RCA WT-524A Transistor Tester

What types of transistor testers did they have in 1971? Unlike the little automated Chinese testers of today, they were types that used an analog meter that required a measure of sophistication to use. For example, the Heathkit IM-36 Transistor Tester required the user to have at hand the specs for the transistor being tested. Then after obtaining the results, the user would use calculations to determine functionality. This tester looks like it could be used for rudimentary transistor analysis. With vacuum tube testers, the user had to rely on tables provided by the tube tester manufacturer for the test equipment’s settings for testing each type of tube. However for transistors, because there were so many different types of semiconductors being made, the transistor tester manufacturers couldn’t keep up with the proliferation of the many and varied types of semiconductors.

To further explain the WT-524A transistor tester, I’d like to describe the various types of testers that are available today:
There are three basic types of transistor “testers.”

• Quick-check in-circuit testers
• Service type testers
• Laboratory-standard testers/analyzers

Most transistor testers are also designed to test diodes.

Note. Curve tracers are the most accurate at indicating a transistor’s performance.

Quick-Check In-Circuit Testers

An in-circuit tester is used to test whether a transistor that has been performing properly in a circuit is still functioning. Basically, this type of tester merely indicates whether the transistor is dead or still operational. The advantage of these testers is the transistor does not necessarily have to be removed from the circuit.

These testers are in essence dual diode checkers.  Most modern meters have a feature enabling the technician to check each half of a transistor. However, the term in-circuit is a bit of a misnomer. If other components in the circuit affect the transistor's function in so far as the test is concerned, then these checkers won't give valid or accurate results. 

Service Type Transistor Testers

These devices usually perform three types of checks:

• Forward-current gain (H_FE) or the beta of a transistor.
• Base-to-collector leakage current with emitter open (I_CO)
• Short circuits from collector to emitter to base.

Some service testers include a working/not working feature, indicating a pass when a certain H_FE is reached. These are useful, but fail on some functional low beta transistors.

Today most testers such as the Atlas DCAs will identify transistor elements i.e. emitter, base, and collector.
Then there are testers that include all these features plus they can check transistors and diodes in and out of circuit.
A transistor’s H_FE can vary fairly widely with I_C, so measurements with a service type tester give readings that can differ quite a bit from the H_FE of a transistor's real life operation. Meaning, these testers are useful, but can't be regarded as giving accurate in-circuit operational H_FE values.

Laboratory-Standard Transistor Tester and Analyzers

Common Base Transistor Circuit

Common Emitter Transistor Circuit

Common Collector Transistor Circuit

These types of testers including curve tracers are used for measuring transistor parameters dynamically under various operating conditions. With a calibrated meter, these analyzers have the necessary controls for making the proper voltage, current and signal settings including the ability of checking high power transistors and rectifiers. Because of these capabilities, the readings they give are real-world.

The important characteristics measured include:

• I_CBO collector current with emitter open (Common base)
• ac beta (Common emitter)
• R_IN ( (Input resistance)

The WT-524A Service Type Transistor Tester

From the October 1973 issue of Electronic Servicing

In cleaning up, fixing and testing the WT-512A, I did compare both its functionality and results with an atlas DCA55. Both of these "testers" are considered to be service type transistor testers.  The DCA55 has a micro processor which enables it to be fully automatic.  The WT-512A is purely analog and requires some understanding of transistors and a bit of skill to use.

I’m not sure where or from whom I got the WT-524A. I did find an old listing on a site that auctions government surplus for one that looked exactly like the one I have when I received it. My WT-524A had these user-installed Velcro pads on the back that held the WG-454A and WG-455A socket adapters. It also had two black stick-on cable hooks which held the power cable and the WG-460A test leads.

I purchased the WT-524A because I wanted a method of testing and analyzing transistors. However, from reading the scanned manual, it seems the WT-524A was primarily used for testing transistors rather than analyzing them.

But…

In 1971, I attended night school at the Ohio Mechanics Institute in Cincinnati. There I was attempting to study electronics. In the semiconductor lab, we did use a Tektronix curve tracer to analyze transistors. But because there was only one curve tracer and 30 students, each of us used several types of metered devices to analyze transistors. One of these meters was – I think, an RCA WT-524A. My memory is a bit foggy, but with the adapters, this WT-524A sure looks like what I used then.

However, I can’t seem to find any documentation on the web on how to use the WT-524A to analyze transistors. The large meter does have unitized scales for G_M, I_C / I_CEO, I_D / I_DSS, I_CBO, B, Beta, etc. So, I presumed when I bid on it, I could vary the parameters to analyze transistors, FETs, etc.

Before retiring, my career was software development, which naturally didn’t involve scientific electronic analysis of components. So it’s been a long time since I played around with semiconductors in so far as a design/analyst prospective. Now I want to study electronics and once again play around with transistors.

Simplified schematic of the WT-524A transistor test circuit.

Basically, an analyzer has to be able to vary parameters to determine beta, active region, saturation, cut-off, performance, etc. The WT-524A manual says that it can accurately determine beta. But, in comparing the readings of the WT-524A to the Atlas DCA55, I find the resulting beta is somewhat different between the two testers. I did calibrate the WT-524A according to the manual. But unfortunately, I cannot easily determine what I_B, V_BE and I_C values are the WT-524A uses during the test. The Atlas DCA55 does display these values.

WT-524A Transistor Tester                  WV-98C Senior Voltohmyst

Again my memories of 1971 are vague. It could be that we had some kind of test rig with transistor sockets where we used an RCA Senior Voltohmyst WV-98C to measure parameters. This meter does look almost exactly like the WT-524A.

Anyway, the WT-524A sure looks like a serious and expensive piece of test equipment. I’d hate to think RCA manufactured something this sophisticated merely to just do a simple test.

The manual states: “The WT-524A provides an accurate indication of transistor gain and leakage.”

At the top of this page I asked, “What types of transistor testers did they have in 1971?“ I now ask, “What kind of transistors did they have in 1971?” The answer is, relative to today’s cost, very expensive transistors.

From the 1971 Allied Radio Shack catalog.

It is true that you could have gone to Radio Shack in 1971 and purchase low quality power transistors for $.89 each. But what cost $.89 in 1971 would cost $5.48 in 2017. Also, if you were to buy exactly the same product in 2017 for $.89, then in 1971 it would cost you $0.14. That's about what you can pay for each on eBay for a bag of a dozen transistors from Chine.

Today high quality class A power transistors, i.e. a 2N3055 NPN Audio Amplifier Power Transistor, will cost you on eBay $1.28 each. In 1971, they cost $2.83 or $17.41 in 2017 dollars.

In 1972 my Radio Shack Realistic STA-120B receiver cost me $259.00. In today’s dollars, it would be $1594.00 for this OK receiver. There were much better name-brand receivers out there. In 1976 if I would have waited and spent three times more and bought the king of all receivers, a Pioneer SX-1250, it would have cost me $900 or $3,940 in 2017 dollars.

In 1971, US-made transistors used in domestic TVs and radios cost quite a bit more. Then there were industrial transistors. Unlike vacuum tubes that could easily be pulled out and tested, diagnosing by unsoldering and soldering in components was not – fiscally lucrative. This is where these types of in-circuit transistor testers like the WT-524A were – and still are useful. Sure they’re not perfect. But with an experienced technician, they can and do give a good indication of failed components.

The point being, electronic components in the burgeoning solid state electronics market were a lot more expensive then and it took relatively expensive test equipment like the WT-524A to diagnose problems and failures.

So, I’ll keep it – and continue learning how to use it. Along with my signal tracer, the WT-524A did enable me to diagnose several pieces of vintage solid state equipment including tracking down an elusive problem with my Kenwood HR-9400.

SteveS December 2017