Okay, here's my report on the loss of center frequency on a 7L13 plugin (digest #7424), albeit a bit late. I had to put together a parts order (Thanks to the tip from 'crippo2', Vernonia Northern, right next door here in Oregon has many Tek parts in stock.) and they just got here today, very quickly I might add. 'David' Hess's advice on control line 'E', it should have been appx. 4 volts, and was at nearly 7 volts. Thanks David for your tips, they were very welcome. U2145 was indeed out to lunch, it's plus and minus supplies were fine. Due to internal issues it allowed pin 6 to swing above the expected 4 volts. R & R the LM311 and viola, the DVM circuit was once again functional. This was the second issue this module had since its arrival from a seller on Ebay. I just love the 'phrase' "We don't know how it works, so it's being sold As-Is", translated: "It's broke, and it don't work". HiHi The first was a shorted tantalytic in the +15 volt rail. Once that was replaced, the machine worked but no center frequency display, LED or on screen. So... a sincere Thanks to David for your help, you were spot on. I kept thinking something downstream was causing the issue, compounded by my non-familiarity with the DVM to begin with. I was thinking 'Frequency counter', when I was dealing with a Digital Voltage Meter, ya live and learn.
73,
~Michael - AF7U
toggle quoted message
Show quoted text
--- In TekScopes@..., David <davidwhess@...> wrote:
On Mon, 11 Mar 2013 06:35:34 -0000, "Michael" <af7u@...> wrote:
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.
U2110 (LM308 style low bias current bipolar operational amplifier) is
configured as a differential amplifier. The signal ground that the
tuning voltage uses may not be at quite the same potential as the
signal ground that the readout uses so the differential amplifier
subtracts the difference. The differential amplifier will also
subtract low frequency noise between the grounds.
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?
Those voltages all sound good to me and they are mostly the ones I
would have asked you to measure. In the worst case they add up to a
calibration problem and not a complete failure.
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?
E is 7 volts? That is quite a trick since the pull-up resistor
(R2149) is connected to +5V and the output of U2145 (LM311 style
comparator) is wired open collector (pin 1 is the emitter of the
output transistor and wired to ground) so it can only pull down and
not pull up. On the receiver side, U2010D is part of a 74L04 and also
has a 5 volt supply voltage so even if it failed, it can not pull up
to 7 volts
U2145 (LM311) is probably bad but you might want to look for a bad
socket or open trace. Nothing should be able to make its output go to
7 volts except an open on pin 4 or failed part.
The operational amplifiers and comparator are all TO-99 8 pin metal
cans which are difficult to find these days. If you can not find
metal cans, I would try replacing them with 8 pin DIP parts which
conveniently have the same pinout. I have done this before by cutting
the DIP leads short and soldering little wires to them. Cutoff 1/4
watt resistor leads work well.
Texas Instruments still sells TO-99 LM311 comparators but they are $10
each.
U2010 may be damaged as well. I think it can be replaced without
problems with a 74LS04 or even a 74HCT04 which are both more common.
If you have to order the LM311, pick up an LM308 and 74L04 replacement
as well just in case. The LM308 operational amplifiers can be
directly replaced with the Linear Technology LT1008. An LT1012 or
lots of other devices can be used if you pull the compensation
capacitor. JFET or MOSFET input devices should work as well.
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?
I think the ramp they talk about is the output of U2140 (another
LM308) which is configured as an integrator. The frequency (and
slope) of the ramp is proportional to the measured voltage.
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.
I had to add a few pages of the 7L13 manual to my book of stuff I do
not have so it took a bit longer to respond to your post. The 7000
series spectrum analyzers look like fun. I will have to keep an eye
out for one.
