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On Mon, Apr 8, 2024 at 06:54 PM, Adam R. Maxwell wrote:

Indeed, a pair of NOS 151-0220-00 transistors in Q309/Q317 did not fix the
problem. I'm curious to see if the faster transistors I've ordered will do the
trick, although if so I suspect it would just be papering over problems with
some other marginal component(s).

Hi Adam,
Your suspicion is valid. How do transistors from bad unit perform in the good 7B70? If they work fine then they are not the root cause of the failure.

Since faster transistors helped, most likely the issue is the signal rise-time or amplitude at the bases.
- What amplitude do you see at the left side of R300/R306 for good and bad unit?
- DC voltage at base was slightly higher than what schematic showed. What voltage do you see at the emitter of Q170/Q171, which are the main current source to TD stage.
- In some plugins resistors going high in value was the issue. If you pull out Q170 and Q172 (if they are socketed) you can measure R173/R175/R176/R177/R178/R179.

Ozan

On Apr 8, 2024, at 19:20 , Ozan via groups.io <ozan_g@...> wrote:

Your suspicion is valid. How do transistors from bad unit perform in the good 7B70? If they work fine then they are not the root cause of the failure.

Indeed, the slow transistors from the bad unit work fine in the good one. I should have thought to check that myself.

Since faster transistors helped, most likely the issue is the signal rise-time or amplitude at the bases.
- What amplitude do you see at the left side of R300/R306 for good and bad unit?

With 1kHz/1Vpp sine applied to EXT and conditions specified in the manual:

Good R300 LHS:



Good R306 LHS:



Bad R300 LHS:



Bad R306 LHS:

- DC voltage at base was slightly higher than what schematic showed. What voltage do you see at the emitter of Q170/Q171, which are the main current source to TD stage.

Good Q170E/Q171E: -1.35/-1.34V to chassis ground
Bad Q170E/Q171E: -1.32/-1.33V to chassis ground

- In some plugins resistors going high in value was the issue. If you pull out Q170 and Q172 (if they are socketed) you can measure R173/R175/R176/R177/R178/R179.

They are socketed, so that's easy. Let's see if tabs survive the listserv:

Bad Good
R173 202 216
R175 8.2 8.5
R176 4.32k 1.21k
R177 8.1 8.7
R178 inaccessible need to find
R179 198 215

* Bad plugin is 173,xxx serial, Good is 237,xxx serial.

* The R176 value is correct; the old one should be a 4.3k 1W 5% carbon comp, and is supplied by -50V. The new/good one should be 1.21k and -15V supply.

* R178 on the old one might require dismantling it to get to the main interface board.

* R175/177 should be 7.5 ohms 5% nominal, per the parts list. Ironically, the bad one is closer to spec.

thanks,
Adam

Hi Adam,
My comments are below:

On Tue, Apr 9, 2024 at 08:22 AM, Adam R. Maxwell wrote:

With 1kHz/1Vpp sine applied to EXT and conditions specified in the manual:

Good R300 LHS:

Good R306 LHS:

Bad R300 LHS:

Bad R306 LHS:

All the waveforms show phase inversion at sine wave peaks, usually it happens when BJT transistor base is driven beyond forward biasing B-C junction. I don't see this effect in the expected waveforms, although the plot at that point is very fuzzy. If you look at earlier nodes you will probably see the same inversion, and the plots look better.

Good Q170E/Q171E: -1.35/-1.34V to chassis ground
Bad Q170E/Q171E: -1.32/-1.33V to chassis ground

These voltages are about ~ 400mV-500mV higher than the schematic values. Although we expect some difference 400mV+ is too much in my opinion. I did a back of the envelope calculation and I also come up with closer to schematic voltages. When you consider common mode rejection of Q150/Q160 it needs a large error in the emitters of Q150/Q156 to create a 400-500mV error in emitter voltages of Q170/Q171.

Since both plug-ins show the same DC operating point my guess is one of the supplies inside the plug-in (or mainframe) to be off. Most likely -15V or -50V is off as it is one of the difference in the plug-ins.

If supplies are good:
If you start measuring base/emitter/collector voltages going towards input you would find where it deviates from schematic values.

Both plug-is may have the same issue but a power supply problem at the mainframe is more likely. One design may be more tolerant than the other (-15V/-50V difference). I think you mentioned mainframe is a new acquisition too.
Ozan

On Apr 9, 2024, at 14:31 , Ozan via groups.io <ozan_g@...> wrote:

Since both plug-ins show the same DC operating point my guess is one of the supplies inside the plug-in (or mainframe) to be off. Most likely -15V or -50V is off as it is one of the difference in the plug-ins.

That sounds logical, but all the LV supplies are within spec. I do get ~1.8V at emitter of Q170 if I disconnect the EXT trigger input. I can't make sense out of that 400-500mV difference.

If supplies are good:
If you start measuring base/emitter/collector voltages going towards input you would find where it deviates from schematic values.

I've taken measurements and annotated the schematic (low serial version), which includes a better waveform from my hardcopy manual. The waveform at base of Q170/Q171 is the same as the schematic in both plugins,

Red annotations are the bad plugin, green are the good plugin. Both under the same input conditions, same settings, and same mainframe slot.



thanks,
Adam

Hi Adam,
The voltage difference between schematic values and what you measure could be because of how your meter reacts to average value. Which equipment do you use for measuring DC voltages? You could check with an oscilloscope since 400mV delta in average value is large enough to see on an oscilloscope.

I have more comments and will follow up with a direct message but I wanted to send a quick note: base voltages of Q181/Q182 shows an issue. This voltage is generated by a resistor divider (R181. R182, value of R182 is 3.3k, not 3.3ohm as schematic shows) and only connection is to the bases. With no signal are the bases of bad one still 0.4V lower than good/schematic voltages? If that is the case Q181/Q182 may be faulty.

If at DC voltages look good without signal, then clamp diodes CR181/CR182 may not be doing their job and letting Q181/Q182 saturate.
Ozan

Adam,

Your voltages at TP78 sounds way off! There is no way a 10 ohm 1/4W resistor would last with 65V across it! Is it possible you typed in the wrong voltage at that test point? The 49,9V would be correct. I am suspect of the 4300 ohm resistor. In mine, I put in a 2W (15mm body length). I wonder about the two 200 ohm resistors, R173 and R179. If carbon, check by removing a transistor at one end of each then measure the resistance. I have seen these resistors out of tolerance. Also see what C178 is rated at. If a 50V type, replace with a higher voltage type. I have seen the rated voltage of a condenser be the same as the voltage across it in other Tek. gear. Check R175, R177, R181 and R182. The voltages being off make me suspect one or more of the resistors needs to be replaced.

Mark

Adam has one newer, one older revision of the plugin where supply of R176 changed from -50V to -15V and resistor was updated accordingly. Colors show measurement between those two units.
Ozan

On Apr 10, 2024, at 14:57 , Ozan via groups.io <ozan_g@...> wrote:

I have more comments and will follow up with a direct message but I wanted to send a quick note: base voltages of Q181/Q182 shows an issue. This voltage is generated by a resistor divider (R181. R182, value of R182 is 3.3k, not 3.3ohm as schematic shows) and only connection is to the bases. With no signal are the bases of bad one still 0.4V lower than good/schematic voltages? If that is the case Q181/Q182 may be faulty.

You were correct: replacing Q181/Q182 brings the voltages much closer to the schematic values! I've redone the comparison graphic with updated values, and I'm thinking this stage looks fine now. Also added a note about the supply voltage.



I had previously tried replacing Q181/Q182, but since that didn't fix the overall problem, I reverted to the old ones without checking the DC values. Interesting lesson there.

thanks,
Adam

I started poking at this 7B70 again, now that the 7704A seems to be working reasonably well. I finally realized that it's triggering on false events, so I don't get a proper free-running trace.

With the mainframe set to RIGHT (empty) vertical, and 7B70 in B horizontal slot, trigger set to INT/AC coupled/AUTO PP or AUTO, setting the trigger pot to about 9:00 will cause the trace to flicker as it triggers on nothing. Probing around, I see a square pulse on base of Q309, and collector of Q181/Q182, but no waveforms earlier in the amplifier (e.g., base of Q170). The pulses are ~70 ms apart and 100 mV amplitude.

This is a low serial 7B70, so does not have the 6.8 pF cap across CR182/CR181. Adding a 10 pF cap there just moves the problem to another setting of the trigger pot. I immediately suspected the +15V supply to Q309 and VR304, but see nothing in it that lines up with the pulses; IIRC there's a 25 kHz or so waveform that I assume is the switching supply.

What would cause a ~14 Hz false trigger?

thanks,
Adam

Adam,

AUTO PP is not the best way to obtain a free running sweep (and not intended to be so) since some noise as tigger signal is already treated as "adequate". I tried a 7B80 in a similar way and at all position of LEVEL I get a flickering, rather dim trace because of "false" triggering.
AUTO should work well to get a free running trace when LEVEL is set outside the range of the trigger signal. With just noise as trigger signal that range is very small, so for merely a very small range of LEVEL settings you get false triggering and a flickering, dimmer trace. For other settings you get the brighter free running trace.
I understand that (in your case) 9 o'clock position for AUTO but not for AUTO PP.

Albert

On Tue, May 21, 2024 at 11:22 AM, Albert Otten wrote:

AUTO PP is not the best way to obtain a free running sweep (and not intended
to be so) since some noise as tigger signal is already treated as "adequate".
I tried a 7B80 in a similar way and at all position of LEVEL I get a
flickering, rather dim trace because of "false" triggering.

Well, *that's* a great data point! Maybe I should RTFM for the 7B70 and study what it says about operation in more detail; coming from the 475 world, AUTO PP is a bit unfamiliar, but I assumed it would be a sensible default as the uppermost setting. Since my other 7B70 and 7B85 don't behave this way, I further assumed this 7B70 was misbehaving.

AUTO should work well to get a free running trace when LEVEL is set outside
the range of the trigger signal. With just noise as trigger signal that range
is very small, so for merely a very small range of LEVEL settings you get
false triggering and a flickering, dimmer trace. For other settings you get
the brighter free running trace.

The range is very small for AUTO to trigger on noise (or whatever it is), and larger for AUTO PP (say 8:00 to 10:00 for AUTO PP, whereas AUTO has to be right on 8:30). It's annoying as the trace sometimes is so dim that it's invisible when I expect it to be free running. For the other timebase plugins, I can very rarely see a blip of the trigger light if I rotate it slowly, but it won't stay on like this one does. If I need to adjust my expectations, that's probably easier than adjusting the plugin :).

thanks,
Adam