Ancient Civilization Artifact - Rotel RB-990bx

[Jorj]

Just found one of these on eBay and had it delivered.

http://www.rotel.com/product/rb-990bx

Production range is from 1992-1997, and the serial numbers do not contain date stamping, nor did they track shipment info, so it's in a 5 year range.

The fascinating part is that as one should do with 20+ year old electronics, I popped off the cover to have a peek at the condition of things, and was simply astonished. It literally looks brand new. Now, I know that you've heard that before, but I'm going to add some pics to show what I mean. The white plastic parts are still white as snow, no age yellowing. The bus-bar jumpers are still gleaming copper. The screw heads show no signs of corrosion. The bare aluminum heat sinks are oxide free, and the thermal pads behind the transistors are as white and fresh looking as a recent manufacture item.

I've not powered it on yet because I've heard repeatedly in audiophool circles that leaving caps unused for years is not the best for them. Many seem to recommend some regiment of variac controlled voltage into the system to 'recondition' the caps, which sounds to my non-electronic-engineer ear like a mother blowing cigarette smoke in a baby's ear to cure ear infection, or just plain old snake-oil inspired audiophoolery.

In this photo, you can see that the only visible issue is the age-darkened glue they used to fix the caps. Note the shiny jumpers. Even the clear plastic shroud over the power switch\relay is still clear.

Transistors....heat sinks....push connections....all look like new.

What say the electronics pro's on the forum? Fire it up, or do I need to fret about prepping what looks to be a never-used amp that is 21+ years old?

 

[RayDunzl]

Reading a bit...

Most of the concern seems related to tube amps, and the higher internal voltages involved. Bad short cap smoking HV transformers, moisure absorbing trasformer potting, that sort of thing.

I might buy a Variac just to have one in my kit. Cheap insurance.

I have some old things here I might like to play with myself.

The next question would be, what harm can come from operating something at too low an input voltage?

 

[RayDunzl]

Free Shortcut:

Put 12 volts DC on the big caps and see if they like it or not.

Proceed with whatever result you get.

 

[Jorj]

Reading a bit...
<snip>
The next question would be, what harm can come from operating something at too low an input voltage?

Thanks Ray, appreciate the input. I have the exact same worry. Undervolting a CPU, I get. Undervolting a nice, high end toroid xfmr and I've no real idea about what the downstream effects are, or if that is actually anything other than just a whimsical red-herring like silver-plating OFC.

 

[RayDunzl]

Undervolting a nice, high end toroid xfmr and I've no real idea about what the downstream effects are

Probably nothing.

My Acurus A250 has no relays (or other noticeable protection) in it, so when it gets powered off, the caps slowly bleed down, and eventually (30 seconds or more at low volume) the output section goes to sleep and the speaker finally stops playing.

A Krell 250A in the bedroom does the same, but faster.

Conclusion: No danger to common circuitry components from low-voltage (at the filter cap) operation.

The switch just cuts the input to the transformer. The rest of the circuit is on its own.

Frequency is likely a more important consideration than low voltage to a power transformer. It's just big wire, but you don't want to heat it up too much (insulation fail).

250W, 8 ohm:

44.72 RMS x 1.414 = 63V minimum on the caps.

As I remember I measured closer to 90V, but that memory is foggy right now. The caps have to have some voltage headroom to bleed down (supplying power to the amp) between recharge pulses.

Acurus A250

 

[restorer-john]

I presume you don't have a variac. The classic way to fire up anything you are unsure of, is through a Dim Bulb Tester. That is no snake-oil audiphoolery. A variac doesn't have the characteristics of a DBT.

The DBT will allow the amplifer to be eased on at a lower voltage initially (the flash and faint glow of the DBT) and IF there is a problem, the bulb will light up, dropping the supply voltage across it and massively limiting current through the amp. This way, you do no MORE damage IF there is a fault, and can determine correct functionality of the amp.

A DBT will cost you a few dollars to build and will arguably be one of the most useful devices on your bench if you get into vintage gear.

It is based on the simple fact that a tungsten filament has a very low cold resistance which increases dramatically as it heats up due to current flowing through it.

 

[Jorj]

I presume you don't have a variac. The classic way to fire up anything you are unsure of, is through a Dim Bulb Tester. That is no snake-oil audiphoolery. A variac doesn't have the characteristics of a DBT.

The DBT will allow the amplifer to be eased on at a lower voltage initially (the flash and faint glow of the DBT) and IF there is a problem, the bulb will light up, dropping the supply voltage across it and massively limiting current through the amp. This way, you do no MORE damage IF there is a fault, and can determine correct functionality of the amp.

A DBT will cost you a few dollars to build and will arguably be one of the most useful devices on your bench if you get into vintage gear.

It is based on the simple fact that a tungsten filament has a very low cold resistance which increases dramatically as it heats up due to current flowing through it.

No, I never had the need for a variac, although they are not expensive. I was just looking at the DBT. For 200 watts into two channels (DIN 330 watts each), what size bulb would I expect to use?

Is there any real science to the idea of reforming caps by slowly upping the voltage over hours?

Thanks for the info and help!

 

[Blumlein 88]

No, I never had the need for a variac, although they are not expensive. I was just looking at the DBT. For 200 watts into two channels (DIN 330 watts each), what size bulb would I expect to use?

Is there any real science to the idea of reforming caps by slowly upping the voltage over hours?

Thanks for the info and help!

I don't know about science. At one time I did mess about with lots of vintage gear. Old paper caps, xfmr windings that could absorb moisture, Quad ESL-57's (original Quads) with beeswax on the parts so on and so forth. I never used a dim bulb tester, but it seems like a good idea for sure. Ramping up voltage slowly does seem to have merit for the old tube gear. I don't know that it would for later SS gear. It definitely would not hurt.

I had an old power supply from Heathkit that among other things let me have 1/4, 1/2 and full power on a 120 ac outlet. That was sufficient for powering up old gear that had been sitting for years. Another trick I used was to series a big capacitor with the load so most of the voltage drop was across it. If you aren't familiar and comfortable with electronics I would suggest the $99 variac Ray had a link for at Amazon.

Now if I had the Rotel, I would plug it up, let it idle a few minutes. If all was good and no noxious odors, no burning smell, and no smoke, I would connect up some speakers and let it rip.

 

[Don Hills]

"Reforming" electrolytic caps of that recent vintage is unlikely to be necessary. It's usually recommended for much older caps that have not been powered in many years, such as old tube radios that have been in storage since the 50s.
I agree with the use of a DBT, I have recently completed testing of several dozen (!) pieces of audio equipment of unknown condition and it came in useful on at least 2 occasions (shorted power ICs in 80s vintage amplifiers).

 

[RayDunzl]

I forgot about using a light bulb as a variable resistor.

what size bulb would I expect to use?

You can calculate and decide... 100W bulb limits to 0.83A if your gear is a short.

http://www.sengpielaudio.com/calculator-ohm.htm

A bulb would allow more current for a cycle or two till it heats up, then some equilibrium is reached (dim bulb) between the loss across the bulb and the gear.

$99 variac Ray had a link for at Amazon.

I bought one... $79

(been thinking about it forever)

https://www.ebay.com/itm/Variac-Var.../302385304098?_trksid=p2349526.m4383.l4275.c1

Might try using it with a fan that circulates air into my hot afternoon corner for daily use... Plus whatever tool-value it presents.

 

[Don Hills]

One thing that is worth checking visually is to see if there is an X or X2 capacitor across the mains terminals or across the power switch. They often have a clear plastic body. The plastic cracks with age. Moisture can ingress while in storage, causing high leakage and breakdown. The usual symptom is a loud bang 10 minutes to an hour after powering on, with shredded foil sprayed over the internals of the device.

 

[Jorj]

<snip>
Now if I had the Rotel, I would plug it up, let it idle a few minutes. If all was good and no noxious odors, no burning smell, and no smoke, I would connect up some speakers and let it rip.

It was cheap enough, I'm not out much if it pops a cap, so I think I'll let it warm up and see what happens. There are a few bits in there I'd be sad to try to replace, like those impressive 15000uF BHC slit-film caps. I've looked and did not see much out there for direct replacements-in-kind.

Now if Parts Express would just hurry up, I'd have my spades soldered on my speaker wire and see how many neighbors I could upset. Actually, I blame the wind. The east coast had strong winds on Friday. So strong, the Federal government (and shipping companies) could not function. Oh, wait....heh.

It's still amazing to me that the stuff inside this thing is so pristine. I've had zip ties on things for ten minutes and they were all yellow and cracking, it seems, but not inside that amp. Time stood still in there. Rotel time machine?

 

[Jorj]

One thing that is worth checking visually is to see if there is an X or X2 capacitor across the mains terminals or across the power switch. They often have a clear plastic body. The plastic cracks with age. Moisture can ingress while in storage, causing high leakage and breakdown. The usual symptom is a loud bang 10 minutes to an hour after powering on, with shredded foil sprayed over the internals of the device.

Don,

I assume you mean this guy, which is the snubber cap, I think? It is hard to really look at without pulling that board out, but I got my little mirror out and it looks like the rest of them, condition-wise anyway. It is light blue, rectangular with a clear plastic coating. There may be some cracks, but it also may be a trick of the light through that clear plastic.

I got sick of my own nervosa, plugged it in and turned it on. No fireworks and no funny smells. Protection turned off in around 5 seconds.

 

[restorer-john]

The Rotel is hardly even vintage, let alone an ancient civilisation artifact In my collection, it would be regarded as a relatively new amplifier.

You can date the components by looking at the stamps/printing on the cap sleeves and the output transistors.

The design is typically Rotel, classically well designed. Bear in mind the 'protection' relay you heard click, merely mutes the input to the power amp during switch-on and a DC condition and doesn't disconnect or protect your speakers should there be a problem. I would definitely run the amp with some sacrificial speakers until you have confirmed the DC offset is reasonable and the bias current has been set as per the service manual. (page 4 attached)

(the warm up at full power is excessive, just warm it up at a typical listening level and then short the inputs or disconnect them whilst doing the bias setup. Use minigrabbers and your DMM)

They are a very good amplifier and you will enjoy it.

 

[Jorj]

The Rotel is hardly even vintage, let alone an ancient civilisation artifact In my collection, it would be regarded as a relatively new amplifier.

They are a very good amplifier and you will enjoy it.

Oh, I know they hardly qualify as ancient, but as a snot-nosed teenager I used to drool on them (the ML and Mac stuff was pure fantasy) at the local Zephyr Audio and dream of affording them. I'm also self identifying as old now that I have to wear glasses some portion of every day. Grr. I have definitely headed towards a 1992 mid-fi setup for my living room. Gonna strap that amp to some pre-Klipsch, pre-China Energy C-9's and irritate some folks in my condo. Signal chain will change on a daily basis, once I have the room figured out.

Thanks again to the forum for the help and insight! This is a great place to share solid knowledge and avoid so much of the rampant voodoo on teh interwebz.

 

[Don Hills]

I assume you mean this guy, which is the snubber cap, I think? ...

That's the one. The ones notorious for exploding were "Rifa" brand, and were completely clear - you could see the wound foil capacitor inside the plastic. I found and replaced one in one of the vintage amps I checked. Some years ago one exploded in my wife's sewing machine...

Edit: I note yours is across the power switch, as was the one I replaced recently. They can also be found connected across the mains input.

 

[Jorj]

(the warm up at full power is excessive, just warm it up at a typical listening level and then short the inputs or disconnect them whilst doing the bias setup. Use minigrabbers and your DMM)

Yes, I'll definitely check the bias a few times, after measuring DC offset just to be sure I don't wreck speakers. Bias moves with old VRs often take time because the pots are so touchy, so that will take a bit of time. TPs seem to be nicely accessible, which is not always the case with Chinese manufactured mid-fi stuff.

Any other tests I ought to run before I put this nondescript black box in service?

 

[restorer-john]

Unless you are going to run a full gamut of measurements on the unit, you can likely put it into service after the basics listed above.

I always like to confirm input RCA jacks are well soldered with no evidence of dry joints or cracks as that can often be a source of great pain down the track. They are usually lightweight in construction and frequently pulled on.

Check any fuses are original correct values. Check any mains cables are correctly routed and safely anchored. Visually inspect all electrolytics for evidence of brown glue corrosion around their bases (if they are glued down). Look for any bulged capacitors or any significantly heat affected components or PCB discolouring.

 

[Jorj]

Not going down the full test suite route, but I've done everything except checking the RCA connections, which is a valuable tip. All the caps look great, and only one wire retention crimp looks like the glue has affected it.

 

[Jorj]

Since I'm new to the game of using practically antique gear (I know...stop yelling....some of you are older than me!), and I'm not an electronics engineer, I needed to educate myself on some of the terms that get thrown around so loosely in forums. I understood the rough idea of bias and offset (as it pertains to amps), but lacked background. I found this article which does an excellent job of explaining why bias is important, why high wattage AB amps have some advantages over smaller ones, and what issues are common with VR pots used for bias adjustment. Good stuff.

http://www.tnt-audio.com/clinica/bias_e.html