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HNY to everybody!

In my SC-502 scope transistor Q855 died. It is a GE X44HR242 in TO220 case,
but I was unable to get its specifications
in order to find a suitable substitute.
Thanks for your help

Giovanni

From Tek specs, it's a D44H11 NPN, currently available from STMicroelectronics, OnSemi, etc. at price of approx. 90 eurocents each
Icmax = 10 A
Bvceo = 80V
hFE 60 @ Ic = 2 A
Vcesat = 1 V
ft min = 50 MHz

Cheers,
Max

Hi Giovani,

Look for the Tektronix part number in the service manual, it should be something like 151-0426-00.
Then use the semiconductor cross reference (big PDF file):
And it is indeed D44H11 as already said.
What's great with Tektronix is that they use mostly still available/made components.

Best regards,

Can't help you with data on those GE transistors but WB Parts seems to have has a pile of them in stock:


They were used by the military and exist under several different nomenclatures.

From my experience, the GE "pink" transistors were not particularly stellar performers, having fairly low gain and only moderate bandwidth. They may have been a high reliability part but that's about all I would speculate as being good about them.

In lieu of obtaining an original part, if unavailable, consider the following procedure.

I've never replaced one of those GE transistors with a like part (and never had to look back), originally preferring to select a suitable RCA device, back when they were still available. Today I would probably use a transistor by ON Semi/Fairchild or perhaps TI, again, selecting one to match parameters required by circuit.

I don't have an SC-502 to probe and the manual does not provide any waveforms, so I'm shooting from the hip here:
Q855 appears to be the transformer drive transistor in the DC-DC converter that provides most of the internal operating supply voltages. With an input voltage of about 33Vdc and a .3A fuse inline, Q855 does not have to be a very robust part. The 470pf C-B capacitor tells us circuit bandwidth demand is also not terribly high. I might try an On Semi fabbed TIP41, not for its power capacity but because it has the least gain found in that package/device category, perhaps being in the same realm as the GE part. For more gain in an otherwise also over specified part, the On Semi TIP 31C might be a good option.

Beyond this, someone else with experience troubleshooting an SC-502 might have more/better information to offer.

Thomas Garson
Aural Technology, Ashland, OR
By my calculation, the dynamic range of the universe is roughly 679dB,
which is approximately 225 bits, collected at a rate 1.714287514x10^23 sps.

Can't help you with data on those GE transistors but WB Parts seems to
have has a pile of them in stock:

Why bother trying to find unobtainum parts when Tektronix specifies "official" modern/current production substitutes (see crossref document)?

FAN to you, too, Giovanni!

I agree with Thomas that something rather generic, like the TIP31C that he mentioned, would probably work just fine. Even though it's an obsolete transistor, it still seems to be quite available.

I'm assuming you are in Italy (or at least somewhere in the EU), but if you are in the US, I could easily send you one.

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070

I think this is the right datasheet for the original:



The bipolar transistors optimized for switching power applications tend to have low gain and strong SOA characteristics. There should be plenty of other transistor types that will do - just try to get similar specs and SOA ratings. I think the -0426- was Tek's go-to part for lots of secondary side (low voltage) DC-DC converter use in that era.

Ed

Thank you to everybody!
I have enough information to attempt a transplant.
I'll post here the outcome

Giovanni

I put the BD911(NPN) and BD912(PNP) in mine. D44H10 TiP73 (B or C) and D45H10 TIP74 (B or C) should also work OK

So I replaced Q855 with a BD239C and Q860 (it was blown) with a 2N2369 (it is the equivalent of the original 2N5769).
But no oscillation in the HV supply. All the LVs seem OK. Any suggestion? By the way,
at which frequency does the transformer operate?

Giovanni

The final outcome is negative. Fried Q855 twice, The HV supply won't oscillate and draws a lot of current that
quickly destroys Q855. I give up.
RIP SC502

Giovanni IZ5PQT

If you do a little search, I repaired a SC-502 a few months back. I’ll have to go through my notes, but I do remember something about Q555. Don’t give up, they are very nice and handy little scopes.

I had a similar thing with a 2235 once.
I did a ringing test that showed me the transformer was bad.

Google "ringing test transformer" and you will find info.

Basically it's very simple to do:

Use a low repeat rate fast rise square wave generator and apply this pulse to the primary winding, this can be done in circuit.

Use a scope to look at one of the secondary windings, you should see the ringing being damped out slowly and nicely.
If it damps out in only 2-3 cycles, the transformer has a short.

Good luck,

Leo

Check HV windings and HV cap.

Both subject to failures, so shorting inverter load

See service manual for the operation and frequency of HV section.

Jon

The final outcome is negative. Fried Q855 twice, The HV supply won't oscillate
and draws a lot of current that
quickly destroys Q855. I give up.

Hi Giovanni,

I would be interested to try to debug and repair it.
Would you be willing to ship it to France (I'll pay of course)?
Thank you.

Best regards,

One question & one observation.
Question: Did you verify that there are no short circuits loading any of the transformers secondaries?
Observation: If the transformer in determined to be bad, there's at least one SC502 being parted out on eBay. You might inquire about the availability of a good transformer for purposes of transplant.

Thomas Garson
Aural Technology, Ashland, OR
By my calculation, the dynamic range of the universe is roughly 679dB,
which is approximately 225 bits, collected at a rate 1.714287514x10^23 sps.