Signal generator help

[martyn_mcg]

Hi,

I have a BM5000 that i have recapped and changed the low voltage supply presets but i want to also change and adjust the volume, tone and bass presets on the tone board however i don’t have a signal generator that is required for the adjustment of these.

In the B&O service manual it requires a 1kHz 175mV signal for adjusting the volume levels. I am assuming that 175mV refers to the amplitude or is it RMS?

The very cheap function signal generators available do have adjustable amplitude but they don’t display the value on screen and without some fancy oscilloscope I don’t know how i would set it correctly; I only have a multimeter and a basic component tester.

Can anyone advise how i would be able to calibrate the amplitude output on a signal generator with a multimeter? or even suggest a reasonably cheap signal generator that has the amplitude on screen?

If anyone could advise it would be much appreciated as I cant really justify a high end signal generator and I really like to do the work myself and learn as i go.

Regards

Martyn

[Glitch]

Inexpensive (or cheap) is a very relative term. How much are you looking to spend?

Check out the Digilent Analog Discovery 2. It is an all-in-one package that has a scope, signal generator, logic analyzer, and variable power supply for $400. All of the functionality is limited compared to much more expensive, dedicated bench equipment, but it does a good job at what it does.

Glitch

[Mark-sf]

It generally refers to RMS for such adjustments and you can use a multimeter wit RMS support to measure.

[martyn_mcg]

are you able to confirm that the voltage reading for amplitude in the B&O service manuals is in fact in RMS?

the manuals don’t actually specify and i am assuming that is because all their technicians at the time had the same equipment….

Thanks

Martyn

[Mark-sf]

As a former B&O service tech, I have always used RMS as the AC voltage measurement unless it was annotated as p-p. This is standard across the industry because it is equivalent to a DC measurement for example when computing current draw.

[martyn_mcg]

Thanks for getting back to me.

There are some very cheap signal generators available on eBay and amazon and the amplitude of the sine wave is adjustable but is not displayed on the unit. they are called an FG-100 DDS and are available for about £20.

In theory i should be able to adjust the amplitude and set it using the RMS function on my multi-meter and this should be sufficient to perform the calibration of the volume, tone and bass as per the service manual? i know its probably very crude but given my lack of better equipment it would be better than nothing?

does this sound plausible?

Thanks again for your help,

Martyn

[Glitch]

I found this review while doing a quick search, https://hackaday.com/2018/05/17/review-fg-100-dds-function-generator/

What you propose above will most likely work, especially if all you are hoping for is “better than nothing” on a measurement that isn’t critically sensitive.

Personally, I wouldn’t trust that (or any similar) device unless I hooked it up to a scope and tested it for the intended usage. Please don’t misunderstand me, I actually like those cheap devices. I have a bunch of similar devices in my “parts box” ;-). I’ve also found a few “gems” that are very useful. Just keep in mind there is a usually a very good reason that they are dirt cheap.

Buying decent quality tools makes any hobby more enjoyable. Junk tools can be very frustrating, especially when one is learning.

Do you trust that your multimeter is accurate enough to calibrate your signal generator?

Glitch

[Mark-sf]

Such a device will work for this particular setting assuming your multimeter supports RMS readings to 1K. An alternative would be to use an app on your phone or tablet such as AudioTools that actually provides a cleaner output and offers more functionality.

[martyn_mcg]

Thank you for the replies,

I purchased one of the little signal generators and hope to give it a go soon, i will test the amplifier as it is and see if the voltages i get are close to the specification in the service manual to prove it out before i swap out the old presets just to be sure it works as i hope.

Thanks again,

Martyn

[Author]

Posts