Calculating voltages when using a DBT?

[ghazzer]

Assuming that a receiver/amp is working correctly, no input, and the volume is at zero, is there any way to calculate (approximately) the power supply and rail voltages when connecting it through a DBT?

I know that this would be very helpful to me, just as an indicator of relative health. It would give me warm fuzzies when first connected, or when changing from say a 40W bulb to a 75W bulb.

Known values would be the total wattage of the receiver, “normal” voltages shown in the schematic, and the size of the incandescent bulb in watts. DBTs with CFLs, LEDs, halogen, or other bulbs are out of the picture, for now.

 

[Blue Shadow]

You can find out the voltage the DBT supplies to the UUT and I would think that the transformer and the power supply would use that voltage with the same ratios as it does mains power. You give it half voltage in and get half voltage out is what I think. I'm gonna check back for those that will come by with the correct answer. Of course, losses might be a different percentage but only a few volts, maybe.

 

[whoaru99]

I think the potential roadblock to a pre calculation is understanding the reduced voltage load presented to the DBT, and the resistance of the DBT at that load. I think those two things would be necessary to calculate the input voltage divider.

I also think the listed/back panel wattage of the receiver is a red herring for this purpose because it typically does not reflect idling power consumption.

 

[awillia6]

I also think the listed/back panel wattage of the receiver is a red herring for this purpose because it typically does not reflect idling power consumption.

Agree on listed wattage irrelevancy, but note that detailed spec section on gear in owner/service manual often lists idling power which IS of use in setting internals (i.e. check that Kill-A-Watt meter shows close to rated idle power with biasing correctly set, or vice versa if no bias info given.)

 

[gadget73]

Never calculate what you can measure, especially when its easy to measure like rail voltage.

Honestly a variac with a 0-100 scale is the easier way here. 50% input voltage should be darn close to 50% rail voltage. It won't be exact, and some things behave poorly at significantly reduced input voltage but its a decent ballpark.

 

[ghazzer]

If you just started troubleshooting a receiver, put it on a DBT with a 40W bulb and fired it up, it is very easy to measure rail voltages. My main question is how would one determine if the measured voltage is what it "should be".

Not everyone has a variac. I have a $5 one from a yard sale but it is worth more as a door stop.

Blue Shadow may be closer to a useable answer. If you can measure the "line" voltage between the DBT and the UUT you should be able to use a simple ratio to determine expected dc voltages inside the receiver. I wonder if that would be linear?

 

[Blue Shadow]

Yes, I was waiting for folks to say that isn't right but maybe it is close enough for some beginning troubleshooting. Since you know the voltage that you are supplying to the uut, by measuring it either at the other outlet on your DBT or from the unswitched outlet on the back of the receiver, you can see if the rail is a similar percentage of that as it is to the line voltage/schematic rail voltage. Within a few volts is all you can get but that might be what you are trying to learn.

 

[gadget73]

the ratio method works, and should get you close enough to use. What I've found with split rail supplies though, if its equal but opposite polarity most likely its just fine. When there is a fault you lose one side or the other, or it will be very uneven. Max voltage will be less than the filter cap rating, but usually not by a whole lot. Often I've found its within 10v or so of the cap's marked voltage.

 

[whoaru99]

Is DBT often used when measuring the voltages on the secondary side of an amp/receiver?

I've generally looked at it more as a go/no-go gauge to check for presence of a major problem, e.g. shorted output(s).

 

[merlynski]

The Physics of the incandescent light bulb: It has a rising resistance as it heats up which makes for a great power-on ballast in a Dim Bulb Tester, but it is a poor choice as a current limiter during functionality testing.

 

[BinaryMike]

Not only is the resistance of an incandescent lamp nonlinear with temperature, but ANY significant resistance in series with the AC line tends to invalidate operating voltage readings. Some of the signal processing circuits are designed for constant current flow over a wide supply voltage range and others are allowed to vary wildly. Bottom line; unless you understand circuit operation in detail, operating voltage readings taken with reduced supply voltage have little value. It's best to clear major faults and get up to normal operating operating voltage as soon as possible in the troubleshooting process.

One of the sneaky tricks used by experienced technicians to troubleshoot high-current faults is to set a variac for just enough line voltage to get a fairly heavy but not damaging fault current to flow, and then feel components to detect overheating. This procedure can very quickly close in on the failed components, especially in SS output stages. You definitely need an ammeter in series with the variac. A 1A range is about right for this job, so the ammeter has to be protected and needs a bypass or high current range option.

 

[ghazzer]

Hmmm, I didn't get those same conclusions from the article. Sounded like there are many changes occurring in a light bulb at turn-on but it stabilizes PDQ.

My original thoughts were to be able to make a quick assessment of a totally unknown UUT by putting it on a DBT and having a reasonable expectation of what the voltage(s) should be. If they weren't reasonably close to expected values I would assume that there is a P/S problem or a loading issue, and it may be wise to disconnect power until one does a little more investigating.

Now it is sounding as if the correct MO is to bring the UUT up on a DBT just long enough to see that the light gets bright, then dims to some degree. Repeat that with progressively higher wattage bulbs, then take out the DBT and proceed with a different course of troubleshooting.

The more I learn, the less I know . . .

 

[merlynski]

The more I learn, the less I know . . .

Ah, yes, the essence of wisdom!
My Son asked me one time how I "got so wise," I told him 'I made a lot of mistakes!'

Now it is sounding as if the correct MO is to bring the UUT up on a DBT just long enough to see that the light gets bright, then dims to some degree. Repeat that with progressively higher wattage bulbs, then take out the DBT and proceed with a different course of troubleshooting.

What I have learned from the AK gurus this is correct. If the power supply voltages are "balanced" and within -20% (roughly, no load, no input, volume turned all the way down, etc.) with a dimmed bulb (100 watt or so) you can go to full mains voltage and begin serious functional trouble-shooting. If you do not have a dimmed bulb you still have a problem.and need further diagnosis before going to full mains..

 

[triode17]

Assuming that a receiver/amp is working correctly, no input, and the volume is at zero, is there any way to calculate (approximately) the power supply and rail voltages when connecting it through a DBT?

I know that this would be very helpful to me, just as an indicator of relative health. It would give me warm fuzzies when first connected, or when changing from say a 40W bulb to a 75W bulb.

Known values would be the total wattage of the receiver, “normal” voltages shown in the schematic, and the size of the incandescent bulb in watts. DBTs with CFLs, LEDs, halogen, or other bulbs are out of the picture, for now.

If the receiver is "working correctly", why do you need a DBT to begin with? Plus, just because a unit has it's proper rail voltages, doesn't mean it's working OK. What I do, is connect the amp to a load , a signal gen and a scope. Then, I turn up the volume control and watch the scope as i Variac it up. I want to know it's acting like an amp. first and foremost. Then, I will measure the rest of the voltages, etc.

 

[ghazzer]

My original thought was to predict what the "correct" voltage levels should/would be with a given wattage bulb. If values are within some tolerance you could proceed. If not, there is likely a problem with the transformer or P/S (or the way you set up the DBT).

 

[merlynski]

Is DBT often used when measuring the voltages on the secondary side of an amp/receiver?

I've generally looked at it more as a go/no-go gauge to check for presence of a major problem, e.g. shorted output(s).

^^ I agree with this, IMHO a DBT is a pass/fail indicator, it is not directly useful for more exacting troubleshooting, there are too many variables. If the bulb flashes bright then goes dim you (probably) do not have a major overload failure, go to full mains voltage and perform more definitive troubleshooting with power on. If the bulb stays bright you (probably) have a major overload failure, immediately turn power off and perform more definitive troubleshooting without power input. There are many posts on AK with specific trouble-shooting tips and procedures for both conditions from excellent experienced techs.

 

[ghazzer]

Yes, I have been reading those posts on AK for many years, and have already had conversations with these gurus. Thanks.

I'm guessing that your answer to my question is "no".

 

[rcs16]

I have not read the whole thread. The DBT acts as a safety or overload fuse. The lamp is is a PTC resistor. As more current flows, the element heats up and the resistance increases.
The issue is if the lamp wattage is to low and you have a heavy load, if it is too low, the cold R will be too high, thus the voltage drop will be too much and you could stress/damage the load equipment with a low ACV supply.
the trick is to only use it momentary to determine if you have a shorted load/equipment. I have a bypass switch on my tester to easily bypass the lamp and also have a switch for AC.
The VA rating on the back of the equipment tells you what you need to know and what should be the minimum lamp wattage.
For 20VA you would be able to get away with a 40W lamp.
For >100VA, use a 150W incandescent lamp. Only use a incandescent lamp as any thing electronic does not behave as a lamp will.