EMT verb amp

[sr1200]

A friend has an emt reverb unit, stereo, and one if the channels starts to get a hum after a while of use. Any have any thoughts as to what might be foing on.  Were going to open the unit up today. To my knowledge its never been recapped and the tubes have never been changed. Any input wiuld be awesome!!

 

[abechap024]

Recap!!


At least it would be the first thing I would try.

 

[sr1200]

Opened it up today... my god what a mess in there.  Channel 1 has a buzz on it, doesn't sound like a ground. I noticed the big transformer on the top of the unit (when standing on end as it does in the chassis) gets REALLY hot.

 

[mjrippe]

Replace the power supply filter caps.  Hopefully it's not too late.  Sometimes a hot power transformer is a sign of shorted windings.

 

[fazer]

Just a thought on the hot transformer. 

I know old guitar amps get hot because they are running on 120v however when origin was for 115v in the old days.  You can use a buck boost transformer to lower that voltage .  Buck boost is 120 to 6v / 12volt ct and you wire the 6volt inline with the 120v if you use one phase you get 120 minus 6 and the other phase gives you plus 6 boost.  hence boost buck.    As mentioned it could be leakage in the windings (short) but hopefully not. 

 

[mjrippe]

"REALLY hot" sounds like more than a 115/120v issue.  It is either failed caps in the PSU or shorted windings in the transformer.  If you can't keep your hand on the transformer for a count of ten, there is probably something very wrong.  Has the fuse been replaced with a higher value?  It should blow before the transformer gets this hot.

 

[sr1200]

Fuse is properly rated. Lets put it this way, 10 min after power off and removal from the chassis, it was still quite warm to the touch (this is the #119 transformer if anyone is familiar with the schematic)

 

[mjrippe]

Hmm, I have several EMT schematics but none of them has a transformer marked #119.  In any case, replace the PSU filter caps and go from there.  If you get stuck, my shop is in Brooklyn and you can PM me.

 

[sr1200]

@mjrippe  thanks for the input and I might give ya a shout about it thanks much

SSL tech had posted this schematic for the unit which coincides exactly (the 119 and everything) with the unit in question.

http://danalexanderaudio.com/EMT/EMTStereoTubeSchem.jpg 

All the way at the bottom.  The unit I'm looking at has these little circle stickers actually they look painted on) yellow red and black denoting the parts.  The part no on teh transformer is kind of worn away, but this is it.  I notice the paper surrounding it is starting to pull away from the widings and the plastic inside of it looks like its starting to disintegrate as well.   Caps first... and praying!

 

[mjrippe]

Oh cool, those little circles are so blurry in my schematics that I couldn't see the "119" by the transformer!  Anyhow, I hope the recap helps and the transformer is OK.

 

[sr1200]

Eek,  was gonna ask if your schematic was better than that one.  Im having a hard time reading the values. Next to the nd1v65 cabt tell if thats a 540 or 340u. Looks like 340 to me and the /50u below it. Theres no voltage rating there but above i see 250 volts. So im guessing i should go that or higher for the voltage values on those caps.

 

[mjrippe]

The filter caps on my other EMT 140 schematics are dual 50mfd at 350v.  I would use 47mfd 450v replacements.

 

[sr1200]

only issue is finding something that would fit the spot for it... really odd shape for this thing. 

 

[mjrippe]

It is common pracice to leave multi-section caps in place and wire new caps under the chassis.

 

[sr1200]

Im going to have to get a pic of it next time im over there.  There's like NO room left in this thing, it is PACKED! lol
In one of my other posts someone sent me to this http://www.die-wuestens.de/eindex.htm site that looks like its got the right kind of cap (complete with the weird screw on part).  Might have to go that route.

 

[mjrippe]

Ok, or remove the original and replace with two modern caps.  New caps are much smaller.  Some purists remove the outer casing of multi-cap cans and re-stuff them with modern caps.

 

[sr1200]

Might be a good idea considering the lack of space available.  Im hoping the polarization will be apparent since just about all markings on this thing have been almost worn away.

 

[mjrippe]

Refer to the schematic if in doubt.  Multi-cap cans use the can as (-) and the terminals are (+). 

 

[sr1200]

Actually, now that im thinking about it, the filter cap might not be the problem if only 1 channel is getting the buzz/hum.  If the PS filter cap was bad, wouldn't both channels be suffering?

 

[mjrippe]

It is possible, though usually both would have some hum.  In any case if they are original it would not hurt to replace them. 

Hum can come from a lot of places on an old tube amp - including the tubes!  If you have the tools, trace through the circuit and find where it starts.