES bipolar capacitors.

Non-signal Path Capacitors:
The electrolytic capacitors in the bias circuit, C611 and C612 are 10µF electrolytic capacitors and their replacements were low ESR types of the same capacity.

The output stage of the RA-970BX is a fully complementary high-current design with a total of 8 output devices, each rated at 15A.

 

[theophile]

Cyril Bateman:

https://linearaudio.nl/cyril-batemans-capacitor-sound-articles

 

[Goldie99]

Yes, the white sleeves and the gray glue (resembles silicone RTV) are original; this arrangement is to thermally couple the differential transistor pairs (for improved DC stability).

Thanks - I should have realised from the fact that there they're encapsulated in pairs - for some reason I missed that (probably too focused on the Sony where that wasn't the case).

.... back to 'lurking'

 

[Leestereo]

Yes, it sounds 'tight' in the bass, but not extended. Run a linear frequency response plot at 1 watt and half power from 120Hz down to 0Hz and you'll see what I mean...

Haven't confirmed the frequency response on my bench, but managed to locate the measurements that Stereophile published for the RA-970BX in the December 1996 issue; essentially a "flat" frequency response from 10Hz-50kHz into resistive and simulated speaker loads.

 

[Leestereo]

Attached is power output at clipping test I did years back on a RA-930AX (rated 30wpc), It came in over specView attachment 787481

From the December 1996 Stereophile issue, the power measurements of the RA-970BX confirm that, like the RA-930AX, Rotel's output specification of 60 WPC is rather conservative; its more like a 100 WPC unit when playing music. The measurements also confirm that is indeed a "high current" design.

 

[Leestereo]

The RA-970 does not use selector switches that connect via a steel ribbon cable, but the cleaning procedure is the same: apply contact cleaner (e.g., DeOxit) via openings in the the housing and cycle through the switch positions a number of times.

 

[Leestereo]

Replaced/Upgraded Components

A total of 40 components were replaced in the RA-970BX restoration/upgrade. Of note are the 14 "audiophile approved" Black Gate capacitors; their use in this relatively inexpensive amplifier illustrates Rotel's commitment to provide high-end sound at affordable prices. But however good the Black Gates were, they are now 20 years old and nothing lasts forever (especially small electrolytic capacitors); the miniature WIMA film capacitors that replaced them are definitely an upgrade (that will not deteriorate with time). It is by implementing current technological advances (such as stacked film capacitors) that the sound and reliability of these units can be improved to better than when they were first manufactured.

 

[Volgeluk]

Hi Leestereo and fellow Rotel-lovers

Thank you very much for this informative post.

I also have a RA-970BX which has unfortunately been very little used over the last 2 decades.

Lately the amp plays perfectly but after a while both channels starts fading out and completely loses sound except for a sound like a soft rush getting louder (Pardon my lack of a better description - English not my native language) None of any knobs or functionality works at this point. When I switch it off, its perfect again after a bout 5 minutes, only to repeat fading sequence

I am not technically clueless but far from good, so I opened the amp up. I immediately saw that the caps C909 and C910 had leaked.

I can get replacements here in South Africa for these caps at RS Components but it also appears that the two transistors - Q901(2SD600KE/KF) and Q902(2SB631KE/KF) is faulty. My problem is that I cannot find direct replacements at all, RS included. Can you kindly recommend an alternative pair(s) that will be suitable since I see you guys are excellent with this stuff and I am not sure which factors is the most important influencing transistor replacements?

Based on my meagre fault-description - is there any other likely damages that you should think I must attend to

I will also try to replace all the components like you did although I struggle a bit to deduct the exact components from your description. Especially it seems that I cannot acquire WIMA 2.2 uF caps.

Can you please recommend alternatives that is perhaps more available at places like RS? Unfortunately I live in a country which I love but is still a 3rd world country where finding replacements or materials is a challenge...

Thank you very much...

 

[eradoncic]

Hi Leestereo and fellow Rotel-lovers

Thank you very much for this informative post.


I can get replacements here in South Africa for these caps at RS Components but it also appears that the two transistors - Q901(2SD600KE/KF) and Q902(2SB631KE/KF) is faulty. My problem is that I cannot find direct replacements at all, RS included. Can you kindly recommend an alternative pair(s) that will be suitable since I see you guys are excellent with this stuff and I am not sure which factors is the most important influencing transistor replacements?


Can you please recommend alternatives that is perhaps more available at places like RS? Unfortunately I live in a country which I love but is still a 3rd world country where finding replacements or materials is a challenge...

Thank you very much...

Ypu should be able to find easy 2SA1492/SC3856 from Sanken
These are frequently used in other Rotel models (mainly power amps), have not to wide bandwith (prevention of oscillation) and cheap.
Change all at once.

 

[Pulp Speaker]

On the bench for restoration and upgrade is a Rotel RA-970BX integrated amplifier. Although consensus is relatively rare in the audio press, the RA-970BX has managed to garner praise in the Stereophile Recommended Components list, the HiFi World Buying Guide and What HiFi? magazine; it is also very well regarded by enthusiasts/owners on Audioreview.com.

Despite being a relatively inexpensive, mass market consumer product, the RA-970BX is built with premium quality parts: large toroidal transformer, British-made "slit-foil" power capacitors, Roederstein metal film resistors, Vishay metal foil resistors, Black Gate coupling capacitors, and polystyrene capacitors.

A couple of pictures of the stock unit (with the top off):

Are you still doing upgrades on the Rotel RA-970BX? I have one and would like if you could do an upgrade like what you have shown here. Thanks John

The restoration/upgrade plan:
1. Replacement of the small power supply electrolytic capacitors with low ESR types (increasing capacity where appropriate and physically possible).
2. Replacement of the reference zener diodes in the regulated power supply with precision 2% variants.
3.Replacement of all of the signal path electrolytic capacitors with miniature stacked film types (whenever possible) or bi-polar electrolytic types.
4. Replacement of any components damaged by the board glue (becomes corrosive as it ages).

 

[theophile]

I think that the following was intended to become part of Pulp Speakers post:


Are you still doing upgrades on the Rotel RA-970BX? I have one and would like if you could do an upgrade like what you have shown here. Thanks John

 

[Leestereo]

@Pulp Speaker: send me a PM regarding your RA-970BX.

 

[enimatek]

Replaced/Upgraded Components

A total of 40 components were replaced in the RA-970BX restoration/upgrade. Of note are the 14 "audiophile approved" Black Gate capacitors; their use in this relatively inexpensive amplifier illustrates Rotel's commitment to provide high-end sound at affordable prices. But however good the Black Gates were, they are now 20 years old and nothing lasts forever (especially small electrolytic capacitors); the miniature WIMA film capacitors that replaced them are definitely an upgrade (that will not deteriorate with time). It is by implementing current technological advances (such as stacked film capacitors) that the sound and reliability of these units can be improved to better than when they were first manufactured.

Wow
Truly impressed by what you've done here, I hope your rotel is still alive and kicking today.
You have inspired me to do the exact same with my Rotel RA-970BX

I got this beauty about a year ago, and it worked fine for the most part. When powering on for the first time in the day the right speakers don't work, but they kick in once things warm up a bit (usually up to 5m it would be back alive). I've been told and I kind of started to notice myself, the left channel has a bit more balls than the right, regardless of how well I try to flatten the balance/volume knob.
Lately instead of the usual right channel silence, its doing this distorted noise, sort of like a bad cable would. Except its not a bad cable, it does it regardless of input selected (some are empty) and regardless of volume level. Also the tone knob does a loud thump if mucked about with in these beginning 5 minutes or so.

I happily lived with it until I read through your thread, and now the idea of doing a bit of work on it is really tempting, I like to tinker with things like this.
I've never done any soldering, although it seems pretty straightforward, I also don't really know much about electronics, so when I opened my rotel, it looked a bit like yours, however I found it a bit overwhelming to look at
I am more worried I won't be able to find the parts that you used in your upgrade journey, I'm guessing you do these restorations/repairs quite often, so you'd have spare parts, whereas I would have to buy a capacitor kit, multimeter, soldering kit etc.

I'm wondering if you think it's worth spending just about £100 on tools/parts to restore mine or do you reckon its had its time? I mean it still runs damn well driving my speakers, the sound is great, its just cranky in the morning I suppose, but then who isn't!


Anyway, I hope you are well, let me know what you think

 

[MJR]

Leestereo, Do you have a parts list with purchase source for this Rotel repair? I have the same amp (slowly giving up its life) and would like to attempt to restore it. Many thanks.

 

[Karl vd Berg]

Despite being a relatively inexpensive, mass market consumer product, the RA-970BX is built with premium quality parts: large toroidal transformer, British-made "slit-foil" power capacitors, Roederstein metal film resistors, Vishay metal foil resistors, Black Gate coupling capacitors, and polystyrene capacitors.

That's what NAD should have done with their late 90s / 2000s low/mid amplifiers...Many cheap electrolytics used in many rather good amps.

 

[MJR]

Thanks, but the question was if you have a list of these parts that can be purchased and from whom. I am assuming that based on your detailed report of your rebuild that you might have this info. Thanks in advance for any details you can provide.