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Hello everyone,
This post is a follow up from post #7360, and my 7L13 with center frequency LED readout all zeros. I am a bit further along with my investigation. I now understand the operation 'somewhat' more. I have looked over the diagrams and taken some voltage readings, I also understand the basic DVM logic and readout operation which sends data to the LCD and the CRT. I have an article from a Jan, 1975 '73' magazine which talks about the Fairchild 3814 digital voltmeter. I must admit I am not too clear on the 'Analog' section of the meter. On board 'A2100', diagram 12, the input to pin 2 of U2110 comes from plug P2540 pin 4. Looking over the A2500 board shows that pin 4 goes to chassis ground. Why would pin 2 from U2110 go all the way back to A2500 to reach chassis ground.

Anyway, the A2000 board seems to be working, albeit the frequency of the clock oscillator is not 200kHz as the diagram shows, it is running at 153.7kHz, though amplitude is good at 5 volts, I would think that would affect calibration but not operation. Back on A2100, the tuning control voltage at R2104 is good, it swings from + - 9.6 volts. The output of U2110 swings + - 10.7 volts. The DCPL Supply rails 15, -15, and 5 volts are all good. The reference voltage at the cathode of VR2115 (Zener) is 3.1, not the 3.3 volts is shows. Voltage at TP 2100, (output of U2130) is 11.65, a bit higher than the listed 11.3 volts. The voltages around the N-channel FET (Q2120) is Drain = 3.37 volts, Source = -3.1 volts, and Gate = -3.5 volts, are those in the ball park?

Needless to say there is NO signals present, waveform, 1, 2, 3, and 4 are all missing. Lines 'E' and 'F' are both quiet, the voltages on 'E' are 7.08 volts, on 'F' the voltage is 5.12 volts which swings a bit higher when I adjust center frequency, up to 5.44 volts. Of course 'E' has no signal as would be expected without drive from U2145 there is nothing to trigger the multivibrator U2030 on board A2000 sending a signal back up line 'F'. I keep thinking of this in terms of a frequency counter, I keep thinking there should be a driving input signal. I try to keep in mind it's a voltmeter, and keep in mind the circuit description given in the manual. It describes a ramp signal coming from the output of U2110, I don't understand that statement. The 'Tune' voltage is just a DC voltage, plus to minus. The minus input of U2120 goes to chassis ground??? What 'RAMP' are they speaking about?

Lastly the manual's circuit description on page 5-10 talks about a positive going ramp from U2110, aren't these just DC levels, what 'Ramp' are they talking about?

Okay, I see there are a lot of questions here. I had better post this and hope for the best. It's a great 7L13, it works well, but without a 'Center' frequency display, it's pretty much just a toy. Also, I do appreciate the feedback~!~ I am not accustom to working on test equipment, my experience has been mostly all in Hi-Fi, but this has been very interesting.

Thanks again VERY MUCH...
~Michael - AF7U







--- In TekScopes@..., "Michael" <af7u@> wrote:

I sure appreciate the input, I will give everything a go once I get it on my bench. Should be this weekend or first of next week. Thanks again,
~Michael



--- In TekScopes@..., "Christopher Hilton-Johnson" <chj@> wrote:

Also make sure that the 2 boards, A2100 DVM analog board and A2000, DVM
logic & readout board are only connected/touching where they should.
There needs to be a small space (say 1mm or less) between each - a piece
of thin card between the two will suffice. If the boards touch the
display can freeze, you can lose the units digit on the SA (although the
7Kxx readout still works). OP is spot on - also remove clean & reseat
the A2000 board actives.

Beyond that you need to start looking at the A2100 board & follow the
volts! The manual gives an excellent description of what should happen.
On the basis that neither readout works, it is more likely the A2100,
analog board you need to look at.

Chris HJ



My 7L13 display was cured by pulling and reseating the DIPs on the RO
board. IC pins were visibly tarnished. Symptoms not as complete as
yours, but AFAIR the display would intermittently 'freeze' while tuning
continued.

--- In TekScopes@... <mailto:TekScopes%40yahoogroups.com> ,
"Michael" wrote:

Hi all,
I have a 7L13 Spectrum Analyzer out in the field, it's not in front of

me right now but I will pick it up later this week. I've seen the unit
and the 'Center Frequency' LED read-out is stuck, and reads '0000', also
the same value is being sent to the CRT display. The analyzer is sitting
in a 7000 series mainframe and the mainframe works fine in every other
way. In fact, the spectrum analyzer works fine, just the 'Center
Frequency' display stays on zeros. The 'Tuning' works and adjusts
correctly. It is used with a TR-502 and that combination also works, you
just don't know where you are frequency wise. You can place a frequency
counter on the AUX out of the TR-502 and manually scan the bandwidth and
read the frequency on the counter. It appears everything is fine, just
the readout is non-functional, or at least stuck on zeros. I thought it
was curious the display in the 7000 is also reading zeros too. I will
have it in front of me next week but I thought I would see if anyone had
any thoughts before I open it up.

Thanks in advance,
~Michael - AF7U

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