Dim Bulb Tester

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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.  

 

Good Radio on DBT

Bad Radio with a short circuit.

 

 

 

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.

 

IMG_2462.JPG (201300 bytes)Basic parts.  Add a two gang electrical enclosure, two gang cover plate and a power cord. IMG_2463.JPG (137023 bytes)I had these parts left over from a remodeling project.   IMG_2464.JPG (127897 bytes)

I purchased several of the plug in lamp sockets to use multiple wattage lamps.  I have this unit attached to the wall with dry wall screws.  

 IMG_2476.JPG (149770 bytes)Snap off this strap on the copper colored screws or the HOT side of the outlet (short slits). This puts the outlet into series.  ONLY connect wires to this side of the outlet. One screw to the switch and the other wire to the Neutral side of the power cord.  

This makes for a compact and simple DBT inside a Double Gang box.
IMG_2478.JPG (187371 bytes)Leave this strap on the silver screws or Neutral side of the outlet (Long slits). IMG_2481.JPG (171478 bytes)I used a salvaged power cord.  Connect the thin blade (hot) to the switch.   IMG_2480.JPG (71657 bytes)This is a Variac or adjustable autotransformer.  This and a voltmeter and amp meter replace the DBT. IMG_2496.JPG (195418 bytes)I use a DPST switch in my DBT.  This disconnects both hot and neutral wires to the power cord.  IMG_2497.JPG (209702 bytes)There are 4 screws on the DPST switch. 

 

 

This is the circuit that I use in my DBT. 
The DPST switch completely disconnects the radio and load lamp from the live power outlet.

 

 

 

Make A DBT with an Extension Cord and a lamp socket.

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.... 

I would NOT recommend this DBT for repeated use or regular duty on a work bench.  Build a more robust tester with stronger parts like the one above.  It is great for the occasional or once or twice use.  

IMG_2484.JPG (208782 bytes) Two basic parts are needed.   IMG_2486.JPG (216357 bytes)Locate and identify the HOT wire.  It is typically in line with the THIN blade. IMG_2488.JPG (193924 bytes)Carefully cut the Zip Cord wires apart.  Do not slice off the insulation or nick the wire.  IMG_2489.JPG (201472 bytes)   IMG_2491.JPG (188312 bytes)Cut the HOT wire and strip back the insulation.  
IMG_2492.JPG (168944 bytes)Find the HOT screw (copper colored).  Connect the wire from the male bladed end of the extension cord here.   IMG_2493.JPG (185050 bytes)Zip tie the wire to the lamp base to provide strain relief. IMG_2494.JPG (185656 bytes) IMG_2495.JPG (164851 bytes)Install appropriate bulb wattage.  About 1.5 - 2 times wattage of the radio.   

 

 

 

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. 

 

IMG_2498.JPG (190537 bytes) Typical TV repair shop isolation transformer.  The bottom two outlets are the isolated outlets.  The top are not. 

There are much simpler isolation transformers with a singe outlet.  

 IMG_2499.JPG (101616 bytes)The DBT plugged into the isolated outlets of the isolation transformer.

The Varriac (below the DBT) must also be plugged into the Isolation Transformer to provide electrical isolation.   

 

Click on the picture to see "Phil's Old Radio's" example of a DBT: 

   Phil also has included a schematic. 

 

I hope this helps a bit.  

 

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