One more Eico ST70 rebuild thread

Hum is constant regardless of volume level. Adjusted hum pots best I could.
Doesnt seem to make a sound on "shorting"
 
both resistors are on the money.
Two channels w/loud hum
Even louder hum when I touch inputs, if this is what you mean by shorting. About the same volume, different tone

hum does get louder with volume
 
Scott
Dont know exactly what you did but...tacked the 1Ms back on and: :banana:

Sounds Great*

Very minimal hum. ALL tone controls and switches operate properly, even loudness switch.

:beerchug:

:bowdown:


ITS ALIVE and to think that I about gave up on it just yesterday. No way I could have done it without the AK gurus

*for an mp3 thru some cheap wharfedales
 
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Must have been a bad connection or something. Have you gone through and cleaned the tube sockets? Since the 5AR4 was crusty, the others may be too.
 
I know my Eico ST-70 had dirty loose sockets 30 years ago when I got it, i tightened and cleaned the sockets and it works fine now.
 
IDK. I think working the phase slide switch had something to do with it.
The mod seemed to work as I have heard that the loudness switch was inaudible.

How do I trace down the hum? Could it be tubes or likely in a circuit?
Advice on pulling off the knobs? Are they friction fit? Dont want to break them or the pots.
 
Generally hum comes from a few places. Power supply (As i understand it you have replaced all the electrolytic caps in the power supply) so for now we'll leave these out of the picture. Ground loops, Basically signals should be grounded at one end and not the other. A common ground usually at the power supply If shielded wires used for signals lose their ground connections, (therefore if there is a compromised connection of a shield/drain wire you will pick up hum) they become antenna and actually attract noise (hum) rather than protect from it. PIck up . Also if multiple ground paths are available it raises the noise floor. Basically if you signal cables run in close proximity to ac it picks up the ac as signal and amplifies it as if it was your source. As a rule you try to keep ac away from signal wires. Have it only cross at a perpendicular if necessary.
A tube can have hum but it'd be at least a little unusual to have same noise in both channels.
I think it'd be prudent to make sure tube sockets are clean and tight, That all hardware is tight (and no corrosion on chassis underneath). Look to see that ac filament winding are twisted and away from signal cabling . As Scott said before, if shields are off of tube shields you can pick up noise. Even a dimmer on the same circuit in the house can cause noise.
I wish i could say look here (this is where your problem is). Hum can be a bear to hunt down even for an experienced tech. I'm hoping if you eliminate some of the more obvious stuff, it will at least narrow it down to a few things. Just the fact that the 1M resistors apparently had a bad connection (cold joint, dirty loose socket?) Only proves sometimes its the details that make everything work right.
Anybody else feel free to correct , add or refine methodology.
 
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Will work on it. Once I buttoned it up, the hum got to a manageable level. It first started singing with Dave Mathews and Mick J. wild horses. Memorable. With some knobs, it will be in the main room soon.
But in the meantime....mp3 here in my little corner of the universe.
eico rocks.jpg
 
Well they are stuck and I guess will remain. Any more pulling and i'll pull the stem out of the pots.

But I know what you mean on the hum issue. I think when I get it on a good source that it wont be audible. Except to those guys that can tell the difference in the type of resistor used in a circuit.
Not me. Lucky to hear the highs.
 
After a big push like this, it often pays to live with an amp for a while. Just get used to how it sounds in normal use. Sometimes such use shakes out new issues that need attention. My ST40 needed to have all the switches, pots and tube sockets thoroughly cleaned. Congrats on completing another restoration, and hats off to your team!
 
Getting used to the sound. In fact the hum all but disappeared when set up on a dac and quality speakers. This amp has more highs than my other two and sounds incredible. But they all do. Still have the old tubes in.The only curious thing about it is that at a low volume, flipping the loudness switch to ON will lower the volume considerably. At higher volume levels, no appreciable change in volume is heard.
Any harm to switch the wires on the pot and see what it does? May be due to the line stage mod?

BTW, there is one of these bidding in a couple hours on "that" site with all tubes and knobs, currently not unreasonable (except maybe the shipping).
 
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The loudness circuit is faulty on the ST40. It may be the same on the ST70. The ST40 has a well documented fix.
 
The loudness circuit is faulty on the ST40. It may be the same on the ST70. The ST40 has a well documented fix.

Was it the same symptoms? Does everyone else with a st70 have this issue? Because I was thinking that the mods I made may have been the reason. I'll look it up, not that it really bothers me as the tone controls all work nicely.
 
The circuit is not faulty per se, it is just designed to work in a different way.
The ST-40,ST-70, ST-84, and HF-85 all have the same system IIRC
It is the same thing you describe.
 
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The loudness mod. I think it is the same for the other units.
The real problem with the HF-85's touchy gain (and it is surely touchy as designed), is -- once again -- the controversial loudness switch design. When you look at the gain through out the line/tone stage circuits, they are all really pretty normal. The gain of the tone stage itself is <1 at a flat setting, which is true of almost all feedback type tone circuits. In this case, it is more precisely about .90. The line stage has a typical gain of about 14.5, which together produces an overall gain of about 13. This is very similar to that of say a Dynaco Pas preamp, that has an overall line/tone stage gain of 10. The big difference between the two however, is that when the loudness switch in the Pas preamp is turned off, the volume control loudness tap is shunted to ground through it's 4.7K loudness switch resistor, where as in the Eico, that control tap is left open circuit when the loudness switch is off. This has a huge effect on how the control operates through out the first half of it's rotation. This is also why there is such a big drop in gain in the Eico when the loudness switch is engaged.

The easiest solution is to use the same loudness components that Eico used, but connect them as Dyna did in their preamp. That is in essence, that one end of the 10K resistor should be grounded, the other end should go to one end of the .025 cap, whose other end goes to the loudness tap. Then, the loudness switch should act to short out the cap when turned "off", and allow it to be in circuit when turned on. That means that only two of the switch terminals will be used for each channel. If you do this, you will find that all the touchiness of the control is gone, and the loudness switch will operate "normally", without the large change in gain associated with the original design.

Dave
 
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