Silent Marshall AS50D Acoustic Amp

[Kidderman]

I have a Marshall AS50D solid state 50-watt combo which has stopped outputting. The amp is less than two years old, has been well looked after and has had moderate home use only. It has not been subject to any impacts or been dropped. The on/off switch illuminates, however there is total silence when connecting a guitar or mic to either of the two channels. There is no noise or hiss when turning all controls to maximum. I've pulled the chassis and carried out a visual inspection. Everything appears to be as it should be. There are no loose connections. The fuse is intact. The speaker wires are connected properly. I have limited knowledge of electronics, but am hoping that there maybe a relatively simple diagnostic test which would enable me to determine the extent of the problem and whether or not I could solve it myself. I have a multi-meter, but wouldn't know what or where to test. If any members could offer suggestions-either through direct experience or general knowledge-I'd be grateful to hear them.

[DrGonz78]

Have you tried cleaning the send/return jacks? Not sure on this amp if that would cut out signal path completely if they have dirty contacts in those jacks. Try plugging in the guitar to the return jack and see if you get any sound. Also, try plugging the send jack into another amp to check if that first preamp stage is passing signal. This schematic might be close to what you have...?

[Kidderman]

Have you tried cleaning the send/return jacks? Not sure on this amp if that would cut out signal path completely if they have dirty contacts in those jacks. Try plugging in the guitar to the return jack and see if you get any sound. Also, try plugging the send jack into another amp to check if that first preamp stage is passing signal. This schematic might be close to what you have...?

Thanks for the post. I'll do as you suggest and post the outcome. The AS50R schematic is helpful, as I can't locate one for the AS50D. The main differences between the two amps is that the AS50D supplies phantom power to the mic input and has digital reverb.

[Enzo]

FIRST, does the speaker work? Pull the wire off the speaker and briefly touch a 9v battery terminals to the speaker terminals. A working speaker will pop or thump when you touch them. If it sits in silence, the speaker is bad.

[Mick Bailey]

After checking the speaker I suggest this as stage 2;

The mains transformer has a higher than usual failure rate in these amps. The switch illuminates because its mains powered off the supply and only tells you that the switch is working. To eliminate the possibility you need to measure the AC voltage coming off the secondary. There are three wires that leave the transformer together - 2 red, 1 black. Measure from black to each red. Set your meter to 200v AC range. If you like you can unplug these connectors and insert the probes under the plastic insulator. and switch on the amp with your hands clear. Just make sure nothing gets shorted out.

Don't go near the primary (the bunch of white, blue, brown, black wires on the opposite side).

[Kidderman]

After checking the speaker I suggest this as stage 2;

The mains transformer has a higher than usual failure rate in these amps. The switch illuminates because its mains powered off the supply and only tells you that the switch is working. To eliminate the possibility you need to measure the AC voltage coming off the secondary. There are three wires that leave the transformer together - 2 red, 1 black. Measure from black to each red. Set your meter to 200v AC range. If you like you can unplug these connectors and insert the probes under the plastic insulator. and switch on the amp with your hands clear. Just make sure nothing gets shorted out.

Don't go near the primary (the bunch of white, blue, brown, black wires on the opposite side).

Hi Enzo, I've tested the speakers with a 9v battery as you suggested. A thump can clearly be heard from both. I've identified the wires and terminals as you've indicated. Before I carry out the tests as suggested, I have two questions: Is it safe to power up and test without the speakers being connected? The speaker wires from the chassis are so short that the chassis has to be in place in the cab for them to be connected to the speakers. Secondly, what do I have to do to ensure that nothing gets shorted out?

[Kidderman]

Hello again DRGonz78. I'm trying the diagnostics as suggested by Enzo before trying anything else, but thanks again. At this stage, it's not looking good. (please see replies to Enzo's posts).

[Kidderman]

Enzo, having done some research and established that most solid state amps don't need a load, I've gone ahead and tested as per your instructions. I set my meter at 200v AC. The black wire is indicated on the board as being T1, the red wires T1 and T2 respectively. I left the connectors on the board, lifted the plastic insulators and set the meter probes on each connector to test. After powering up, I held the black probe to the T1 connector and then held the red probe to the T1 and T2 connectors in sequence. In both tests, there was no reading from the meter. If I've tested correctly, I guess that indicates a significant problem.
Please advise further.

[Mick Bailey]

Will the real Enzo please step forward...

You need to double-check the transformer. I never rely on a single measurement due to the relatively high cost of a replacement part. I've always found the primary to be open when these fail. To check this remove the power cable, switch the amp to the 'on' position and measure the resistance between the L and N pins of the mains socket. You should 'see' the primary resistance (I think its 65 Ohms). There's a catch, though. Sometimes a transformer will only fail when the full mains voltage is applied so this will tell you if the transformer is bad, but not necessarily if its good.

Also, don't assume the fuse is good just by looking at it - remove it and measure it.

Please post whether you have a universal transformer fitted - it has black/white/brown/blue wires on the primary.

[Kidderman]

Sorry Mick, should've gone to Specsavers. I've attached photos of the mains transformer and the power socket on the back of the amp. I'm not sure whether the wire colours and configuration make it a universal transformer. Please advise. I've checked with Marshall spares. The transformers for these amps are readily available at a cost of £22.00.
Please look at the photo of the mains socket. Which pins are L and N? Also, how do I set my multimeter to measure the resistance between the two pins? (my experience is limited, to say the least). Please also advise as to how to measure the fuse. I have found two packs of fuses which look identical to the original. They are glass, 20mm long and 5mm in diameter. However, one pack is marked 2A, the other 4A. I can't find specific information as to the required fuse rating for this amp,
so I'd be grateful if you could help on that one as well. Cheers.

[Mick Bailey]

L & N are on the left and right. The middle one is earth. Don't know what multimeter you have, but if its manual then set it to the lowest range for testing the fuse and 200 Ohms for the mains socket.

That's a universal transformer P/N TXMA-90008 (Mains AS50R/AS50D). If it does need replacing it's imperative that you connect it as per the existing one. Make some notes and take some clear reference pics.

Did you have the amp from new? When you register some Marshall amps you get an extra year warranty - 2 years in total.

[Kidderman]

It's a manual multimeter. Lowest range of resistance measurement appears to be 200 Ohms. I tested the fuse. The reading fluctuated between 1.0 and 1.5. Is that what you'd expect? Tested pins L and N after putting power switch in the 'On' position as you instructed. Swapped red and black probes over. There is no reading. Would that indicate a definite need to replace the transformer?

[Kidderman]

I had the amp used. However it's in pristine condition inside and out, so much so that it can't have had much use.

[DrGonz78]

Uh... I am a little confused here...

I set my meter at 200v AC.

Now I see your profile includes "UK" as your location, I would think setting meter to 200vac would just show "OL" if checking for 230vac line voltage.

[Kidderman]

I have no idea, the more technical stuff is beyond me. I'm simply following advice given as far as I can within my limited experience in order to try to establish where the problem lies before deciding how best to solve it.