One more Eico ST70 rebuild thread

NJ
Voltages at R68 and R99 was -56v and -122.
But....catastrophic failure!
A 40/500 cap blew. Missed me but shrapnel of paper and oil hit about everything in sight. Not to mention the smoke from the explosion and power transformer. Guess that when they go...they go.

Its just not my day. And someone said I was just about home.

Assuming all else is fine (smoking transformer not a good sign), I have a 50/50/500 can that someone once said was ok in lieu of the 40 and 20/500 I mounted under the chassis. Should I go that route or order a 40/500?

Rwood
What did the readings on pin 6, without tubes, amp on tell you?

old...and wore out

popped cap.jpg
 
Sorry to hear you got zapped. DC is awful. Just ask George Westinghouse.

If you had the amp on end so you could turn the pots, I don't think you have to do that - if I recall a small flat blade screwdriver will work on the back side of these pots. Safer for these tests to keep the amp upside down. And one hand in your pocket, like Allanis Morrisette. Seriously, I hope you are OK and that your amp is, too.

Ch1 -14v, -19v
Ch2 -16v, -16v

If you can't get Ch 1 to balance out, then you've got some bad resistors in that circuit - power down the amp and ohm them out.
Ch 2 looks good but if you have the range I would start them at about -20 VDC on each (for both channels actually) which might start out biasing the tubes cold, but then you gradually lower the DC voltage while you are monitoring current draw on the cathodes, and shoot for your 38mA. Return to your balance pot, which will now allow you to compensate for mismatched tubes. Do this on both sides and I believe your power supply voltages will fall into place.

Always better to start out biased a little cold and then gradually increase the draw, than vice versa, as you could damage the tubes if its too hot.

But. Be. Careful. (As if you need us telling you now)
 
A quick diversion....here's mine from a couple nights ago. A pair of '63s:

67DAF502-2BCE-4733-94C2-081334B1ABA2_zps4rvjwdab.jpg



and here's how it started out:

056_zps78cc6ccc.jpg
 
Mine looked better when i started.
I guess i'm lucky. I haven't had a cap go into meltdown on me (failed yes). Maybe you were too much of a load for that cap to take:D
 
Sometimes its not your day. Maybe you should take the rest of the night off. I think you earned it. Sometimes , for some unexplained reason, everything just falls into place tomorrow.
 
NJ
Sent PM, cant let the ex know where Im hiding.
Ears ringing and face stil tingling. One hand on the chassis, the other on something that bites. Chest a little tight. May need to take a break. Hope the data yields something.
Next time i'll measure thru the socket.
I don't know where my ex is (i prefer it that way). Since i blocked her on my cell i haven't heard from her in some time. :biggrin:
 
If you feel up to it and still have a working meter, i have some unplugged, powered off tests for you (tomorrow is fine, or the day after). With output tubes pulled, Do you know how to bleed down the power supply? Today was exciting enough. If not please ask
I'd like you to give resistance measurements of R67,68,69,70,71,72,73,74,75,76,77,78, and to beat a dead horse R81,82,83,84
Should look something like this

R67, 68,69,70 150K

R71,72,73,74 4.7K

R75,76,77,78 100K

R81,82,83,84 10Ω
 
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Thx. I'll check them. Have plenty of time as the cap took out a leg of the pot. VOM works (contrary to popular belief) but stand broke off. Was able to pull out the snapped banana jack and all is good.

Anyone have a good source for a 50k pot, 7/8" diameter snap in?

You are right about giving up for the night. Was replacing power cord on the Scott and it bent all new ecl86s when it slipped off the foam pad. Three strikes.

Nightcap is coming early.
 
After seeing what Rwood started with my problems look minor. So proceeding. Though I do covet those knobs.

Does the 40uf cap blowing up help identify the circuit with the problem? Or the PT smoking?
Or voltages at R68 and R99 being -56v and -122v just prior to cap failure?

Maybe this should have told me something:
http://audiokarma.org/forums/index....2/&temp_hash=8f78c87d31278a5d15090a7a42133698

NJ
Will check resistors today. Can they be checked in circuit or will I have to pull some?
 
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Caps exploding usually mean they were in backwards or over-volted. Possible it was just a dud part but I'd confirm polarity first. if that is in the bias circuit, the + side typically goes to chassis.
 
^^^
Good point
Did you originally change the cap in question.
The bias caps are (+) to ground. Those voltages sound normal. (I wish i could find that sams again.
It had voltage labels everywhere.
If that cap was original it is 50 something years old and not out of character to fail and should have been changed as part of this project with his siblings.
I'm pretty sure with the tubes pulled you should be able yo to read those in circuit.
If the resistance is higher its bad (a lot higher)
If I'm wrong and resistor reads lower lift a lead to double check. +/- 10% is fine
 
As far as your PT
That failing cap likely introduced a short to the transformer. I'm sure it didn't appreciate that.
I can't promise but I'm cautiously optimistic that its ok
 
It was a new cap replacing the 40uf side of the 500v multi-cap. Wired neg to ground. I would assume that the tabs on the can are + and the - side of the cap was the chassis tab.
 
Okay
Good news (maybe)
Bad news
Go to bottom right corner of schematic
Follow blue off PT, CR 1, to junction of 50uf 150v cap and r98,r99
They go to what look like c40 and c39(?)
All three of the just mentioned caps should be (+) to ground. The bad news is if you installed reversed already you should replace them even if they're new. The good news is once you do , (assuming nothing died during the cap event ) is (I'm reasonably confident here) that your bias issue should be cured and probably your voltage issues now the output tubes will be drawing current. I'm keeping my fingers crossed.
 
More bad news for me...those all have pos to ground. Being a swapper, I make sure I replace exactly how it was. Now when I make mods...thats a different story. But those caps did take some abuse: before
cornercaps.jpg
after
popped cap.jpg
 
So you're saying that wasn't one of the 3 caps mentioned?
What does it mean that they took abuse?
But, those three caps have positive to ground? If that's so that's good. We can look elsewhere:thumbsup::idea:
 
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