Tektronix 7704A power supply question

Yes, Jim, I'm aware of the differences between the two models. I threw in that comment because a reference to the 7704 was made in a previous post, just in case Joerg wanted to see the schematics for both models for comparison.

--

Dave M
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The self-start circuit (basically an RC discharge) should be disabled in operation through an active drain or rectifier discharge of the 'C' in the circuit.

RL

CR3046 or cr3050 are supposed to keep C3048 discharged when the unit is running.

RL

Hmmm..my manual is titled 7704A. Useful?

Hi Joerg,

Service manual for the 7704A is here:

2 files about 25MB each, so I didn't see if the schematics were in them. Good luck.

Thanks. Dave pointed me to the link where I could get it. Confirmed the suspicion that the neon bulb is just there to warn of high voltages.

--
Regards, Joerg

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Luckily it doesn't have a neon bulb in place of CR3040 like some older machines. If it acts up again I'll probe around that area but so far the scope purring alone just fine.

--
Regards, Joerg

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According to the schematic it's just a signal lamp in an oscillator setting (R, C, neon lamp), to warn of remaining charge in the big primary 'lytics. So there is no connection between this and anything that can tick. But tick it does, exactly in rhythm with the neon lamp. Oh well.

When I removed this power supply it was in a sorry state. The circuit board was truly jammed in, the two big caps bent and pressed against the aluminum panel with the large round holes instead of resting inside the holes. The FR-4 also bent with the plastic standoff dangling in the air. Oh man. Now it sits there like it should.

--
Regards, Joerg

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I admire your faith. I've seen some horrendous "workmanship" hidden behind cal seals.

The neon is really a little relaxation oscillator to indicate that the high-voltage reservoir capacitors are charged.

It isn't really a switcher in the strict sense of the word. It's actually sort-of-sinewave, resonant around 20kHz. If it ticks, that's usually a sign that it's going into current limiting, or could be under-loaded. They need about 40 watts of load spread around the rails to work properly. The control chip actually gets its Vcc supply from the current feedback (dunnask me why), so it won't work at all on insufficient load, and will repeatedly trigger its startup circuit which uses a sort of diac thingy.

Try Manuals Plus

for the manual.They have the service manual in stock for 65 bucks. You really do need it. The 7704A and

7704 are *different*, especially around the PSU The 7704 has the CRT HV supply integrated into the main PSU, the 7704A has a separate HV unit "upstairs" in the removable display section, driven from the main PSU up two coaxial cables.

A lot of apparent PSU problems involving ticking turn out to be shorted capacitors, especially tantalums, usually on the interface PCB, where they're a bitch to get at ;-(

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
                                             (Stephen Leacock)

Like some oil change places? I always do that myself. Same for lab gear maintenance, where I give the units extra TLC. EPROM re-seating and such.

Must be a bit higher than 20kHz, else the shepherd would leave the lab and give me "the looks". She always does that when one of my switcher test beds goes into hiccup mode.

Thanks to Dave M I now have the schematics. Luckily there seem to be not tantalums so I'll just keep using the scope until something happens.

--
Regards, Joerg

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The flashing of DS3019 should not produce audible effects. The firing of diac CR3040 to discharge C3048 will.

RL

Maybe the neon lamp and the diac have caucused into some kind of collective bargaining process ;-)

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Regards, Joerg

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I could do with a lab-trained dog. I'm down to about 12kHz, now. In my teens I could hear 24.

Just dug the 7704A service manual out. That says 25kHz. In the past, I've tested the transformers out of circuit by resonating the primary. Using the series capacitor from the PSU, if memory serves, driving with 20 volts, I got around 30kHz with a Q of about 16. Can't find my notes any more.

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
                                             (Stephen Leacock)

I'm down to can't hear the wife unless she's right in my face ;-)

(Almost totally deaf in the left ear... you know... the one I damaged while designing a hearing aid chip ;-)

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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 I love to cook with wine     Sometimes I even put it in the food

I can only hear that high stuff on one ear, the other one always kind of rings a bit, courtesy of a kablouie event while in the army. Well, at least it missed us by about 10 feet.

Just looked in the schematic. The waveforms are lighter colored, hard to read, but for both REG OUT and MONO RAMP on U3105 (the Tektronix custpm PWM chip) it says 20usec which would be 50kHz.

--
Regards, Joerg

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One of my cow-orkers in college could hear somewhat above 30kHz. I used to drive him crazy with Waveteks. ;-)

At least you heard that one.

The sergeant heard it first. Ouieeeee ... sound did not change ... "Oh S..t! Down!" ... *BOOM* ... splat

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Regards, Joerg

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Jim Thompson wrote in news: snipped-for-privacy@4ax.com:

Uh,the 7704A transformer doesn't determine the resonant frequency. there's a series-LC circuit in series with the XFMR that determines the Fr.

that's why it's a SERIES-resonant power supply.

IIRc,it's a .03 uf/600v high current cap,can't recall the inductor's uH. it's buried in the middle of the PS,too.

(and there was a problem with some of the .03uF caps being faulty...all throughout the 7K series using the series resonant PS )

--
Jim Yanik
jyanik
at
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Subsonic? Where was this?