#1
I'll sum this up Quick amp turns on fine until i flip the standby on then i get a fairly loud hum through the speakers took it into my tech he said it was the transformer and there is nothing he can do so long story short how can i get rid of this hum?
#2
Honestly that sounds like a cop-out to me... The only way the transformer could be at fault is if it is extremely leaky (possible but not very likely) or if it is picking up a stray magnetic field from another appliance (surely someone will correct me if I'm wrong). This might help.

http://www.geofex.com/ampdbug/hum.htm
#3
Quote by Invader Jim
Honestly that sounds like a cop-out to me... The only way the transformer could be at fault is if it is extremely leaky (possible but not very likely) or if it is picking up a stray magnetic field from another appliance (surely someone will correct me if I'm wrong). This might help.

http://www.geofex.com/ampdbug/hum.htm

Thanks, i'll try and run some of those tests and see if i can find the root of it.
#4
Quote by Androidjoey
I'll sum this up Quick amp turns on fine until i flip the standby on then i get a fairly loud hum through the speakers took it into my tech he said it was the transformer and there is nothing he can do so long story short how can i get rid of this hum?

He can't replace a transformer in an amp, but he claims to be a tech? I think you have a guy who knows how to use Deoxit and bias tubes, but calls himself a tech.

+1 to Invader Jim

Mojotone should have a Transformer if you need one, but I would take your amp to a better tech for a second opinion
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Last edited by Robbgnarly at Oct 4, 2013,
#5
I agree - a transformer swap is super easy. If you went to a mechanic and he said that you need new tires but there was nothing he could do to help you, you'd probably go to a different mechanic.

Take it to someone else. There are two transformers in your amp, if he just said "the transformer" that doesn't even help much. Get a second look, see if that's actually what's wrong, go from there.
#6
Quote by Roc8995
I agree - a transformer swap is super easy. If you went to a mechanic and he said that you need new tires but there was nothing he could do to help you, you'd probably go to a different mechanic.

Take it to someone else. There are two transformers in your amp, if he just said "the transformer" that doesn't even help much. Get a second look, see if that's actually what's wrong, go from there.

Well this was my second opinion, both people i went to said it was the transformer. and i'm not gonna pay 200 bucks for a replacement, so i've started testing possible things that might be the cause so far power tubes and pre amp tubes are just fine and all the jacks are good is there a way i can test the transformer myself to see if it's really the problem?
#7
Try moving the amp into a different room away from any tv, computer, monitor, flourescent light, etc. and see if the hum persists. If not, the amp was picking up noise from something aforementioned or possibly (not likely) even the particular branch circuit you were plugged into.

If you know what voltage your pt is supposed to supply you could measure it to verify that here are no shorted turns in the coils. This is a high voltage area and you could kill yourself if you aren't careful...

If that checks out, unplug the amp, discharge the filter caps, and remove the pt. Get an ohm meter and check the power transformer for leakage. To do this you'll look for very high (ideally "infinite") resistance from the coils to the iron core. You may be able to leave the pt in and just measure the voltage between the core and chassis ground (it should be zero or very near). Someone correct me if I'm wrong.

Note that the core of a transformer is cut into thin sheets and each sheet coated with varnish and reassembled. The varnish insulates the sheets from each other to minimize losses and improve efficiency. Having said that, I have very rarely seen a transformer where the sheets were actually varnished individually and insulated from each other; they cut the core into sheets them put them right back together and varnished only the outside of the laminations, totally negating the point of slicing up the core in the first place. To actually contact the core you'll need to break through the varnish layer or the test would be invalid. Check the primary, secondary, and filament coils. Probably wouldn't hurt to test the output transformer as well.
#8
Quote by Invader Jim
Try moving the amp into a different room away from any tv, computer, monitor, flourescent light, etc. and see if the hum persists. If not, the amp was picking up noise from something aforementioned or possibly (not likely) even the particular branch circuit you were plugged into.

If you know what voltage your pt is supposed to supply you could measure it to verify that here are no shorted turns in the coils. This is a high voltage area and you could kill yourself if you aren't careful...

If that checks out, unplug the amp, discharge the filter caps, and remove the pt. Get an ohm meter and check the power transformer for leakage. To do this you'll look for very high (ideally "infinite") resistance from the coils to the iron core. You may be able to leave the pt in and just measure the voltage between the core and chassis ground (it should be zero or very near). Someone correct me if I'm wrong.

Note that the core of a transformer is cut into thin sheets and each sheet coated with varnish and reassembled. The varnish insulates the sheets from each other to minimize losses and improve efficiency. Having said that, I have very rarely seen a transformer where the sheets were actually varnished individually and insulated from each other; they cut the core into sheets them put them right back together and varnished only the outside of the laminations, totally negating the point of slicing up the core in the first place. To actually contact the core you'll need to break through the varnish layer or the test would be invalid. Check the primary, secondary, and filament coils. Probably wouldn't hurt to test the output transformer as well.


You've not seen Edcor transformers :P Complete pain in the ass to take apart.

OP as the others have said, it sounds like crap. Do be extremely careful as Jim said if you decide to look at the high voltage portion of the amp.
#10
My friend had an Ampeg SVT-3 that was not working right. He took it to the authorized Ampeg repair center in our area. They charged him $250 and did not fix it. They claimed they re soldered the board, but there was something wrong they could not figure out.

My brother-n-law looked at it after the fact for free. Not one board had been removed or any solder added to anything. It was 2 pre-amp tubes that were bad, and that is is. My brother charged him $50 to fix it that was including the new pre-amp tubes.

The tech was contacted and told that it was evident he never did anything to the amp and my friend would like his money back. They told him the $250 was the bench fee. They are no longer an Ampeg/Crate repair center after Ampeg was contacted.
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate
#12
it really sounds like the filter cap to the power tubes is busted. is isnt filtering out the AC signal and is causing the amp to fart out. thats my guess at least.
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