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 Post subject: Eico 221 VTVM resistor replacements
PostPosted: Feb Thu 14, 2019 1:35 am 
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Location: Opelika, AL
I'm ordering some parts I need to finish a few long ago started projects, and need some help with this one.

One the schematic for this there are a bunch of resistors that are non-standard values, and I'm trying to figure out how critical they are to circuit function.

1st question is R7, which is listed as 5M. this feeds into the zero adjust circuit for AC volts. I'm wondering if 4.7M is ok here, and also what the worst case voltage drop across this resistor would be. My amatuer hour calculations showed something like 1KV could be dropped by this resistor when measuring 1000V AC. If someone else can look at this and tell me if I'm correct in this assumption, it would be most appreciated. I did the math based on peak volts of 1.4KV, and about 225µA of current flowing there.

The other points of interest were the C wafer of the range switch, where R23, R24, and R25 don't have any tolerance listed, but seem to be an integral part of the resistor network for the voltage divider. Are these critical tolerance components? R23 and R25 are non-standard values. If they are out of tolerance and not replaced with exact values shown in the schematic, how do they effect the voltage measurements? Part of me thinks that they are non-critical, since they don't form part of the voltage divider, but rather look like they're there to limit current in the circuit rather than serve any purpose in the actual voltage measurement.

Last one that is non-standard is R9, listed as 5M, with no tolerance. is 4.7M ok here for this grid (leak?) resistor?

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 Post subject: Re: Eico 221 VTVM resistor replacements
PostPosted: Feb Thu 14, 2019 6:51 am 
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Location: Portland Oregon
Can you fit in 2-10Mohms in parallel to make a 5 Mohm?

This resistor is not as important as R18 and 10% is probably OK. What is the spec for the resistor? Personally, the 1% metal film resistors are really cheap and I just use these for everything on my instruments. They are more stable than carbon and you aren't worried about the potential small changes of inductance potentially introduced by these resistors.


Last edited by TechyMechy on Feb Fri 15, 2019 6:46 am, edited 1 time in total.

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 Post subject: Re: Eico 221 VTVM resistor replacements
PostPosted: Feb Thu 14, 2019 8:25 am 
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Location: Mpls, Minnesota
Mouser and Digikey both have 5 meg resistors.

Dave


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 Post subject: Re: Eico 221 VTVM resistor replacements
PostPosted: Feb Thu 14, 2019 1:07 pm 
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R23, 24, 25 will be critical to the accuracy of the instrument. R7, since it's in a circuit with an adjustment, probably not so much. The grid leak 5M also probably not so much since there is a balance adjustment in that tube circuit. Original tolerances should be on the resistor body.... you may find that they were 10% or even 20% resistors, except for the voltage divider chain which will be important to keep the same

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 Post subject: Re: Eico 221 VTVM resistor replacements
PostPosted: Feb Thu 14, 2019 2:23 pm 
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Can someone give an explanation of what is happening with those three resistors and how they effect accuracy? I don’t have the guts of this thing in front of me at the moment, so can’t check the tolerances, though the other Eico VTVM I have (a 249) has all 5% resistors aside from the voltage dividers.

My 249 has enough bad (drifted high out of spec) carbon comp resistors that I’m planning on replacing all of them. I haven’t decided if I’m doing that with this one yet, but leaning towards it, since some of the parts are not super easy to reach without taking others loose too, and I’d ideally like to repair once and be done, rather than mess around in there multiple times.

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 Post subject: Re: Eico 221 VTVM resistor replacements
PostPosted: Feb Thu 14, 2019 3:14 pm 
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r23-25 are 20% tolerance ones at the moment in this particular meter. No idea what they "should" be. I know I can just replace with tight spec stuff and be done, but I'd also like to know how this part of the circuit is working, and gain some knowledge, in addition to having a VTVM that is working at rated spec or better.

I'm also still curious about the voltage seen by r-7, and whether my assumptions there are correct that the worst case scenario is about 1KV. Selecting a resistor with this rating is easy enough, but I've got to be sure to put it on my order if I need it. My understanding of how to do this is to calculate the current through all of the resistors in series, and then use that to calculate the voltage drop across that particular resistor. Is this the correct way to figure this value In this instance?

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 Post subject: Re: Eico 221 VTVM resistor replacements
PostPosted: Feb Thu 14, 2019 3:19 pm 
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Location: Opelika, AL
TechyMechy wrote:
Can you fit in 2-10Mohms in parallel to make a 5 Mohan?

This is an important resistor, IMHO


2 10M resistors would probably work in the places where there they call for 5M. All is point to point wiring, plenty of space to be had to piggy back things if necessary. As easyrider8 (dave) pointed out, 5M are available. I was just wondering how much changing to 4.7 would effect the circuit negatively. It's easy enough to simply replace parts, but I'm always trying to learn why certain parts work better, where in a circuit tight tolerances are actually important, etc, etc. Since I'm mostly self taught, with a lot of help from my late great uncle Fred, and many of the kind people here, there are still some giant gaps in my understanding/knowledge... Trying to make those holes smaller as I fix things.

-jon

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 Post subject: Re: Eico 221 VTVM resistor replacements
PostPosted: Feb Thu 14, 2019 7:49 pm 
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It is your meter, going outside the specifications will yield unexpected results. Correct resistors are cheap, a little home work will find them most everywhere. How can accuracy be maintained if it is not put in initially?

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 Post subject: Re: Eico 221 VTVM resistor replacements
PostPosted: Feb Thu 14, 2019 8:07 pm 
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Location: Mpls, Minnesota
If you want to know how a VTVM works download and read this book.
http://mirror.thelifeofkenneth.com/lib/ ... 1_text.pdf

Dave


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 Post subject: Re: Eico 221 VTVM resistor replacements
PostPosted: Feb Thu 14, 2019 8:51 pm 
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Quote:
Since I'm mostly self taught, with a lot of help from my late great uncle Fred, and many of the kind people here, there are still some giant gaps in my understanding/knowledge... Trying to make those holes smaller as I fix things.

Read everything you can find.

As far as resistors, in some circuits (such as voltage dividers in VOM and VTVM's,) they are critical and will make your readings with that device less precise, or even useless. In other circuits, there is a tolerance that's accepted.

That is why there are resistors made in 20% tolerance, 10% tolerance, 5%, 1%, 0.1%, and if you really want to spend a TON of money, even better can be had. Or using today's $100 testers, you can hand pick your desired ohmage (ohmmage?) out of a batch with accuracy that was impossible "back in the day".

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 Post subject: Re: Eico 221 VTVM resistor replacements
PostPosted: Feb Thu 14, 2019 9:49 pm 
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R23, R24, and R25 are in series with the input grid of the 6SN7. They're chosen to keep the grid resistance relatively constant for all voltage ranges. The idea is to compensate for grid current and keep the bridge balanced when changing voltage ranges. This isn't usually a big problem and most VTVM's don't have this extra circuitry.

RRM


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 Post subject: Re: Eico 221 VTVM resistor replacements
PostPosted: Feb Fri 15, 2019 12:32 am 
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Location: Opelika, AL
thanks RRM!

I made a simulation of the bridge using n type jfets in icircuit, and changing the values of those resistors didn't change the current across the resistor I was using to simulate the meter. In a perfect world, there should be no current flowing from the grid, correct? what would happen if these resistors were left off in the actual measuring circuit in this VTVM? and why don't most VTVM's have this extra circuitry?

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 Post subject: Re: Eico 221 VTVM resistor replacements
PostPosted: Feb Fri 15, 2019 2:54 am 
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Keeping the grid resistances fixed is in the realm of good practice. Whether it's really necessary in a particular case depends on the design and parts used. Most VTVM's don't have this kind of compensation because grid current in the bridge tube is expected to be so small that it will not cause enough meter deflection as to be noticeable. Some designs grounded one grid, others used fixed resistances chosen to sort-of balance things.

RRM


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 Post subject: Re: Eico 221 VTVM resistor replacements
PostPosted: Feb Fri 15, 2019 4:05 am 
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So is it fair to assume that the 5M value for the grid leak was at least slightly capricious on the designer's part, and that what is currently "standard" at 4.7M is plenty good enough to keep the meter in spec? And the same would go for the 5M resistor that is R7? I have read a few times that often times these values were picked due to practicality of cheap parts rather than because one value was inherently better than another. I'd still love to know a little more as to why certain values are chosen for these grid resistors amongst others, if someone has it in them to explain. is 5M inherenently better than 4.7, and if more is better, why not 10M?

I know compared to an HP 410B or some nice piece of genrad kit, that whatever I do to these consumer grade VTVM's is basically polishing a proverbial turd, but there is much to learn from studying them, and messing about with upgrading them to better components. even if what one learns is that the limiting factor in the precision of the circuit lies elsewhere than some particular resistor value.

I just found these pages again showing descriptions of the circuits used in VTVM's from march/april 1959 popular electronics. very helpful descriptions of the circuits and how they work:

http://www.rfcafe.com/references/popular-electronics/vacuum-tube-voltmeter-april-1959-popular-electronics.htm

http://www.rfcafe.com/references/popular-electronics/vacuum-tube-voltmeter-ohmmeter-5-1959-popular-electronics.htm

The circuits shown show the two principal designs that I have looked at basically the eico 221 and the eico 232/249/everyone else circuits. Hopefully these are helpful to other curious souls out there.

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 Post subject: Re: Eico 221 VTVM resistor replacements
PostPosted: Feb Fri 15, 2019 4:17 am 
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Location: Sunbury, Ohio 43074
Quote:
is 5M inherenently better than 4.7, and if more is better, why not 10M?

ummmmmm no, more is not inherently better. Neither is 5M vs 4.7M. Unless you thoroughly understand the circuit, and can analyze it from the standpoint of someone actually engineering the instrument, sticking with the original design values would seem the more prudent course of action.

5M vs 10M would most likely render ANY circuit inoperable. By that logic, why not 1M instead? You can't just randomly decide to select a specific part value without a full understanding of the device in question.

If you're talking about $$$, more is probably better. But not resistors. :)

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 Post subject: Re: Eico 221 VTVM resistor replacements
PostPosted: Feb Fri 15, 2019 5:34 am 
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Location: Opelika, AL
Barry H Bennett wrote:
Quote:
is 5M inherenently better than 4.7, and if more is better, why not 10M?

ummmmmm no, more is not inherently better. Neither is 5M vs 4.7M. Unless you thoroughly understand the circuit, and can analyze it from the standpoint of someone actually engineering the instrument, sticking with the original design values would seem the more prudent course of action.

5M vs 10M would most likely render ANY circuit inoperable. By that logic, why not 1M instead? You can't just randomly decide to select a specific part value without a full understanding of the device in question.

If you're talking about $$$, more is probably better. But not resistors. :)



Yes, of course. this is all crystal clear, and I'm not just dropping in random parts. I'm also perfectly capable of ordering and installing the correct parts from the schematic. I am asking if anyone can tell me why they chose X vs Y in this situation so as to learn something. I am not an EE, and I don't even play one on TV, so how else to learn but to ask questions of folks who know how the instruments were engineered??? (and of course read things and hope to gain understanding).

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 Post subject: Re: Eico 221 VTVM resistor replacements
PostPosted: Feb Fri 15, 2019 5:59 am 
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Location: Mpls, Minnesota
anchorman wrote:
I just found these pages again showing descriptions of the circuits used in VTVM's from march/april 1959 popular electronics. very helpful descriptions of the circuits and how they work:

If you really want to know how VTVM's work download the link I provided in an earlier post. This is the book by Ryder

Dave


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 Post subject: Re: Eico 221 VTVM resistor replacements
PostPosted: Feb Fri 15, 2019 6:40 am 
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Location: Portland Oregon
Hey Dave,

I started to read the book you referenced from Ryder. I'm a good 60+ pages in...

It really is a fairly easy to understand, complete read. Thanks for the pointer.

One thing I have problems with with Ryder is that he flips between conventional current (holes) and electron current (correct). For vacuum tubes, no matter how hard I try, it is much easier to think of the circuits as electron transport (like in Marcus and Levy books).

Thanks again!


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 Post subject: Re: Eico 221 VTVM resistor replacements
PostPosted: Feb Fri 15, 2019 7:01 am 
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Location: Opelika, AL
Hi Dave,

I have downloaded, and started to read, thanks for sharing that! Nothing I've come across yet talks about why 5M is preferred over 4.7M resistors in grid leak and before the AC balance pot on the 6H6/6SN7 version of these meters (eico 221). the 12AU7 versions that are all close variations on a particular PP circuit very different values in this place eico 249 - 4.7M for the grid leak resistor, the heath IM-11 has 10M grid leak, knight kg625 has 6.8M. I'm guessing that it is something to do with available cheap parts for one design or the other. Not sure if it is related to the tubes used, or other parts of the circuit that I'm not taking into account. I did skim past a part that talked about RC constants for ac measurements, and also something talking about high value of grid leak being problematic from a stability standpoint, and low value caused some other issue. The RC constant is looking like what I need to look at closer, which would also explain why these meters also have vastly different AC blocking caps, with the eico 249 having the largest, and the heath IM-11 the smallest. It's late, and there is work in the morning, so I'll have to read more later.

If anyone wants to pipe in and help a guy out with a clear explanation, please feel free!

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 Post subject: Re: Eico 221 VTVM resistor replacements
PostPosted: Feb Fri 15, 2019 7:05 am 
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well, scratch that - the grid leak on the triode isn't related to the resistor involved in the RC part of the AC section... the 5M from r7, however is?

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