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On Sat, Jan 2, 2021 at 01:43 PM, David Campbell wrote:

5. Possibly the main point, there is **** NO such thing as grid bias.**** No
such thing as base bias in transistors either. Control is exerted from GRID to
CATHODE. A grid cannot control the space by itself. It is an electro-static
couple with the Cathode! Base bias is meaningless without the emitter.

David,

This may be the source of confusion for those of us not as well versed in these issues. Regardless of your fine technical explanation, TEKTRONIX still calls R1375 "GRID BIAS" on the schematics. That being said, who am I to argue the point? If some EE or Designer at the place that made the instrument calls it "GRID BIAS", then for continuity we poor and less educated souls in the field tend to use that name. Right or wrong, that is the name that shows up on the schematics. Another one of the many error and omissions we find in various schematics.

I appreciate your explanation of what is supposed to go on here!

Sincerely

--
Michael Lynch
Dardanelle, AR

I also would be interested in a set of boards if they aren't too pricey.

A source of the mechanical parts is plug-ins for the 5000 series scopes e.g. 5103, 5440, which use exactly the same form factor as the TM 500/5000 plugins, but with different keying on the edge connector as they are NOT electrically compatible.
The plugins for those scopes are frequently available inexpensively as they seem to be not very popular now.
Personally I like them for the large screen (same size as the 7603) and lack of fan. The low bandwidth of the 510x is adequate for audio, I use one with my audio analyzer to monitor both audio channels as well as the distortion trace simultaneously.
The frame rails with the mounting tabs for the PCB are identical and if you're lucky the infamous plastic front bezel will be intact and the side panels will be present. The front panel will have holes for the specific plug-in, but it is easy to make a new aluminum backer plate and front panel.
-Mac

" unable to get a visible spot on screen at the +15 V intensity level as indicated in the service manual. I had to crank the intensity up nearly to maximum, and then the trim pot (R1375) for grid bias had to be cranked to one extreme to see anything on screen."

Having done so much tube type TV work that I went cross-eyed.. LOL.... I got burned repeatedly back in the day with similar problems, so dont feel bad, this is normal...

1. Z-Axis is intensity modulation, which jives with lack of such related to "cant see anything on the screen". Thats on top of the regular CATHODE bias. Theres regular intensity, then on top of it, Z axis modulation OF that intensity.
The basic intensity must be correct first.

2. With all due respect, theres a lot of wild and frustruating guessing going on. The answer is never 'lift components and make measurments in good circuits"
Circuit analysis is required.

3. Bad Z axis amp. WHich transistor?, theres 7 of them. If its not being driven in Z axis modulation, that circuit might not be relevant. It doesnt take Z axis drive to put a spot on the screen.

4. The bad amp may not be related to the dim CRT. Very complex circuits here. I cant tell you how many times Ive chased the wrong problem!

5. Possibly the main point, there is **** NO such thing as grid bias.**** No such thing as base bias in transistors either. Control is exerted from GRID to CATHODE. A grid cannot control the space by itself. It is an electro-static couple with the Cathode! Base bias is meaningless without the emitter.

MUST measure the CATHODE potential.

The Cathode is NOT GROUNDED, therefore, its potential works with, or against., the G1 bias.

Further, these are not just DC bias circuits, they are pulse driven from T1320. The CRT Cathode receives pulses from T1320, thats not a filtered DC line thru CR1321.

There is a sensing feedback from the CRT cathode back to Q1306 probably to control intensity,
that circuit should control the SMPS associated with T1320. That transformer generates K, G and Focus potentials.

6. In tubes, the voltages on the elements between G1 and A can have a major effect on K-P current. Not as much, depending on voltage, typically as the G1, since G1 is closest to K,
but G2 can have a major effect. That CRT has FOUR GRIDS and all 4 and the Cathode MUST have proper voltages:

G1 control grid
G2 ~ 50V fixed bias
G3- Focus, ~ 2Kv
G4- Astigmatism, ~ 20-100V?

ALL of them have an effect on anode current, thus, brightness..

7. Low anode voltage can cause loss of brightness. That takes a HV probe.

Ignore the Z axis circuit, yes, I know thats what was "blown up" but troubleshooting MUST begin with verifying all the correct CRT voltages including pulses.
Not an easy problem to tackle.

On Sat, Jan 2, 2021 at 02:32 AM, Jeff Dutky wrote:

VR1374 is not shorted (I didn't check that it's breakdown voltage was 82 V, as
I sadly have no way to do that).

Jeff,

Can you measure the voltage drop across the diode? That is partially how I discovered the "bad" VR1374 on my 475. You should measure 82V across the diode. Here is my conversation with Chuck Harris regarding the problem:


ME: I must admit, I do not fully understand the DC Restorer. I have been reading these and other posts on the subject.

CHUCK: Have you checked that VR1374 has 82V across it? That 82V puts the grid
bias control into the range where the grid is in the cutoff region for
the CRT.

ME: YES! Testing across VR1374 I get about 66VDC. This seems at odds with your statement quoted above. This is with the + lead on the band side and the - lead on the opposite end of the diode. Reversing the leads gives a negative voltage of similar potential. So I have "Low" voltage and something like you are describing below, correct?

CHUCK: High voltage zener's and the capacitors that parallel them often start
to break down at too low of a voltage... they become leaky... and that
would put your CRT grid into the blindingly bright range.

ME: I was expecting 82Volts but perhaps I am not measuring this correctly? From your statement, 60-66VDC would put me in the "blindingly bright range", correct?

ME: CR1373 and VR1374 check "good" with a conventional diode check, using a DVM. Both show infinity one direction and .576 VDC in the other.

Like I stated previously, the scope works OK for the first 15-20 cold minutes of operation. After that it all goes nuts."

END OF QUOTED CONVERSATION.

MORE FROM SAME THREAD (after testing):

ME: "I may have answered my own question. After writing that post, I thought that I might actually be following or understanding Chuck's explanation and that I might have a bad 82V Zener or a bad capacitor. Since I had a parts scope (the one that the CRT came from) I went out and pulled C1373 and VR1374 from that unit and installed them into my "malfunctional" unit. I am testing now, but the issue seems to be resolved. In addition, I put the unknown or "Bad" VR1374 into my Type576 and 577 curve tracers. In both cases, the component would show a normal Zener curve, then suddenly break down into a crazy looking curve (looked almost like a tunnel diode curve). Not sure what was going on, however, both the 576 and 577 showed the same results."

END OF QUOTED CONVERSATION

This scenario above may or may not be relevant. But if a "Low" voltage (less than 82V) is "blindingly bright", then does it not make sense that "High" voltage (more than 82V) would cause the display to dim? I can be way wrong, but you see my line of thinking?

Good luck!
--
Michael Lynch
Dardanelle, AR

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)?

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.

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,

On 2020-12-14 9:04 a.m., toby@... wrote:

On 2020-12-14 12:17 a.m., Chuck Harris wrote:

I'm pretty sure it is something put together
by Tektronix's graphics department.

Yes, and I don't think it's consistent either; after all, it spanned
several decades. It would probably be possible to find a suitable free
bold, compressed font that resembles it.

Turns out I have considered something suitable for binder spines before:

View/Reply Online (#146250): /g/TekScopes/message/146250

I also took a shot of my own shelf with some sample setting:
/g/TekScopes/album?id=258709

Of course Fjalla isn't the _same_ font as any of the binders, but the
binders aren't consistent either, so it might help someone get close to
the original look. You could also try horizontal scaling as desired (I
couldn't do that in GIMP?!)

--Toby

--Toby

-Chuck Harris

snapdiode via groups.io wrote:

Cough, cough... Can someone perhaps identify the font(s) used on the plastic CombBind spines of the manuals?

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

Can anyone shed light on what is involved in repairing a unit showing that error?

Thanks,
Reg

Ah yes, the switch is physically constructed as two gangs of 12 contacts, with two oppositely opposed wipers per gang.
You can insert some pins to physically limit rotation so the two wipers never pass more than 6 contacts each and that effectively divides each of the 2 gang of 12 contacts with two wipers into 4 gangs of 6 with their own wiper.

It had me confused at first and was a little difficult to find, but it is electrically a 4P6T switch.

Well, at least it's working as one in my application. :)

Thank you Harvey! That is pretty much what I had surmised but I was not sure. So, I have three function generators and a few DMM etc... My RTB also has a generator and frequency counter and more. Clearly, I paid for my RTB, but it seems to have almost everything I've needed to learn and explore SO FAR!

Clearly I laud the effort that went into the project.

Again, thank you Harvey! larry

I also would be interested in a set
Stan

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

ADT name to the list as well, please.

Hi all,

I'll add my name to the list as well. You have to grab these things as they go by!

Cheers
Phil / W1PJE

Jared,

Your manual lists S3 as 4P6T. Entering that part number in Digikey shows it as 2P6T. Is there a typo somewhere?

Tom,

Thanks for setting me straight. Yes, when I draw the simple schematic it's pretty obvious what's happening, and why the measured resistance of the pot has a humped profile (when you reach the middle of the range you get current flowing preferentially through the other leg). Damn. I was really hoping that I had this licked (even though it meant pulling the scope completely apart).

So I'm still at a loss to explain what's happening here. I guess I'm going to have to put everything back together, power it up, and probe around the live circuit with a scope, and compare against the sample traces in the schematic.

Tally-ho! I guess?

-- Jeff Dutky

Hi Jeff,

I misunderstood -- I thought that all of your pot measurements were done with at least 2 terminals isolated. If you did not isolate the terminals, your measurements don't really point to anything specific.

To understand why, draw a schematic of the pot (two resistors in series, with their junction brought out as the wiper connection). Now represent the (still-connected) other stuff as resistances that connect each terminal of the pot to the other two terminals of the pot. If you stare at that for a second, you'll realize that an ohmmeter will be measuring things besides the pot itself. You'd only be able to indict the pot for sure if you measured higher than 25k. Measurements that are below 25k would be expected, as all that other stuff is in parallel with the pot.

So, you can't conclude that the pot is dead. Indeed, I would say that your pot is most likely just fine. Its resistance is varying as you rotate the wiper, and the values are below 25k. That behavior is inconsistent with the standard failure modes of a pot.

--Tom

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

Please add me to the list! it is an interesting project!
Thanks
Mike