Park (Marshall) G10R amp power light on but no noise

craigdylan

Hello all

I have just started playing again after almost 20 years of not touching any of my guitars. I've been playing mainly my Eko acoustic but today decided to try my Fender Jag-Stang through my old, but virtually unused Park Marshall G10R amp, which I'm unable to get to work. When I plug the amp in to the mains power socket the red power light comes on but there's no feedback noise or any noise whatsoever, even with the volume turned to maximum. Likewise when I connect it by jack cable to my guitar. I have checked the open back of the amp and both the red and white cables are plugged in to their respective sockets correctly and there's no visible signs of wiring damage. In fact, the amp looks brand new.
Any ideas about this?

Many thanks 

ICBM

Does it work through headphones?

If so there's a good chance it's a corroded switch contact in the headphone jack, which disconnects the speaker. It will need cleaning or probably at worst re-tensioning.

If not, it's either the speaker or the power IC most likely.

craigdylan

Thanks ICBM. I must admit, that doesn't all make sense to me! But, I'll start off trying with headphones. I'm unsure what re-tensioning, in the context of an amp, would involve.
It may be the power as I'm not sure I'm using the original/correct mains power lead, though the red LED power light is on.
Thanks again 

ICBM

craigdylan said:

Thanks ICBM. I must admit, that doesn't all make sense to me! But, I'll start off trying with headphones. I'm unsure what re-tensioning, in the context of an amp, would involve.
It may be the power as I'm not sure I'm using the original/correct mains power lead, though the red LED power light is on.
Thanks again 

It can't be anything to do with the power cable or the amp's internal power supply since the red light is on - on this amp, it's driven from the 'circuit' side rather than the mains input side, so the transformer and all the other bits are working fine.

The problem is that no sound at all is coming out, which leaves the speaker, the power IC ('chip' - it's a single one in these amps) or the connection between them, which is in the headphone jack. It is *just* possible that the problem is earlier in the signal chain, but if so I would expect faint hiss to be audible from the speaker.

The reason I think it's most likely to be the headphone jack is that the amp hasn't been used for a long time, and sometimes a spot of corrosion will form in it and do exactly what you've found. Re-tensioning it just means bending the springy bit (technical term ) of the headphone jack slightly so it makes a better contact, but you need to be careful not to make it worse, it's quite easy to if you're not familiar with doing it. Often cleaning it - sometimes even just putting the headphone plug in and taking it out, if it disturbs the corroded spot - can be enough.

Funkfingers

The amplifier has a semi-open back. It would take seconds to eliminate the loudspeaker from enquiries. Test the voice coil for continuity with a resistance meter. (One probe to the positive spade connector contact. The other to the negative.) A meter reading of zero Ohms means that the voice coil is good. A reading of infinity means that it is blown.

ICBM

Funkfingers said:

The amplifier has a semi-open back. It would take seconds to eliminate the loudspeaker from enquiries. Test the voice coil for continuity with a resistance meter. (One probe to the positive spade connector contact. The other to the negative.) A meter reading of zero Ohms means that the voice coil is good. A reading of infinity means that it is blown.

You need to disconnect at least one speaker connection, or a shorted power IC would give you an apparently 'good' speaker reading even if it's blown.

The correct reading won't be zero ohms, it will be either about 3 ohms (4-ohm speaker) or about 6 ohms (8-ohm speaker). A reading of zero means the speaker is shorted - rare, but not unknown.

Funkfingers

Probably 'bout time I replaced ye olde analogue deflecting needle multi-meter.