Bass amp dead - swap out power module?

[www68]

Hi there,

I have a dual channel 300w/8ohm 15inch 7-band eq bass combo amp (not D-class) - perhaps 6 years old - which I suspect is a badge engineered unit. Certainly not a premium name - 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 / amp? 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 below 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.

[glebert]

Hi, welcome to MEF!

This to me looks like this module contains the power supply and the power amp section for this amp. The four legged chip mounted to the heatsink (furthest right in the picture) looks like a bridge rectifier. I'm guessing that the 10 pin black connector in the lower left of the picture is where the signal from the preamp comes in but also sends the voltage rails to power the preamp (usually something like +/- 15V). The other transistors on the heatsink are presumably the power transistors that send power to the speaker. As you noted, it would be hard then to find a module that 1.) takes the same AC voltage that your transformer puts out 2.) sends out the right voltages to the preamp and 3.) has the same connector (and pinout). Many of the class D modules that you can get for cheap require DC voltage rails, so you would have to make a separate power board that takes the AC from the transformer and creates the high DC rails for the power transistors (maybe +/-75V) and also the low voltage rails for the preamp. There are some class D modules that have the power supply and power amp together (search ebay for "class D power amp with switching power supply), but that still leaves you with possibly having to figure out how to mate the connectors of the preamp and power amp regarding signal and voltage rails.

Basically I think it would be easier to fix this module, even without a schematic, than trying to make some other module work in it. If it were me the first thing I would be doing was measuring voltages and seeing if I can find the high and low DC rails. Oh, and not letting out blue smoke.

Is this the model of amp? https://www.creativemusic.com.au/pro...-amp-300-watt/

Assuming that is is, one test that many do on an amp that is not passing signal is to plug an instrument straight into the effects return jack. Usually that has the effect of bypassing most or all of the preamp. If you get sound out when you do that it helps with diagnosis. I always do this test if the amp powers up but doesn't pass signal.

[Jon Snell]

Well, simple enough to repair but the SAP15s are obsolete,
https://pdf1.alldatasheet.com/datash...KEN/SAP15.html
A suitable replacement darlington pair can be purchased to get it going again if it is actually faulty.
All values are printed on the PCB so a good engineer can get it going.

This amplifier was probably made by Samson Electronics, part of the Loud Technologies Group.

[www68]

Howdy Jon - thank you for your thoughts.

I only came across the fx-return trick this evening while hunting around on the net, so I will try that tomorrow and see what happens.

That link you provided is to the very same amp that I am attempting to resurrect - that is about the only reference for the model I can find apart from an old Pro Music Australia (wholesaler that I have been talking to) catalogue that I also found - which is why I suspect it was a generic badge engineered model.

You are correct - the black 10 pin connector takes +/- 15v to the preamp and presumably carries signal from the preamp bus to the amplifier. The speaker itself connects to the centre area of the board using spade connectors, and the white six pin connector on the far right receives power from the transformer.

Based on your thoughts I might try to hunt down an electronics nerd, as opposed to a repair shop, and see if they can check the components across the board - my limit of experience and knowledge of electronics stops at wiring up guitars/basses and repairing leads.

The combo amp itself very much resembles a Laney RB4, and even the newer Livero B300 http://www.lirevo.cn/detail.aspx?cid=231- however these are only single channel amps.

Thank you again - I'll see what happens next...

W

[glebert]

The combo amp itself very much resembles a Laney RB4, and even the newer Livero B300 http://www.lirevo.cn/detail.aspx?cid=231- however these are only single channel amps.

Ooh, now that makes it interesting. One channel vs. two is irrelevant for this since those are specific to the preamp. I couldn't find a picture of a Laney RB4 module but did find one for an RB8. It looks really similar, but not identical, to yours.

[Enzo]

Sure looks like the same module.

Surely you can check for main power rails. I may assume the speaker relay does NOT pull in? In which case, check for DC on the amp side of the relay.

[g1]

Attaching RB8 schematics. Pg.3 is version with SAP output devices, probably very close to the RMS branded amp being discussed.
Pg.4 is newer version using different output devices. Did not check if there are other changes as well.