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Eico ST40 Balance Control

Sure, I only mentioned it so the OP has an idea what is involved in testing it. Essentially the hope is to provide information in order to charge the bullshit filter. I've been fed and have heard of people being fed some absolute nonsense from people who don't really understand tube gear.


Maybe matched output transformers assuming that an original isn't going to turn up?
 
It's funny because I really didn't hear a huge difference in volume other than the balance control weirdness. In fact yge amplifier sounded really great. Just in case I need to go shopping, is it really necessary to purchase two OPT's or can I cheat and just get a single replacement?
 
Dandy said:
All quite within the capabilities of a tech. And why two 'matched OPTs'? Is it not possible to get a replacement that matches the one good one you already have?
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Sorry, just saw this now. This is his exact quote:

I see that there are NOS transformers available to purchase online but most of them are used pulls from other equipment and usually the transformers are matched for per unit production at the factory
There is a chance that any used-pull transformer may have much different LCR readings to the one that is still working and thus you will not achieve an AC balance at OP transformer level. The output signal will not be the same. It is totally your choice if you want to use a pull or a aftermarket type... there are many aftermarket types and some are much better built than the originals. If you choose aftermarket types then you will need to purchase 2 pcs. You are welcome to acquire the parts yourself.
 
Thanks for the full story. In my book, once you have to purchase two replacement OPTs, you're replacing the amp. I still don't understand him when he says that the NOS transformers are actually used pulls from other equipment.

Would it be an option two aftermarket replacements and the resell the good original to recover some of the cost?
 
Dandy said:
Thanks for the full story. In my book, once you have to purchase two replacement OPTs, you're replacing the amp. I still don't understand him when he says that the NOS transformers are actually used pulls from other equipment.

Would it be an option two aftermarket replacements and the resell the good original to recover some of the cost?
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No, there's no darn way I'm going to sink that much money into this thing. I'm still holding out that the OPT isn't badly damaged because it was definitely still doing its job when it entered the shop.
 
s-petersen said:
Secondary taps are the leads going to the speakers, primary go to the tubes
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Right. OK. So if these are damaged they can't be repaired? I just want to be absolutely sure before shelling out money to have one or both transformers replaced. Because I honestly think I'll end up selling it "as is" or parting it out before do this. My question is if there are no open windings or shorts how are the "damaged secondary leads" affecting the sound? Correct me if I'm wrong but it's possible that there could be some damage to these leads with no noticeable affect. I'll direct all these questions to my tech tomorrow.
 
The speaker leads connect to the secondary winding inside the transformer

It may help if you ask the tech how he tested the transformer to arrive at the defective diagnosis. Did he compare it to the other one?
 
The secondary winding to the speaker is tapped at 4, 8 and 16 ohms. He's saying that there is a problem with one of those connections. If so, you could ask which one, and are the others okay? (And sorry for the missing words in my last, late night, post.)
 
Transformers are usually not matched, other than both having the same part number. They pulled them off a shelf, there was not any special involved testing process to make magic happen in gear like an Eico. Maybe on really esoteric high end stuff you might see that claim, but anything sold as a kit was not that high brow.
 
Just thinking about transformers. If, hypothetically, the OP were to buy new aftermarket replacements, would he go for ones with UL taps?
 
Dandy said:
Just thinking about transformers. If, hypothetically, the OP were to buy new aftermarket replacements, would he go for ones with UL taps?
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Dandy said:
The secondary winding to the speaker is tapped at 4, 8 and 16 ohms. He's saying that there is a problem with one of those connections. If so, you could ask which one, and are the others okay? (And sorry for the missing words in my last, late night, post.)
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Apparently they're all FUBAR'd.
 
I'd get it back since it was working and get someone else to look at it.

Even better learn to diy that way you know what's going on.
 
I think I'd forgotten how ugly this amp really is! I hooked it up and yes, there is a very definite volume difference between the two channels. I'm pretty sure I'll be shopping for a new OPT this week and I'm gonna take gadget's advice and buy a single one.
 
I think I'd still go back in and confirm the diagnosis myself. Even without extra involved test gear, you can compare the transformers against one another to see if they come out close. Checking the DC resistance across the windings might even be enough to tell you about a serious problem.
 
I have just acquired two identical Eico ST-40 amps. Believe it or not both work fine, except one weird issue. Both have the same quirky issue. When you turn the balance all the way to the left or right both channels continue to play. The only difference when moving the balance is some bass and treble contrast differences. Since both amps act the same I am wondering if someone may have bridged these to mono and were using them as two mono amps for stereo. So the questions are: can these be bridged and if so what is the process to unbridge? If they can not be bridged, what would be causing the failure to balance left to right like any other traditional stereo amp?
 
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