Hi there,

I am hoping someone can help me. I have just installed some new Trimpots into my circuit for an old B&O Beomaster 1200.

I know that the system works as I have had it working, and I cam changing the trimpots because they are very old and I am getting a crackling scratchy noise through the speakers.

I have attached some images from the manual with regards to adjusting them.

My confusion is when taking the voltage reading across the resistors.

Am I right in thinking that I am to take the Voltage measurement across the corresponding resistors within the images. So in image 1. I adjust the trimpot so that the reading across resistor 456 measures 33volts?

The same goes for image 2 and 3. Adjusting the trimpots so that the resistors 423 and 421 measure 7.5mV?

Image 4 confuses me a little as it asks for an old Tube reading for a maximum VTVM. can I measure this with a multimeter? I presume I am taking the VTVM reading from resistor 190?

I did try to take a reading resistor in image 1, all powered off, but I didnt get any reading at all? Maybe I have my multimeter set up wrong?


Thanks for any help.

All the best
Matt

 

Your manual is not exactly a model of clarity, but my understanding is that you should measure voltage
1) between the right-hand end of R456 and ground (chassis),
2) across R421 and across R423.

A good quality multimeter on a voltage range should have an input impedance of at least 1 megOhm, so would substitute for a VTVM in this application

 

Your manual is not exactly a model of clarity, but my understanding is that you should measure voltage
1) between the right-hand end of R456 and ground (chassis),
2) across R421 and across R423.

A good quality multimeter on a voltage range should have an input impedance of at least 1 megOhm, so would substitute for a VTVM in this application

Ha yes I totally agree, its from the 1970's and I guess things were a little more 'general' those days. Thanks for the advice with the VTVM reading as well. Ill set it to that.

Thanks again

 

The large challenge is getting around the change in voltage caused by shunting of a multimeter. At some point the manual does appear to say VTVM, which means a quite high input resistance meter. So if you are using a typical hand-held battery powered multimeter it should have no effect on the voltages read.

 

REALLY!! Th input resistance of most digital multimeters is at least one MEGOHM. Those analog multimetrs might have as high an input resistance as 5000 ohms. So using the digital meter is even better than using the VTVM..

 

[/QUOTE]The first kit I built eons ago was an Eico VTVM that had an input resistance of 1 megohm, which was typical for a VTVM.