Alesis RA-150 Schematic...or suggestions

[HiString]

Just wondering if anyone has the schematics for an Alesis RA-150 amp as so far, approaches to the local Alesis distributor have been ignored.

The problem...........:

For some time, my RA-150 has been subject to Channel 1 failing after being turned on for periods ranging from 10 to 30 minutes.

Observations..........:

A quick boost of signal from the mixer usually causes the amp to start working again, however it is usually only a matter of minutes before it again fails and any further attempts to "kick start" it are futile. My most common solution has been to turn the amp off for a short while and then turn it back on but this is only a temporary fix as it soon fails again. Also, on a few occasions, just removing any signal and turning Channel 1 down has been temporarily successul.

A couple of days ago I removed the top cover and turned the amp on and within a few minutes it was obvious that components on the smaller pcb of Channel 1 were considerably warmer than those on Channel 2. On running the amp with the top cover removed it was nearly an hour before Channel 1 failed and I have used the amp over the weekend and confirmed that without the top cover it takes considerably longer for failure to occur. Obviously, the better ventilation is responsible.

If anyone has any advice, etc., it will be much appreciated.

 

[crm0922]

is the amp going into overcurrent protection on the affected channel? Maybe it has an indicator or something.

If you run a table fan on it, does it run even longer?

Can you swap any internal cards around to learn any more?

Chris

 

[HiString]

Thermal protection on these amps would shut down the whole amp.........it was one of the first things I checked in the manual.

I could try a fan but it will be hard to know whether it makes any difference, as with the top off, the amp does run for much longer but it's near impossible to determine a "bench mark" time to judge things by.

Changing pcb's around doesn't appear to be an option.

ChrisO

 

[crm0922]

Not thermal protection. Overcurrent for things like shorted output leads or too-low impedance speakers attached.

You aren't running too low or different impedances on the two channels I assume.. ?

Chris

 

[HiString]

No Chris, everything is as it should be. I posted the question on the off chance that someone may have recognised this behaviour and been able to narrow down my search to certain (types of) components.

 

[Boswell]

I've seen amplifiers with that sort of behaviour. It usually turns out to be a dry joint that fails after thermal expansion of a component and/or board. I've also had resistors whose end caps come adrift and capacitors that go open circuit when hot.

Try putting a low level signal through the affected channel only with the top off, and gently but firmly prod the components on the PCB for that channel with a wooden rod. Include the connectors and inter-board wiring in your mechanical manipulations.

A can of freezer spray with a long thin delivery tube is a helpful diagnostic tool in these circumstances, as it can identify individual components where percussive diagnosis is less localised.

 

[HiString]

Guys, thanks for the comments, etc.

Earlier tonight I went "probing" taking temp readings on various components comparing between the two channels..........nothing particularly outstanding there BUT on the power supply pcb I found one smallish electrolitic running at >70 deg C., whereas it's mate (there's two identical circuits in this part of the board) is running at 59 deg C.............suspicious? Whether this is part of the thermal protection circuit or not I'm not sure, I have emailed Alesis directly to see if I can get any schematics, etc., although knowing their reputation I'm not holding my breath :grin:



Update..........:

Less than 4 hours after contacting them and about 40 minutes after I posted the above Alesis emailed a copy of the service manual including schematics, etc.

 

[kilmister]

Old thread but I continue in here.

I have RA150 in my bench but now its PSU faulted so I'm looking schematics for it. Mailed to Alesis but regarding older posts up there I'm not hoping too much.
It's also helpful if anyone can tell values for R401 and R402.

Peace,
Paavo

 

[abbey road d enfer]

A quick boost of signal from the mixer usually causes the amp to start working again, however it is usually only a matter of minutes before it again fails and any further attempts to "kick start" it are futile.  My most common solution has been to turn the amp off for a short while and then turn it back on but this is only a temporary fix as it soon fails again.  Also, on a few occasions, just removing any signal and turning Channel 1 down has been temporarily successul

I've had the same problem on an RA300. The cause is oxidisation of the connectors. I have cleaned them with a sharpie, but I think I'll have to hard solder them one day or another.