Hi Ozan,
I'm not sure but I think I may have found problem with the LEVEL/SLOPE control. I checked the base voltage of Q224/Q234 with the SLOPE in + and - mid-positions. In both cases, Q224's base is ~ 0.78V and Q234's base is ~ 1.62V. I checked the voltage at the "CQ" point (junction of R248/R260/CR245/C248) does not change when I rotate through the 12:00 position and I think S260 is not working properly (not opening).
I know that when I was watching the signal at TP262, rotating the SLOPE control from -, through mid-position, and then to +, a - slope would cause the square wave to appear, it would go to a steady-state (SLOPE pointing to 12:00) and then the square wave would reappear as the slope control was further rotated to the + side. I don't know whether S260 was actually working when I was looking at that trace or whether the pot was causing the square wave to reappear strictly due to the resistance changes.
I'm not very fond of that particular type of control. There is very little, if any, indication when the maximum or minimum is reached and it is very easy to rotate it continuously in either direction. I have a 7B80 that has a separate rotary switch that gives a very positive "click" between slope directions which works very well.
I removed the S260 assembly last night but I'm not sure which contacts are associated with S260 and which ones are associated with the various stacked pots. I'm unsure how to disassemble the stacked elements to determine if the switch is physically damaged so I'm a bit stuck. If anyone on the list knows which contacts are which on that assembly, please let me know. Until I can get this verified, I'm not sure of whether it makes sense to continue with the rest of the troubleshooting.
Thanks,
Barry - N4BUQ
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----- Original Message -----
From: "Ozan" <ozan_g@...>
To: "tekscopes" <[email protected]>
Sent: Wednesday, November 17, 2021 9:09:11 PM
Subject: Re: [TekScopes] 7B50 - No Trace
Hi Barry,
My comments are below:
On Wed, Nov 17, 2021 at 04:03 PM, n4buq wrote:
...
I presume you want "-" slope so as to disconnect the CQ connection point from
the rest of the circuits.
There is a subtle reason. If you are interested (otherwise skipping is fine): We
want to turn off the current to CR262 to see if it goes to low voltage state.
With -'ve slope Q244 path is selected, to turn off Q244 we turn on Q224 (and
turn off Q234). Base of Q244 is at a known voltage ~ Vbe+little offset (you
measured 0.786V which is fine). This way we can predict most of other voltages.
When +'ve selected we need to turn on Q234 to achieve the opposite effect but
its base voltage depends on the level knob so voltages are a little harder to
predict accurately.
We expect :
At base of Q224 ~ 0.6V [0.786V]
At base of Q234 at least 1V below base of Q224 [1.586V -- presume this is a
problem?]
Perhaps I got it wrong, does turning LEVEL knob all the way CCW decrease Q234
base voltage or increase it? How about all the way CW? We want to make base
voltage of Q234 below base voltage of Q224. When not in P-P Auto mode Level
knob should be able change Q234 base voltage above and below 0.7V.
Top of R224 ~ 10V ( +/1V is OK) [9.12V]
Base of Q244 ~ 1V below Top of R224. [9.12V]
This one and next one are swapped because of base voltage of Q234.
Base of Q254 ~ About same as top of R224. [8.22V]
These look OK except they are swapped. Otherwise one is at 9.12V and the other
is at ~9.12V-1V.
This should turn off all the current in Q244.
Because of the above reason instead of Q244 turning off now it is pulling ~ 10mA
(full on).
Measure voltage across R262 to find out current (Voltage at TP262/2k =~ 6mA
expected). [1.462V/100 ~= 15mA]
R262 passes sum of R261 and collector current. R261 current is 11.1/2k, plus
collector current of 10mA =~ 15mA. Matches the expectation given the swap.
TP262's voltage is 11.1V which, if R261 is close to 2k, that's approximately
6mA as you note. Resistance across R261 checks around 200 ohms which I
presume is due to parallel resistance somewhere(?).
Yes, there is R267+R264 path to +15V supply.
Measure voltage across R267 (should be same as voltage across CR262), also
note
the resistor value (it is selected) and find the current through R267.
[0.477V and 3.0 ohms!]
R267 is marked as 39 ohms. If, indeed, it is now 3 ohms, then that would be
160mA which doesn't seem correct at all. I measured several times with the
probes in both directions. With the + probe at the junction of CR262, I get
2.8 ohms. With the - probe at that junction, I get 3.0 ohms. I don't know if
that's significant but I did notice that. Is this possibly due to CR262 or
could C267 be leaking?
This is bit of a good news. It says the TD is mostly working, when you use a
ohm-meter that doesn't pass enough current TD will look like a short circuit.
It is the low voltage state (I don't know if there is a better official term).
It's a dogbone and I didn't think those were
troublemakers but I suppose anything's possible. I think I can lift the end
of R267 to confirm it's true value if you think that's necessary.
I don't think it is necessary yet. This part of the circuit is best left
untouched.
If all the R262 current is going through R267 then TD is open circuit.
With the weirdness around measuring R267, I'm not sure if this points to the
TD or what.
We couldn't turn off the current to TD. It doesn't say much yet. Only news is
most likely TD is fine.
I'm going to hold off checking the TPs below until I hear back from you
regarding the above as it appears there are problems with those readings. The
TPs below are on the side that's difficult to get to (I don't have an
extender) and if I put that plugin in the left (vertical) bay such that I can
access them better, I'm not sure if that would yield meaningful readings on
Horizontal plugins do generate sweeps/waveforms in vertical slots if externally
triggered. However, it is simpler to attack one problem at a time.
Ozan
