EMT 140 with Solid State 162 Reverb Amp...

[timjimbaker]

Hey guys, I've been working on my stereo EMT 140 lately. I started by removing the plate,  I cleaned some duct tape residue that was on it from what i assume was from shipping it at some point. I got the bit of rust that was on it cleaned off and I buffed the whole thing to a shine. Then, I hung and tensioned the plate which is a harrowing task. I had gotten a few extra clips from dan alexander but I broke a few in the process of tensioning it. Jim Cunningham's operation seems to be defunct now. If anyone knows where to get more clips these days, I'd really appreciate a heads up.

So I hung the cleaned plate, tensioned it (not perfect but pretty good, scared to break another clip) and got the driver and pickups remounted. At that point the plate was working but it was noisy and had quite a (60 cycle) hum to it as well. I decided a recap was in order. I recapped the all of the electrolytics except the large metal can cap because I couldn't easily find a source for one. The recap got the amp sounding better, it removed the hum in one channel entirely and brought the noise floor down a bit. After that I swapped the 709c opamps out for some 5534's. That made a big difference in the quality of the sound and brought the noise floor down even more.

The unit is sounding pretty damn good now. The only issue is the ground hum on one of the channels. I opened the box up last night and checked all the connections. I figured maybe a shield got disconnected somewhere inside. Everything looks good. I also isolated the inputs to make sure the hum wasn't being introduced from somewhere else. I'm pretty sure the hum is coming from within the amp. I've had the unit connected to my console a bunch of different ways and the hum always stays the same.

Does anyone have any suggestions on what my next move should be... Any advice would be greatly appreciated. Also, I was wondering if someone might be able to take a high res shot of the component side of the 162's pcb. Mine seems to have had some work done on it. It would be nice to compare it with another 162. The last thing is that the schematics I have for my 140 are difficult to read. I got them from AnalogRules.com. While I really shouldn't complain about free shematics, does anyone have a 162 schematic that is easier to read.

Thanks for your time... Regards, Tim.

 

[pucho812]

replace the can cap. Even better replace the can capacitor with individual electrolytics if there is space.

 

[timjimbaker]

Thanks for your quick reply... I finally found a schematic that I can read. There is a link to it on another post:

http://technicalaudio.com/pdf/EMT/

Please bear with me pucho812, I don't know too much about this stuff. If the problem was in the can caps, wouldn't the hum be evident in both channels? Correct me if I am wrong but I believe this cap is in the power supply. Wouldn't it feed the voltages with the hum into both channels?

Another question I forgot to ask in my first post... There is a jumper on the back of the amplifier to ground the unit to earth or to the chassis. Which is a better bet?

Thanks, TJ.

 

[QUEEF BAG]

try both earth and chassis, which ever works best

 

[jensenmann]

http://www.martinsound.com/

Martinsound sells clips and a driver adjustment ruler.

 

[Sookie]

Hi Tim,

I was curious if you managed to trace your problem.
I have the same here with a 162 transistor EMT.
I could help you with some pictures of inside if you still need them. Although there has been some small work done here.

Mine has the bias current slowly moving up as well... What could that be?
And both output sides have different current draw.

Do you have to change something else if you swap the uA709C for 5534's?

 

[timjimbaker]

Sookie... I've been meaning to update this thread so I am glad you replied to it... Thanks. The hum on my 162 was pretty bad and driving me crazy. I checked it over and over, recapped most of it and swapped out those opamps. The lead on one of the pickup connectors broke from opening and closing the door on it so many times. I didnt notice and one channel went down. I thought I blew it up. I pretty much gave up and brought it to a friend's house for him to have a look at. He used to repair MRI machines and speculated that the problem wasn't in the amp but with interferance as the 140 is basically a giant antenna. I thought he was crazy as I was sure the problem was in the 162. He told me to open up the 140 and check every single connection. I got the 162 home, found the broken lead and repaired it, found a ceramic cap that had come loose from an input to ground (I don't know how I missed that one), opened up the 140 and found some loose bolts around the pickups, I tightened them. It is important to note the bolts were only slightly loose, the connections were made, they just weren't that tight. I put the 162 back in the 140 plugged it in and it was dead quiet... It was a pretty humbling lesson in EMI. Check those connections!

My plate is sounding awesome, it's not quite tight enough but I am too scared I will break another clip.

Mine has the bias current slowly moving up as well... What could that be?
And both output sides have different current draw.

Sookie, I think that is a bit above me, I don't really know what would cause that... Hopefully someone else can give you some advise on that.

Do you have to change something else if you swap the uA709C for 5534's?

Yes, there is another thread that covers how to do this mod... It's really easy though. Just remove the uA709C's. The traces for the 709c's were actually for 8 pin DIPs. My 709c's were the metal can kind and there was an adaptor to bend the pins on it into a DIP format. I have heard some of the 162 are designed for the metal cans and you either have to bend the pins of the DIP or get some kind of adaptor... I soldered in some sockets to make changing ICs easier and to avoid overheating my 5534's. I then just soldered a 22pf cap across pins 5 and 8. My pcb is numbered but make sure you get the orientation right (sorry to state the obvious).

here's that thread too...

http://www.groupdiy.com/index.php?topic=28895.msg349808#msg349808

Sookie, good luck! Having a quiet and working plate is the best. Well worth the time and effort.

Best regards, Tim.

 

[Sookie]

Hi Tim, thanks for your update!

I had noticed the fragile lead situation already and checked and resoldered. One was almost breaking. The unit is recapped now too.

Mine has the 709c's metal cans with the adapters too. I also had sockets installed for comparing etc.
So swapping with 5534's only needs this adding op the caps? Nothing to remove?

Well, testing time again before I switch to 5534's. I'll update on the situation.

 

[Sookie]

So hooked up and tested.
Amp input and output voltages nicely in balance now.

Is already much quieter. But the noise on the 2nd channel is still there... it's just a lot of hiss. It is not a hum. It's like a white noise.
Do these 2N3053's get noisy? I notice on that channel some have been replaced before. Anyone?

 

[AjStorm]

Hi, for those of you scared to break clips, I have a few sets of clips and springs left on eBay. They're new old stock, or you can email me here :[email protected]

 

[thestudio]

The ones for $240 ........crazy.....