RCA RT2770 Reciever (Home Theater?) Need help!

[OldManRiver]

Re: RCA RT2770 Reciever (Home Theater?) Need help!

the r202 is RESISTOR 8.2ohm +-5% 5W. confirmed for rca

siski123,

Please post pix of your unit and show the Model # of the PWR brd as there are several version of this brd out there and seems RCA engineering is beefing up this snubber network, but uping value on R202 is not the fix, it is the voltage rating of C211 that needs upping!

Cheers!

OMR

[OldManRiver]

Re: RCA RT2770 Reciever (Home Theater?) Need help!

pics of my resistor:

HCC,

If there is no white band then this is 10 ohm 1 Watt resistor.

Cheers!

OMR

[OldManRiver]

Re: RCA RT2770 Reciever (Home Theater?) Need help!

I may be a novice but I no what a snubber is.

Anyway I have a whopper 8.2 Ohm resistor on my other RT 2770 (bare in mind it's got a different lay out ). Would that work in place of the burnt one of the other board?

It in the same basic area that the burnt resistor on the other board is right by the 3-pin heat sink mounted diode.

OH, and fyi, my board still works (Well probably not now since the resistor wiggled free of it's soldered terminal) but that slightly burnt resistor haunt's me .

I'm also thinking about upgrading all the 63v 1000uF caps for 100v 1000uF caps (Extra 27 volts of overvoltage cushioning ) along with the 2, 200v 330uF caps, to 250v 330uF caps. I blast my systems constantly, when they work, lol

Wolf_Punch,

Per other entries and pix this would be 8.7 - 190 ohm, but betting on 10 ohm 1 Watt 5% resistor. Also from your comment and others looks like RCA engineering, not understanding the problem of the under rated voltage on C211 has upped the wattage on this resister to 5Watt, but not necessary if C211 is replaced with new cap, same mfd rating, but 35-50 KVAC rating.

Cheers!

OMR

[OldManRiver]

Re: RCA RT2770 Reciever (Home Theater?) Need help!

8.2 ohms is too low, much too low.
You need about 1~100kohm 3W.

Depends on the output voltage of that PSU and breakdown voltage of diode.

If output is 50V and breakdown is 200V, worst case is 250V across resistor.

So calculate (MaxVoltage^2)/Resistance <= 1.5W (half rating of 3W to limit dissipation)
and that will give you a decent lower bound... Example 47kohm works here.

tom66,

This is a "snubber network" C211 and R202, so only handles or "clamps" the high voltage spikes produced by inductance rings (by loading) on XFMR T103, when power switching MOSFET V103 switches on. Therefore it is only there, C211 calculated for resonance of frequency to kill/clamp all spikes exceeding the voltage rating of fast response dual diode stack D207, as fail safe to not compromise it. These spikes, can only be seen on scope with high voltage probe, exceed 20K volts, usually 30-45 K volts. Never test this part of the ckt with a VOM or other type of meter as will blow it immediately.

When the "snubber network" cap is under rated, high voltage exceeds it's rating, punches "holes" in the dielectric and starts arching. This high voltage short/arch is what causes the excess load on the resistor burning it out.

Because voltage accross R202 is generated by a spike, which is less than 2% (usually .2%) of the output load, this resistor can be rated extremely low (ohms and watts) and still work right, if cap is correctly rated voltage wise.

Hope that explains it better!

Cheers!

OMR

[OldManRiver]

Re: RCA RT2770 Reciever (Home Theater?) Need help!

Also, btw, if I remember correctly, that snubber cap is a high voltage,blue epoxy coated, 2200pF 1KV ceramic capacitor. If I'm not correct on the ratings, then the marking (if you need correct me) are
222K 1KV, and another letter, but I don't think the other letter really means much, as a guess it's probably just to denote the size.

WP,

Wow no wonder C211 is blowing. This has to be rated well above 20K volts.

I typically use 35-50KV caps on these ckts as the voltage spikes from the XFMR inductance loading is always over 20KV, usuall over 30KV.

Cheers!

OMR

[OldManRiver]

Re: RCA RT2770 Reciever (Home Theater?) Need help!

All,

I reached out to RCA support on this. Got a response, but they are still looking up the values by schematic spec.

Will let you all know what this are and what I recommend for replacement.

Since I repaired switching power supplies for 2 years, back in mid 90's, hope you found my inputs helpful.

Snubber network failures made up over 70% of all failures in these switched PS units, so really know this scenario well.

Cheers!

OMR

[LLLlllou]

Re: RCA RT2770 Reciever (Home Theater?) Need help!

Ooooooh.... *idea

would a wire wound resistor of that resistance be a good idea for it's more efficiant cooling, I could heatsink to the case, no?

You definitely don't want to use wire wound in this application. You're dealing with transients with harmonics up into the megahertz. The inductance of wire wound resistors will be several dozen to several hundred times as bad as a metal film or (preferably) a carbon composition resistor. Wire wounds are for low frequency use, unless you take it's stray inductance into account when designing the circuit.

[budm]

Re: RCA RT2770 Reciever (Home Theater?) Need help!

Snubber design.

[budm]

Re: RCA RT2770 Reciever (Home Theater?) Need help!

By the way, I would like to see the size of the 50KV cap (at what capacitance?). This is the Diode RC snubber.
Here is 30KV
http://www.mouser.com/ProductDetail/...LxRas5T50vJHKv

50KV:
http://www.mouser.com/Passive-Compon...9x6m?P=1yznciv

[SneakyB]

Re: RCA RT2770 Reciever (Home Theater?) Need help!

I am having the same problem with my RCA as well... same resistor is blown.

But, I haven't seen any mention of speakers not working.. I can't seem to get the FR speaker to work on this unit... and the SR speaker only works on some of the surround settings. The sub is clearly not working at all...

Has there been an actual resolution to this problem here? Or was it just left open with a bunch of possible fixes?

"RCA RT2770' uses Class D amp as you can see from the picture and the spec. No audio amp uses Class B, too much distortion, most uses Class AB amp, or more expensive one will use Class A. Where did you get that Class B info from? the Class B label on the back is for FCC Class B compliance because it uses switching power supply and switching amplifier.
Learn more here about Class D output filter network.
https://cdn.badcaps-static.com/pdfs/...974574fbd7.pdf

The link for the Filter Design on the TI website is now
https://cdn.badcaps-static.com/pdfs/...3806b8959c.pdf
Just an update.

Thanks for all the info here, budm..!

[SneakyB]

Re: RCA RT2770 Reciever (Home Theater?) Need help!

By the way, I would like to see the size of the 50KV cap (at what capacitance?). This is the Diode RC snubber.
Here is 30KV
http://www.mouser.com/ProductDetail/...LxRas5T50vJHKv

50KV:
http://www.mouser.com/Passive-Compon...9x6m?P=1yznciv

And the caps from the Mouser site at these ratings are pricing at around $30.00 US each?

[Eel_dahc]

Re: RCA RT2770 Reciever (Home Theater?) Need help!

Old thread but i could read my resister with a meter.Just found one of these in the trash. It works as in turns on but I do not have the correct speakers for it. Front left/right work. Subwoofer doesn't put out anything, but it might need the special subwoofer control or because I'm using line in with my phone? But anyway, the r202 is bleached out, can't tell the colors, it was loose on one solder point perhaps that saved it from blowing? Tests as 24ohms. Didn't see in any posts whether your units powered up or not. My unit is a RT2870