Rotel RB850 Rebuild 2019

gni · 2019-01-31 3:30 pm

jean-paul · 2019-01-31 3:49 pm

Why remove perfectly fine film caps like the 150/330/1000 pF caps? The 1 uF electrolytic caps C613/C614 can also be replaced for film caps with same value and voltage ratings like Wima MKS2 or the like. MKS/MKT 1 µF 50V will fit exactly. That will make things better. While you are at it you can also replace the "open" input caps for molded 1 µF 50V MKT caps too. Same caps as C613/C614 so to speak. C607/C608 are best replaced by bipolar electrolytic caps (for instance 100 µF 16V Nichicon MUSE bipolar caps) as the signal is AC there. Polar caps were chosen there as they are cheaper.

The 10,000 µF caps are best replaced by very good quality caps that fit exactly on the PCB. I have good experiences with Nichicon KG.

The connector block on one of the pictures has no place in quality audio devices.

gni · 2019-01-31 4:03 pm

Now on to the questions. Thanks jean-paul for jumping in. You have started where I was going with the capacitors.
Identification, polarity & suitable replacement (not high end). The electrolytic caps have polarity markings. The
schematic has an 'S' in a circle for the blue caps and the axial cap. Is that a brand, type or polarity mark? The blue
caps don't seem to have a polarity mark even when examined with a magnifying glass. The power supply caps had
the brown substance on the board. Was that glue or leaking? I have had this amp since the mid 1990s and I don't
remember the brown gunk or discolored blue caps. They used to be a beautiful blue. The amp is about 25 years old
and if I'm going to do some repairs, I should do all the caps. Any help would be grateful.

C601-602 1 100V silver cube [SIGNAL INPUT]
C603-604 100P axial dip silver [to ground before LTP]
C605-606 1000P radial blue dip [FEEDBACK EQ]
C607-608 100 6.3V radial electo [FEEDBACK EQ]
C609-610 150P radial blue dip
C611-612 330P radial blue dip
C613-614 1 50V radial electo [BIAS]

sgrossklass · 2019-01-31 4:06 pm

gni said:
Power Supply Capacitor on Board (possible leaking)
http://groundnoiseindustries.com/diyaudio/rotel_rb850/Rotel_1009.JPG

Power Supply Capacitor on Board (possible leaking)
http://groundnoiseindustries.com/diyaudio/rotel_rb850/Rotel_1010.JPG

Looks like the customary fixing glue to me. Arguably better scraped off to avoid potential corrosion damage. Originally translucent, can turn a dark brown over time.

Have you read the long RA-820AX (I think it was - or 830AX?) project thread here? That was quite inspiring.

jean-paul · 2019-01-31 4:07 pm

The brown stuff is cheap glue that is not the best material for that purpose. I use acid free silicone stuff specifically for electronic purposes. Don't use normal silicone, you will regret it.
All the polar caps are to be replaced as they have had their best time. The other questions have been answered already.

You write "not high end" and that is Ok but if you replace caps in an amp from the nineties I would not skimp on part quality and buy known good stuff.

*The "S" probably means styrene for polystyrene. These are fine caps and don't wear out when they are the molded or dipped version like Rotel has used. In fact you will have a hard time finding replacements of the same or better quality.

gni · 2019-01-31 4:39 pm

An externally hosted image should be here but it was not working when we last tested it.

Thank-you. I appreciate the detailed advice. I thought the PS caps were leaking. I'm replacing the PS caps and electrolytic for sure. I planned about $60 total in caps. The power supply will set me back about $35 with shipping. I can put back the others (blue and silver axial and cube). I was worried that they have darkened over time (heat?). The C611/612 were different physical sizes but same value. . .someone was just grabbing parts and stuffing it looks like. I always thought the blue ones were tantalum. THANKS -- I Really Appreciate Your Help! ! !

jean-paul · 2019-01-31 4:48 pm

Forget planning costs as quality is not cost driven. Choose the best you can find for a good price and have a renewed amp that will last many years to come. Over the next 15 years you will enjoy the amp without issues when buying good stuff. Like I said before, replacing polystyrene caps is totally unnecessary and expensive. The resin has changed color but when there are no cracks in the resin you are doing useless replacement. C603 is an example of a polystyrene cap that has an "open" contruction and replacing those makes some sense as dust and dirt creep in and they are hygroscopic.

The "open" cube caps are to be replaced. You will experience them falling apart when resoldering. When not they will start to cause crackling in a while. Replace them for dipped/molded MKS/MKT caps of the same physical size. We are talking about 4 pieces molded/dipped MKS/MKT 1 µF 50V in total which cost peanuts.

If you want advice but don't wish to listen then asking advice is superfluous.

gni · 2019-01-31 4:55 pm

sgrossklass said:
Looks like the customary fixing glue to me. Arguably better scraped off to avoid potential corrosion damage. Originally translucent, can turn a dark brown over time.

Have you read the long RA-820AX (I think it was - or 830AX?) project thread here? That was quite inspiring.

Thanks, I will search for that one. I want to do this right the first time. My picture links in the first post show the before and after cleaning the board.

Before Cleaning & After PS Caps Removed
http://groundnoiseindustries.com/diyaudio/rotel_rb850/Rotel_1021.JPG

After Cleaning. . .
http://groundnoiseindustries.com/diyaudio/rotel_rb850/Rotel_1022.JPG

jean-paul · 2019-01-31 4:57 pm

gni said:
I want to do this right the first time.

gni said:
I planned about $60 total in caps.

Contradictory! 😀

gni · 2019-01-31 5:21 pm

I planned about $6 USD to $8 USD each for the 10000 50V power supply caps and about $2.50 USD each for the 14 smaller caps -- some a little more, some a little less. I thought that was reasonable.

If you want advice but don't wish to listen then asking advice is superfluous.

I'm taking in all the advice and did not mean to offend in any way. I think what I've heard is really
showing me the path to a successful repair and enjoyment. I'm catching up to the replies and
sometime answering to previous statements. I've concluded that I will have to mount the new
PS caps differently and that will be fine. I'm running on the United States power at 120VAC 60Hz.

Cleaned Board topside:

An externally hosted image should be here but it was not working when we last tested it.

jean-paul · 2019-01-31 5:24 pm

Must be a cultural difference or something but I would first look what I could find in quality parts and only then choose the best bang for the buck. Are you a banker? 😀

6 to 8$ is not much for a 10,000 µF cap....better choose wisely otherwise you will receive the Teapo curse. A long term investment for a capital good so to speak. Nichicon/ELNA/Kendeil (to name a few) cost way more but will deliver. I think you want your audio device to deliver.

I have bought these in the past (NOT from the same seller) and they were quite OK for the money but they were not the best. They fit nicely in your budget. Snap in types generally have better specs and they do cost more but they won't fit on the PCB. I leave the judgement if these FW fit on the PCB to you. I would choose snap in types and make the PCB accept the snap in caps. Done many Rotel amps in the past, it is not hard. You will never regret buying qood quality snap in caps.

4PCS NICHICON 10000uF/50V 50V 10000UF FW Audio Electrolytic Capacitor | eBay

*Change the voltage setting of your RB850 to 120V as your mains voltage may be close to that.

gni · 2019-01-31 6:10 pm

I've been looking through the Mouser and Digi-Key catalog to find caps. I haven't bought anything
from Ebay in years. I was burned twice. But, it gives me direction! The following is my first
shopping list.

Power Supply Filtering
UKW1H103MRD Nichicon | Mouser

Input Cap (C601/602) 1µF Poly
MKS4D041003F00KF00 WIMA | Mouser

Input to Ground Cap C603/603 100pF Poly
FKP2F001001D00JF00 WIMA | Mouser

Feedback Cap C605/606 1000pF Poly
MKS4O111003C00KSSD WIMA | Mouser

C609/610 150pF Poly
https://www.mouser.com/ProductDetai...=sGAEpiMZZMv1cc3ydrPrF2CPLAojp4yWpbiNq0LvxQ8=

C611/612 330pF Poly
https://www.mouser.com/ProductDetai...=sGAEpiMZZMv1cc3ydrPrF2CPLAojp4yWTWX/7Epl5Ow=

Feedback Cap C607/608 100µF Electro
https://www.mouser.com/ProductDetai...Jlu2/0JHpU9kDT%2bJMhjhF4x9B3WwUAqae%2bZapAw==

Bias Cap C613/614 1µF Electro
https://www.mouser.com/ProductDetai...=sGAEpiMZZMtZ1n0r9vR22e0BBN1kF1oiC776pDlgO/Q=

jean-paul · 2019-01-31 6:39 pm

Sorry but of the links I clicked I saw some caps with a different pitch/footprint and one of them was electrolytic while you could use superior film caps for that one.

Reread tips and choose caps with exactly the same pitch. So use 5 mm caps where 5 mm types were used. We are not the meltglue,"extension wires" or "cap freely dangling in the air" types here 🙂 Choose film caps over electrolytic caps where possible i.e. with small values. Use superior film caps like polystyrene or polypropylene for very small values like the 330 pf etc.

These seem to be OK for your purpose:

Power Supply Filtering
UKW1H103MRD Nichicon | Mouser (again, snap in types are better)

Input to Ground Cap C603/603 100pF Poly
FKP2F001001D00JF00 WIMA | Mouser

C609/610 150pF Poly
https://www.mouser.com/ProductDetail...pbiNq0LvxQ8=

C611/612 330pF Poly
https://www.mouser.com/ProductDetail...X/7Epl5Ow=

Please find bipolar Nichicon ES (MUSE) 100 µF 16V for this one:

Feedback Cap C607/608 100µF Electro
https://www.mouser.com/ProductDetail...52bZapAw==

Bipolar (= no plus or minus) elecrolytic caps used to be too large and costly in the past but they are smaller nowadays and they are superior for feedback circuits where AC is. Like these:

Nichicon Aluminium-Elektrolyt-Kondensatoren - Radial bedrahtet | Mouser Deutschland

So enough of my time, good luck with the recap of the RB850.

gni · 2019-01-31 7:07 pm

Thank-you! You have given me a real push in the right direction. This has been a real education. I will do more research to get my list together before posting. Cheers from California!

jean-paul · 2019-01-31 7:14 pm

Mmm I just thought you may not know differences in film caps. It is not hard but marketing names and prejudice are abundant as this is audio. In general cheapest film caps are polyester, then follows polycarbonate, polypropylene, polystyrene and there are a few "newcomers". This goes too far for now but remember: electrolytic caps (with their limited lifetime) should be avoided when possible which is nowadays no issue with values till approx. 10 µF 50V available in 5 mm pitch. These are way smaller than in the time the RB850 was built as technology has improved.

In general:

- almost any film cap performs better than an electrolytic cap.

- Bipolar electrolytic caps are simply 2 polar caps in series in one casing. They are better when AC is around like music. The feedback loop of a power amp is best done with bipolar electrolytic caps. Too bad 100 µF is needed otherwise a film cap would have been an even better choice.

- Filtering should better be done with any of the types better than polyester. Please note you choose FKP(polypropylene -> OK) where you had slightly superior polystyrene.

gni · 2019-02-01 4:29 pm

I'm stuck on using a bipolar electrolytic for the 100µF cap (C607/608) in the feedback filter. The original schematic called for a polarized electrolytic -- is that an electrically sound practice? [answered: bipolar electro more expensive so Rotel used cheaper polarized]

Thanks for the 'quality' list of film caps -- stlil searching for better quality parts.

The C603/604 shows no polarity on the schematic and the silk-screen shows a polarity mark. I posted an image earlier in the thread.

The silver input cube cap (C601/602) still vexes me: it is 1µF, should I use a film cap here also? [answered, yes]

As a side note, I've considered putting in a couple of input isolation transformers. Would I remove the C601/602 cap but leave the rest of the input resistors/caps? My understanding is the input cap prevents DC from entering the amp and being increased and showing up as DC on the output.

NOTE: I just reread the whole thread and I think most of these questions have been answered in the first few posts. If it is indeed true that you can pretty much use a bipolar replacement for the called for polarized cap then I'm closer to understanding. Otherwise, I shouldn't be messing around in electronics.

Found this discussion about polarized and non-polarized capacitors: I want to know the main advantage of polarized capacitor. If non polarized can replace it then why are we using polarized capacitor.

Apparently the politically correct crowd doesn't like using the term 'bipolar' so I will use 'bi-polar' or 'bypolar' or 'bipolar', because I'm not politically correct.

Polarized

higher Distortion
smaller in size
cheaper
Can not be used in AC without DC polarizing bias
Value are relatively high

Non-Polarized

lower in Distortion
huge in size
More expensive
AC and DC
All values

jean-paul · 2019-02-01 5:07 pm

gni said:
I'm stuck on using a bipolar electrolytic for the 100µF cap (C607/608) in the feedback filter.
The original schematic called for a polarized electrolytic -- is that an electrically sound
practice? [answered: bipolar electro more expensive so Rotel used cheaper polarized]

Bipolar electrolytic caps are a strange breed as they are 2 polar electrolytic caps in series. Normally one would not like that but they are handy in feedback circuits of power amps as that is where AC and DC are and thus they have a technical advantage there. As every penny counts manufacturers can get away with polar caps as voltages are low there. You could not do that trick with power rails 🙂

If one could one would use film caps but as stated they are too large in 100 µF. Then bipolar electrolytic caps are of good use. It is an improvement that was omitted because of cost thinking like you have as a habit 😀

For other applications like decoupling quality polar electrolytic caps do good work. For coupling manufacturers often used electrolytic caps in the past (cheap, large values in small casing etc) but since film caps have shrunken one can use superior film caps in 5 mm pitch as "elco-replacement" withput penalties. On the contrary...

C603/C604 are small value styrene caps with very good properties for that function at that spot.

Sorry but I think your knowledge is not enough (yet) to start adding transformers. Don't leave out the DC blocking caps! Why would you make matters more complex and risky? If you want to experiment please do that in an external case with 4 x RCA connectors and measure/listen what it brings.

gni · 2019-02-01 5:23 pm

Why could one not string in series ten or so 10µF film caps to get the 100µF value necessary for that location in the circuit? Not series, parallel.

Just saw the problem: 50µF film caps cost around $12 EACH and 2200pF cost about $1 (about 45 needed to get to 100µF). It would require a whole circuit board to hold them.

I need to measure the pitch of the circuit board holes for the parts. At a glance, C601-C604 are 10mm [axial] and C605-C614 are 5mm [Radial]

Making your own 100µF electro would require two 220µF back to back.

Grrrr. . . part selection isn't like going to the market to shop for oranges.

jean-paul · 2019-02-01 8:00 pm

Your calculator needs a new battery.

45 x 2200 pF = 99000 pF = approx. 100 nF = 0,1 µF standard value.

Making your own bipolar electrolytic caps (please use complete information as "your own 100µF electro" is half the info) makes no sense as it would be physically too large and thus mechanically instable. You also don't want long lead wires as they might function as antenna in sensitive circuits. You want lead wires to be short as possible and physically small parts secured on a PCB not 2 back to back "dangling in the air" caps. Just find the Nichicon ES 100 µf 16V and be done with it. You could use 1 µF 50V MKT/MKS for C605/C606 in parallel to the 100 µF instead of the 1000 pF. Then you can buy 6 pieces 1 µF 50V to make things simpler. These were added to improve high frequency behavior of the electrolytic cap slightly. Modern 5 mm 1 µF MKS/MKT or 10 nF MKP/FKP are so small that they will fit at that spot without penalties.

gni · 2019-02-02 12:00 am

Oops, I forgot I needed a bunch more zeros -- skipped 'nano' on my way to micro!

1,000 pF = 1 nF
1,000 nF = 1 µF
1,000000pF = 1 µF

I didn't realize that some extra lead distance could create that much of a problem. I think my ideas are inconsistent with good circuit design.

Here is my possible order from Mouser. Life in a compromise; most of the film caps don't come in the MKS series.
All prices in U.S. Dollars

http://groundnoiseindustries.com/diyaudio/rotel_rb850/Mouser_Order_Test_1.pdf

Capacitors Replacement

Power Supply Filtering
Replacement: 10000 µF 50V 20% Nichicon Snap-in #LKG1H103MESCBK ($8.94 each)
LKG1H103MESCBK Nichicon | Mouser

C601-602
1 µF 100V Film? silver cube [SIGNAL INPUT]
Replacement: 1 µF 100V 5% WIMA Polyethylene Terephthalate (PET) #MKS2D041001K00JSSD ($0.74 ea)
MKS2D041001K00JSSD WIMA | Mouser

C603-604
100 pF ???V axial dip Styrene [to ground before LTP]
Replacement: 100 pF 100V 2.5% WIMA FKP2 Series (PP) #FKP2D001001D00HSSD ($0.40)
FKP2D001001D00HSSD WIMA | Mouser

C605-606
1000 pF 50V radial Styrene Blue [FEEDBACK EQ]
Replacement: 1000 pF 100V 1% FKP2 Series (PP) #FKP2D011001D00ESSD ($2.05 ea)
FKP2D011001D00ESSD WIMA | Mouser

C607-608
100 µF 6.3V radial electo [FEEDBACK EQ]
Replacement: 100 µF 16V 20% bipolar electo Nichicon ES (Muse) #UES1C101MPM ($0.65)
https://www.mouser.com/ProductDetai...=sGAEpiMZZMtZ1n0r9vR22e0BBN1kF1oieqGePpsa0Vg=

C609-610
150 pF 50V radial Styrene Blue
Replacement: 150 pF 100V 2.5% WIMA FKP2 Series (PP) #FKP2D001501D00HSSD ($0.42)
https://www.mouser.com/ProductDetai...67c4jEJ147xUiJo5Ug0DQZGNDDYkW49tqZerMRxhxiY/r

C611-612
330 pF 50V radial Styrene Blue
Replacement: 330 pF 100V 2.5% WIMA FKP2 Series (PP) #FKP2D003301D00HA00 ($0.25)
https://www.mouser.com/ProductDetai...IBEflOBIyW/LXDcYg3Y8n25Ni%2bE6AVMjnExqMiAtB6O

C613-614
1 µF 50V radial electo [BIAS]
Replacement: 1 µF 100V 5% WIMA Polyethylene Terephthalate (PET) #MKS2D041001K00JSSD ($0.74 ea)
MKS2D041001K00JSSD WIMA | Mouser

Search

Amplifiers

Source & Line

Loudspeakers

Design & Build

General Interest

Live Sound

Member Areas

Site

Featured Vendors

Members Market

Vendors Market

Vendors

Search

Rotel RB850 Rebuild 2019

gni

jean-paul

gni

sgrossklass

jean-paul

gni

jean-paul

gni

jean-paul

gni

jean-paul

gni

jean-paul

gni

jean-paul

gni

jean-paul

gni

jean-paul

gni