Rotel RA-940BX distorted sound

[holmzy]

Hi guys I failed miserably on a teac amp recently and decided to try this rotel out.

The problem is the sound on both channels is distorted. you have to turn it up max and all you hear is crackles on speakers and via headphones sounds half decent but at high volumes struggles and starts distorting.

I have service manual and its really confusing. The voltage seems right for one side but the main voltage is wrong but everything checks out ok?!?!

The part is also not the same on sm is 802 bridge diode. but its actually an PBPC803 installed but it checks out ok. everything there seems to check out diodes, resistors, caps. sm manual states 41v but im getting 36v. on the other end it states 17v and i am getting 17v

[stj]

Re: Rotel RA-940BX distorted sound

that description sounds like a chip is bad.

[holmzy]

Re: Rotel RA-940BX distorted sound

op amp chips? I have tested them. the voltage coming out of them are fine and sound also is fine.

[stj]

Re: Rotel RA-940BX distorted sound

got a schematic? - other than on that xp desktop i mean!

[holmzy]

Re: Rotel RA-940BX distorted sound

lol yes

[holmzy]

Re: Rotel RA-940BX distorted sound

As you can see right have very little dc off set with the bias turned all the the way down and left has a little more. I think the setting is millivolts. so right is 0.004 v and left is .040 v

The one things I did notice was when headphones connect dc off set set at highest rating I could hear half decent sound. When dc off set was set lowest like now I could only hear distortion via headphones. Alo I get 0 volts across the test terminal for bias adjust, adove dc adjust. dc off set adjust is closest to big yellow cap.

[R_J]

Re: Rotel RA-940BX distorted sound

You did check f601 and f602? they are both good correct?

[holmzy]

Re: Rotel RA-940BX distorted sound

Hi yes I changed fuses twice. once with 6a then with 4a. no difference!

[stj]

Re: Rotel RA-940BX distorted sound

about the 17v and 41v - did you check both + and - lines wih the meter grounded to the frame?

[holmzy]

Re: Rotel RA-940BX distorted sound

yup. gives me 36v. according to sm should be 41. the main bridge diode gives me 36v. should be 41v bridge diode gives me >560 reading on diode setting and does not appear to be faulty.

[holmzy]

Re: Rotel RA-940BX distorted sound

the 17v line gives me 17v just fine

[stj]

Re: Rotel RA-940BX distorted sound

+17 and -17?

[holmzy]

Re: Rotel RA-940BX distorted sound

Q903/4 have 36v going in and 17 and 17.5 as per sm manual. its suppose to be 41v but everything else checks out.

I just measure the transformer and it said 27.1v! the markings on the unit itself say 28.3v. so it was never going to be 31.7v input ac!?!?

[stj]

Re: Rotel RA-940BX distorted sound

what country are you in? what's your mains voltage - both official and metered?

[holmzy]

Re: Rotel RA-940BX distorted sound

You may be on to something here. Im in UK. the back states for 240v. but inside is set on 220V. The ac input measured is indeed 240v

[holmzy]

Re: Rotel RA-940BX distorted sound

I have just seen that ac4 is actuay connected directly to ac3. so should be already set on 240v right?

[stj]

Re: Rotel RA-940BX distorted sound

well mainland Europe was 220v, and the u.k. was 240v
they were supposed to settle on 230v but there is no timeline for this.
what does your meter find on your sockets?
mine is 235v right now. (just checked)

[stj]

Re: Rotel RA-940BX distorted sound

btw, if the unit was wired for 220v your secondary voltages would be high - not low.

[holmzy]

Re: Rotel RA-940BX distorted sound

inside the unit itself where the mains wire goes in measure 240 exactly. could it be the bridge rectifier? its a PBPC803 on the sm manual its suppose to be KBPC802. could that be limiting the voltage? I really dont understand what could be causing the power loss!

[stj]

Re: Rotel RA-940BX distorted sound

it's the transformer - maybe the manual is wrong.
unless it's not being smoothed much because the main caps are worn.
where your getting 36v, set your meter to AC and check again - it should only show the ripple.

the rectifier is not the problem - the difference is just the maximum voltage 300v vs 200v