• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Another Eico ST 70 thread

Unless you use some kind of buffered probe for inputting the scope signal,your sound card can only handle 5v or so for the input.
I have tried some of those programs,so far the ones I have seen are frequency limited.You can't beat a cheap $50 scope,with a PC
 
That was what I was thinking.( talking about). I didn't realize there was a program (software) that you could run right into your soundcard. I actually saw an app for a smartphone that was a scope, but I can't take that seriously
 
I have seen a usb o-scope ,but I think that was limited in voltage also,"we" need at least 600v to be safe in tube equipment in case of mistakes.

I already blew out a 1000v meter,and a mp3 player(signal source)working on this stuff.The meter was measuring 460v on an output tranformer(1000v setting)when a bad solder joint on the tube socket caused a spike and killed it.

The Mp3 player I was using,I tried to inject a signal into the circuit
through a cap,but it must have sent a spike into the player and wiped it out.
 
I killed a cheap digital meter on an amp. I was in a thousand volt range measuring 400 something. Nice arc! blew fuse in meter but was still dead after replacement. I suspect true impedance of meter never could of handled voltage (i'm glad it was a cheapy).
 
OK I was looking at our Peco conversation and the schematic you posted in post 134 which I agree is accurate based on the link we were looking at. My problem, The schematic of the peco in the amp doesn't show that 1k resistor. I'm thinking that's wrong and that resistor in reality doesn't exist. Any thoughts?
I'm even open to the possibility that danno 2000's values are right? (aside for the 1k values are pretty close)
 
I would try it per the schematic and see how it sounds,and disconnect the resistor and see if it is better or worse.You could build the boards so the end of the resistor and the junction of the cap and resistor are up in the air so you can bridge them and disconnect them in the amp
 
Waiting for parts to build pecos. I did get and install the 68pfs in the Loudness mod/ fix. The old ones read around 62pfs. Not bad after 50 years (I can't say I notice the difference). To my ears (unscientific I know) what sounds reasonably flat now, treble @ +1 1/3, Bass @-2 1/2. If you get past the concept of it should be flat at 0,0. It actually sounds quite good!
 
I think one of those links say something about soldering resistors across the tone controls to centre the controls.I'm not sure.
 
I'll go do some homework thanks
Your right . The Tronola article for the ST 84 in post 136 addresses it. Thanks
P.S. I think I'll hold off on that until Peco work is done
 
Last edited:
Maybe only interesting development , I think I found out what caused the oscillation. The amp was plugged into same outlet as my soldering station. I hadn't seen the issue in a while. I had been going back and checking to see if issue were cured or just intermittent. Sure enough (again nowhere near amount when I first had problem) maybe a swing of .006-7 volts but it was stabler. I did change two things (which I thought might've been the culprits) I had CL 80 jumped, which gave me spot on voltages. I took jumper out which dropped everything a couple of volts. I re jumped and fluctuation was still there. OK other thing , there was once a .03uf cap from line in (110) to ground which I replaced. So I open that , still there. I'm looking at meters attached to test jacks and I turn off soldering station and low and behold it flattens right out. Maybe one last thing. I find the low level hum disappears if I ground the chassis. I did try reversing plug but that had no effect. So I ordered a 3 wire line cord. I'll make it a permanent fix. Waiting on some resistors for the peco. I have metal films but I thought carbons might be a little quieter.
 
Finally got resistors (from China). Am I the only crazy guy that will measure a roll of a hundred resistors to match them. Supposedly 5% . All tested better than 3%. Matched a set I needed (6) within 1% of spec and as good as my meter is to each other. Built the pecos ( I got 500v mica caps) with the resistors on the breadboard a good deal larger than the Pecos. Shot right now. I'll get it together tomorrow with a review.
 
I usually try to match resistors from a batch,as close as I can,channel to channel.(and verify the color codes because they are poorly printed and hard to read)
 
ok back together plays fine. Seems like you get more bass boost from one bass pot than the other. otherwise (had a lifted solder trace on one board spent a little time finding it. I have on the way a scope and a tone generator. Hoping to set to center tone controls, balance controls and ac balance . Fingers crossed
 
Yeah I was tired last night but was thinking something wasn't right. Again my first experience with the perf board with the trace in a line on the back. I don't know if it was a matter of my iron being too hot, the perf board being crappy (or its just like that and you have to baby it) or the 20 awg wire I was using was too stiff that when I tried to position it, it stressed the solder point but I had more than one pop. I checked point to point and sure enough. Move leads in to nearer open spot on same trace and bass gain balanced. I'd say tone controls close to center but definitely not centered and neutral. I have to wait for the new toys to come. In fairness will a little patience and tweaking bass and treble it sounds quite good and I haven't listened to it on very good speakers. I feel pretty pleased with where I am so far. If any one was curious about the peco values I went with it was the first set I got second opinion from author of the loudness article Dan Nicotera and he confirmed (3) 100ks two 3000pfs and one 330pf like the moderator from the Eico Yahoo group listed in the other article, if anybody needs the exact value connection list ask.
 
Hey got tone generator today (it looks pretty cool) but pretty worthless without scope. Maybe by mon.. I was looking at hf 85 and ST 70 parts lists and books. I think I understand why the pecs are coming in with different values . ST 70 ( probably 84 too)part # is29751. HF 85 is 29571. Could be a typo. It doesn't even show component layout without values in the 85 manual.
Let me try an say this without sounding ignorant (doesn't me i'm not, I just don't want to sound like it). It seems to me the adjustment of the tone controls is an unnatural curve. Granted by nature flat would be natural and anything you change would be unnatural. That being said, based on the effect of other tone controls I've encountered (I grant none are alike per say) it seems the sound is twisted as you move off center at a /an unnatural amount (having difficulty putting into words). The emphasis of certain frequencies is boosted or attenuated in such a way that as you move off center rather than tweaking sound it seems like you're distorting the response. I don't feel like i'm conveying this well but I think you get the gist. I'll look at it with scope and see what it looks like. I suspect the values for the tone control circuit could be massaged to a better effect. I wouldn't know where to begin to experiment with it. Just my thoughts. ...anybody?
 
You must log in or register to reply here.
Back
Top Bottom