Rotel RA820A stereo amp no right channel

[juz571]

Hi,
I am looking for some guidance, if possible? I am an electrician of 30 years, but my electronics knowledge is at a foundation level, lets say!

So I bought this nice old amp from Ebay and it was sold as only 1 channel working. I opened it up and had a quick inspection. Nothing obviously damaged, to the eye, so I started checking fuses. Fuse F601 was popped (schematics attached), so I replaced it and, bingo, we had 2 channels again.
So today I put the lid back on and give it another check. Only 1 channel again. But the fuse has not blown this time. What is puzzling is, that fuse F601 does the left channel, but it's now the right channel that's gone down! (F602 is fine). I've tried with multiple inputs and checked both speaker outputs as well as headphones and there's no right channel on anything

Thanks in advance and any help will be greatly appreciated

 

[kellys_eye]

Did you use the correct rated fuse? 4A is not exactly common and replacing it with something larger is a no-no.

What are F901 and F902 like? I'd check for distorted smoothing capacitors first (visual checks first) then we can start doing some basic test measurements assuming you have a test meter?

 

[juz571]

Did you use the correct rated fuse? 4A is not exactly common and replacing it with something larger is a no-no.

What are F901 and F902 like? I'd check for distorted smoothing capacitors first (visual checks first) then we can start doing some basic test measurements assuming you have a test meter?

Thanks for the reply Kellys_eye. Ummmm......no, I didn't use a 4A as I didn't have one (embarrassed face). I must confess I used a 6A. I know, I'm a fool! The thing is it's the right hand channel that's gone now. F601 does the left channel if I'm reading the schematic right? All fuses are fine at the moment. Yes I have various test meters and an oscilloscope, but no esr meter at present. No obvious signs of any bad caps from what I can see. I've got some 4A fuses coming tomorrow to get that right before I go any further.

 

[kellys_eye]

get that right before I go any further.

Too late!

Usual issues, apart from capacitor ESR, would be a short in one of the amplifier output transistors but we must establish that the sections are receiving the right voltages and that there's no bad ripple (as a dud capacitor would introduce).

Sure you didn't inadvertently cross the speaker connections after fitting the new fuse? i.e. mistaking the left/right channels.

 

[Delta Prime]

I am an electrician of 30 years

6 amp fuse in a 4 amp slot?
Take some photos sparky. I got to see this.

 

[juz571]

Too late!

Usual issues, apart from capacitor ESR, would be a short in one of the amplifier output transistors but we must establish that the sections are receiving the right voltages and that there's no bad ripple (as a dud capacitor would introduce).

Sure you didn't inadvertently cross the speaker connections after fitting the new fuse? i.e. mistaking the left/right channels.

Definitely no crossing of the speakers. I double checked after my last reply. Was 100% the fuse for the left channel that was blown when I got it, and now it's the right channel that has failed. I've checked with speakers in both outputs (main and remote) and on headphones. I turn the balance to the right and there is nothing at all. Not even a faint sound as far as I can tell.

 

[juz571]

6 amp fuse in a 4 amp slot?
Take some photos sparky. I got to see this.

Ok, ok. I get it! We’re not so delicate as you electronics guys! Use what’s at hand and see if it pops again! Photos attached. Hope they are clear enough. The 6A is the white fuse btw

 

[Delta Prime]

Ok, ok. I get it! We’re not so delicate as you electronics guys!

I was just teasing you guys are great thank you for the photos.

 

[Delta Prime]

Just in time for Halloween what a horror story you got to clean all that up with isopropyl 99.9% alcohol. Do not power that unit back up again.

 

[Martaine2005]

Below F601, there’s a resistor. The track looks ‘scraped’ away rather than a blown track. Have you been scraping?.
Also, 30 years sparky? Puts a 6A in a 4A?. To see if it ‘pops’. Electronics is not like a consumer unit.
Ohms law tells you that another 3rd current will be present. Let alone the overheads.
Anyway, the component is open circuit or you’d be blowing the fuse again.

 

[Martaine2005]

Also, just viewing the other photos, there are many points that look like the traces have been scraped away.
I’d definitely clean the boards first with IPA (as suggested) and then check the scraped or broken traces before another power check.

Martin

 

[juz571]

Below F601, there’s a resistor. The track looks ‘scraped’ away rather than a blown track. Have you been scraping?.
Also, 30 years sparky? Puts a 6A in a 4A?. To see if it ‘pops’. Electronics is not like a consumer unit.
Ohms law tells you that another 3rd current will be present. Let alone the overheads.
Anyway, the component is open circuit or you’d be blowing the fuse again.

Hi Martin, no I’ve not done any scraping. I see what you mean about the track though, It doesn’t look good at all. Believe it or not I’ve already given a clean with cotton buds and isopropyl before I took these photos. That’s how bad it was before!

 

[Delta Prime]

Believe it or not I’ve already given a clean with cotton buds and isopropyl before I took these photos

I believe you; but you must know... I am a masochist; show me the solder side of the board.
And I will see if we cannot cure your insanity.

 

[kellys_eye]

That's not the track, that's just the topside print to assist with following the circuit layout - the tracks are under the board.

 

[Delta Prime]

That's not the track, that's just the topside print to assist with following the circuit layout - the tracks are under the board.

Good eye!

 

[kellys_eye]

Check the +37 and -37V rails then measure at either end of F601 and F602, report all you find.

 

[Delta Prime]

And I'm telling you do not power that unit up! Just take the photos. Solder side please...

 

[kellys_eye]

And I'm telling you do not power that unit up!

There's no risk. The damage has been done. This isn't CMOS stuff so the risk of other faults happening are minimal.

 

[Delta Prime]

There's no risk. The damage has been done. This isn't CMOS stuff so the risk of other faults happening are minimal.

Do not presume to tell me what risks are involved.
Go ahead take the advice of an old fart; your words not mine.
Just playing; yeah,what the hell, I admit it you are my Superior; let's see what kind of voltage you have on your rails?

 

[juz571]

OK I'll take some readings when I get home tonight. Thanks all for your assistance on this. I really do appreciate you guys taking time out of your days to help. And I promise I'll never put a oversized fuse in again!