EICO ST40 help please? Only 1 channel operating

Hi guys and gals ,

I plugged in my EICO for the first time and only have one channel operating. All of the 7591 tubes glow. Someone on this site had suggested that one of the output tubes on their st70 had a cathode resistor that was open. I will definitely check for that. Being a beginner at this I was wondering if anyone could point me in the right direction as to how to check. Also any other possibilities and remedies that I could try would be greatly appreciated. I have the Original manual to help me along the way.

Thanks again ,

Rickyo

 

Rickyo-

It could be lots of things. Start with the obvious: Swap your speaker wires from Left to Right. If the problem moves from speaker to speaker, you know your speakers and speaker wires are OK. And can be sure the problem is IN the ST40. Next, power down, let the tubes cool off and swap your power tubes from L to R. If the problem moves this time, you've got a bad pair of tubes.

 

DSC_0198 by Mark Hardy, on Flickr
ST-40 on the right.

Three other things to consider:

* Dirty switches (the selector switches, but also (in particular) the slide switches across the top) can cause all sorts of woes. "Fiddle" with them and see if there is any sign of life from the "dead" channel. If so -- clean the switches.

* The input jack configuration and source selection of the ST-40 and/or the ST-70 is a bit abstruse -- mostly due to support for the interim "AM-FM simulcast" stereo format that briefly filled in the gap from mono FM to mono-compatible MPX stereo FM. Are you sure everything's hooked up right? It usually takes me a couple of tries when I reanimate either of the pair shown above

( The speaker hookups are also, if memory serves, a bit arcane. Are you sure you have both channels properly hooked up?

All three of these suggestions arise from my own bitter experience with the ST-40 and ST-70 that live here (i.e., BTDT!)

 

Just looked at the ST-40 schematic -- looks like the cathode resistors are 125 ohm, 5 watt. I'd probably try to check each with a DMM on either side of the resistor body. You're probably looking for an open circuit (infinite resistance) if the resistor's cooked. Compare the readings for the two channels' resistors (R73 and R74).

http://www.cma4ch.org/chemo/image/hifi/st40-sche12-a3.png

If you do this, make sure that the amp is unplugged and has sat for a while to allow any charge in the larger capacitors to dissipate. Lethal voltages can be found in an energized piece of vacuum tube equipment such as this amplifier.

 

In case the OP doesn't have 'em (which I suspect, and hope, is very unlikely!) the manual (construction and operation sections) are available right here on AK's digital doc base.

http://akdatabase.com/AKview/displayimage.php?album=42&pos=33
http://akdatabase.com/AKview/displayimage.php?album=42&pos=34

I've circled what I believe to be the two cathode resistors in red:

 

nice photo of the back panel of an ST-70. source: https://forum.polkaudio.com/discussion/111045/eico-st-70-vintage-tube-integrated-amp-restoration-upgrade/
I think the ST-40 is extremely similar -- although I must admit that, right this minute, I am too danged lazy to go upstairs and check!

 

Thank you mhardy6647!

The problem was 2 fold.

1. When connecting a source via rca cable to the inputs in the back, they are inserted side by side (ch.1 & ch.2) rather than top to bottom like I had thought initially.

2. Speaker connections are really funky, but figured out where to connect the jumper wire and + - connections.

BTW, I can not believe the power in this unit for 20 wpc! I have paired with 2 vintage Knight kn2380s and it just sounds amazing. Lots of punch and the output tubes really do like the Jensen horns contained in the Knights.

Just one other thing, if you look at the photo below you can see in the first photo there is a green wire on the bottom of an output tube. In the second photo, you can see that that green wire leads to a resistor that someone had cut intentionally for some reason. When I touch that green wire back to the resister I get a hum out of the unit. Otherwise it’s working fine. But it bothers me that there is a cut wire just hanging there. Any thoughts on why someone may have cut that? Wouldn’t that green wire have a reason to be there? Obviously.

Thanks to all for the helpful input,

Rickyo

 

My st 40 is upstairs as well but you are correct the layout on the back is the same.i also agree that those slide switches on top can really create havoc if dirty (I have first hand experience. For future reference here's a picture of the back of my st40.

 

Thanks Guys. Now I would like to do the loudness switch modification. I have enclosed a photo of the existing loudness switch. I’ve seen the box design on how to do the modification. However, does anyone have an after picture of their loudness switch that they could share?

 

Hope this helps.

 

Hope that helps.

 

Oh that WILL help! That’s awesome. Those yellow components are new caps correct? What value?

 

I'll let you know tonight after I get home from work

 

Thanks. It’s so unbelievable that with this modification, this unit will sound even better than it already does.

 

Sorry for the delay but here are the values.

 

.022 400v

 

No worries. It’s the most wonderful and busy time of the year. . I’ll post a pic when completed.

Thanks again for your assistance. Merry Christmas and Happy Holidays.

Rickyo

 

Hi guys,

I’m experiencing a soft and low hum when I turn on the ST40. When I touch the orange cap on the Bass pot in the pic, the hum gets louder. Does anyone know why that could be? Also, when the white wire leading to the cap is touched, the hum will get slightly louder.

I’m hopeful That when I do the modification to the loudness switch, some of the hum will rectify itself some if not all.

Any thoughts.

Thanks.