Testing erase voltage (120khz) question - UK Vintage Radio Repair and Restoration Discussion Forum

pentoad · 7th Apr 2020, 8:07 pm

Will a decent professional DMM ( AC multimeter) show the correct (ballpark) AC test voltage or do you need to use an oscilloscope to do this?
(Not sure if they only work on 50hz)
Is a diode required in the signal path to interpret the results?

kalee20 · 7th Apr 2020, 8:23 pm

Depends on the mu8ltimeter! If a decent professional one, there's likely to be the spec on-line.

120kHz is pushing it, to be honest - I'd expect maybe 10kHz with falling-off in accuracy above. But, check the spec.

Not sure what you mean about a diode in the signal path - the erase head should see only HF AC!

DMcMahon · 7th Apr 2020, 8:25 pm

No a basic DMM typically only goes up to around 400Hz, much more expensive ones will go somewhat higher. But for this high frequency bias you really need to either measure with a scope, remember that a scope will display peak to peak value, whereas bias measurements referenced in manuals typically will be rms.

A typical old Avo multimeter will only go up to around 2kHz on AC voltage, so also is no good for HF bias measurements.

For direct rms reading you really need something like an old V.T.V.M. i.e. a Vacuum Tube (Valve) Volt Meter or a slightly more modern FET VOM.

You do not need a diode.

Ted Kendall · 7th Apr 2020, 8:31 pm

Oscilloscope?

ms660 · 7th Apr 2020, 10:10 pm

I always used a 'scope, you can check the bias frequency as well.

The 'scope is one of the best tools out for tape recorder fault finding and checking, the other for me was the Ferrograph test set.

Lawrence.

Radio Wrangler · 7th Apr 2020, 10:12 pm

Scope with a good x10 probe to avoid capacitive loading. No contest.

I have one multimeter that doesn't roll off until 200kHz which is pretty unusual. It's an old AVO EA113. I wouldn't trust it for this job due to capacitance of the leads.

David

DMcMahon · 7th Apr 2020, 10:49 pm

That Avo has impressive performance David.

I see it is an electronic AVO with FET inputs.

David

Radio Wrangler · 8th Apr 2020, 12:21 am

Have a look at the prices EA113s fetch nowadays. They are invoked in the cal procedure for VCM163s and the audio gang are after them!

It's a rather nice little instrument, runs for years on 4 AA cells, and mine is my main bench multimeter. Big snag is they go loopy if there is strong RF around.

David

TIMTAPE · 8th Apr 2020, 3:07 am

I've always used an oscilloscope. A good test of the erase function is if it (fully) erases tapes. To test the frequency, fr. counters are cheap these days.

garry71 · 15th Apr 2020, 4:41 pm

Would a normal DMM at least show if some sort of voltage was present at the head, if only to eliminate the head itself from being faulty?

DMcMahon · 15th Apr 2020, 11:21 pm

Depends upon the DMM, unless a good one I would say not much of any use, even if you see a voltage you will not know if it is in the right ballpark frequency wise.

I tried my DMM tonight on my TK 14 which is 57kHz bias similar to your 54 kHz on your Ferguson (DMM 400Hz AC voltage spec but from signal generator tests I know it will read reasonable up to to at least 800Hz) but it totally failed to read any AC voltage and it kept auto switching to DC and then only gave erratic very low readings.

garry71 · 17th Apr 2020, 12:02 pm

I wonder if attaching larger value capacitors to the oscillator would bring the frequency down enough for a meter to take a reading of sorts.

Might give it a try as an experiment.

DMcMahon · 17th Apr 2020, 4:40 pm

Garry,

The erase head is very unlikely to be faulty, you could measure its resistance using a DMM to check that it is not open circuit. If it is not open circuit then it should function if receiving good bias/erase signal.

Note - that checking resistance of heads could somewhat magnetise them if an analogue test meter is used and could affect playback sound, then requiring head de-magnetisation.

With respect I think you would be wasting your time adding capacitors, even if you got a reading you would have no idea if the frequency/voltage/waveform shape is good/bad.

You really need to check it with a scope or a very good DMM that can read AC voltage at high frequency.

David

DMcMahon · 17th Apr 2020, 4:43 pm

Timtape had a good input at Post 9.

chriswood1900 · 17th Apr 2020, 5:05 pm

Quote:

Originally Posted by pentoad

Will a decent professional DMM ( AC multimeter) show the correct (ballpark) AC test voltage or do you need to use an oscilloscope to do this?

The simple answer is yes if you use something like a Fluke 85 or similar, they will resolve down to 20mV and read frequencies of 120KHz with enough accuracy (although they are only specified to 100KHz). However as others have said my first choice would be to use an Oscilloscope.

TIMTAPE · 18th Apr 2020, 2:36 am

Pentoad it seems you have now posted three threads on the same A77 machine with possibly the same related issues. Why do you want to test the erase voltage? Is it because the tapes are not fully erasing and you have concluded it is due to low erase current? Is it possible the erase head is - like the other head was - dirty?

Or is another possible contributor a faulty tape dirtying up the tape path? Is it possible you are using a tape too "high energy" for the machines? If so the machine will not be able to fully erase it.

Again the A77 tape path is very hard to see. There is no substitute for making the effort to view it as clearly as possible using a proper inspection mirror and if needed, extra light and magnification. Unfortunately the only way to properly view and clean the A77 tape path is to remove the lower front panel completely.

7th Apr 2020, 8:07 pm	#1
pentoad Hexode Join Date: Feb 2011 Location: Newcastle upon Tyne, Tyne and Wear, UK. Posts: 280	Testing erase voltage (120khz) question Will a decent professional DMM ( AC multimeter) show the correct (ballpark) AC test voltage or do you need to use an oscilloscope to do this? (Not sure if they only work on 50hz) Is a diode required in the signal path to interpret the results?

7th Apr 2020, 8:23 pm	#2
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,087	Re: Testing erase voltage (120khz) question Depends on the mu8ltimeter! If a decent professional one, there's likely to be the spec on-line. 120kHz is pushing it, to be honest - I'd expect maybe 10kHz with falling-off in accuracy above. But, check the spec. Not sure what you mean about a diode in the signal path - the erase head should see only HF AC!

7th Apr 2020, 8:25 pm	#3
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,603	Re: Testing erase voltage (120khz) question No a basic DMM typically only goes up to around 400Hz, much more expensive ones will go somewhat higher. But for this high frequency bias you really need to either measure with a scope, remember that a scope will display peak to peak value, whereas bias measurements referenced in manuals typically will be rms. A typical old Avo multimeter will only go up to around 2kHz on AC voltage, so also is no good for HF bias measurements. For direct rms reading you really need something like an old V.T.V.M. i.e. a Vacuum Tube (Valve) Volt Meter or a slightly more modern FET VOM. You do not need a diode.

7th Apr 2020, 8:31 pm	#4
Ted Kendall Dekatron Join Date: Jul 2008 Location: Kington, Herefordshire, UK. Posts: 3,675	Re: Testing erase voltage (120khz) question Oscilloscope?

7th Apr 2020, 10:10 pm	#5
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Testing erase voltage (120khz) question I always used a 'scope, you can check the bias frequency as well. The 'scope is one of the best tools out for tape recorder fault finding and checking, the other for me was the Ferrograph test set. Lawrence.

7th Apr 2020, 10:12 pm	#6
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,899	Re: Testing erase voltage (120khz) question Scope with a good x10 probe to avoid capacitive loading. No contest. I have one multimeter that doesn't roll off until 200kHz which is pretty unusual. It's an old AVO EA113. I wouldn't trust it for this job due to capacitance of the leads. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

7th Apr 2020, 10:49 pm	#7
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,603	Re: Testing erase voltage (120khz) question That Avo has impressive performance David. I see it is an electronic AVO with FET inputs. David

8th Apr 2020, 12:21 am	#8
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,899	Re: Testing erase voltage (120khz) question Have a look at the prices EA113s fetch nowadays. They are invoked in the cal procedure for VCM163s and the audio gang are after them! It's a rather nice little instrument, runs for years on 4 AA cells, and mine is my main bench multimeter. Big snag is they go loopy if there is strong RF around. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

8th Apr 2020, 3:07 am	#9
TIMTAPE Octode Join Date: Jul 2006 Location: Perth, Western Australia Posts: 1,969	Re: Testing erase voltage (120khz) question I've always used an oscilloscope. A good test of the erase function is if it (fully) erases tapes. To test the frequency, fr. counters are cheap these days.

15th Apr 2020, 4:41 pm	#10
garry71 Tetrode Join Date: Apr 2020 Location: Birmingham, West Midlands, UK. Posts: 88	Re: Testing erase voltage (120khz) question Would a normal DMM at least show if some sort of voltage was present at the head, if only to eliminate the head itself from being faulty?

15th Apr 2020, 11:21 pm	#11
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,603	Re: Testing erase voltage (120khz) question Depends upon the DMM, unless a good one I would say not much of any use, even if you see a voltage you will not know if it is in the right ballpark frequency wise. I tried my DMM tonight on my TK 14 which is 57kHz bias similar to your 54 kHz on your Ferguson (DMM 400Hz AC voltage spec but from signal generator tests I know it will read reasonable up to to at least 800Hz) but it totally failed to read any AC voltage and it kept auto switching to DC and then only gave erratic very low readings.

17th Apr 2020, 12:02 pm	#12
garry71 Tetrode Join Date: Apr 2020 Location: Birmingham, West Midlands, UK. Posts: 88	Re: Testing erase voltage (120khz) question I wonder if attaching larger value capacitors to the oscillator would bring the frequency down enough for a meter to take a reading of sorts. Might give it a try as an experiment.

17th Apr 2020, 4:40 pm	#13
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,603	Re: Testing erase voltage (120khz) question Garry, The erase head is very unlikely to be faulty, you could measure its resistance using a DMM to check that it is not open circuit. If it is not open circuit then it should function if receiving good bias/erase signal. Note - that checking resistance of heads could somewhat magnetise them if an analogue test meter is used and could affect playback sound, then requiring head de-magnetisation. With respect I think you would be wasting your time adding capacitors, even if you got a reading you would have no idea if the frequency/voltage/waveform shape is good/bad. You really need to check it with a scope or a very good DMM that can read AC voltage at high frequency. David

17th Apr 2020, 4:43 pm	#14
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,603	Re: Testing erase voltage (120khz) question Timtape had a good input at Post 9.

18th Apr 2020, 2:36 am	#16
TIMTAPE Octode Join Date: Jul 2006 Location: Perth, Western Australia Posts: 1,969	Re: Testing erase voltage (120khz) question Pentoad it seems you have now posted three threads on the same A77 machine with possibly the same related issues. Why do you want to test the erase voltage? Is it because the tapes are not fully erasing and you have concluded it is due to low erase current? Is it possible the erase head is - like the other head was - dirty? Or is another possible contributor a faulty tape dirtying up the tape path? Is it possible you are using a tape too "high energy" for the machines? If so the machine will not be able to fully erase it. Again the A77 tape path is very hard to see. There is no substitute for making the effort to view it as clearly as possible using a proper inspection mirror and if needed, extra light and magnification. Unfortunately the only way to properly view and clean the A77 tape path is to remove the lower front panel completely.