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Recapping Rotel RB-850s

DoctorRad

DoctorRad

pfm Member
I have a couple of old RB-850s that I've never used in anger that I'd like to get going before too long. They'll almost certainly need recapping, so I dug out the service manual.

Most of the cap aren't electrolytics other than the reservoir caps, but C613/4 caught my eye as they're only 1uF 50V. Do they for some reason need to be electrolytics, or could I for instance replace them with correctly rated polyester caps?

y4m2yA_D6Lw2NlsbEvBuYhgxtsbJ9OBeYchNX8gT_I8aQ2pFwFfPCM0jutSjgpk1pVIubPx9FagWiJxh5rUd31RHEhPJfeaMA5APrDsvRNgppMGroEILeoUpmLR7RbVXg7do_42GHyw8M4zjAsJJ2dxSITaA1mB8KaRE542bs2RMdw4W8EiY42Qj2721nkOBiU9rtQ7Z8QzghlMghcbNXDeow
 
Very pertinent to me as I have one with monstrously leaky PS caps and need a cheap path to restoration! I'll watch this thread!
 
Replacing this little capacitor with a polyester may bring a longer life but won’t make any major improvement on sound quality given its function.
Given the price difference, I would stick to a good electrolytic with a rating of 105C as these amps tend to overheat a bit because the heat sinks are captive in a box, kind of.
 
Gervais Cote said:
Replacing this little capacitor with a polyester may bring a longer life but won’t make any major improvement on sound quality given its function.
Click to expand...
Okay, I'll bite. Given that half-an-hour on Google didn't turn up a comparable circuit with an explanation of the purpose of such a cap... what is its purpose?
 
The purpose of that cap is to lower the impedance of the Vbe multiplier at signal (and above) frequencies.

I changed both the small electrolytics in mine. It made zero difference to the sound, so I put the originals back.

The problem with the RB850 is the non-standard main smoothing caps. It will require some modification to accept standard 2-pin parts.
 
Gervais Cote said:
these amps tend to overheat a bit because the heat sinks are captive in a box, kind of.
Click to expand...

The heatsinks are not the most generous in size but they do have vents below and above so convection/airflow can take place. Actually this is not a bad way to do it because you can allow a higher heatsink temp than with external heatsinks.
At the correct Iq the amp runs cool.
 
FWIW I did more than a recap to my RB850:

IMG-0150.jpg


That's a pair of TGM8 modules in there (currently for sale in classifieds).
 
Yes I would say so. IMO the RB850 in pretty good but does sound a bit grainy in the mid/highs and slightly uneven in the bass. I think it has that 80's sound... pratty!... and if you look at its design features it does have something in common with the "he must not be naimed" company e.g. no local decoupling, no output inductor etc.

The TGM8 is very clear and detailed sounding - more modern I guess.
For my tastes the TGM8 is just a touch too obvious in the presence region. However there are loads of people on DIYAUDIO that love the P3A that is is based on as well as the TGM8. Many years ago I built a P3A and from memory I would say that the TGM8 is significantly more refined in sound.

I'm currently transplanting my Mooly Mosfet amp modules into the Rotel chassis - hence the sale of the TGM8s.
When that is completed, then the Mooly chassis can accept the NCC300s I've had in the waiting room for quite a while.
 
Fair comments... I bought mine close to the bottom of the market because of their "you'd have to spend a lot more to get significantly better performance" reputation, and a recommendation from a friend.

It would be relatively easy to regulate (or even decouple) the supply to everything to the left of Q615 in the diagram above, but I'm not sure it's worth the effort...
 
Back in the day my brother had one of these so I spent a lot of time listening to it... A generally good amp but ultimately lacking in transparency and soundstaging is how I remember it. I thought it remarkably VFM at the time.
 
S-Man said:
The problem with the RB850 is the non-standard main smoothing caps. It will require some modification to accept standard 2-pin parts.
Click to expand...
What's odd about the original caps? I've been thinking of buying (and "tuning") a RB850 as a backup for my accident prone Sugden.

(when noodling around with LTSpice, I found that giving the front end a clean PSU à la Avondale Voyager, things could be improved a lot. But this is likely true of most amps

https://pinkfishmedia.net/forum/threads/martin-clark-was-right.223904/#post-3577330

)

BugBear
 
bugbear said:
What's odd about the original caps? I've been thinking of buying (and "tuning") a RB850 as a backup for my accident prone Sugden.

(when noodling around with LTSpice, I found that giving the front end a clean PSU à la Avondale Voyager, things could be improved a lot. But this is likely true of most amps

https://pinkfishmedia.net/forum/threads/martin-clark-was-right.223904/#post-3577330

)

BugBear
Click to expand...
Well my leaky cap 850 mentioned above is available if you need to find one to start with. Let me know if you're interested (I'm being realistic about the liklihood of me getting the soldering iron out in the near future!).
 
DoctorRad said:
It would be relatively easy to regulate (or even decouple) the supply to everything to the left of Q615 in the diagram above, but I'm not sure it's worth the effort...
Click to expand...

I constructed an LTSpice model of the 850, partially out of genuine interest, but also as a learning exercise in LTSpice. The 850 is a fairly classic, un-adorned, class AB circuit.

I also separated the power supply left of Q615. This allowed me to measure the PSRR of the front end and output stages by injecting ripple onto the separate power rails. (I wouldn't fully trust my results, my knowledge of LTSpice remains marginal).

It also allows me to state that the current draw of the front end is only 18 mA!!

EDIT; anyone who wants my model is very welcome to it. Just ask. I can't see an easy way to post it to the forum.

BugBear
 
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