Swapping out a power module...?

[www68]

Hi everyone,

 

I have a dual channel 300w/8ohm 15inch bass combo amp (not D-class) - perhaps 6 years old - which I suspect is a badge engineered unit. In Australia it was sold under the brand name RMS Amplification. It has some quality components and features, yet was used for one gig then 'died' and then placed in a shed until it was passed on to me. I'm trying to resurrect it and suspect the power module is at fault. 

 

Currently the amp powers up, lights glow, but that's all. Speaker is good, but no signal gets to the speaker, and no signal through the DI output. I have checked the only fuse I can find which is at the power lead input. Continuity along the front panel board is good (input to channel selector through eq, etc), but I wouldn't know how to check the power module board or transformer. I took it to a shop who reported the power module as dead but wanted the schematic before going further. I have spoken to the Aust importer who tried to hunt down a schematic, but the factory will not respond to our requests, and do not keep replacement parts - so that avenue of help is closed.

 

At the moment, if I can find a 300w class D power module and transformer suitable for our 240v, I'd happily swap it over, but the connector to the preamp (front panel) board will most likely be different, and I'm not sure if it would have a low voltage takeoff for the preamp board. Likewise trying to install any other A+B type power module is unlikely to have the connections I want.

 

So - beside turning the cabinet into a passive cabinet, finding a 1U bass head, or screwing in a Warwick Gnome or similar to fill the void, what options would I have to resurrect the head unit? Is there any other trouble shooting tips that I could try (that doesn't involve me going up in a puff of blue smoke)?

 

attached is a pic of the power module (is it usual to use that much hot-melt glue?) in case someone recognizes it - would other bass combo's use one like this? (Behringer? Laney? Peavey?, etc). As seen in the pic, apart from component id's, there is no writing on the powerboard apart from '0704420  2001-0173-4  300w BASS POWER' - which I was really hoping would turn up something at AliExpress or ebay, but nothing...

 

Any thoughts appreciated...... Wayne.

[basstone]

On 16/01/2022 at 12:50, www68 said:

Hi everyone,

 

I have a dual channel 300w/8ohm 15inch bass combo amp (not D-class) - perhaps 6 years old - which I suspect is a badge engineered unit. In Australia it was sold under the brand name RMS Amplification. It has some quality components and features, yet was used for one gig then 'died' and then placed in a shed until it was passed on to me. I'm trying to resurrect it and suspect the power module is at fault. 

 

Currently the amp powers up, lights glow, but that's all. Speaker is good, but no signal gets to the speaker, and no signal through the DI output. I have checked the only fuse I can find which is at the power lead input. Continuity along the front panel board is good (input to channel selector through eq, etc), but I wouldn't know how to check the power module board or transformer. I took it to a shop who reported the power module as dead but wanted the schematic before going further. I have spoken to the Aust importer who tried to hunt down a schematic, but the factory will not respond to our requests, and do not keep replacement parts - so that avenue of help is closed.

 

At the moment, if I can find a 300w class D power module and transformer suitable for our 240v, I'd happily swap it over, but the connector to the preamp (front panel) board will most likely be different, and I'm not sure if it would have a low voltage takeoff for the preamp board. Likewise trying to install any other A+B type power module is unlikely to have the connections I want.

 

So - beside turning the cabinet into a passive cabinet, finding a 1U bass head, or screwing in a Warwick Gnome or similar to fill the void, what options would I have to resurrect the head unit? Is there any other trouble shooting tips that I could try (that doesn't involve me going up in a puff of blue smoke)?

 

attached is a pic of the power module (is it usual to use that much hot-melt glue?) in case someone recognizes it - would other bass combo's use one like this? (Behringer? Laney? Peavey?, etc). As seen in the pic, apart from component id's, there is no writing on the powerboard apart from '0704420  2001-0173-4  300w BASS POWER' - which I was really hoping would turn up something at AliExpress or ebay, but nothing...

 

Any thoughts appreciated...... Wayne.

Looks like a conventional class A/B power amp module to me along with auxiliary low voltage power supplies for the preamp, probably + - 15v DC. It is suspicious that the DI output is not working as that comes from the preamp, so the fault may not be the power amp at all or there may be multiple faults.  It will also be worth checking that the thermal cutout (mounted on the heatsink) has good continuity.

 

If the transformer is good (likely as it lights up) one option would be to replace the power amp if it is indeed the fault, with a readily available MOSFET power amp module. The power output might not be quite the same but I doubt that you'd notice the difference. Class D modules are available, notably Icepower, which are powered directly from the mains and handily have auxiliary  DC power outputs for preamps, however these are not cheap!. You would need to figure out the preamp power requirements and either reuse the existing regulators or provide new and wire it correctly. None of this is technically difficult for an experienced engineer but it might not be cost effective, depending what value you place on the amp.  Your best option of all will be to find a competent repairer who will accurately diagnose the problem and propose the best cost effective fix. There is an index of repairers on the forum, although probably a bit out of date now. Where are you based? I'm a repairer based in Somerset if you're anywhere near to me I'd be happy to have a proper look

[rmorris]

From the OP's post he mentions Australia so not very near Somerset !

I'd suggest find out what's (not) going on with the DI first before getting into power amp territory.

As you say, that should be independent of the power stage (but never say never...)