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475A Z-Axis Amp and Q1338 Confusion


 

I'm still waiting for one component to replace the blown transistors in my sick 475A, so I decided to investigate the similarities/differences between the 475A and a 475 parts scope. The first thing I checked was Q1338, which is not bad in my 475A, but which does not seem to match what is found in the schematic or parts list.

Q1338 is shown as an NPN transistor in the schematics for both the 475 and 475A, and also listed as such in the parts list ("Silicon, NPN, SKA6516) but the parts that I find in the 475A and the 475 both measure as a PNP transistor, and both bear the markings "151367 TI811" which appears to be a Motorola SPS8811 part.

I have found some other Tek service manuals that list part # 151-0367-00 as a Motorola SPS8811, but the both the 475A and 475 service manuals list that same part number as an SKA6516. I can't seem to find any data sheets for either a Motorola transistor with part number SPS8811 or a transistor with the part number SKA6516.

The Semiconductor Common Design Parts Catalog does nothing to clarify matters, listing 151-0367-00 as an A5T3571, an NPN transistor with a minimum hFE of 100 (@ 6V/5mA) and a minimum fT of 1.2 GHz (@ 6V/5mA). I have found a data sheet for the TI A5T3571 which is listed as an NPN transistor having a "minimum calculated f[max]" of 2.2 GHz, so that is at least is the same ballpark as the Tek docs. I have not yet found a source for this part.

The fact that all of the documentation insists that the part is an NPN transistor, while the physical parts that I find on the physical boards test as a PNP transistor is quite confounding. I'm perfectly willing to believe that my component tester is lying to me (it's one of those cheap kits, but it's gotten good reviews from the EEV community on YouTube, so I figured it was better than nothing. Still, maybe it's not ALWAYS better than nothing).

How should I go about testing the the actual parts myself? I know that I can use the diode test function of a multi-meter to check the CB and EB voltage drops, and determine if this is an NPN or PNP based on the polarity, but are there other tests I can/should do?

Again, I'm not currently counting this as a failed part, but the fact that my tester identifies it as something other than what the service manuals say it should be is confounding (and, unless I can explain it, really SHOULD count as a failure). The fact that I don't seem to have an replacements that don't also read as the wrong part (and that I can't seem to find is the only reason I'm not calling this a failed part.

-- Jeff Dutky


 

On Sun, Nov 29, 2020 at 12:26 AM, Jeff Dutky wrote:


The fact that all of the documentation insists that the part is an NPN
transistor, while the physical parts that I find on the physical boards test
as a PNP transistor is quite confounding. I'm perfectly willing to believe
that my component tester is lying to me (it's one of those cheap kits, but
it's gotten good reviews from the EEV community on YouTube, so I figured it
was better than nothing. Still, maybe it's not ALWAYS better than nothing).

How should I go about testing the the actual parts myself? I know that I can
use the diode test function of a multi-meter to check the CB and EB voltage
drops, and determine if this is an NPN or PNP based on the polarity, but are
there other tests I can/should do?
Q1338 *is* an NPN transistor. Are you sure you've got the pinout right?
To check if NPN or PNP with DMM:
With most (not expensive) DMM's: Using your battery-operated DMM, set to 2kOhm range, connect the red (+) lead to the base and the black (-) lead to either the collector or emitter. You should see something like 0.68 V, with a slightly lower voltage with B-C than B-E.
Check the other way around (black vs red) and you'll see overflow.
If so, it's an NPN that isn't completely dead, might even be fine. Be a bit careful with static, it's an HF transistor with small capacitances, so more easily zapped than LF transistors.

Raymond


 

On Sun, Nov 29, 2020 at 01:15 AM, Raymond Domp Frank wrote:


To check if NPN or PNP with DMM:
With most (not expensive) DMM's: Using your battery-operated DMM, set to 2kOhm
range, connect the red (+) lead to the base and the black (-) lead to either
the collector or emitter. You should see something like 0.68 V, with a
slightly lower voltage with B-C than B-E.
Check the other way around (black vs red) and you'll see overflow.
If so, it's an NPN that isn't completely dead, might even be fine. Be a bit
careful with static, it's an HF transistor with small capacitances, so more
easily zapped than LF transistors.
Actually, same as in Diode Check mode, if your DMM has one.

Raymond


Harvey White
 

There are a number of ways to test a transistor (bipolar type).

One is to use an ohmmeter to check the BE and BC junctions.  The problems with this can be that the older meters not only have more voltage than is good for the transistor, and the current used could be more than the transistor would like.

Another option is to use a transistor tester (they're very common on Amazon and EBAY).  They apply voltages to each lead, and measure voltage and current in other leads.  They're not bad, but not infallible.  They're good for lead identification.

Another option is the classic transistor tester, leakage, beta, etc.  It does require you to understand which leads are which, though, for best results.

The last is a curve tracer.  Be prepared to spend money to get one of these unless you can build one.  There are older units, B&K and Heathkit, they'll cost who knows how much, but they ought to work well enough.  Next come the Tektronix plugins, specifically the 7CT1N (if you have a 7000 series scope) and the 5CT1N (for a 5000 series scope).  The prices seem to be unreasonable on these.  There are also specific Tektronix curve tracer scopes and instruments.  They seem to be equally expensive.  The problem with all of these is that while they give you very exact and meaningful data, you have to know how to interpret it.

Perhaps your best bet is the transistor tester, and a DVM with a transistor tester (beta) setting.

Curve tracers are wonderful, but $$$$.


Harvey

On 11/28/2020 6:26 PM, Jeff Dutky wrote:
I'm still waiting for one component to replace the blown transistors in my sick 475A, so I decided to investigate the similarities/differences between the 475A and a 475 parts scope. The first thing I checked was Q1338, which is not bad in my 475A, but which does not seem to match what is found in the schematic or parts list.

Q1338 is shown as an NPN transistor in the schematics for both the 475 and 475A, and also listed as such in the parts list ("Silicon, NPN, SKA6516) but the parts that I find in the 475A and the 475 both measure as a PNP transistor, and both bear the markings "151367 TI811" which appears to be a Motorola SPS8811 part.

I have found some other Tek service manuals that list part # 151-0367-00 as a Motorola SPS8811, but the both the 475A and 475 service manuals list that same part number as an SKA6516. I can't seem to find any data sheets for either a Motorola transistor with part number SPS8811 or a transistor with the part number SKA6516.

The Semiconductor Common Design Parts Catalog does nothing to clarify matters, listing 151-0367-00 as an A5T3571, an NPN transistor with a minimum hFE of 100 (@ 6V/5mA) and a minimum fT of 1.2 GHz (@ 6V/5mA). I have found a data sheet for the TI A5T3571 which is listed as an NPN transistor having a "minimum calculated f[max]" of 2.2 GHz, so that is at least is the same ballpark as the Tek docs. I have not yet found a source for this part.

The fact that all of the documentation insists that the part is an NPN transistor, while the physical parts that I find on the physical boards test as a PNP transistor is quite confounding. I'm perfectly willing to believe that my component tester is lying to me (it's one of those cheap kits, but it's gotten good reviews from the EEV community on YouTube, so I figured it was better than nothing. Still, maybe it's not ALWAYS better than nothing).

How should I go about testing the the actual parts myself? I know that I can use the diode test function of a multi-meter to check the CB and EB voltage drops, and determine if this is an NPN or PNP based on the polarity, but are there other tests I can/should do?

Again, I'm not currently counting this as a failed part, but the fact that my tester identifies it as something other than what the service manuals say it should be is confounding (and, unless I can explain it, really SHOULD count as a failure). The fact that I don't seem to have an replacements that don't also read as the wrong part (and that I can't seem to find is the only reason I'm not calling this a failed part.

-- Jeff Dutky





 

Ramond,

Okay, I have a couple of DMMs with diode check mode. I did as you said, and using that method the Q1338 shows a diode voltage of 0.773 from base to collector and the same (or nearly so) from base to emitter (red always connected to the base, black to either the collector or emitter), which means that this is definitely an NPN transistor, and the cheap component checker was wrong.

Okay, that clears up the core of my confusion. I'm not nearly as concerned with specific part numbers in different service manuals as I am with a completely different sort of transistor in the actual circuit than is specified in the schematic.

As for whether I got the pinout right: I had thought that the component checker didn't care and would try all combinations, but I just tried putting the transistor in with the pins offset by one position (the checker has a ZIF socket where the pins are labeled 1, 2, 3, 1, 1, 1, 1 so you can connect the part either as 1,2,3 or as 2,3,1, or as 3,2,1 if you flip it over), and now it gets correctly identified as an NPN transistor. This is what I get for $15: no manual, and a cargo-cult approach to electrical engineering.

For the record, the other three transistors that I used the component checker to identify as bad, I actually went back and checked them with the multi-meter, so I'm pretty sure that they were, in fact, bad: one was a dead short between all three terminals, the other two were complete open circuits between all three terminals. Clearly something fairly dire happened to the beam intensity amp which blew out two diodes and three transistors. I just hope I've fixed whatever the problem was.

-- Jeff Dutky


Roy Thistle
 

On Sat, Nov 28, 2020 at 03:26 PM, Jeff Dutky wrote:


are there other tests
If you've got one of those DMMs that have a socket, and a Hfe setting... along with a transistor that is packaged so that the pins cooperate (or rig up a kludge)... you measure the current gain... and see if it is in spec. (Many of the very cheap Amazon DMMs have that.)
You can also measure the current gain with your DMM on uA (base to emitter current)... and on mA( collector to emitter current) to see the current gain.
If you know your circuit parameters, you can also investigate/measure to see if the particular transistor in the circuit is meeting those specs.
Do you need a curve tracer? ... well they're kind of cute... the old ones have lots of gravity to hold things down.... and, their probably cheaper than an iPhone.


 

I haven't got either a DMM with the socket and Hfe setting, nor do I have a curve tracer. I'm trying to limit my acquisition of new boat anchors, as I've already filled my work area with 400-series and 2200-series scopes (I started out with a 475 and a 2213 that belonged to my father, then I bought several parts 475s, one of which is a 475A, in order to ensure that I could keep my father's scope running. Then I saw that the 2215 had a neat feature that the 475 lacked, and was pretty cheap. Finally, after losing an auction for a 475A+DMM44 I saw that there was a 2200-series scope, the 2236, with a DMM/Counter-timer built in, and now I am up to my ears in scopes).

I'm a little surprised that I haven't seen an external curve tracer that could be attached to a scope operating in X-Y mode. I know that you can do some kinds of component checks using a scope in X-Y mode, but I haven't gotten around to learning exactly how to do that.

I've been wanting to get a reliable ESR meter, and some of them appear to have component test sockets as well, but the range of choices has been beyond my analysis-paralysis threshold. If someone had a solid recommendation for a reliable, but not too expensive meter with these features, I would be grateful for the suggestion.

-- Jeff Dutky


Michael W. Lynch
 

On Sat, Nov 28, 2020 at 11:38 PM, Jeff Dutky wrote:


I'm a little surprised that I haven't seen an external curve tracer that could
be attached to a scope operating in X-Y mode. I know that you can do some
kinds of component checks using a scope in X-Y mode, but I haven't gotten
around to learning exactly how to do that.
Jeff.

Knowing what I know now, a Curve Tracer would be one of my first acquisitions, if for no other reason than a learning tool as well as to test and match components. I use mine to find replacements for those TEK part numbers that can no longer be had. There are several curve tracer or "octopus circuit" kits that can be had or constructed that use the scope in the X-Y mode. These work to some extent, but fall far short of the capabilities of say a TEK 576 or 577 Curve Tracer. I am certain that there are many here that can diagnose or interpret data sheets and schematics far better than myself; but for me there is no substitute for seeing a part function, real time, in a graphical or visual mode. The value and utility of a dedicated curve tracer, properly and skillfully used cannot be understated.

--
Michael Lynch
Dardanelle, AR


 

On Sun, Nov 29, 2020 at 03:28 AM, Jeff Dutky wrote:


I did as you said, and using that method the Q1338 shows a diode voltage of
0.773 from base to collector and the same (or nearly so) from base to emitter
Hi Jeff,
These are quite fast, low current transistors and their construction/geometry could explain the largish voltages. Still, I'd expect the BE-voltage to be slightly larger than the BC-voltage. OTOH, I don't know the geometry...

A "serious" curve tracer is a treasure: I use my Tek 576 for looking at small and large transistors, (tunnel) diodes, MOSFets etc. It's also quite useful for checking capacitor leakage and breakdown voltage.
It also keeps my house solidly fixed to the ground underneath.

Raymond


 

Roy, Michael, and Raymond,

I have checked on eBay, and most of the curve tracers I found there were DEFINITELY more expensive than an iPhone. All of them were over my impulse-buy threshold (though some not by very much).

I also saw the octopus devices, which were in my impulse-buy range, but they don't seem to produce the kind of output that I expect from what I've seen the Tek devices do in YouTube videos.

There is one (a "D1 577") that is only $400 (Buy It Now, free shipping!) but seems to be lacking a front module to mount the DUT. Still, if this were on my wish list, I'm not sure I would be any kind of impediment. It shows a trace on the screen, and appears to have all its knobs and buttons intact. It's really quite beautiful.

https://www.ebay.com/itm/TEKTRONIX-D1-577-CURVE-TRACER-OSCILLOSCOPE/133554810318?hash=item1f187cadce:g:2PcAAOSw6oFfkjZd

Then there are these kits: https://www.ebay.com/itm/Transistor-Curve-Tracer-adapter-Oscilloscopes-Plus-Power-supply-UnAssembled-Kit/291531434190?hash=item43e0a0a8ce:g:XLYAAOSw~gRVwN4i

But the best deal looks to be this (but it's "local pick-up only"): https://www.ebay.com/itm/Pallet-of-Tektronix-Equipment-Oscilloscopes-Modules-Mainframes-Curve-Tracer/313080719471?hash=item48e510a86f:g:~ggAAOSw7kReut4L

-- Jeff Dutky


Michael W. Lynch
 

Jeff,
Not wanting to hijack the thread with Curve Tracer talk. I was very fortunate to be able to buy a pair of 576's and 577; all at fire sale prices (all under $300 each). I am cheap, so lots of luck involved to get one at a price.that I am comfortable with. Keep your eyes open, you might get lucky.

I used on of those Cheap Chinese Component tester and a DMM for many repairs. They are limited, but can be useful when used within those limits.

I really like to follow these 4x5 discussions as I have worked on many of these scopes and enjoy bringing them back to life. I look forward to hearing about your final success with this instrument.

Good Luck. .

--
Michael Lynch
Dardanelle, AR


 

Michael,

I don't think that discussion of curve tracers is in any way hijacking this thread, as my original confusion stemmed directly from my own ignorance on the subject.

While I don't have any plans to acquire a Tek 576 or 577 (as pretty as the 577 is), I have fallen down a bit of a rabbit hole on DIY curve tracers. It looks like there are actually lots of options that don't require purchasing a dedicated unit.

Maybe, when I've gotten the 475A squared away, I will throw together one of the simpler curve tracers, since it looks like it would be both educational, and very useful in further restoration work.

-- Jeff Dutky


Dave Voorhis
 

On 29 Nov 2020, at 05:38, Jeff Dutky <jeff.dutky@gmail.com> wrote:

I'm a little surprised that I haven't seen an external curve tracer that could be attached to a scope operating in X-Y mode. I know that you can do some kinds of component checks using a scope in X-Y mode, but I haven't gotten around to learning exactly how to do that.
Heathkit and EICO made curve tracers designed to attach to a ’scope in X-Y mode. They strike me as tending to be overpriced for what they are.

On eBay, under the listing "Power Transistor Curve Tracer adapter XY Oscilloscopes NPN/PNP Vce=10V” is a simple and very inexpensive curve tracer kit. A couple years ago, I bought one and assembled it into a case with a power supply and appropriate switches, etc.

For the price it’s a fine plaything, but in practice it's inadequate for anything except maybe limited verification that a three-legged component might resemble a transistor if used in a circuit.

But that got me interested in the real thing, and I’ve subsequently acquired a Tek 577 (nice, though with some storage mode issues that aren’t critical to my use cases) and a Telequipment CT71 which is very capable. The latter sometimes turn up for comparatively reasonable prices as Telequipment tends to be looked down upon, but their curve tracer is very usable.

I’ve also recently acquired a non-working Tek 571. It’s kind of the red-headed stepchild of the curve tracer world, but I’ve always wanted one (I find anything combining one or more CPUs and analog circuitry to be oddly irresistible) but it has serious issues and is currently undergoing surgery on my bench. If you can find a working/repairable one of those, it’ll probably be cheaper than the usual highly-desirable units. Though it appears mainly designed for university student labs and the like, it should be an excellent — and very easy to use — general-purpose semiconductor troubleshooting tool.


Byron Hayes, Jr.
 

Jeff,

The Heathkit Semiconductor Curve Tracer (Model IT-1121) does not have a CRT and works with an oscilloscope having an X-Y function. On eBay there are some "aftermarket" terminals for this unit that make some connections easier. You will have to watch for it, as I haven't seen any currently on eBay.

Byron, WA6ATN

At 09:38 PM 11/28/2020, Jeff Dutky wrote:
I haven't got either a DMM with the socket and Hfe setting, nor do I have a curve tracer. I'm trying to limit my acquisition of new boat anchors, as I've already filled my work area with 400-series and 2200-series scopes (I started out with a 475 and a 2213 that belonged to my father, then I bought several parts 475s, one of which is a 475A, in order to ensure that I could keep my father's scope running. Then I saw that the 2215 had a neat feature that the 475 lacked, and was pretty cheap. Finally, after losing an auction for a 475A+DMM44 I saw that there was a 2200-series scope, the 2236, with a DMM/Counter-timer built in, and now I am up to my ears in scopes).

I'm a little surprised that I haven't seen an external curve tracer that could be attached to a scope operating in X-Y mode. I know that you can do some kinds of component checks using a scope in X-Y mode, but I haven't gotten around to learning exactly how to do that.

I've been wanting to get a reliable ESR meter, and some of them appear to have component test sockets as well, but the range of choices has been beyond my analysis-paralysis threshold. If someone had a solid recommendation for a reliable, but not too expensive meter with these features, I would be grateful for the suggestion.

-- Jeff Dutky