# Calculating voltage drop when using Dim Bulb Tester

Discussion in 'The Projects Forum' started by redrooster01, Aug 28, 2015.

1. ### redrooster01 Thread Starter Member

Jul 15, 2014
85
1
I have an oscilloscope that has developed an arc in the high voltage power circuit and the only way I can do voltage checks without any sparks and damage to diodes is to use a Dim Bulb Tester with a 42 watt bulb in series with the 240v 50hz mains supply.The service manual for this oscilloscope says that V Line 1 is supposed to read between +209v and +231v,with the DBT I get +124v? V Line 2 is between +114v and +126v. I get +76v using the DBT? etc... Is there some way I can calculate the proper voltages from the readings I get on the DMM, as if the scope was plugged into the mains?

2. ### MrAl Distinguished Member

Jun 17, 2014
3,721
789
Hi,

A very long time ago i had an oscilloscope with a sparking problem too. Not sure if it is the same as yours, but my scope would shoot the circuit under test with a spark which of course would destroy some circuits. I lost a very nice calculator that way.

When examining the scope itself, i found that the high voltage transformer was the problem. There must have been a short from one of the windings to a lower voltage winding or something, i never cleared that up, but replacing the transformer did the trick.

I could not get the exact transformer, so i had to use two or three secondaries from lower voltage transformers in series to get to the full required voltage. I cant remember the exact voltages now, but say it was 1000 volts AC for the original. Then i might have connected three 300vac transformers in series (the secondaries in series, the primaries in parallel). That would give close to the 1000vac required and then the scope works again although it takes three rather large transformers. Back then i had a lot of tube stuff rather than transistor stuff, so i had transformers pulled from old tube equipment and they always needed high plate voltages like 300 or 400vdc so the transformers had relatively high output secondaries. The current requirement isnt much however, maybe 10ma or something, but i cant remember that either since this was 40 years ago

Now that i think about it, i think it may have been the high voltage secondary was shorted at least in part to the filament winding. If that was true, i could have used the same transformer but just used a different transformer for the filament winding. If you have this problem you could try that instead perhaps.

3. ### redrooster01 Thread Starter Member

Jul 15, 2014
85
1
Sorry for the late reply,I just found it? It had been moved by a mod,I was given an alert but it was just an obscure exclamation mark? I wouldve thought an alert was an email! I dont think its the flyback transformer because Ive realized that it arcs when a FET blows thats connected to both primary and secondary Ive replaced that same FET 4 times now.Thats why I need to calculate the voltage drop when using a dim bulb tester to find out whats happening? I tried to upload the circuit diagragm but it wouldnt work?