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Bringing a flood-damaged ST-70 back to life

ADClosson

ADClosson

Member
Hi all. I've got a previously restored ST-70 that I want to resurrect after a flood a few years back. As you can see, the output transformers and power transformer have some surface rust. The interior, to my eye, looks remarkably well preserved.

My question: what are essentials steps to take before attempting to fire her up? Should I assume that anything is destroyed or out of spec? Are transformers always damaged by water, or might they function as intended? The choke? I assume the tubes are toast.

Many thanks,

Aaron
 

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I don't see any corrosion. Did you personally experience the flood or was it reportedly flooded? It just looks dusty. Only way to know if parts are damaged is to test. You can check the xformers and chokes using a multimeter. resistors and caps are unlikely to be damaged but tubes might be suspect. They can be tested with a tester. Once your pretty sure the parts are ok, you might bring it up with a Variac.
 
dcgibby said:
I don't see any corrosion. Did you personally experience the flood or was it reportedly flooded? It just looks dusty. Only way to know if parts are damaged is to test. You can check the xformers and chokes using a multimeter. resistors and caps are unlikely to be damaged but tubes might be suspect. They can be tested with a tester. Once your pretty sure the parts are ok, you might bring it up with a Variac.
Click to expand...
Thanks for the reply. I personally experienced the flood. I don't know how long the unit was submerged, but that dust and debris are the result of water engulfing the surface of the amp. I'll try bringing it up on a Variac.
 
I assume all the OEM electrolytic caps and germanium rectifiers were replaced before the flood. How long ago was the ST-70 last used, i.e. in fully functional state? No more than 5 years? If so, just give it a good cleaning, including the tube pin sockets and bring it up slowly on a variac while monitoring the total current drawn from the wall plug, as well as the cathode currents of each PP pair. Unfortunately the stock ST70 does not allow you to measure cathode currents of individual power tubes. Be cognizant that in such a PP pair one tube might conduct more and the mate less to give you the correct total across the 15.6 Ohm resistor.
 
spantou01 said:
I assume all the OEM electrolytic caps and germanium rectifiers were replaced before the flood. How long ago was the ST-70 last used, i.e. in fully functional state? No more than 5 years? If so, just give it a good cleaning, including the tube pin sockets and bring it up slowly on a variac while monitoring the total current drawn from the wall plug, as well as the cathode currents of each PP pair. Unfortunately the stock ST70 does not allow you to measure cathode currents of individual power tubes. Be cognizant that in such a PP pair one tube might conduct more and the mate less to give you the correct total across the 15.6 Ohm resistor.
Click to expand...
Yes, all discrete components were replaced previously and the amp was running perfectly prior to the flood two years ago. I'll acquire some EL34s and give it a try, thanks.
 
Have never thought about the affect being submerged in water would have on tubes.....until now. If the pins are not corroded to the extent the vacuum has been lost or they don't make good connections in the sockets, would not they still be good?? Asking the collective.
 
giving the sockets a good cleaning would be worth doing. I use those little dental brushes to clean the pin connectors in the sockets. Inwoukd think moisture would collect there and cause some corrosion.
Its a nice looking amp though.
I had a CB radio I had left in a friend's shop. We had a flood and the rice fields around that shop turned into a very large lake. About 4' of water got into that shop. We had to take a boat to go find another boat that floated away with the trailer attached to it.
I found the radio after the water went down. It was full of bits of rice straw and silt. I based it out, let it dry, and it fired right up and worked.
 
BmWr75 said:
Have never thought about the affect being submerged in water would have on tubes.....until now. If the pins are not corroded to the extent the vacuum has been lost or they don't make good connections in the sockets, would not they still be good?? Asking the collective.
Click to expand...

The pins or wires that pass through the glass envelope are make of Kovar, whose outer oxides of nickel and cobalt will (usually) remain unaffected for decades of exposure to moisture without failure of their metal to glass seal.

Octals, on the other hand, often have bases of hygroscopic material. Mind, even when used in high humidity environments they will readily dry out under operating conditions, which will eliminate any leakage.

Problems can arise due to octals being submerged, especially for extended periods. The entry of salts (NaCl) or rust (Fe₂O₃) between the base and the bottle can cause conductive paths to form between the pins. If you are lucky, and the tube has not already arced between the pins, which forms a carbonized track, you can usually flush them out with clean water.
 
From all the threads that have been made about bringing old crusty, rusty Dynaco amps back to life jus5 goes to show its hard to ruin the darn things for good. :biggrin:
 
As for tubes and water ,the markings on the tube will be ???? but after a short swim they should be good .
I have seen 9 pin tubes ( ala 12 .. 7 ) with the pins rotted off , but those were "stored " on wet cardboard in a verry damp place.

Regards F.
 
ADClosson said:
Hi all. I've got a previously restored ST-70 that I want to resurrect after a flood a few years back. As you can see, the output transformers and power transformer have some surface rust. The interior, to my eye, looks remarkably well preserved.

My question: what are essentials steps to take before attempting to fire her up? Should I assume that anything is destroyed or out of spec? Are transformers always damaged by water, or might they function as intended? The choke? I assume the tubes are toast.

Many thanks,

Aaron
Click to expand...
Tubes are probably ok.
The "hot" item here is the transformers, they might be soaked inside with water trapped. If you apply power they might go bad.
I suggest placing the whole amp in an owen at 60-80 degrees C for a day. This will remove all water. ( heating and vacuum is better but
seldom available)
7199 only needs to be dry. The octals may have water between the socket and the glass, let them dry in the owen.
The circuit breaker might need some 5-56 and excersice to work.
 
ConradH said:
A good bake shouldn't hurt. The transformers are the only thing I'd be really worried about. Can you remove the end bells to see the internal condition and dry it further?
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I agree 100% with Conrad here!

A look under the end bells should be done before applying power to them, any residual dampness very well could prove fatal to them.
 
init4fun said:
I agree 100% with Conrad here!

A look under the end bells should be done before applying power to them, any residual dampness very well could prove fatal to them.
Click to expand...
That's whey I suggested it earlier. I was given an old portable tube RTR.
It had been stored in an old garage with a leaky roof. I salvaged some tubes, power transformer, and output transformer out of it. The power transformer was rusty and had cloth leads. Once I removed the end bells, I found it was pretty crusty inside. I cleaned it up best I could and let it dry out.
It tested good so I used it in a small single ended 6K6 guitar head amp.
You could bake the whole amp to make sure the transformers are good and dry. Or use a heat gun and let the amp sit for a few days. Letting the amp sit in the hot sun ll day should do it too.
 
Good luck with the ST-70. I think you'll be OK.

After Hurricane Sandy, I lost around 15 antique televisions, along with some mint stereo gear.

I decided to hose out and fire up some of the stuff just to see. I had an early eighties 12" black and white RCA tv and a 1976 NEC brand 13" fire up with a pretty decent raster, amazingly. There are pictures somewhere, I can't remember where I posted them.
 
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