"bg" wrote in news:fgt74f$idp$ snipped-for-privacy@news.xmission.com:
They would have to be matched,else you get an offset.
check this out;
the 155-0078-xx is used in a lot of different scopes,several 7000 series plug-ins,too.
-- Jim Yanik jyanik at kua.net
Both channels on the verticle front end are damaged. One dual FET follower is a definite (Q1297). It is tek part number 151-1090-00. I haven't seen one yet using google. Is there a replacement or two discretes out there that will work? I noticed that the pinouts for the chip following the dual FET (U1290), are incorrect. Doing a diode test on the chip, there are several pins mislabeled. The external circuitry seems to go to the pins as shown on the schematic, but the internal circuitry ohms out to different pins. This would only be an issue if you ohm out the chip like I did to see if the chip was bad. If anyone is interested, I can post what seems to be the correct internal circuitry. bg
"bg" wrote in news:fgvkea$n3d$ snipped-for-privacy@news.xmission.com:
I've seen the input BNC connector cause DC offsets! seems it absorbs contaminants and becomes a weak "battery".
-- Jim Yanik jyanik at kua.net
Now that's certainly not something I'd have expected. I'll keep it in mind should my 465 ever misbehave in that way.
"bg" wrote in news:fh08m1$2og$ snipped-for-privacy@news.xmission.com:
are there input protection diodes on that FET input? did you disconnect the atten PCB and still have the DC? (and then GND the gate and the offset goes away?)
-- Jim Yanik jyanik at kua.net
"James Sweet" wrote in news:A_PYi.24184$Cb.6275@trnddc08:
definitely a weird one. only had a couple of those,as I recall.
-- Jim Yanik jyanik at kua.net
Thanks Jim. I had already been to that site and didn't see it the first time, even with my glasses on! Offset is the issue I am having. bg
I replaced the channel 1 connector, but plugged in one of the power connectors backwards. So I blew one of the dual fets and the other one seems to have a leakage problem. There is DC on the gate lead right after the attenuator. The 1 meg resistor in the attenuator output isn't low enough to pull the gate to 0 volts. Out of the circuit I can't measure anything abnormal with the FET but in circuit the DC is there. BTW , you are a great weath of info for these tek products. bg
I replaced the ch1 connector, but connected a power connector backwards. It blew one of the dual FETS and the other seems to have a leakage problem. There is DC on the gate where the attenuator output connects. The one meg resistor in the attenuator output is not low enough to pull the gate to zero. Out of the circuit, I don't see anything wrong with the fet. BTW , you are a great resource for these tek products, thanks.
"bg" wrote in news:fh2qir$8d2$ snipped-for-privacy@news.xmission.com:
No,the atten grounds it almost directly.Maybe thru a ~47 ohm R. I forget the exact circuit values. Check your schematic.
1 megR is the basic input Z,it's no "ground". You HAVE to see signals across that input Z.Have you done your DC bal adjustments?
-- Jim Yanik jyanik at kua.net
I've disconnected the gate completely from all of the external circuitry and then connected it to ground thru a 1.5 meg resistor , which I assume is what it sees from the attenuator. Seeing as the gate draws no current , there should be no voltage right, or perhaps just microvolts. I get way too much. If I short the gate to ground or use low resistance like about 50 ohms, I can drive it with a generator and all is fine. But with High Z to ground, the offset is too high for the verticle position control to bring the trace back on screen and none of the balance or position pots have enough range. bg
The output of the attenuator has a 1Meg resisitor shunted across it. The gate connects to this resistor with a 470K. I think the short only exists when the front pannel AC/OFF/DC switch is in the off position. It's supposed to short the output of the attenuator. If I put that switch in the off, center position, the trace does come back to center screen. Balance adjustment works, gain adjustment works. The attenuator measures 1 meg in on all ranges. I've ruled out DC in the attenuator by disconnecting the FET from it. I think I just need to get a new fet because everything points to the gate having leakage of some sort. bg
"bg" wrote in news:fh342u$rd9$ snipped-for-privacy@news.xmission.com:
DC-GND-AC. You just said that shorting the FET gate thru the 470K R removes your offset;that tells me the offset is coming from the atten/BNC.
Not relevant. You're dealing with mV offsets.
And grounding the gate,you still get an offset?
The FET gate has an extremely high input Z(100's of Megs),the 1 MegR -sets- the scope Z to the desired input Z.
-- Jim Yanik jyanik at kua.net
With the gate totally disconnected from the rest of the scope, I connected it to ground thru a 470k resistor. There is about 1.5 volts DC on the gate. The spec sheet says this gate should deliver about a 100 pico amp reverse biased with 15 volts. That comes out to 47 microvolts of offset on the gate lead. So this FET has some pretty severe leakage at the gate. Sorry for the confusion. bg
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