Dim-Bulb test limitations!

[Hi Fi and I]

Hi,
What are the limits of a dim-bulb tester?
Let's say I want to test McIntosh MC2300. That amps has some juice I would say.
Would I be able to use dim-bulb with it?
Thanks!

 

[EngineerNate]

Bigger amps just need a higher wattage bulb to perform their function properly.

I'd guess at a certain point you'd start sacrificing the protective ability.

 

[Hi Fi and I]

Thank you!
Could someone go deeper on this topic?
How would one determine what bulb wattage to use for a particular unit?
What units should not be tested with the dim-bulb tester?

 

[Hyperion]

What units should not be tested with the dim-bulb tester?

Valve (Tube) amps are generally better tested using a Variac, but you can use a DBT if you want. Some people have a 'rig' that includes both functions. DBT's should never be used to fault find any amplifiers with VFET OP transistors. I can see how it might give strange results with class D amplifiers but then so would a variac.

Note that the main reason for using a DBT is when you have a runaway current situation, where applying full mains to a unit blows a fuse, or burns components (for example), thus denying you any testing opportunity to find exactly where the fault is. The DBT automatically and instantaneously limits the current making the bulb grow brighter in the presence of a fault, but also mostly limiting damaging current flow while still powering the unit. Choice of bulb wattage is obviously very important, most amplifiers that I come across need a 60w bulb with the higher powered ones needing a 100w bulb, but I might use either bulb wattage with either type of unit.

I also use a DBT as a 'safety net' after any major work on an amplifier, making the first power on after the work 'on DBT' - just in case I have made a mistake, this has saved me a couple of times.

 

[maxhifi]

I never use a dim bulb at all. I just check that all the fuses are in place and correct, and power up - if fuse blows, I replace fuses and power it up with a variac to facilitate troubleshooting the short circuit.

Transistor amps are pretty simple beasts.. usually if fuses blow it's because output transistors have packed it in and shorted. If I get something in with blown fuses, the first thing I do is pull the output transistors and check them, before applying power. If they're bad, then check all the other transistors in the power amps.

I've read lots about dim bulb testers on the internet, but I've never seen a tech use one in a shop. I think it's mostly an insurance against mistakes?

 

[Hi Fi and I]

Before that fuse blows , there could be tons of problems caused!

 

[maxhifi]

Before that fuse blows , there could be tons of problems caused!

Like what? Whenever I get something which eats fuses, it's always been a dead shorted component, which draws enough current to blow the fuse really fast and limit damage to the amp.
I'm sure there's exceptions, I'm genuinely curious where the value is in using a dim bulb tester - has it actually saved anything for you?

 

[audiodummy]

It's saved many fuses while trying to locate damage in semiconductor equipment (switch mode power supplies) for me. Replacing a shorted transistor doesn't mean the "short" is gone - the replaced transistor still causes a "short" because it's still biased that way due to damage in other parts of the circuit.

I don't know how effective DBTs are for tube/valve equipment because the heater/filament draw is significant enough to hide potential problems, and agreed, for these types of equipment a variac and monitoring the current draw may provide more information.

 

[maxhifi]

Ah, I never have had to get into switch mode power supplies.. and if I find a bad transistor, I check every other one which is DC coupled to the one which failed. I do however have spare lamp sockets, and a huge stash of incandescent bulbs I hoarded when the government decided they were about to become illegal to sell.

 

[gslikker]

Myself, I also do not use a DBT, but I can understand people do. It just depends on habits, ways one does things, the equipment to be tested and other means of testing available.

For SOME main power amps boards testing, the following is superior to DBT or Variac...........

 

[audiodummy]

I prefer the DBT over using a resistor mainly because it gives a visual indication on how much current the circuit is still passing.

I also use car bulbs for secondary/low voltages.

 

[Aerobat]

Try a lower wattage first and if the bulb blows go higher.

 

[kaplang]

I use a Variac with amp meter. If while turning up the voltage on the Variac the meter starts to read high I back off the Variac. I have been told several times by some AK members that a DBT is better but I don't think so. It's just another method.

 

[Love Vintage]

Thank you!
Could someone go deeper on this topic?
How would one determine what bulb wattage to use for a particular unit?
What units should not be tested with the dim-bulb tester?

Maybe this will answer your question:
https://antiqueradio.org/dimbulb.htm

 

[Bill Ferris]

Never in 44 years of servicing electronics used, or felt the need to build one, and until I started lurking here over 3 years ago, and finally joining this site a year later, I never even heard of, nor knew any technicians I worked with, or met, who mentioned, or used one.

Not saying DIM`s have no benefit, or value, just turned out, one wasn`t needed in my equipment servicing needs/procedures..

A device that I built, in the late eighties, was in a sense similar, and was to take fairly high in secondary output voltage with a center tap, surplus power transformer connected to a high voltage/current bridge rectifier, two surplus computer grade beer can sized capacitors, and feed those bi-polar outputs through snap in fuse holders that I installed 6/8/12volt fuse lamps for glow observation when connecting this 100 volt DC max power supply to an multi- transistor power amp output section removed, and or electrically disconnected from the chassis, if not removable, for checking it out for shorts, before, and after any rework..

All parts mounted on a 1`X1` scrap board with a secured old scrap rack handle to carry it. from bench to bench, as needed.

I plugged it`s fused 120volt power cord into my variac, and slowly raised the line voltage feeding it, while watching the variac`s amp meter, and the fuse lamps as I raised the power supply`s output +/- voltage, and could easily find any voltage sensitive leaking outputs by feeling them getting warm at well below their operational voltage ratings, or any TO3 style transistor mounting insulation failures/compromise`s ..

It was very handy for reducing the time for checking out all the stressed out/abused rack mount high wattage PA power amps brought to me, by local rock bands, for a quicker turn around, as they usually didn`t have spare backup power amps.

Go with whatever your comfortable with..

Kind regards, Billy Ferris