UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Test Gear and Workshop Equipment

Notices

Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

Reply
 
Thread Tools
Old 19th Jan 2019, 2:38 pm   #1
K100RSBrick
Diode
 
Join Date: Jan 2019
Location: Norwich, Norfolk, UK.
Posts: 4
Default Comark Early 1905 Insulation Meter Checks

I have just inherited a Comark 1905 early insulation meter from my 92 year old dad and would like to know how to check it out on the ranges if possible. Could not locate the instruction manual for it or repair manual. It has 8 C rechargeable battery's in the back and on cal gives a 100% FSD with batts fully charged.
Is there any way you can check the voltages on the output terminals to check correct operation of the 1905 from 25v to 1000v range? I do poses a Fluke 179 true RMS multimeter and a Tektronix 2213A scope.

Any help gratefully received as this is from 1971 era.


K100RSBrick
K100RSBrick is offline   Reply With Quote
Old 19th Jan 2019, 6:44 pm   #2
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 12,976
Default Re: Comark Early 1905 Insulation Meter Checks

Is the tester ac or dc output? Either way I would expect that you could check the voltage up to the max ratings of the Fluke at any rate. The 10M input resistance of the meter should be high enough not to load the tester too much.

I'd be inclined to keep the 'scope away from it unless you've got some /100 probes rather than the usual /10 type.
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is offline   Reply With Quote
Old 19th Jan 2019, 9:39 pm   #3
jimmc101
Heptode
 
jimmc101's Avatar
 
Join Date: Mar 2009
Location: Portsmouth, Hampshire, UK.
Posts: 617
Default Re: Comark Early 1905 Insulation Meter Checks

This thread may be of help COMARK 1905 Insulation Meter

In the case of the early version (which I have) the test voltage is only at its rated value with no leakage.
The voltage across the terminals will reduce to half rated value at half scale.
eg 500v on the 100v range with 100Mohm load.
With the 10Mohm load of the Fluke the voltage should drop to around 90.9v.

Note for the 100v, 50v and 25v ranges use the 1000v, 500v and 250v scales and divide the reading by 10.

I believe that the later version is the same but I have never investigated this.

Jim
jimmc101 is offline   Reply With Quote
Old 20th Jan 2019, 1:48 am   #4
Bazz4CQJ
Nonode
 
Bazz4CQJ's Avatar
 
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 2,274
Default Re: Comark Early 1905 Insulation Meter Checks

There's also another thread, which I started https://www.vintage-radio.net/forum/...ad.php?t=83980.

As above, there are two versions on the 1905; one uses discrete components and one uses an op-amp. Mine is the former, and was working well well when I first got it, but then developed a fault. Fortunately, there is a known issue with that version, as Bill WME correctly advised me (see thread).

I'm pretty sure (?) the output is DC, the oscillator being rectified by a voltage doubler to charge caps. I vaguely recall checking mine just by stringing resistors across the output and comparing against the instantaneous meter readings 'on test'.

B
__________________
We know there are known unknowns. But there are unknown unknowns – ones we don't know we don't know.

Last edited by Bazz4CQJ; 20th Jan 2019 at 1:56 am.
Bazz4CQJ is offline   Reply With Quote
Old 21st Jan 2019, 2:00 pm   #5
K100RSBrick
Diode
 
Join Date: Jan 2019
Location: Norwich, Norfolk, UK.
Posts: 4
Default Re: Comark Early 1905 Insulation Meter Checks

Hi jimmc 101 and Bazz4CQJ

My Dad's Comark 1905 has discrete components inside and not the later op amp. It has been looked after with no scratches or dents on the bodywork.
Have connected my Fluke 179 true RMS multimeter across the terminals with the results below.
Cal FSD
25v gives 20.73
50v gives 34.33
100v gives 51.4
250v gives 73.1
500v gives 85.6
1000v gives 93.8

The meter does seem to show 10MegOhm but would like to know if the readings are what should be expected for this meter? Thanks for all your help on this old meter but would like to use it for checking shorts or leakage on heating elements.

K100RSBrick
K100RSBrick is offline   Reply With Quote
Old 21st Jan 2019, 11:04 pm   #6
WME_bill
Octode
 
Join Date: Oct 2007
Location: Exeter, Devon, UK.
Posts: 1,207
Default Re: Comark Early 1905 Insulation Meter Checks

Comark 1905
The voltages across the test terminals should be close to that specified for the range with an infinite load. On the 1000v range, the impedance is 100Mohm fed through 2.2M, so a 10M voltmeter across the 100M will read 10/12.2 of the 1000v. The differences will be worse on the lower voltage ranges.
But your very low voltages on all the ranges suggest that the transformer has failed.
I assume as you talk of a RMS AC voltmeter that you are measuring the DC voltage.
If you are actually finding AC, then you have more troubles with the high voltage rectifiers and the EHT smoothing capacitors as well.
Have you checked it over with the circuit diagram.
I will get mine out of the attic and see what the voltages are, though that won't give you vital information. The results you have show clearly enough that it need some work done on it.
wme_bill

Last edited by WME_bill; 21st Jan 2019 at 11:14 pm.
WME_bill is offline   Reply With Quote
Old 21st Jan 2019, 11:15 pm   #7
Bazz4CQJ
Nonode
 
Bazz4CQJ's Avatar
 
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 2,274
Default Re: Comark Early 1905 Insulation Meter Checks

Posted in parallel with Bill.

As I understand these meters, they charge up a capacitor to the nominal voltage and then when you press TEST it discharges across the load. Consequently, the output voltage will be constantly changing and could give you odd results on a digital voltmeter, though the readings you quote do seem much too low.

I haven't used mine in ages and it has no batteries in it at the moment and I have no C cells in the house.

Do consider trying it by putting some resistors across the output.

At least it does seem like your oscillator is working. When I had to do the repair work on mine, I took the precaution of replacing all the high voltage caps (see the circuit diagram) as the condition of those is really very important for this instrument. As Bill comments, so too is the transformer.

B
__________________
We know there are known unknowns. But there are unknown unknowns – ones we don't know we don't know.
Bazz4CQJ is offline   Reply With Quote
Old 21st Jan 2019, 11:30 pm   #8
jimmc101
Heptode
 
jimmc101's Avatar
 
Join Date: Mar 2009
Location: Portsmouth, Hampshire, UK.
Posts: 617
Default Re: Comark Early 1905 Insulation Meter Checks

The circuit is arranged such that the sum of the current through R10 and the meter is just sufficient to start to turn off TR1 and hence reduce the drive to the transformer via oscillator formed by TR2/3.

When calibrated this current is 10uA assuming that the meter resistance is much less than 2.5M (R10).

When the load across the test terminals is open circuit the meter current is of course zero and all of the 10uA must flow through R10.
On the 25v range the -ve test terminal is connected to the 'top' end of R1 giving a test voltage of 25v (10uA x 2.5M). As a load is applied and current flows through the meter the current through R10 to keep the total constant.
jimmc101 is offline   Reply With Quote
Old 22nd Jan 2019, 12:04 am   #9
jimmc101
Heptode
 
jimmc101's Avatar
 
Join Date: Mar 2009
Location: Portsmouth, Hampshire, UK.
Posts: 617
Default Re: Comark Early 1905 Insulation Meter Checks

Sorry I hit send too soon, this is the full post

It is with some trepidation that I have to disagree with Bill, but I am sure that your results indicate that your meter is working correctly.

Referring to the circuit of the early meter...

The circuit is arranged such that the sum of the current through R10 and the meter is just sufficient to start to turn off TR1 and hence reduce the drive to the transformer via oscillator formed by TR2/3.

When calibrated this current is 10uA assuming that the meter resistance is much less than 2.5M (R10) (so that negligible current flows through the resistor).

When the load across the test terminals is open circuit the meter current is of course zero and all of the 10uA must flow through R10.
On the 25v range the -ve test terminal is connected to the 'top' end of R1 giving a test voltage of 25v (10uA x 2.5M). As a load is applied and current flows through the meter the current through R10 to keep the total constant.

On the higher ranges more resistors (R 9, 8, 7, 4, 5, 6) are added in series with R10 to give a total of 100M on the 1000v range.

If the feedback via TR1 was perfect so that the current was always exactly 10uA (after calibration) then the open circuit voltage across the test terminals would be that given by a 10uA current source shunted by a resistor (RS) equal to the range (V) x 100k.
eg for the 1000v range 10uA x 1000 x 100k = 1000v

Adding a load across the terminals drives current through the meter and the current through R10 must decrease to keep the sum constant

This load (RL) changes the voltage to 10uA x (RS//RL).

Calculating the result 0f a 10M load on each range gives
25v______20.0v
50v______33.3v
100v_____50.0v
250v_____74.3V
500v_____83.3v
1000v____90.9v

Remember these figures assume perfect feedback (infinite loop gain) in real life the actual figures will be slightly different and I'm sure your results show that everything is working as it should.

As a final confirmation my meter gives very similar voltages to yours.

Jim

Last edited by jimmc101; 22nd Jan 2019 at 12:10 am. Reason: Typos
jimmc101 is offline   Reply With Quote
Old 22nd Jan 2019, 1:08 am   #10
Bazz4CQJ
Nonode
 
Bazz4CQJ's Avatar
 
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 2,274
Default Re: Comark Early 1905 Insulation Meter Checks

Thanks for that explanation Jim, I had never quite figured what's actually going on. I'm sure that it is self-evident from your post, but I cannot quite see how to calculate what the applied voltage would be at the terminals, on a particular range, and for particular MOhm reading. That would be interesting to know, particularly when testing caps. Could you elucidate? .

B
__________________
We know there are known unknowns. But there are unknown unknowns – ones we don't know we don't know.
Bazz4CQJ is offline   Reply With Quote
Old 22nd Jan 2019, 5:31 pm   #11
WME_bill
Octode
 
Join Date: Oct 2007
Location: Exeter, Devon, UK.
Posts: 1,207
Default Re: Comark Early 1905 Insulation Meter Checks

Comark 1905.
I stand corrected, Yes, Jimmc101 looked at the circuit more carefully than I did, and the feedback does indeed reduce the voltage under load, and he has calculated the voltages on the various ranges. Many thanks.

I quote from the manual: OUTPUT VOLTAGE
If the "cal" control has been set correctly, the output voltage is within 5% of the nominal at infinity ohms. Due to action of the stabilising circuit, the voltage falls linearly from this point to zero at zero ohms.

That feedback also protects the expensive 10 ua meter. With the cost and rareity of that meter, I can see why for the later models, Comark used Op-amps and a 500ua meter.
And if I mention it now, the Avo Valve tester people will be wanting to use it, suitable shunted, to replace a failed Avo 30ua meter.

wme_bill
WME_bill is offline   Reply With Quote
Old 22nd Jan 2019, 10:54 pm   #12
jimmc101
Heptode
 
jimmc101's Avatar
 
Join Date: Mar 2009
Location: Portsmouth, Hampshire, UK.
Posts: 617
Default Re: Comark Early 1905 Insulation Meter Checks

Quote:
Originally Posted by Bazz4CQJ View Post
Could you elucidate?
I'll try...

This is how I originally did it...
If RS is source resistance of meter where RS = Range x 100k
and RL is load across terminals
then
RP = RS//RL = RS*RL/(RS+RL)
and voltage across terminals is given by
VT = 10uA*RP

However, having thought about it, it is easier to apply a Thévenin equivalent which gives a voltage source equal to the Range (VRange) and a source resistance of RS = Range x 100k
The load across the terminals (RL) now forms a voltage divider of RL/(RS+RL) and hence VT = VRange * RL/(RS+RL)


A quick way of estimating the test voltage voltage (VT) for a given reading is to note the meter deflection in terms of percentage FSD (say D) (eg 50% for half way across the scale) and use
VT = Range * (100 - D) /100

So on the 1000v range 100M gives 50% FSD and the voltage is
VT = 1000*(100-50)/100 = 500v
on the same range 10M gives about 90% FSD so
VT = 1000*(100-90)/100 = 90v
etc

I made an error in my last post, the table should read

25v______20.0v
50v______33.3v
100v_____50.0v
250v_____71.4v (My excuse is the actual value is 71.43 an I dropped a digit)
500v_____83.3v
1000v____90.9v



Finally WME_bill's post made me think about the meter movement and its vulnerability especially as I sometimes use mine with an external power supply instead of batteries.
I shall place additional protection on my to do list.

Jim
jimmc101 is offline   Reply With Quote
Old 23rd Jan 2019, 12:29 am   #13
Bazz4CQJ
Nonode
 
Bazz4CQJ's Avatar
 
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 2,274
Default Re: Comark Early 1905 Insulation Meter Checks

Many thanks for that Jim. I'll try and find a few minutes to plot the numbers.

As the owner of an Avo VCM which has one dodgy 50uA meter (needs a corrective op-amp to drive it), it does seem as if Comark used some better quality meters, as they seem to be surviving better than many of the Avo ones.

Is the use of the usual pair of "side by side" silicon signal diodes (or maybe Schotky's?) across the meter likely to offer useful protection?

I don't use my 1905 very often, but it is one of those instruments which (having paid 50p or £1 for it at a rally) I feel is very nice to have around. I also have an old analogue Comark thermocouple meter (similar size and style) which has proved very reliable and useful, and that too was picked up at a rally for next to nothing.

B
__________________
We know there are known unknowns. But there are unknown unknowns – ones we don't know we don't know.
Bazz4CQJ is offline   Reply With Quote
Old 23rd Jan 2019, 2:48 am   #14
Bazz4CQJ
Nonode
 
Bazz4CQJ's Avatar
 
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 2,274
Default Re: Comark Early 1905 Insulation Meter Checks

I've plotted the way the voltage presented by the Comark varies as a function of the load, and I think the results are interesting if anyone is using this meter for testing high voltage caps.

Fig1 just shows how the applied voltage (or test voltage) fall in line with the expression VT = Range * (100 - D) /100 , where D is the % meter deflection from the RHS of the meter. There is no linear scale on the 1905, so those are "by eye" measurements, but the line looks good.

Fig2 shows how the voltage falls (the same behaviour applies to all ranges, but the 1000V range is shown) quickly when there's a significant load on test, or even the effect of connecting a "high impedance" DMM on to the 1905. I forced Excel to show a smooth line, and it has put a wiggle in there.

The bottom line here is that if you use a 1905 on the 250 or 500 volt range to test a HT capacitor, it may well be that you only have a fraction of the nominal voltage being applied, so the results need to be considered carefully.

B
Attached Files
File Type: doc a Fig1.doc (23.5 KB, 18 views)
File Type: doc aFig2.doc (23.5 KB, 13 views)
__________________
We know there are known unknowns. But there are unknown unknowns – ones we don't know we don't know.
Bazz4CQJ is offline   Reply With Quote
Old 24th Jan 2019, 6:17 pm   #15
K100RSBrick
Diode
 
Join Date: Jan 2019
Location: Norwich, Norfolk, UK.
Posts: 4
Default Re: Comark Early 1905 Insulation Meter Checks

Thanks Bazz4CQJ and Jimmc101

Have really learnt something about the Comark 1905 from you and the Vintage Gear Forum. My dad could not locate any of the paperwork after so many years so you were my last hope having trawled the internet for information on the unit.
Will check out the condition inside of the components and will now go very carefully if I check out HT capacitors with this meter. Perhaps I should use my old Peak Atlas ESR60 for them?

Have also a Comark electronic thermometer type 1602 Cr/Al with a range from 0* C to 1000 *C which seems to work ok and very accurate against my Fluke 179 and its Fluke Type K thermocouple. But again he has no paperwork for the unit to read or check calibration.

K100RSBrick
K100RSBrick is offline   Reply With Quote
Old 25th Jan 2019, 8:02 pm   #16
Bazz4CQJ
Nonode
 
Bazz4CQJ's Avatar
 
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 2,274
Default Re: Comark Early 1905 Insulation Meter Checks

I had my 1602 calibrated by RS a while ago and and it sailed through. I think that both my Comarks were dumped by the RAF when they acquired more modern digital stuff and I was lucky to stumble on to them at rallies. Of course, back in the days when I was a student, the old Comarks were regarded as 'A1' pieces of kit, so I was delighted to find them. As yours were owned by your dad, that's an additional reason for appreciating them!

Incidentally, when I first had problems with the 1905, and before I got the circuit via the forum, Comark advised that they no longer supported this old kit and couldn't help out in any way.

B
__________________
We know there are known unknowns. But there are unknown unknowns – ones we don't know we don't know.
Bazz4CQJ is offline   Reply With Quote
Old 25th Jan 2019, 10:40 pm   #17
WME_bill
Octode
 
Join Date: Oct 2007
Location: Exeter, Devon, UK.
Posts: 1,207
Default Re: Comark Early 1905 Insulation Meter Checks

Comark. 1905 and1600 series.
As well as posting the 1905 Insulation manual for this forum originally, you will be pleased to know I also have details about the 1600 series of thermometers.
They all use a clever simple design of chopper DC amplifier. The manual is very much a mix and match of the different parts. DC input chopper, temperature compensation, networks for the different types of thermocouple. Meter appears to be 500ua.
Thanks to Bazz4CQJ for his history. My notes say the unit was 1970 - 1971. By 1976, the firm had moved from Rustington Sussex to Welwyn Garden City. My di-azo circuit for the Op-Amp version is dated 1981, so they still have been going then.
The thermometer unit I find very useful. My wife borrows it under careful supervision for marmelade boiling as well.
wme_bill
WME_bill is offline   Reply With Quote
Old 26th Jan 2019, 12:15 am   #18
Bazz4CQJ
Nonode
 
Bazz4CQJ's Avatar
 
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 2,274
Default Re: Comark Early 1905 Insulation Meter Checks

Any thoughts about adding any form of protection to the meters Bill?

B
__________________
We know there are known unknowns. But there are unknown unknowns – ones we don't know we don't know.
Bazz4CQJ is offline   Reply With Quote
Old 29th Jan 2019, 12:22 pm   #19
K100RSBrick
Diode
 
Join Date: Jan 2019
Location: Norwich, Norfolk, UK.
Posts: 4
Default Re: Comark Early 1905 Insulation Meter Checks

Thanks for all the information again. Will be setting up some ice, boiling water and a higher temp from my Antex work station to check the calibration of the Comark 1602 electronic thermometer. My dad had put 8 GP rechargeable batteries in the back of the Comark 1905 so it will be interesting to take the back of the 1602.

Have found it so useful as a needle moving up or down is so much more informative than a digital thermometer changing figures quickly. Just placed the soldering iron bit on the end and rotate the iron for the maximum temp reading. Both meters have been well looked after with no cracks or scratches on the cases. Is there any calibration notes for this meter still floating around as I had also contacted Comark without any luck.

KRS100Brick
K100RSBrick is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 7:19 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Copyright ©2002 - 2018, Paul Stenning.