Re: Continuing Problems with 7704A Display


Hi John,

I plan to recheck the voltages in this entire circuit. Hopefully something will become apparent. As you say, with 3178V (presuming I didn't measure something incorrectly), it doesn't make sense that I can get -2960V (or a lot more) at TP4221. I'll follow up with anything I find.

Sorry I didn't remember who had asked about that. It's a pretty long thread now...

Barry - N4BUQ

----- Original Message -----
From: "John Kolb" <>
To: "tekscopes" <>
Sent: Saturday, October 16, 2021 12:14:53 AM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
Hi Barry,

That was me. The purpose was to make sure the voltage from T4201 and the
voltage doubler wasn't to low to be able to reach the -2960V at the test
point. Instead, 3178V looks to high. The Collectors of Q4105/4115 can
only range from 0 to 150V (maybe 154 or 156) and 150 subtracted from
3178 is still -3028. The Voltage at the top of of C4208 should be
between 0 and + 150 with respect to ground, and the bottom should be the
same reading as at TP4221. About the only thing that could make them
different would be R4221, 100 ohms, turning into a very high resistance.

John KK6IL

On 10/15/2021 3:14 PM, n4buq wrote:
Hi Ozan,

I checked the differential voltage between -54V AND +54V and it was 108.5V. I'm
not sure why the schematic shows 109.1V but I presume that happens to be what
someone measured during development.

BTW, I don't remember who asked, but I just measured the voltage across C4208
and, unless I did something wrong, that shows to be 3178V.

Barry - N4BUQ

----- Original Message -----
From: "Barry" <>
To: "tekscopes" <>
Sent: Friday, October 15, 2021 1:27:12 PM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
Hi Ozan,

I'm getting 153.5V at C4201. If 156V is expected, then that might be a symptom
of other issues.

I disconnected P4212 and P4213 and measured the voltage across the inner
conductors. That shows 118.2VAC. I measured that with my Fluke 27 which
_should_ be okay at that frequency (the meter is spec'd up to 30kHz).

I haven't measured the differential voltage between P313's pin 1 and pin 2 but
I'm pretty sure the individual voltages are correct with respect to ground.

I'm not sure what that all might indicate but it's what I measured.

Barry - N4BUQ

----- Original Message -----
From: "Ozan" <>
To: "tekscopes" <>
Sent: Monday, October 11, 2021 10:55:26 AM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
Hi Barry,
There is a possibility that T4201 is not generating high enough voltage. As
others pointed out even when collector of Q4115 is as low as it can go you are
not able to get low enough (or high in absolute sense) cathode voltage.

What voltage do you measure across C4201 at +150V terminal? +156V is expected.

T4201 voltage depends on what comes from J4212 and J4213. If you have access to
those terminals each should show about ~ 100Vpp 20kHz signal according to the

54V setting that matters for T4201 is in the lower unit, sheet <8>, it is the
+54V adjustment R3143. Differential voltage at connector P313 pins 1 and 3
should be 109.1V according to the schematic.

741 could still be bad since you are measuring 0.8V at pin2. If R4120 measures
470k (i.e. it is not faulty) there is too much current coming out of pin2.
However, replacing the 741 may not fix the real problem.

On Mon, Oct 11, 2021 at 08:29 AM, n4buq wrote:

+54V was about +53V. I set it properly and tweaked -50V (was -50.1V).
Virtually no change with the ability to set -2960V and the voltage at
Q4105/Q4115 is still at about 0.5V.

I presume that since the HV multiplier is a X14 and the anode voltage is
supposed to be 21kV, then the input to the multiplier (from
T4201) should be 1500V RMS @~20kHz. So far, I haven't found this stated
empirically so would like confirmation on it. Furthermore, from there, the
doubler circuit should yield 3kV. Is that correct?

Until I get the replacement 741, I'm not sure what else I can check.

Barry - N4BUQ

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