Lanzar Optidrive 4000D Gain & BB pots behaviour

[Custom Chris]

I own a Lanzar Opti4000D amp. This has the same board as the Soundstream XXX6500.

The problem is, when you adjust the gain or bass boost up more than a quarter turn, it makes loud erratic bass, even with nothing playing.

It only does it with some RCA's plugged in.
I have tried 3 different RCA sources.
Tried a different load rating and subs.
The amps RCA shields read just over 1Kohm
Voltages remain stable.
It does this at the same dial position no matter on the input voltage (on both pots).
Output transistors and Power mosfets read OK.

Any help or suggestions would be much appreciated

[Perry Babin]

Did you confirm that you have ~0 ohms between the RCA shields and the secondary windings of the small transformer?

You should read 0 ohms if you read 0 ohms to R70 but check this to make sure. Do this with the amp off and preferably after the rail caps have had time to discharge.

[Custom Chris]

Thanks. I have it discharging now.

[Custom Chris]

RCA shields to transformer = 47.0 Ohms
RCA to R70 = 47.0 Ohms
RCA to R70 (after 1k) = 982 Ohms
Transformer to R70 = 0 Ohms

It would appear that the RCA shields travel through RF53 which is a 47 Ohm resistor, hence the reading. The other end of RF53 reads 0 Ohms at R70.



The end of RF53 with the brown disc capacitor attached to it is 0 Ohms to R70. The end nearest reads 0 Ohms to RCA shields.

[Perry Babin]

The schematic diagram for the RCAs isn't very good. It appears that all of that is OK.

Can you borrow an oscilloscope?

[Custom Chris]

Yeah, I know, there seems to be bits missing. I can't find another gut shot clear enough to see if the 2 brown caps are supposed to be there. It looks like they were soldered on by hand...
I'm not sure I know anyone with a scope, well, local anyway. I can ask about though.
Does it need to do anything other than one that would be used for setting gain?

Is there anything else I could check in the mean time?

Thanks Perry

[Custom Chris]

I just plugged a 8ohm load in it (house speaker) and it was fine?
So I tried a 2ohm load and it's still putting out 'a mess' noise...

[Custom Chris]

OK I have plugged it back in the car so I could drive a 2ohm load fairly loud, safely.

I measured the VAC and it was consistent with the music until I get to the 'dirty' point. Then I got a few high readings, then it went out of range.
I also tried VDC, this just had a low reading to nothing...

So, just to recap. It seems like the amp behaves like this with low loads on it. I didn't get any problems when running an 8ohm load on it last night.

[Perry Babin]

Without being able to see the distorted signal, this will be difficult to troubleshoot.

[Custom Chris]

No scope yet I'm working on it though... So, in the mean time,
I took the amp out of the car again and found this:



CA3 appears to have leaked, not seen one of these leak before like this. I can't find it on the schematics. Any idea what it runs with?
It reads 105K63 I know the 63 is the voltage rating but, not sure on the first part. Is it a 10uf?