One more Eico ST70 rebuild thread

What does it mean:idea: that they took abuse?
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If you look at the second pix, the two 50uf caps are covered with paper and foil. In the line of fire from the cap that blew-between the two leads of the empty cap. Unbelievable the amount of paper, foil and oil that blew.
 
When you get back to where we were (and you can check those resistors in the meantime). If we determine the resistors are in spec and you have the bias source voltages shown (keep in mind they'll vary with pot position. I would look at the coupling caps C19,20,21,22. If you're slick with a desoldering tool (of your choice) and you're gentle you can swap the caps channel to channel to see if your problem moves. (if you have .1uf caps they have to be rated 400v or higher) you could try them instead of swapping (i was trying to help you avoid a mouser order) You should be able to tell which channel has the problem so you only need two to test. You could just tack them in for a test you don't have to wrap for testing
 
Did you say you did mods from tronola article? If so you would have change those caps to .047s already which, would somewhat take air out of the theory that the caps were leaky:idea:
 
Its hindsight here and now. I know i and a lot of others take pictures (lots and lots of pictures). A digital camera is your friend. Overall pics, in close. different angles of same area. I find it priceless when doing restore work. Especially if documentation is scarce and you're unfamiliar with a piece. It doesn't help here and now but hopefully will in the future. (points of attachment, routing, polarity,value.....) keep in mind for the next one. Also as i think Dandy suggested, do a little, test. Do a little, test. If you goof its easier to back track. Another common work tool. Do one channel at a time and then the other. You have an example until the end for reference.
 
Thx for the advice. This unit was barely playing when I bought it. Have some photos I relied on to swap parts.
I only did the line stage mod. In regard to testing after I replace the cap and pot, I'm a bit hesitant to light the fuse again. Hopefully get time to check the resistors and find issue.
 
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Thx for the advice. This unit was barely playing when I bought it. Have some photos I relied on to swap pars.
I only did the line stage mod. In regard to testing after I replace the cap and pot, I'm a bit hesitant to light the fuse again. Hopefully get time to check the resistors and find issue.
Since you are doing your own troubleshooting it pays to get yourself a variac and a SSR for the tube rectifier. That way you can safely do testing at a lower voltage and not jeopardize your tubes or other components while you take voltage readings or check for excess current draw.
 
I understand the frustration.
Don't worry about the setbacks. We all get them. Its a good learning experience. I think you're much closer than you might believe right now.
We'll do our best to avoid explosions and shocks in the future.
You probably have a new respect for high voltage dc now. Not necessarily a bad thing.
 
Since you are doing your own troubleshooting it pays to get yourself a variac and a SSR for the tube rectifier. That way you can safely do testing at a lower voltage and not jeopardize your tubes or other components while you take voltage readings or check for excess current draw.

Variac was set at 117v during testing
SSR beyond my pay grade unfortunately.
So I could have been testing at 60v? Would the AC voltage reduction be directly proportional to the reduction in DC voltages?
 
While awaiting parts, I was going to replace the power cord with a three wire with chassis ground. After researching threads on this site, it seem that it may not be a good idea.

Another question for a small Scott unit that I replaced the cord with a three prong prior to researching. For appearance sake, I used 18G as it is only a 12 wpc unit. Is 18G ok and should I have grounded the chassis? It already had a cap from PT to neutral.
 
Its been written about and you should do some research. Why don't you hold off on the cord until we hash out existing issues. I have read if it wasn't designed for a grounded cord it could introduce new hum issues.
18g is fine
 
You could temporarily tack in a couple of 1N4007 diodes. Voltage won't be quite proportional but it would tell you if there are obvious shorts or opens.

The thing with tube rectifiers is that under about 80 volts power input, they tend to do virtually nothing. If it wants 120 in, set it at 60 and you're liable to get nothing at all, just because the rectifier tube isn't hot enough to work.
 
Should I have grounded the HH Scott? I remember that to eliminate hum, I had to flip the plug a certain way. I wired it neutral to utility plug and black to switch then ground to chassis.
Dont want to derail this thread but since we are on the subject of power cords.

I did research but there appears to be two sides on this issue.
 
You could temporarily tack in a couple of 1N4007 diodes. Voltage won't be quite proportional but it would tell you if there are obvious shorts or opens.

The thing with tube rectifiers is that under about 80 volts power input, they tend to do virtually nothing. If it wants 120 in, set it at 60 and you're liable to get nothing at all, just because the rectifier tube isn't hot enough to work.

Misunderstood SSR. Are the diodes in lieu of SSR? And where do they tack in?
 
Variac was set at 117v during testing
SSR beyond my pay grade unfortunately.
So I could have been testing at 60v? Would the AC voltage reduction be directly proportional to the reduction in DC voltages?
If you go up to about 3/4 wall voltage that should have the tubes operating at their low end and they will pass music. I usually to a thorough check with the variac set at 75% with music playing through the speakers. It should sound good at that level, just down in power. You can easily make your own ssr with an empty octal socket and 2 1n4007 or other rectifier of your choice.
 
And....... if you do use the two diodes/octal socket like Primo suggests, you can make your tests with a lot less than 3/4 wall voltage because you will get instant high voltage from volt 1. You won't be needing enough voltage for the 5AR4 to conduct, cuz you're not using it. Checking negative bias voltage, functionality of bias pot adjustments, all can be done at lower voltage. As can plate voltage to preamp tubes. I didn't realize you had a Variac or I would have suggested this.

I made my SSR out of a broken octal tube, soldering the two diodes right to the pin leads in what had been the tube. This came back to bite me, literally, as I was doing THE SAME tests you were doing on the ST70. I brushed my hand on one of the diode ends on the top side of the amp and zappp I got my high voltage. What I have done since then, since this amp is right on the bench with the soldering station etc, is I solder the diodes temporarily from pins 4 and 6 to pin 8 of the rectifier socket. That way its there with all the other high voltage stuff that I am used to staying clear of. I remove them when I am ready to pop in the rectifier tube.
 
Misunderstood SSR. Are the diodes in lieu of SSR? And where do they tack in?
ssr is solid state rectifier. You can get an empty octal socket for a couple bucks from epay or a tube supply store like Angela or Antique Electronic Supply and just put 2 diodes of over 1000v rating and at least 1A from pins 4 to 8 and 6 to 8. the empty octal socket is like a cup so you mount your diodes safely in the base where they wont be touched unless you stick your finger down there.
 
You can also temporarily tack them across the bottom of the socket, just remember to remove them when you're done testing. Voltages at full line voltage will be higher with diodes, but you can test at low voltage with the diodes where you can't do that with a tube rectifier.
 
All good alternatives.
I was trying to keep it quick and simple. Its reusable, dead to the inadvertent top side touch.
Not too pricey either though you can buy a bunch of diodes for 10 bucks
 
:yikes:

10 ohm....10k ohm

Guess it makes a little bit of a difference.
Are my problems symptomatic of R82, R83 and R84 being 10k in in of 10 ohm?

R81 is a strange looking resistor that looks to be labeled 9 ohm and it reads 9.6 ohm so I left it in place.
Its the large white one upper left in this photo.
9.6ohm.jpg

Maybe this is why one channel was playing? And why I have a pack of 10 ohm resistors left over.

And cause a cap to blow? :dunno:
 
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