Mystery ST-120 Problem, Solved

I listened to my ST-120 this morning for 4-5 hours. Switched it off and back on about one hour later. After it re-warmed a bit I decided to check the bias voltage. The two left tubes read a little high at .525v (.500 normally). Then probing right front tube bias and 0.00v! I looked at the tube and the heater was stone cold!

See if you can deduce the reason why after the following symptoms.

- No visible heater glowing on right front KT-120
- Right rear KT-120 heater looked OK
- So do the right phase inverter and center tube heaters.
- The Rt front tube heater pins 2 - 7 measured 1.9 ohms DC and no different than another KT-120
- Swapping right front/ rear tubes and no help
- Pulled the tube and measured heater voltage (6.6v AC) on right front tube socket pins 2 & 7. (This one had me fooled)
- Retention all octal socket pins (no help)
- Threw a KT-88 in right front socket and (no help).

Time to open up the patient and have a look. And what I found was a classic cold solder joint on R/F pin 2 socket. Made enough contact to measure voltage with a high impedance meter. The down stream small signal tubes had a connection via enough solder to bridge the two wires. But the wires were losing contact with the socket pin. Some wire redress and solder and good as new.

Some sloppy wiring visible.
In many years and thousands of items, I have never done a complete examination and *not* found a soldering fault of some nature - including in kits of my own building.

It has worked for almost 5 years. But I thought later that probably installing tubes over the years moves the pins around and a weak solder joint will eventually fail.

Just curious but that red wire about an inch away from the socket pin you point out looks like the insulation is melted away exposing copper.

Good eyes... The insulation melted on the red wire is due to another repair on the socket replacing the 10 ohm cathode resistor.

I have since started using Kapton tape to mask components in delicate, cramped area. Kapton tape doesn't melt and provides a shield from wandering soldering tips.

Peter W. wrote:Some sloppy wiring visible.
In many years and thousands of items, I have never done a complete examination and *not* found a soldering fault of some nature - including in kits of my own building.

The reality we older geezers must face is that our eyesight ain't what it used to be, no matter how well-corrected it is by lenses & etc. Much brighter lighting in the work area than what used to be adequate will help reduce the slop factor. So will a second pair of younger eyes checking your work.

The reality we older geezers must face is that our eyesight ain't what it used to be, no matter how well-corrected it is by lenses & etc. Much brighter lighting in the work area than what used to be adequate will help reduce the slop factor. So will a second pair of younger eyes checking your work.[/quote]

Three articulated lights over the workbench - one with a 4x optical grade magnifier. And point-source LED clamp-on light at hand. Yes, light is good!

I hear you Deepee and Peter.  My old eyes need help.  I bought this microscope a few years ago and the best investment for working on electronics especially fine pitch parts.

arledgsc wrote:Good eyes... The insulation melted on the red wire is due to another repair on the socket replacing the 10 ohm cathode resistor.  

I have since started using Kapton tape to mask components in delicate, cramped area.  Kapton tape doesn't melt and provides a shield from wandering soldering tips.  

Have you considered PTFE (aka Teflon) tubing?

Yes I have Telfon wires and are wonderful. Easy to strip insulation, and resists burning/ melting. Good stuff. Parts Connexion has a good house brand PTFE hook-up wire. Ag plated, lots of gauges. and by the foot.

http://www.oldworldaviaries.com/text/styles/teflon.html

Also cats & dogs.

Peter W. wrote:http://www.oldworldaviaries.com/text/styles/teflon.html    

Also cats & dogs.

It's why I only cook with cast iron. Could be problematic as an insulator, though . . .

Cold solders are a pain. I found one myself after using my amps for over a year. I noticed that one of the power tubes was not turning up. Re flowed the solder and good to go!

Can't recall if it's in the VTA Destruction Manuals or just sort of passed-along wisdom from guys on this board, but even if they're working great, after the first year or so of use, turn them upside down and re-flow the solder, esp. around the sockets and ground lugs.

I had the same right front tube socket not light off the filaments this morning.  Pins 2-7 measured 6.6Vac.  Must be pin contact so I cleaned the socket pins with TechSpray flux remover and it appears the pins had some oxidation.  Re-tensioned the pins again.  Working OK now!  

But you can see see the oxidation removed on the end of the pipe cleaner.   Pins swabbing will be on my next maintenance schedule.

I've gotten into the habit of crimping tube tabs after inserting the wire and then soldering it. Extra mechanical strength helps. Trade off is that it can be a bit of a PITA if you have to change the wiring,but how often does that happen?