Date   
Re: Slightly OT. What is the grime I always see in HV areas?

Craig Thibodeaux
 

Evidence of past HV.

Very normal….indicative of a dirty environment, not an equipment problem in its origination…maybe in other ways.

Used to have a projection TV repair business specialized in Rear Projection…..3 tubes…..dirty house, smokers, real nasty TV….

Craig
KM4YEC

On May 10, 2018, at 12:17 PM, lop pol via Groups.Io <the_infinite_penguin=yahoo.com@groups.io> wrote:

Sorry for the dumb question.


Re: Slightly OT. What is the grime I always see in HV areas?

Dave Voorhis
 

Electrostatically-attracted dust.

On 10 May 2018, at 17:17, lop pol via Groups.Io <the_infinite_penguin=yahoo.com@groups.io> wrote:

Sorry for the dumb question.

Slightly OT. What is the grime I always see in HV areas?

Brendan
 

Sorry for the dumb question.

Re: Diagnosing faulty Transistors with a Curve Tracer

Dave Hills
 

I still have and occasionally use an RCA WT-501a that I acquired new in my teens. I served me well until I could acquire a surplus Tek 577. Although it is limited to 1Amp Ic, the theory and circuits are thoroughly discussed in the RCA user manual, and from that one could easily construct a modified test rig for much higher collector currents/voltages. Not a curve tracer by any means, but certainly more informative than an ohmmeter junction test. Heathkit, Sencore and others made similar Beta testers as well. The RCA manual is here: https://bama.edebris.com/manuals/rca/wt501a/

Dave

On Thu, May 10, 2018 at 05:33 am, Bert Haskins wrote:




On 5/10/2018 4:16 AM, M Yachad wrote:
Ed
No, not quite.

I don't have a curve tracer.

The current practice is to replace ALL the audio power transistors in the
output circuit in one shot, discarding the faulty ones, along with the
possibly still-good ones (because there is no 100% way to determine a
good/defective status, with all my testing toys, out-of-circuit).

Because Sanken (the preferred audio transistor manufacturer) is now long out
of production, it makes $ and sense to discard and replace ONLY the faulty
units.

So, would a curve tracer be able to give a definite go/no-go diagnosis?

On the curve tracer, what would a good curve look like, versus a faulty
curve?
Comparative sketch or photo would be fine.

Menahem
Back in the mid-1960s I built a high current curve tracer just for
evaluating/ matching power transistors.
It was quite easy to do.
The problem at that time was beta compression and low current tests were
just about worthless.

This could turn into a very long story and there is lots of info on
basic beta testing on the net so check this out and get back to us if
you still have questions.

-Bert


Re: Diagnosing faulty Transistors with a Curve Tracer

Tam Hanna
 

Menachem,
Rig up a Solartron 7150 and a HP 6624A with a workstation via GPIB. Small bit of Visual Basic code, and the target is dead.

Especially in your case, where a generic curve tracer is imho overkill.

Tam
--
With best regards
Tam HANNA (emailing on a keyboard-less handset, sorry for spelling mistakes and brevity)

Re: Diagnosing faulty Transistors with a Curve Tracer

 

On Thu, May 10, 2018 at 05:32 am, Chuck Harris wrote:


one of the labs I taught
had the kids make a simple curve tracer out of an HP flat
bed chart recorder, two power supplies, and a couple of
resistors.
That sounds pretty cool. (or kewl in today's lingo)

Re: Diagnosing faulty Transistors with a Curve Tracer

 

On Wed, May 9, 2018 at 11:14 pm, M Yachad wrote:


So if there is a way to positively identify one failed transistor, then I do
not need to replace all 4 of them, with dubious units.
If there are 4 in one channel it is inadvisable. If you are talking 2 per channel it is not as bad.

When there are 4 per channel and they are in parallel each "bank" must match, that is all on the positive sides matched and all on the negative side matched. However the positive ones do not need to be absolutely matched to the negative ones.

From polarity to polarity the negative feedback will take care of gain variations within reason no matter how the output circuit is configured. But transistors in parallel which are supposed to share the current are a different story. They must be very closely matched or failure will occur. That is unless you limit a 200 watt amp to about 35 watts, and even sometimes even less because of power dissipation, it is not linear with output level. In fact amps run hotter at one third power than at full power. While it may help that all the transistors are at the same temperature, the power dissipation must still be within the Pd/temp derating limit line. Even at the same temperature if one of two transistors is carrying 1/3rd of the current and the other 2/3rds then the latter will have to dissipate twice the power. Fine if it is within the derating limit. If not, poof.

This critical matching is not as critical when the outputs are arranged in series, amps can go either way, the big Ampzilla has them in series, most Phase Linears have them in parallel. The former must share voltage equally, the latter, current. Sharing the voltage is easier to achieve without much designing because they drive one pair, usually the outer (closer to the rails) with a voltage divider. If they use commutators, the commutators are non-critical but the actual outputs are as described.

Note however that when there is a shorted output say on the negative side, the transistor(s) on the positive side will have their limits exceeded. That is because while the shorted transistor is going to pull the DC on the output its way, the circuitry is going to do everything it can to pull it the other way. This means large collector current but at full voltage, for which it is not designed. That voltage must drop as the drive increases or dissipation can go sky high so fast that the junction is overheated and fried by the time you even begin to feel it on the heatsink, let alone have a chance to cut power. While a DBT will minimize this on the bench, the unit was not on the bench nor a DBT when it failed. (if anyone needs info on the DBT - Dim Bulb Tester say so)

So, the increase in reliability afforded by replacing less transistors is not a sure shot by any means. I have seen amps destroyed by not changing all the transistors. well not quite, the next time around it got to part of the current limiting because it blew open some emitter resistors. One more time and it may have destroyed some germanium transistors and in THAT circuit, modifications would be just about impossible. Figuring that out would take me a very long time, I am not technically an engineer, I am a good hack actually. A REAL engineer could do it but a kid fresh out of college could not, take my word for it, I know of what I write here. This type of circuitry is not their strong point to say the least. And remember, not doing it right you are risking the earlier stages in the amp and some of that is much more critical than the output transistor parameters.

I use a few different transistors when I can't get the original, available or valid subs available. For example for TO-3 2N3773 and 6609 are good up to a certain power. Beyond that you have 2SB555/7 and 2SD425/7, and then comes the MJ15024 and it's comp. (150222 I think) Them babies are good up to about 180 volts and new ones are about $ 6 each. For the large plastic there are the 2SC(or is it D ?) 5200 and its comp. They say you can even use them for 2SC3281 and 2SA1302 but I have never tried them. For the 2 screw flat jobs, 2SC2292s(?) and comps are actually still available. Little bit pricey but those are very tough transistors. They can handle 200 watts 8 ohms with only one pair. However, short the speaker wires and the current limiting is not good enough.(I know this unfortunately from home experience)

In outputs, the gain curve is really not that critical unless it is an audiophile amp of special design with low or no feedback. Those are rare. Some amps using a shunt bias design are critical in forward Vbe characteristics, there is nothing that can be dome about that in some of them. Push comes to shove you just have to raise the value of the emitter resistors and that will make it current limit before it was supposed to unless you do a little more reengineering.

Old amps are a problem, but replacing less transistor is not a very good answer in my considered opinion.

Re: 453 calibrator

Dale H. Cook
 

At 02:49 PM 5/9/2018, Lop Pol wrote:

BTW if anyone needs the 453 above SN 20,000 manual I can send it to you. I can't remember where I got it. It was a 6 part .rar download.
You got it from my domain - it is a scan of my physical copy, with the prints that use blue ink scanned in color.

Google has disabled the older CAPTCHA that I used on my domain, and I have been so busy that I haven't upgraded it. I hope to upgrade it this weekend and will let the list know that the download is again available.

Dale H. Cook, GR/HP/Tek Collector, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcity/radios/index.html

Re: Diagnosing faulty Transistors with a Curve Tracer

Bert Haskins
 

On 5/10/2018 4:16 AM, M Yachad wrote:
Ed
No, not quite.

I don't have a curve tracer.

The current practice is to replace ALL the audio power transistors in the output circuit in one shot, discarding the faulty ones, along with the possibly still-good ones (because there is no 100% way to determine a good/defective status, with all my testing toys, out-of-circuit).

Because Sanken (the preferred audio transistor manufacturer) is now long out of production, it makes $ and sense to discard and replace ONLY the faulty units.

So, would a curve tracer be able to give a definite go/no-go diagnosis?

On the curve tracer, what would a good curve look like, versus a faulty curve?
Comparative sketch or photo would be fine.

Menahem
Back in the mid-1960s I built a high current curve tracer just for evaluating/ matching power transistors.
It was quite easy to do.
The problem at that time was beta compression and low current tests were just about worthless.

This could turn into a very long story and there is lots of info on basic beta testing on the net so check this out and get back to us if you still have questions.

-Bert

Re: Diagnosing faulty Transistors with a Curve Tracer

Chuck Harris
 

Absent special test equipment, test circuits, etc. ,
you cannot test transistors for proper operation.

DVM tests on the various junctions do not constitute an
accurate test on power transistors, as they do not test the
transistor for gain under normal operating conditions.

So, you have a couple of options:

1) buy a 576 curve tracer.
2) build a simple common emitter test circuit, and feed
base current, and observe the changes in collector
current... essentially a manually driven curve tracer.
3) swap transistors.

There is plenty of literature available on the net that
can tell you what the curves look like. Google is your
friend.

Back when I was in graduate school, one of the labs I taught
had the kids make a simple curve tracer out of an HP flat
bed chart recorder, two power supplies, and a couple of
resistors.

-Chuck Harris

M Yachad wrote:

Ed
No, not quite.

I don't have a curve tracer.

The current practice is to replace ALL the audio power transistors in the output circuit in one shot, discarding the faulty ones, along with the possibly still-good ones (because there is no 100% way to determine a good/defective status, with all my testing toys, out-of-circuit).

Because Sanken (the preferred audio transistor manufacturer) is now long out of production, it makes $ and sense to discard and replace ONLY the faulty units.

So, would a curve tracer be able to give a definite go/no-go diagnosis?

On the curve tracer, what would a good curve look like, versus a faulty curve?
Comparative sketch or photo would be fine.

Menahem

1A4 added signal

snapdiode
 

When adding channels 1 and 2, is there a way to get that added signal on the front panel signal output jack?

Re: Diagnosing faulty Transistors with a Curve Tracer

EricJ
 

Try an octopus circuit with your oscilloscope.
--Eric
Sent from my Samsung Galaxy S8.

-------- Original message --------From: M Yachad <@yachadm> Date: 5/10/18 3:16 AM (GMT-06:00) To: TekScopes@groups.io Subject: Re: [TekScopes] Diagnosing faulty Transistors with a Curve Tracer
Ed
No, not quite.

I don't have a curve tracer.

The current practice is to replace ALL the audio power transistors in the output circuit in one shot, discarding the faulty ones, along with the possibly still-good ones (because there is no 100% way to determine a good/defective status, with all my testing toys, out-of-circuit).

Because Sanken (the preferred audio transistor manufacturer) is now long out of production, it makes $ and sense to discard and replace ONLY the faulty units.

So, would a curve tracer be able to give a definite go/no-go diagnosis?

On the curve tracer, what would a good curve look like, versus a faulty curve?
Comparative sketch or photo would be fine.

Menahem

Re: 7T11 sampling timebase - how does it behave?

 

HankC,

IIRC, the 7T11 has a Random Sampling mode; the dots on screen are not sequential.
The 7T11 does indeed have a second "Random Sampling" mode, which is very different from the regular mode and more difficult to adjust and use.
The dots on the screen are sequential (otherwise, you wouldn't see the waveform) but their acquisition is not.
The regular mode is much more like the way that the 7S14 does its acquisition.

Raymond

Re: Diagnosing faulty Transistors with a Curve Tracer

 

Ed
No, not quite.

I don't have a curve tracer.

The current practice is to replace ALL the audio power transistors in the output circuit in one shot, discarding the faulty ones, along with the possibly still-good ones (because there is no 100% way to determine a good/defective status, with all my testing toys, out-of-circuit).

Because Sanken (the preferred audio transistor manufacturer) is now long out of production, it makes $ and sense to discard and replace ONLY the faulty units.

So, would a curve tracer be able to give a definite go/no-go diagnosis?

On the curve tracer, what would a good curve look like, versus a faulty curve?
Comparative sketch or photo would be fine.

Menahem

Re: Diagnosing faulty Transistors with a Curve Tracer

Ed Breya
 

Do you mean that you can pull a transistor that's apparently failed in-circuit, and it tests out normal on the curve tracer, but if you put it back in, it still doesn't work? What happens if you pull it and test again afterwards?

One thing that can cause problems is degradation of the transistor's actual durability (SOA) versus the stress applied. Usually, SOA failure would be catastrophic in a power circuit, so the part would suffer permanent damage that would show in further testing, but maybe not, depending on the circuit. I assume you're talking about audio power amplifier applications, so the DC SOA curve is what counts. If you set up the curve tracer testing to push a questionable part to just below its specified or empirically determined SOA limits, or slightly above the estimated circuit conditions, it may show that something's going on.

Ed

Re: 7T11 sampling timebase - how does it behave?

Craig Sawyers <c.sawyers@...>
 

IIRC, the 7T11 has a Random Sampling mode; the dots on screen are not sequential.
Instead, the sampling window is intentionally jittered horizontally.
This allows you to see the trigger point without the need for a delay line, which would reduce the
bandwidth.
Keep an eye out for a Tek Concepts book on sampling scopes.

HankC, Boston, WA1HOS
I have a complete set of Concepts books, including the one on sampling, which I have read cover to
cover several times. But random sampling on the 7T11 is even worse than sequential sampling regarding
getting a useable trace. Often the dots are widely spaced and (of course, given the mode) jittering
around. Identifying anything that looks like a rising edge is usually a forlorn task.

Craig

Diagnosing faulty Transistors with a Curve Tracer

 
Edited

This is an issue which has been cooking in my thoughts for a while.

On older discrete power amplifiers, which have suffered failure, the main power transistors test OK on all usual devices, when out-of-circuit. But when in-circuit, the amplifier fails, and replacement of those transistors solves the problem.

So normal testing is NOT able to identify the failed transistor.

Why does this bother me, if all I need to do is throw in a new transistor?

Because the overall majority of these transistors are now NLA, and the widely advertised replacements are almost guaranteed to be counterfeits.
So if there is a way to positively identify one failed transistor, then I do not need to replace all 4 of them, with dubious units.

Any ideas?

Menahem Yachad
CondorAudio

Re: 453 calibrator

 

On Wed, May 9, 2018 at 01:15 pm, Richard Knoppow wrote:


On my schematic BA and BB are shown right on the edge line of the board. They
look like they are connected together but are not.
You nailed it. you beat me to it. I did attribute it to a mistake but it is not. I am too aware of mistakes on prints, thy occur quite a bit in consumer. You see a wire that says 12 volts and then a inch or two away it says 0 volts. And it is no mistake like the one voltage is something else, I have seen them where it is simply not possible. And they don't really care.

Anyone who doesn't see it - observe the dot that indicates a connection there at BA and BB. Look at the other dots. This one is bigger. Well it is bigger because it is not a dot, it is a circle. The coax is grounded on that side but not on the other. This is not that uncommon.

I have to admit it fooled me up until very recently. I have shit eyesight, I thought I was used to it and I am to a point, but still I missed this.

That doesn't change the fact that I dismissed it as a mistake for troubleshooting purposes up to this point, and the resistors were the wrong value.

This is nothing. Poor eyesight contributed to me frying a 7603. I switched two of the plugs on the Z board and dumped 130 volts into the unobtanium horizontal IC. Luckily I had a parts donor handy. If not I would have been guilty of criminal involuntary scopicide. So this is nothing.

Re: 7T11 sampling timebase - how does it behave?

 

IIRC, the 7T11 has a Random Sampling mode; the dots on screen are not sequential.
Instead, the sampling window is intentionally jittered horizontally.
This allows you to see the trigger point without the need for a delay line, which would reduce the bandwidth.
Keep an eye out for a Tek Concepts book on sampling scopes.

HankC, Boston, WA1HOS

Re: 7T11 sampling timebase - how does it behave?

 

Dennis,

As an extensive user of the 7S11/7T11 (I own 6 7S11s 4 7T11s, and 2 7T11As) they are awful to use
I don't fully agree with you on this, Dennis. Whereas the 7S14 behaves almost exactly like an analog 'scope, as you describe, the 7S11/7T11 behaves very differently. Although it (actually, mostly the 7T11) does remain a bit finicky to use, the most important thing I guess before using the 7S11/7T11 is realising it's not a general purpose 'scope setup, taking a deep breath and letting go of the assumption that the time settings are some sort of equivalent to normal time base setting (s/div.) with an ordinary time-expanded window in it, like with a second (delayed) time base. I do agree that triggering effort, noise level etc. do remain less pleasant than when using a 7S14 or a 7104 equipped as you mention. OTOH, no amount of effort will show an 8 GHz waveform on the 7S14 or 7104 - or a 50 ps step with any fidelity. I get quite good results with a 7S11/S-4/7T11A in a 7854, using digital storage and filtering, though I haven't exactly tried things like your 10 ps/div or 2 mV/div. I might give that a try though, one of these days with the 7854.
Not only have I had fun playing with the 7S11/7T11 but I have had results as well; one does get used to them. As an example, I certainly prefer adjusting the fast edge of a PG-506 using a 7S11/7T11 over a 7104 or 7S14... Signal fidelity (shape and speed) cannot be matched by either of the two latter setups. And I cannot judge the performance of my home-made TD-pulser without the 7S11/S-4/7T11 setup.

Raymond