Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Dec Sun 08, 2019 6:08 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 23 posts ]  Go to page 1, 2  Next
Author Message
 Post subject: Heathkit VTVM Resistors source
PostPosted: Nov Tue 19, 2019 12:24 am 
New Member

Joined: Dec Wed 26, 2018 3:46 pm
Posts: 19
Hi all,

Been trying to find a good source for the weird resistor values that Heathkit used for their VTVM's. In my units most of the precision resistors are out of spec. It takes forever to find a few and it always ends with that a supplier does not have them all. Which means it has to be ordered for several sources and the order turns into a big mess.

Does anyone know a source that has ALL resistors to for example IM-28, IM-13? I have a few of those. I am located in Europe but order from USA will not be an issue. Or how do you do it?

Many thanks in advance!


Top
 Profile  
 
 Post subject: Re: Heathkit VTVM Resistors source
PostPosted: Nov Tue 19, 2019 12:33 am 
Member

Joined: Nov Wed 30, 2016 7:35 pm
Posts: 5362
Location: Sunbury, Ohio 43074
Bart, most will just use two resistors In series or parallel to give you the precision value you are looking for. Finding the exact oddball resistors that any of the manufacturers used is generally an exercise in futility.

You could also buy a few dozen of whatever you can find that is closest, and hand pick one that is exact.

How far off are they? In my experience the Heath gear used resistors that don't generally drift much in their divider chains. Remember that the accuracy of the meter is not exactly lab grade either. ........ A resistor replacement jihad shouldn't be required generally speaking.

_________________
Preserving the hist. of electronics, one boat anchor at a time! :)
https://www.bbtvtestequipment.com


Top
 Profile  
 
 Post subject: Re: Heathkit VTVM Resistors source
PostPosted: Nov Tue 19, 2019 12:39 am 
Member

Joined: Sep Tue 30, 2014 6:08 am
Posts: 4863
Location: Norfolk, VA
What values?? Some info would help....

_________________
Brian
"Capacitor Cosmetologist since 1979"
USN Retired 1984-2006 (Avionics/Cal)


Top
 Profile  
 
 Post subject: Re: Heathkit VTVM Resistors source
PostPosted: Nov Tue 19, 2019 12:44 am 
Member

Joined: May Thu 04, 2017 1:08 pm
Posts: 290
Location: Raleigh, NC area
Barry is on the mark. Just measure the ones you have, see if they are in spec. If not, either parallel another resistor with the ones which drifted high (I don't think I've ever seen low ones), or put a few new values together. This is some of what I had to do with mine, which was an awful piece of junk from the auction site, but is quite serviceable now:
Attachment:
image.jpeg
image.jpeg [ 463.55 KiB | Viewed 316 times ]

Edit: That picture is from a V7A, but you probably get the point.


Last edited by DaveInNC on Nov Tue 19, 2019 1:21 am, edited 1 time in total.

Top
 Profile  
 
 Post subject: Re: Heathkit VTVM Resistors source
PostPosted: Nov Tue 19, 2019 1:10 am 
Member

Joined: Jun Sat 15, 2019 7:43 pm
Posts: 260
Attachment:
IM28resistors.jpg
IM28resistors.jpg [ 276.97 KiB | Viewed 308 times ]


Top
 Profile  
 
 Post subject: Re: Heathkit VTVM Resistors source
PostPosted: Nov Tue 19, 2019 1:13 am 
Member

Joined: Jan Tue 10, 2012 8:39 am
Posts: 787
I find it amusing that the parts list says 'precision' but that's meaningless without a number.


Top
 Profile  
 
 Post subject: Re: Heathkit VTVM Resistors source
PostPosted: Nov Tue 19, 2019 1:26 am 
Member
User avatar

Joined: May Wed 18, 2011 2:40 am
Posts: 4235
Location: Littleton, MA
bart wrote:
Been trying to find a good source for the weird resistor values that Heathkit used for their VTVM's. In my units most of the precision resistors are out of spec. It takes forever to find a few and it always ends with that a supplier does not have them all. Which means it has to be ordered for several sources and the order turns into a big mess.

Does anyone know a source that has ALL resistors to for example IM-28, IM-13? I have a few of those. I am located in Europe but order from USA will not be an issue. Or how do you do it?

I just did both an IM-28 and am in the middle of doing an IM-13. Yes, Heathkit did use an oddball set of values in the range divider for these: 21.6 KΩ and 68.3 KΩ, 216 K and 683 KΩ, up to 2.16 MΩ and 6.83 MΩ. Note that the 0-15 scale on the meter is shorter than the 0-50 scale, unlike the IM-11 and most other VTVMs.

These range resistors are not E96 values, so I compromised by using the closest E96 values:
21.5 KΩ, 68.1 KΩ, 215 KΩ, 681 KΩ, 2.15 MΩ, 6.81 MΩ. These are within 0.5% of the values specified by Heathkit. You could do better by making up series and parallel combinations.

I had to use both Mouser and DigiKey to fill my stock of VTVM resistors, although I was stocking up for more than just those Heathkits, I have a number of other VTVMs to restore. I don't think either supplier has a complete enough stock to cover all the values needed for an IM-13/IM-28. Like you, I'd sure like to find a good reliable supplier of axial-leaded precision metal film resistors that stocks all the oddball E96 values.

_________________
Steve Byan http://www.byan-roper.org/steve/steve-at-play/


Top
 Profile  
 
 Post subject: Re: Heathkit VTVM Resistors source
PostPosted: Nov Tue 19, 2019 2:55 am 
Member

Joined: Jul Mon 01, 2019 4:42 pm
Posts: 200
Location: St. Louis, MO
Barry H Bennett wrote:
Bart, most will just use two resistors In series


What Barry said right here. I had to do this with my own Heathkit VTVM. Half of the original precision resistors were out of spec. The good thing about those old point to point wired switches is that there is plenty of room between the 2 tie points to mount a pair of resistors in series. Be creative. There are a lot of common values that will add up to your weird values. I remember adding a 680k and 220k to make 900k on my IM-11.

Oh, and watch the slack. Replacing 60 year old components with modern ones usually means thinner wire leads, which means that new point to point mounted parts have the capability to move more once soldered in place. Plan ahead to prevent parts shorting together.

edit: Also, I remember there being a LOT more lesser values available for precision resistors than higher ones. If you're adding up to a big value resistor in series, get close with a common lesser value, then find a smaller value to add up to your final value. For example, I replaced a 7m resistor with a 6.8m and a 200k.


Top
 Profile  
 
 Post subject: Re: Heathkit VTVM Resistors source
PostPosted: Nov Tue 19, 2019 4:20 am 
Member
User avatar

Joined: Sep Mon 16, 2013 2:42 am
Posts: 3547
Location: Tucson, Arizona U.S.A.
Another problem is the voltage rating of the resistors. Some of the resistors in a VTVM can have substantial voltage across them when measuring high voltages; voltages much higher than the ratings of many of the easily available resistors. The easiest fix for this is to use several resistors in series to divide the voltage among them. Unless all the resistors are equal value, the voltage will divide unequally with the smaller resistors taking less of the voltage. But this is easy to calculate.

_________________
Jim Mueller


Top
 Profile  
 
 Post subject: Re: Heathkit VTVM Resistors source
PostPosted: Nov Tue 19, 2019 12:40 pm 
Member

Joined: Jun Fri 19, 2009 6:34 pm
Posts: 8920
Location: Long Island
Most of the precision resistors in the middle vintage Heathkit VTVMs were carbon film 1% types. Like all film and carbon resistors they are subject to long term as well as short term drift, including that due to temperature and humidity. For that reason I always let instruments like this run in their cases for about a week to re-settle the resistors before testing them. (Obviously any funky electrolytics or dead selenium rectifiers have to be gotten out of the way first). If the resistors are still out of spec after that, the drift is permanent and they have to be changed if they are affecting the accuracy of the meter on some or all of its ranges.

One thing to note is that the inherent drift has to be taken into account when choosing replacements. Mixing precision carbon film and metal film resistors might give you more drift in some cases since the resistors won't change in the same direction together any more. If you only need to change a resistor or two, it would be better to use carbon films to match the remaining resistors. However, if all the resistors are to be changed, you could use metal films throughout.

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


Top
 Profile  
 
 Post subject: Re: Heathkit VTVM Resistors source
PostPosted: Nov Tue 19, 2019 1:16 pm 
Member

Joined: May Thu 04, 2017 1:08 pm
Posts: 290
Location: Raleigh, NC area
Chris108 wrote:
One thing to note is that the inherent drift has to be taken into account when choosing replacements. Mixing precision carbon film and metal film resistors might give you more drift in some cases since the resistors won't change in the same direction together any more. If you only need to change a resistor or two, it would be better to use carbon films to match the remaining resistors. However, if all the resistors are to be changed, you could use metal films throughout.

Oops. Oh well. I'll remember that for the next one, I guess. Good advice, there, along with running the instrument for a while to settle the old carbons. I didn't do that, either.


Top
 Profile  
 
 Post subject: Re: Heathkit VTVM Resistors source
PostPosted: Nov Tue 19, 2019 1:54 pm 
Member

Joined: Jun Sat 09, 2007 8:14 am
Posts: 3911
Location: Florida
Resistors for the usual voltage divider networks can be made up from series combinations of "1,2" 1% values (10k, 20k, 100k, 200k, 1m, 2m). I fold them in a zig-zag fashion and use heat shrink to make a single package.

For the resistance function I rewire using values corresponding to the mid scale value for each range (1 meg for Rx100k, etc.) instead of chains made of of "9's". The only exception is 9.2 for RX1 to account for internal and lead resistance.

RRM


Top
 Profile  
 
 Post subject: Re: Heathkit VTVM Resistors source
PostPosted: Nov Tue 19, 2019 6:21 pm 
Member

Joined: Sep Tue 30, 2014 6:08 am
Posts: 4863
Location: Norfolk, VA
Me, I'd scour the RN65 precision resistor offerings at Mouser. Despite what their drill-down menu shows, they are all 1/2W resistors at room temps. The military requires two temps - 25 and 125 degrees celcius, and Mouser often coughs up the wrong (125 degree) wattage rating. By the MIL-R spec, they are all 1/2W resistors. The 125 degree point is a degrade (failure) point. If you can accept 2% resistors, the CMF20's or RL20-series are acceptable as well. Even the military sees them as acceptable for RN65 replacement when a value is NLA or requires a 100-resistor ($$$$) minimum. (That is my day job, finding obscure discrete parts......)

Mouser lists some 400+ RN65 series resistors - two 18 ohm in parallel will give you the 9.1ohm value almost immediately. If replacing all, you'll be looking at 25-30 dollars of resistors. Other vendors (Surplus Sales of Nebraska, West Florida Components, Talon) also have precision resistors - great for finding that odd value that Mouser doesn't stock.

_________________
Brian
"Capacitor Cosmetologist since 1979"
USN Retired 1984-2006 (Avionics/Cal)


Top
 Profile  
 
 Post subject: Re: Heathkit VTVM Resistors source
PostPosted: Nov Tue 19, 2019 7:17 pm 
Member

Joined: Jun Sat 09, 2007 8:14 am
Posts: 3911
Location: Florida
A Heathkit VTVM.

RRM


Top
 Profile  
 
 Post subject: Re: Heathkit VTVM Resistors source
PostPosted: Nov Tue 19, 2019 7:57 pm 
Member

Joined: Nov Wed 30, 2016 7:35 pm
Posts: 5362
Location: Sunbury, Ohio 43074
Take a million dollars and spend a million days and you’ll still have a vtvm that’s +/- 3% at best....

_________________
Preserving the hist. of electronics, one boat anchor at a time! :)
https://www.bbtvtestequipment.com


Top
 Profile  
 
 Post subject: Re: Heathkit VTVM Resistors source
PostPosted: Nov Tue 19, 2019 9:51 pm 
Member

Joined: Jul Mon 01, 2019 4:42 pm
Posts: 200
Location: St. Louis, MO
Findm-Keepm wrote:
you'll be looking at 25-30 dollars of resistors


Keep an eye out at Mouser. I remember for some of them, it was actually cheaper to buy 10 than to buy one... Gotta love quantity discounts.


Top
 Profile  
 
 Post subject: Re: Heathkit VTVM Resistors source
PostPosted: Nov Tue 19, 2019 10:08 pm 
Member
User avatar

Joined: May Wed 18, 2011 2:40 am
Posts: 4235
Location: Littleton, MA
Barry H Bennett wrote:
Take a million dollars and spend a million days and you’ll still have a vtvm that’s +/- 3% at best....

You can do considerably better than that, if you have a good 4.5 or 5 digit voltmeter as a reference and a stable power supply to calibrate against. I got my IM-28 to better than 1%, except for the 1.5 volt range:
Image

_________________
Steve Byan http://www.byan-roper.org/steve/steve-at-play/


Top
 Profile  
 
 Post subject: Re: Heathkit VTVM Resistors source
PostPosted: Nov Tue 19, 2019 10:17 pm 
Member

Joined: Nov Wed 30, 2016 7:35 pm
Posts: 5362
Location: Sunbury, Ohio 43074
Steve, you get a gold star! So, how many million $ and million days did it take? And how can one do better than the linearity of the meter movement and dial scale itself ? ;-)

I applaud your tenacity but personally, I'd rather trust the DMM for that kind of accuracy. Way better actually. But I sure like to see that needle kick!! I still use my Simpson 270 for go-no go transistor tests. And a quick capacitor check for any obvious bad guys.

_________________
Preserving the hist. of electronics, one boat anchor at a time! :)
https://www.bbtvtestequipment.com


Top
 Profile  
 
 Post subject: Re: Heathkit VTVM Resistors source
PostPosted: Nov Tue 19, 2019 10:45 pm 
New Member

Joined: Dec Wed 26, 2018 3:46 pm
Posts: 19
Hi all and thanks everyone for your replies. I wrote a post but it seems to have dissapeared!? Anyways I would like to qoute all but that would be a too long post :)

Interesting to see Stevebyan, I will start restoring and create a similar document with the results. Really good job there!!

The RN65 seems to be the way to go (21.5k etc). Would be so nice though to order all in one batch. I guess I lost patience a bit there for a while. :)

Thanks all for the tips! I really like these units and especially the IM-13 and up with the bigger meter :)


Top
 Profile  
 
 Post subject: Re: Heathkit VTVM Resistors source
PostPosted: Nov Tue 19, 2019 11:36 pm 
Member

Joined: Jan Tue 10, 2012 8:39 am
Posts: 787
Years ago HP solved the meter linearity issue by automating the scale printing for individual meters. I don't know the dates they did that or for which models.

Bob


Top
 Profile  
 
Post New Topic Post Reply  [ 23 posts ]  Go to page 1, 2  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: Majestic-12 [Bot], Notimetolooz and 10 guests



Search for:
Jump to:  




























Privacy Policy :: Powered by phpBB