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7B80 sweeping off the screen


ik1zyw@...
 

Thank you Roger and Albert for the excellent advice.
I have the rackable 7603 and side-probing on the live system is not possible. But if the middle bay is left empty then signals can be brought outside easily, it just needs some planning since you connect test leads when the unit is on the desk and then carefully plug it in.

It turns out Q424 was bad. Socketed transistors are suddenly cool! The original part is 2N4122 which I replaced with a BC557 which has lower f_max but the same pinout. Other transistors mentioned by Roger are OK.

Now I will move to the calibration part of the 7B80 manual because the sweep is compressed: who knows what has been done to that 7B80 between 1974 and 2007 before I got it.

Thank you!
Paolo


Roger Evans
 

Albert,

Thank you for that particular insight, my suggestion was just to have a single plugin (the 7B80) in the 7603 and move it to whichever slot gave best access for probing. Your suggestion is much better since you are using the working parts of the 7603 to diagnose the part which is not working. I have previously suggested that if you have one channel not working on a normal double channel (not mainframe plus plugins) scope you can use the working channel to probe the non working channel from the input connectors as far as the channel switch.

Roger


Albert Otten
 

On Sat, May 23, 2020 at 12:42 PM, <ik1zyw@...> wrote:


First of all, on the "+ sawtooth" output in the back of the mainframe, the
working 7B53A produces a down sloping negative signal, sweeping between -4.5V
(trace @left) and -5.5V (trace @right). Measured on the low-ish impedance
analog voltmeter.

OTOH the 7B80 produces an up going positive "+ sawtooth" signal between +1V
(theoretical left) and +10V (theoretical right). Its frequency follows
time/div changes. Trigger is honoured (no sweep in single-shot for example).

Which reading is correct?
Hi Paolo,

The 7B80 reading is correct. If you change the Hor Pos control you might notice that the signal is actually between 0 V and +10 V. Just checked this in my similar 7633 (with 7B80 and 7B53A) and the Calibration procedure also mentions that 10 V amplitude.

In addition to Roger's comments (if this is not already what he meant), note that you might also use the 7603 as ordinary 'scope with 7B80 in the Left bay and some 7A in the middle bay and the 7B53A as time base. Trigger the 7B80 externally with some Line frequency signal and set the 7B53A also for Line triggering. Now you can probe the 7B80 internal circuitry and display wave forms via the 7A module. (Of course this makes it impossible to probe the rhs of the 7B80 though.)

Albert

Albert


Roger Evans
 

You can operate the 7B80 in any of the three slots of the 7603 and it will drive the vertical deflection if you put it in a vertical slot. So if you insert only the 7B80 and take all side panels off both the mainframe and the plugin you have some options for probing but you will need to run in auto trigger mode to have the time base sweeping with no trigger. If you don't have a second scope but do have an analogue multimeter you can run on the slowest possible sweep speed and the voltages should largely follow what is in the manual but obviously on a slower timescale.

If you probe test point TP345 (waveform 4) you can confirm that the sweep generator is producing the correct sweep voltage range and polarity. Measuring the waveform at the collectors of Q448 and Q458 (waveforms 8 and 9) you should see if either half of the paraphase amplifier is causing the problem. Then you need to follow the signal through Q428 etc to track down the culprit. If the transistors are socketed ( mine aren't) I would just begin by testing Q424, Q428, Q434, Q438.

Regards,

Roger


ik1zyw@...
 

Hello.
My R7603 mainframe came with a non-functional 7B80 plug-in. Back then I bought a 7B53A and happily used the scope until few weeks ago when I came across the 7B80: time to fix it, or find a new home. I am asking for guidance because it doesn't seem too broken: it triggers, but no trace is drawn on the visible area of the screen. I have no other oscilloscope, only analog and digital multimeters. Frequency counter, signal generator and VNA won't be useful in this mission.

First of all, on the "+ sawtooth" output in the back of the mainframe, the working 7B53A produces a down sloping negative signal, sweeping between -4.5V (trace @left) and -5.5V (trace @right). Measured on the low-ish impedance analog voltmeter.

OTOH the 7B80 produces an up going positive "+ sawtooth" signal between +1V (theoretical left) and +10V (theoretical right). Its frequency follows time/div changes. Trigger is honoured (no sweep in single-shot for example).

Which reading is correct?

If I turn CRT intensity to the max I see the green halo coming from the right and it flashes off when the beam is swept back. In these conditions, pressing the "Position" button shows the cursor leaving a trace entering from the RHS of the CRT and a dot is positioned a couple of divisions right of the timing value compressed position. So the beam is trying to sweep somewhere behind the second vertical plug-in bay :)

I have run the 7B80 base without Q344 and Q346, therefore having the sweep signal (TP345) pass through R343 and off to "+ sawtooth" output. The result is a negative sloping signal between -2V and -3.5V on the "+ sawtooth" output, so much closer to 7B53A's values, but not enough to see a trace.

So far I have tested Q344 (hFE~=50), Q346 (hFE~=50), Q458, U416, all apparently good.
Not owning a plug-in extender I found a trick to measure voltages inside the 7B80 in-place, and +50V, +5V and -15V picked near Q344/Q346 stage measure OK, few 10's of mV low and no sign of AC.

Why on Earth do I get a reverse and amplified sweep?! If voltages were somehow reversed the whole plug-in would be totally dead. Or the 7B80 isn't meant for a 7603 mainframe?!

Simply put: help!
Paolo