sci.electronics.repair@googlegroups.com

Google Groups

Phil Allison <pallison49@gmail.com>: Jul 16 05:36PM -0700 Chris wrote:   -------------- > measuring the total resistance of a circuit with a) a DMM and b) an old > analog meter with a physical needle. And this doesn't only happen at high > impedance points, either. What could account for this?   ** The fact you are totally clueless ?   Semiconductors are not resistors and there is no right value to measure.   DMMs are designed NOT to cause didoes or BJT junctions to conduct when using the ohms ranges.   Also, DMMs ohm ranges are very sensitive to any residual DC or AC voltage on a component while analogue meters are much less so.   Interesting fact: you can measure the resistance of a loudspeaker voice coil with either type, but not if the room is full of loud bass noise.   Think about it.     ...... Phil

Ralph Mowery <rmowery28146@earthlink.net>: Jul 16 11:01PM -0400 In article <okgudk$djj$40@dont-email.me>, curd@notformail.com says... > decent gear would provide expected voltage readings for 20k OpV analogue > meters, the AV0 model 8 specifically IIRC, which saved the technician the > extra bother of scratching his head when checking hi-z parts of a circuit.   Yes, I have seen some schematics specify the voltage was measured with a certain ohms/volt meter. It does take some guess work or head scratching out of the servicing. Makes a big difference when measuring the screen votltage that is fed with a high value resistor and you use a vtvm or a 20 k ohms/volt or a 5 k ohms /volt meter.   I don't think I have done any trouble shooting on a tube circuit in 20 or 30 years. Last thing I recall even looking at with tubes was my Heathkit sb101 that was many years ago. Only other thing I have with tubes are a couple of RF amplifiers and they use voltages that I don't work on with any power on them.

Jon Elson <elson@pico-systems.com>: Jul 16 11:12PM -0500 Chris wrote:     > I've got four DMMs and two analogs. The DMMs agree with the other DMMs > and the analogs agree with each other. But the different types don't > agree with each other! I have some older DMMs. I get odd resistance readings the first time I set it to Ohms scale. By wiggling the plugs on the probes at the meter end, and occasionally working the range selector dial around a few times, I get a stable Ohms reading of about 0.4 Ohms with the probes shorted. Then, I get more sensible readings on circuits. So, these meters get poor contact on the range selector switch and the bannana jacks. So, that is one thing to check for.   Second, most DMMs use very low voltage to meke Ohms measurements. Analog meters often used 9 or even 22 V batteries for the Ohms range, to push enough current to move the needle on high resistance circuits.   If there are any seminconductors in your circuit, a DMM likely will not give enough voltage to forward bias any junctions, while an analog meter will.   Jon

oldschool@tubes.com: Jul 16 11:28PM -0400 On Sun, 16 Jul 2017 23:12:42 -0500, Jon Elson <elson@pico-systems.com> wrote:     >If there are any seminconductors in your circuit, a DMM likely will not give >enough voltage to forward bias any junctions, while an analog meter will.   >Jon   Sounds to me like you need to spray some contact cleaner in your test meter switches, and clean the plug contacts too.

Phil Allison <pallison49@gmail.com>: Jul 16 09:38PM -0700 Jon Elson wrote:   ----------------- > more sensible readings on circuits. So, these meters get poor contact on > the range selector switch and the bannana jacks. So, that is one thing to > check for.   ** A little WD40 or similar will fix that in seconds.   A short squirt under the edge of the switch followed by a few complete rotations is all you do. Clean up any excess.   Wet a cloth to do the 4mm plugs and a cotton bud to do the sockets.     ..... Phil

oldschool@tubes.com: Jul 17 12:07AM -0400 On Sun, 16 Jul 2017 16:30:03 -0700, Jeff Liebermann <jeffl@cruzio.com> wrote:   >These were probably the most common VOM's available. At one time, I >had about 5 of them. They've been replaced by more modern meters, but >I still keep one of them on the shelf, just in case.   Yep, I've seen several of them models. I just never owned any of them. I generally used one of my VTVMs (Heathkit or Eico) for most in circuit testing on tube gear. I have always been pretty satisfied with my older Radio Shack VOMs. I blew up a few of them many years ago, but I learned what NOT to do, and to pay attention and make sure I dont have the meter on the ohms scale when I measure voltages.   I also have a GB Instruments model GMT-19A VOM sitting right next to me, that I use for darn near everything, and it's held up well for many years.   I am not real fond of digital meters. I find them confusing, since they tend to pick up stray voltages and they also take awhile to "settle" on a reading. I have some of them, and I use them for some things, but my analog meter are usually the first ones I grab.   Seeing those Simpson meters makes me want to look for one to buy though. Just seems there should be one on my bench.......

Mike Coon <gravity@mjcoon.plus.com>: Jul 17 09:08AM +0100 In article <okgudk$djj$40@dont-email.me>, curd@notformail.com says... > decent gear would provide expected voltage readings for 20k OpV analogue > meters, the AV0 model 8 specifically IIRC, which saved the technician the > extra bother of scratching his head when checking hi-z parts of a circuit.   For amusement I fetched the Avo Model 7 that I inherited from my father (goodness knows when he got it!). The plate on the back specifies 1.5V cell for < 1M, 2x4.5 for 1M, and external voltage sources for 10m and 40M ranges.   It also weighs 5lb (2+kg). I guess in those days servicemen were MEN!   Mike.

Cursitor Doom <curd@notformail.com>: Jul 17 08:27AM On Mon, 17 Jul 2017 09:08:01 +0100, Mike Coon wrote:   > It also weighs 5lb (2+kg). I guess in those days servicemen were MEN!   Yes, and built like a tank! I have a Model 7, too, IIRC it was made in 1943 and is still going strong. I'll wager when it was first produced, techs of the day marvelled at how light and portable it was. ;)

tabbypurr@gmail.com: Jul 17 02:50AM -0700 On Monday, 17 July 2017 09:31:17 UTC+1, Cursitor Doom wrote:   > Yes, and built like a tank! I have a Model 7, too, IIRC it was made in > 1943 and is still going strong. I'll wager when it was first produced, > techs of the day marvelled at how light and portable it was. ;)   My 1920s v/i meter weighs a small fraction of that. Avos were high impedance accurate bench meters.     NT

oldschool@tubes.com: Jul 17 05:00AM -0400 On Sun, 16 Jul 2017 21:24:23 -0000 (UTC), Cursitor Doom   >They've been exported all over the world for decades!   >They typically use one 15V battery (about the size of a AA cell) and a >single 1.5V D cell.   This is one I've never seen. I have seen Simpson and Triplett vom's but not this one.   I dont think I'd want any of these that need these special batteries. I have looked at some of the specs for the older Simpson 260 meters and some of the early models had oddball batteries too. The series 5 has D and AA batteries, and the series 6 an above have D and 9V.   I do have to ask, why these meters used BOTH the D cells and other batteries as well. (In other words, more than one kind of battery). Why didn't they just use one battery or one set of batteries for the whole device?   My Radio Shack and GB meters only have 2 AA cells, and work fine.

Cursitor Doom <curd@notformail.com>: Jul 17 10:17AM On Mon, 17 Jul 2017 02:50:34 -0700, tabbypurr wrote:   > My 1920s v/i meter weighs a small fraction of that. Avos were high > impedance accurate bench meters.   "High impedance" back then, yeah. :)

Cursitor Doom <curd@notformail.com>: Jul 17 10:24AM On Mon, 17 Jul 2017 05:00:55 -0400, oldschool wrote:   > batteries as well. (In other words, more than one kind of battery). Why > didn't they just use one battery or one set of batteries for the whole > device?     Dunno. I'm sure someone here will, though. Fortunately, although 15V batteries are largely unobtainium these days, 10x1.5V AAA cells will get you there and the battery compartment is capacious enough (AVOs are anyway) to house them all comfortably.

tabbypurr@gmail.com: Jul 17 05:39AM -0700 On Monday, 17 July 2017 11:21:54 UTC+1, Cursitor Doom wrote:   > > My 1920s v/i meter weighs a small fraction of that. Avos were high > > impedance accurate bench meters.   > "High impedance" back then, yeah. :)   They were, you could always get lower resistance meters for a lot less. Why it took so long for analogue meters to get FETs I don't know. My 1920s meter is moving iron, so the resistance is dreadful & it's nonlinear. But its worst shortcoming is that the case is the -ve electrode, you hold it in your hand and it's bare metal. AND it's low resistance, so measuring radio/TV HT was always a fun experience. Maybe they figured if they killed their customers there wouldn't be any requests for refund.     NT

tabbypurr@gmail.com: Jul 17 05:40AM -0700 > batteries as well. (In other words, more than one kind of battery). Why > didn't they just use one battery or one set of batteries for the whole > device?   A 1.5v cell is high capacity, high current & cheap. 15v batteries are low capacity, low current & never cheap.     NT

Jeff Liebermann <jeffl@cruzio.com>: Jul 17 07:26AM -0700 >batteries as well. (In other words, more than one kind of battery). Why >didn't they just use one battery or one set of batteries for the whole >device?   The Rx10K scale needs high voltage and low current to measure large resistors. The Rx1 scale needs low voltage and high current to measure the small resistors. One could use the high voltage 15v battery for measuring low resistances, but you would be replacing 15v batteries at an a alarming rate. Be thankful that the ancient designers of the VOM did not include a different battery for each resistance scale.   -- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558

Jeff Liebermann <jeffl@cruzio.com>: Jul 17 07:59AM -0700 On Mon, 17 Jul 2017 10:17:54 -0000 (UTC), Cursitor Doom   >> My 1920s v/i meter weighs a small fraction of that. Avos were high >> impedance accurate bench meters.   >"High impedance" back then, yeah. :)   Not high enough. If you want to measure really high resistances, such as insulation leakage, you need a Megger (which is actually the name of the company that makes them but has become somewhat of a generic term for high voltage resistance testers): <https://www.google.com/search?q=megger+meter&tbm=isch> If you want to see if you really have water in the coax cables, you need one of these insulation testers.   I have an old and ugly meter, which has a hand crank generator. It produces enough voltage to have given me a rather nasty shock. It's fairly difficult to electrocute oneself while operating the crank, but I managed. Some modern Megger models still have such hand cranks generators: <http://www.tequipment.net/Megger212160.html> <http://us.megger.com/extended-range-insulation-resistance-testers-210170-and-210600-> These small testers will deliver 1000V in order to measure up to 2000 Mohms. Now, does anyone still want to complain about 15v batteries?   -- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558

oldschool@tubes.com: Jul 17 12:26AM -0400 On Sun, 16 Jul 2017 18:46:16 -0400, Ralph Mowery >than a new item. Like one older battery drill. The batteries would not >hold a charge. For slightly more than one battery, I bought another >drill of better quality that had 2 batteries.   This is a hobby for me too, but I dont even want to work on the extremely small stuff. I'd leave the hobby real fast. I find stuff that small very nerve wracking to work on, and since my eyesight is not real good, I cant see the stuff too well. Not to mention that for most of that stuff, you cant even get parts since they dont ID the parts anymore, and even if they did, as you said the parts cost more than the item.   I enjoy working on tube stuff. I will tackle some early transistor stuff if it's worth fixing, but I usually only work on stuff that I enjoy doing.   I only tore this solar charger apart to save the solar cell panel. I know the unit was trash before I even opened it. None of the chips on it even have any numbers. Before I even spend an hour on that thing, I'd sooner buy another solar phone charger for around $25. (not that same brand though). But I dont really need one anyhow. Those $6 battery packs work just fine and I can charge them on my computer, with my AC phone charger, or in my car with my car phone charger. I dont even use my phone real much, but it always seems to need charging when I do need it. Those power packs work just fine.

tabbypurr@gmail.com: Jul 17 02:40AM -0700 > that stuff, you cant even get parts since they dont ID the parts > anymore, and even if they did, as you said the parts cost more than the > item.   Surface mount parts are trivial to get for nothing.     NT

Ralph Mowery <rmowery28146@earthlink.net>: Jul 17 09:54AM -0400 In article <e9eomc5eq6g0lb2hta7pfc9llnnrqo64lt@4ax.com>, oldschool@tubes.com says... > charger, or in my car with my car phone charger. I dont even use my > phone real much, but it always seems to need charging when I do need it. > Those power packs work just fine.       I did see a typo in my price list. Put in 800 instead of just 80 like I should have. It will take about $ 350 to get set up for the SMD repairs.   Sometimes it is fun or interisting to take broken items apart to see what is in them and maybe salvage some parts.   Many of the SMD parts are really very inexpensive. The older tube parts are getting very expensive, and some that were common years ago are getting hard to find.

"David Farber" <farberbear.unspam@aol.com>: Jul 16 06:02PM -0700 > combined with the small size of these parts, is one reason I prefer > working on old tube gear. Maybe someone else on here can point you in > the right direction regarding that transistor. I cant....   It does a few fancy things. There are three selectable speeds which can be selected by successive presses of the power button. The last keypress of the button selects a random mode where the speed and the direction of the motor varies.   I've done some more checking and both of the Y1 transistors, which are NPN, have failed. One is open B-E and the other is open B-C. I can't remember many, or even any transistors that have ever opened B-C. I'm going to check it a few more times to make sure I'm making solid contact with the terminals when using my DVM. I'll still haven't been able to positively identify the specs of those transistors.   Thanks for your reply.   -- David Farber

oldschool@tubes.com: Jul 16 11:48PM -0400 On Sun, 16 Jul 2017 18:02:52 -0700, "David Farber" >when using my DVM. I'll still haven't been able to positively identify the >specs of those transistors.   >Thanks for your reply.   Those transistors are the speed controls. I wonder if they might be SCRs or Triacs, (generally used in motor speed controls and light dimmers), rather than common transistors. It kind of sounds like your motor failed, or shorted until you tore it apart, and that wiped out the transistors.   But like most everything these days, they dont identify the semiconductors, because they want you to toss the whole device and buy a new one. All I can suggest is to try some new transistors and see if they work. Or try some triacs. (At least you know they are NPN). Since I mostly work on old tube gear, I cant even suggest what sort of transistors to try. Maybe someone else on here can shed some more light on the subject.   I suppose you could also buy another of these cat toys and measure the parts on the new one so you know what these parts show for measurements. I recall seeing those things advertised on tv a few years ago. From what I recall, they were not real expensive.   (Or just buy a plain old live mouse, which have built in speed controls, and use non-electronic sensors to run away from cats) :)   Good luck with it.

tabbypurr@gmail.com: Jul 17 02:38AM -0700 > Those transistors are the speed controls. I wonder if they might be SCRs > or Triacs, (generally used in motor speed controls and light dimmers), > rather than common transistors.   It really would make no sense to use those in this circuit   > It kind of sounds like your motor > failed, or shorted until you tore it apart, and that wiped out the > transistors.   I doubt it. These toys are chinese junk construction, transistors hopelessly underrated.   > parts on the new one so you know what these parts show for measurements. > I recall seeing those things advertised on tv a few years ago. From what > I recall, they were not real expensive.   It's hardly difficult to measure the motor to see what tr specs are required   > (Or just buy a plain old live mouse, which have built in speed controls, > and use non-electronic sensors to run away from cats) :)   I guess I'm not a fan of unnecessary torture.     NT

Phil Allison <pallison49@gmail.com>: Jul 16 05:28PM -0700 Ralph Mowery wrote:     > > work just fine.   > My oven will not regulate that low, so I put the wet equipment in a car > that is out in the sun in the summer.   ** The trick is to bring the oven up to about 100C, then turn it off.   Place the item inside and close the door, it will heat to about 75C and be dry as a bone in 15 minutes. Take it out and let it cool.   75C is not hazardous to electronic components.     .... Phil

ABLE1 <someone@nowhere.net>: Jul 16 08:38PM -0400 On 7/16/2017 8:28 PM, Phil Allison wrote:   > 75C is not hazardous to electronic components.   > .... Phil   More CHOICES............... I love it!!!   Thanks Phil,   Les

You received this digest because you're subscribed to updates for this group. You can change your settings on the group membership page. To unsubscribe from this group and stop receiving emails from it send an email to sci.electronics.repair+unsubscribe@googlegroups.com.