Got a Fosgate 225.2 from a guy who said it works but gets hot (hot is pretty subjective on these). Checked it over and didn't find any problems other than maybe needing to reflow the RCA block. No excessive DC on the outputs, but powered up it drew about 1.2A...a bit high. Marked the bias pots and turned them all the way down, powered back up and it drew 0.98A - still a little high I would say. Adjusted the bias pots for 0.05A rise on each, so now it's at 1.08A idle.
At this point I connected speakers and RCA and the amp has clean output, and the amp draw drops to 0.95A at idle. That seems strange. If I unplug either RCA the draw goes back up to 1.08A, and I get low level static hiss on the right speaker with either RCA disconnected, which I assume means I have a broken RCA shield on the right channel. But why the change in idle amp draw?
At this point I connected speakers and RCA and the amp has clean output, and the amp draw drops to 0.95A at idle. That seems strange. If I unplug either RCA the draw goes back up to 1.08A, and I get low level static hiss on the right speaker with either RCA disconnected, which I assume means I have a broken RCA shield on the right channel. But why the change in idle amp draw?
I can't find R33. Do you know where on the board it's located? I don't have this particular schematic. I see R33 on the 200IX schematic, but not on the 500a2 schematic. I think this 225.2 is more like the 200x2 than the 200IX.
See attached.
It's behind RV102.
These amps (ones with flat pots) have a problem with oscillation if both the gain and treble are set high. You can generally see it clearly on a scope.
It's behind RV102.
These amps (ones with flat pots) have a problem with oscillation if both the gain and treble are set high. You can generally see it clearly on a scope.
Thanks. R33 reads 3.08k ohms in circuit.
Gains are about half way and I had both treble & bass pots all the way down since starting
Gains are about half way and I had both treble & bass pots all the way down since starting
The diagram says it's 10 ohms. Photos I have show a 1 ohm. Either is better than 3k.
Do you see any oscillation on the bridging terminals of either channel?
Do you see any oscillation on the bridging terminals of either channel?
Yeah, it's 1ohm (marked 1R0). Why might it go bad?
I still don't have a scope. I've been looking. Got any rec's for one that isn't 30 years old?
I still don't have a scope. I've been looking. Got any rec's for one that isn't 30 years old?
What's your budget?
I can't stand the donkey-kong displays of most LCD based scopes. Siglent (for some of their scopes) have decent displays. I'd still prefer to have virtually any CRT based scope in good working order. That's a personal preference.
If 12v was driven into the shields, that could burn that resistor.
I can't stand the donkey-kong displays of most LCD based scopes. Siglent (for some of their scopes) have decent displays. I'd still prefer to have virtually any CRT based scope in good working order. That's a personal preference.
If 12v was driven into the shields, that could burn that resistor.
Budget is TBD...just moved, looking to buy a house, etc. I saw a Tek 2465 for what seemed like a good price, but then I read they had some unobtainable parts, and their age turned me off.
Pulled R33 and it continued to read over 3k ohms. I didn't have any 1ohm resistors so I put in a 10ohm for the time being. Replaced the scratchy gains and reflowed the RCAs and some other parts. Now have 0.92A idle after bias adjustment without RCAs connected. Drops to 0.89A when they are connected. This is just an RCA to 1/8" miniplug, so the grounds are shared.
Pulled R33 and it continued to read over 3k ohms. I didn't have any 1ohm resistors so I put in a 10ohm for the time being. Replaced the scratchy gains and reflowed the RCAs and some other parts. Now have 0.92A idle after bias adjustment without RCAs connected. Drops to 0.89A when they are connected. This is just an RCA to 1/8" miniplug, so the grounds are shared.
...and still getting some static hiss out of the right channel. Maybe on the left but I can't tell for sure. The right is only audible from a couple feet away.
Amp seems to be operating fine and doesn't get hot so far. I haven't found anything else out of spec. I guess I'm just curious why there is a little more noise out of the right channel than left when no RCA's are hooked up.
If the noise is an issue that you want to find, you'll have to trace it through the circuit to find where it's entering the circuit.
If there is no noise with RCAs plugged in, it may not be a problem that needs to be resolved.
The decision will be yours and/or the owner's on which way to go.
If there is no noise with RCAs plugged in, it may not be a problem that needs to be resolved.
The decision will be yours and/or the owner's on which way to go.
Okay. I'm the owner now, so more of a curiosity and wanting to know if I do want to sell it down the line. Makes me wonder if it's just normal and part of why RF added their "TOPAZ" components on the next generation. I don't get any noise from the 250a2 I just repaired a couple days ago.
Went ahead and picked up a Siglent SDS1052DL+ scope. I haven't used a scope in about 15 or so years 🙂
