Help required for Racal 9520 Frequency Meter - UK Vintage Radio Repair and Restoration Discussion Forum

MikeCross · 6th Jan 2018, 10:50 am

I use this meter to set up the speeds on my Revox A77 recorder.
However I now have doubts about it's accuracy.

Can anyone help with a schematic or calibration instructions for this unit.
I have owned this unit for many years so perhaps it's due some tlc.

Many Thanks

Station X · 6th Jan 2018, 12:10 pm

I'm not familiar with this particular Racal counter.

The accuracy of a counter depends on the accuracy of its internal crystal controlled clock which generally runs at 5 or 10MHz. In the case of the 9520 this crystal is in a glass envelope. It would require a massive absolute error in the crystal's frquency to make any difference to audio frequency measurements which I assume is what we're talking about here.

The crystal clock may be brought out to an external connector so could be checked with another counter of known accuracy.

EDIT. I see this counter has a test button. Pressing this should result in the display showing 1kHz. Does that work?

barrymagrec · 6th Jan 2018, 4:31 pm

Speed accuracy on a tape machine is not going to be much better than +/_ 0.2%, this was the old broadcast spec as required by the BBC. No crystal oscillator is going to drift anything like this much even over many years.

A crude way to check the calibration is to measure the frequency of a signal generator while tuning it for zero beat with a known broadcast station.

MikeCross · 10th Jan 2018, 9:18 pm

Thank you for your help.
Having replaced the power supply caps, all is now ok and very stable.

Revox calibrated and ready for this year's projects.

Mike

6th Jan 2018, 10:50 am	#1
MikeCross Tetrode Join Date: Sep 2016 Location: Tring, Hertfordshire, UK. Posts: 87	Help required for Racal 9520 Frequency Meter I use this meter to set up the speeds on my Revox A77 recorder. However I now have doubts about it's accuracy. Can anyone help with a schematic or calibration instructions for this unit. I have owned this unit for many years so perhaps it's due some tlc. Many Thanks

6th Jan 2018, 12:10 pm	#2
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Help required for Racal 9520 Frequency Meter I'm not familiar with this particular Racal counter. The accuracy of a counter depends on the accuracy of its internal crystal controlled clock which generally runs at 5 or 10MHz. In the case of the 9520 this crystal is in a glass envelope. It would require a massive absolute error in the crystal's frquency to make any difference to audio frequency measurements which I assume is what we're talking about here. The crystal clock may be brought out to an external connector so could be checked with another counter of known accuracy. EDIT. I see this counter has a test button. Pressing this should result in the display showing 1kHz. Does that work? __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

6th Jan 2018, 4:31 pm	#3
barrymagrec Octode Join Date: Dec 2017 Location: Morden, Surrey, UK. Posts: 1,557	Re: Help required for Racal 9520 Frequency Meter Speed accuracy on a tape machine is not going to be much better than +/_ 0.2%, this was the old broadcast spec as required by the BBC. No crystal oscillator is going to drift anything like this much even over many years. A crude way to check the calibration is to measure the frequency of a signal generator while tuning it for zero beat with a known broadcast station.

10th Jan 2018, 9:18 pm	#4
MikeCross Tetrode Join Date: Sep 2016 Location: Tring, Hertfordshire, UK. Posts: 87	Re: Help required for Racal 9520 Frequency Meter Thank you for your help. Having replaced the power supply caps, all is now ok and very stable. Revox calibrated and ready for this year's projects. Mike