TV-7 micromhos conversion calculator

TV-7 tube tester micromhos calculator
Meter: µmhos

DIRECTIONS:

  1. Enter your meter reading (0 – 120) into the “Meter” box.
  2. Choose your Range Switch position in the drop-down box.
  3. Press the “Calculate Gm” button and read your micromhos score.
  4. Press “Reset” button to clear your entries or to start another calculation.
  5. Invalid entry ( meter > 120 ) will automatically reset the calculator.

NOTES:

  • calculator works for all TV-7 models: TV-7/U, TV-7A/U, TV-7B/U, and TV-7D/U
  • calculator requires javascript
  • Range A on TV-7 is only used for emission testing of diodes and rectifiers — no mutual conductance reading exists. Therefore, I have omitted Range A from the calculator. The Range A meter reading is an arbitrary emission score that is evaluated in relation to the “Minimum Value” notation in the setup book.
  • Range B signal voltage is 5.0v ac with bias 0 to -40 vdc
  • Range C signal voltage is 5.0v ac with bias 0 to -40 vdc
  • Range D signal voltage is 1.0v ac with bias 0 to -40 vdc
  • Range E signal voltage is 0.5v ac (500 mV) with bias 0 to -40 vdc
  • Range F signal voltage is 0.5v ac (500 mV) with bias 0 to -4 vdc.  Range F is 0 to 30,000 micromhos, not 60,000 as reported elsewhere.  Click to “Read the rest of this entry” below for details.
  • ©2014 TubeSound

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repair broken TP-Link TD-8616 ADSL modem

If you are trying to fix a TP-Link TD-8616 modem, chances are good that this is your problem.

On May 21, 2012, I purchased from Newegg a new TP-Link ADSL TD-8616 DSL modem to replace a very old Westell modem that was supplied by Verizon. At that time, the reviews for this product were overwhelming positive. Less than two years later, this TP-Link modem is already broken, and I see that recent reviews are more up and down.

Simply put, this modem had garbage capacitors, which is a common quality-control manufacturing problem with modern electronics.

Symptoms: for the past several months, connection was erratic, sync erratic, speeds fluctuating. Unable to connect to modem admin via TCP-IP.

Repair was as follows: My TP-Link TD-8616 was version 6. It has 7 capacitors, of which 6 were badly defective, brands were “GJT” (which were labeled “Low ESR and 105°”)  and “Leaguer” (which were not specified to be Low ESR but were marked as 105°). They were real junk.  Defects were very high ESR in all, and in a few, low capacitance, and one of 100uf caps was open. Here are photos as a few examples:

100uf high-esr and low capacitance

a 470uf “GJT” capacitor with bulging vent, high-esr and low capacitance.  The other caps that I removed from the modem are piled on the face of the meter.

Very high esr in this 100uf "Leaguer" cap

Very high esr (20 ohms) in this 100uf “Leaguer” cap

All the other caps, except for one, tested similarly bad ESR — extremely bad. These were easy to troubleshoot in-circuit with my ESR meter, so repair was easy to diagnose and fast to perform. The values of these caps (two of 470uf and 5 of 100uf) are all values that I keep on hand, so the modem was repaired in less than a half hour.  Since all seven caps were from same manufacturing lot, and therefore junk, I replaced all of them even though one was still good.

ESR is “Equivalent series resistance” and is measured in ohms. As a comparison, I will show you how a “good” and “better” capacitor will test for ESR. First of all, you should understand that ESR must be compared ( vs. )  a new cap of SAME CAPACITANCE value. In other words, you cannot compare the ESR of a 100uf to that of a 470uf. Second, if you are using an ESR meter, do not rely on the “ESR charts” they give you. They are very loose guidelines and not necessarily accurate. Your best comparison will always be to compare the old cap vs a new cap of same capacitance.

Comparison of new 100uf caps:

a typical general purpose brand new 100uf capacitor being tested for ESR

a typical general purpose brand new 100uf capacitor being tested for ESR

This photo shows testing a typical new “general purpose” 100uf capacitor.  ESR is 0.72 ohms, and in my experience should be less than 1.0 to be of reasonable quality.  This cap is perfectly fine for most uses, but will not work well in a circuit that requires a low ESR capacitor.

Rubycon ZL-series low ESR 100uf capacitor

Rubycon ZL-series low ESR 100uf capacitor

This photo shows testing of a Rubycon ZL-series 100uf low ESR capacitor. As you see, it tests much lower ESR than a “general purpose” capacitor, testing at 0.10 ohms. This cap would be a good choice for low ESR circuits.

One final note: don’t get caught up in thinking that you should always buy low ESR caps for all of your projects. Some regulators (low dropout regulators) require a certain amount of ESR in order to work properly. (Often, if you examine the datasheet of the regulator, it will specify a minimum and maximum ESR for the capacitor.) Thus, installing a low ESR cap will actually cause that kind of circuit to not work at all, or be unstable. In most cases, I only use low ESR caps when they are required, such as in switch mode power supplies.

the Crosley Auto Expressionator circuit

In the 1930s, Crosley designed a tone-volume expander circuit that would, in theory, add fidelity to the music that you were receiving from over-the-air radio AM transmitters.  The circuit was said to boost the bass and expand the volume.  It was used in some of their better 1930s Crosley console radios that are popular with radio collectors today.

I have no personal experience with the circuit because I do not service or work on Crosley radios, but a good friend of mine that services antique radios says that the circuit adds nothing of practical value and is tantamount to an early example of tech-snakeoil.  Looking at the schematic, I am inclined to agree, but without any actual hands-on experience with the circuit, I would not want to prejudge it.

For those of you either servicing one of these radios, or who enjoy reading about esoteric circuit designs of yesteryear, [ this article ] from National Radio News, July 1936 issue, will be a good read.  I scanned this article and cleaned it up with Photoshop as a courtesy to my readers.

the Reprocessed Tube Racket

I found an interesting tube-related article in the Dec-1955-Jan-1956 issue of NATIONAL RADIO-TV NEWS.  The article exposed a supply-chain problem during the 1950s where unscrupulous vendors would buy “pull-outs” (duds or near-end-of-life tubes) from repair shops, then manipulate the tubes in a manner that would make them look reasonably new.  The tubes were then resold to distributors and repair shops, who assumed that they were buying “factory seconds” or possibly even fresh inventory.

The defect rate was enormous.  The first stealth order of 21 tubes showed 14 tubes to be defective; the second stealth order had 17 defects of 20 tubes purchased.

No doubt that some of these gems are still floating around today.  While the article seems to suggest that many of the tubes were resold with their original brand name intact, I suspect that most of the “off-brand” or “private label” tubes were using the same unseemly source (pull-outs) for their inventory.

When I test vintage tubes labeled as “Standard Brand”, “Rad-Tel”, “Atomic”, etc…, the defect rate is far too high (in my opinion) for them to have been “factory seconds” or “quality used”.  That doesn’t mean that all of them are duds, of course, but that they need to be carefully checked before selling or using.

I would point out that the advice at the end of the article is not completely relevant today, almost 60 years later.  It is not uncommon for the tube designation to be hard to read, or completely missing, because some labeling can be EASILY wiped off while cleaning the glass envelope.  Furthermore, it is common to find NOS tubes with oxidized tube pins that must be cleaned — sixty years and varying storage climates will “do that.”

[ Here is a PDF ] of the article that I scanned & cleaned up with Photoshop for good readability.

PACO SA-50 Tube Amp Rebuild

I find it exciting to discover a nice audiophile amp from a nondescript brand.  In this article, I chronicle my rebuilding of a PACO preamp-amplifier model SA-50.

PACO is the Precision Apparatus Company, best known for building quality test equipment such as tube testers, signal generators, and oscilloscopes.  The “PACO” branded gear (as opposed to gear that used the full name of “Precision Apparatus”)  was sold as a build-it-yourself kit.  During this rebuild and subsequent troubleshooting, I found two connections that were never soldered, and one loose solder joint.  Those issues are extremely common to find when servicing vintage kit-built gear.

I do not have any paperwork or background information about this model.  I did find a schematic for a model SA-40, which appears to be largely the same circuitry.

The SA-50 is a stereo preamp-amp with these design provisions: Read the rest of this entry »


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