Re: Continuing Problems with 7704A Display



BTW, you asked about the 741 voltages. I just measured again (with the new chip installed) and Pin 2 shows 0V but Pin 3 is at 1.1V. From what I understand, that's an indication that the chip is pegged trying to increase the negative voltage as far as it can but it can't get it down to -2960V.

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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