Please note. This is homebrew test gear. It meets no safety standards. But has a specific use to me in this hobby.
I periodically receive questions about Dim Bulb Testers (DBT), their application, how to make one. The most difficult thing to explain is how a good radio or a radio with a serious problem should look on the DBT.
I will not go into detail on when to use a DBT or include a step by step procedure to build one. There is plenty of documentation on the web about these topics. I hope to include a few tips that I use when building my DBT. Email me if you have any other questions that you feel I may be able to answer.
See how a Dim Bulb Tester operates.
My Dim Bulb Tester
The trick to a DBT is the current limiting characteristics of the incandescent lamp. And it provides an intuitive and visual indication of current flow. No interpretation of a voltage and current meter needed! Simple and sweet. I use it religiously before and after each and every restoration. It has saved me time and headaches.
This is a picture of how my DBT is wired.
Karen, a Journeyman electrician, emailed me. She pointed out a modification she made to the wiring of the Dim Bulb Tester to properly accommodate equipment dependant on a polarized plug. See below her email explanation and pictures.
Thank you to Karen. Keep us posted on your Hammond and Guitar amp repairs.
Make A DBT with an Extension Cord and a lamp socket.
I would NOT recommend this DBT for repeated use or regular bench duty. Build a more robust tester with stronger parts like the one above. It is great for the occasional or once or twice use.
The objective is to insert the lamp in the HOT side of the extension cord. This places the current limiter (lamp) in series with the load (radio). Use the radio's power switch in place of the toggle switch in the above unit. Or use the all ready included "Twist on Twist Off" switch. The lamp its self. But don't burn you fingers....
AA5 and HOT chassis radios and the DBT.
AA5 (All American Five) tube radios, "Hot Chassis" (no power transformer) and other radios that have no electrical isolation (i.e., Zenith Transoceanic) from the line (power cord) are dangerous when opened up on the bench.
When a hot chassis radio is open on the bench during servicing present additional hazards. The use of the DBT or a Variac must be incorporated with an Isolation Transformer.
If the hot chassis radio is closed up in "user ready physical status" then I do not use an isolation transformer with the DBT.
Click on the picture to see "Phil's Old Radio's" example of a DBT:
I hope this helps a bit.
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