Tektronix 7A13, technical questions - Page 1
Forum section: electronics / projects

author: PartialDischarge
author_rank: Super Contributor
timestamp: July 17, 2018, 06:57:28 pm
content: I'm currently studying the inner workings of some Tek 7000 plugins and since I'm missing a big thread about these great pieces of technology I'll start with one about the 7A13.To the point, question 1) It did strike me the AC coupling capacitor at the BNCs, as it has a big metallic body, the input is soldered right there, where I expect this point to be critical in parasitic capacitance, maybe it has to do with the fact that both input capacitors in CH1 and CH2 must be closely matched. Picture and schematic:The second question relates to the first amplifier stage where according to the manual NPN Q152 is used as a switch, in a saturated state. Could they have used a JFET instead and why they didn't?

author: Wimberleytech
author_rank: Super Contributor
timestamp: July 17, 2018, 10:48:49 pm
content: I will hazard this guess.The dimensions of the cap were dictated by capacity and voltage (and perhaps ESR).The cavity where it fits and connects to the switches with shortest possible trace (minimize inductance, etc.) forced the odd orientation and connection.

author: David Hess
author_rank: Super Contributor
timestamp: July 17, 2018, 11:49:13 pm
content: Quote from: MasterTech on July 17, 2018, 06:57:28 pmThe 7A18 and 7A26 dual trace amplifiers use similar input coupling capacitors configured the same way. The shielded outside of the capacitor is driven to help prevent EMI pickup. In the case of the 7A13 and presumably 7A22, those capacitors are matched to 0.4% as shown in the parts list to preserve AC common mode rejection.Note that those capacitors are oil filled plastic film units for outstanding performance. At least I think they are oil filled; I have never cut one apart.QuoteIt is kind of a weird design, isn't it?The differential clamp circuit which is not shown in your excerpt allows Q152 to disconnect the signal if it becomes 1.2 volts positive than the other side and CR150 disconnects it if it becomes 1.2 volts (?) negative compared to the other side.My guess is they could have used JFETs but FET Vgs varies over a much wider range than bipolar Vbe making matching more difficult although this would only have affected the clamp levels. JFET transconductance is also lower making for a softer clamp and lower frequency response. JFETs are also more expensive and noisier; the general rule was to use them only as required.Note that the 7A13 vertical amplifier was specifically designed for fast overload recovery time because of its use as a differential comparator.The guys over at TekScopes2@groups.io and TekScopes@groups.io may have more suggestions.

author: PartialDischarge
author_rank: Super Contributor
timestamp: July 18, 2018, 06:33:24 am
content: Quote from: David Hess on July 17, 2018, 11:49:13 pmIt definitely looks like one, same as oil filled resistors.Just visited tekgroups and found a recent upload by Kurt Rosenfel, Tektronix transformers design and manufacturehttp://w140.com/tekwiki/images/3/3d/Tektronix_transformers_design_and_manufacture.pdf

author: duak
author_rank: Super Contributor
timestamp: July 18, 2018, 04:05:21 pm
content: C1 is mounted the way it is to minimize stray capacitance to ground or any other part of the circuit. It also seems to minimize the loop area of the input attenuator circuit, although because this is a high impedance circuit, the increased inductance doesn't matter as much.C1 is most likely a high quality film cap with low Df and soakage for good pulse performance. It could have oil in it but I have no idea. They could have used a ceramic cap to minimize size but it would introduce distortion or error.

author: PartialDischarge
author_rank: Super Contributor
timestamp: July 19, 2018, 10:14:52 am
content: hmm, is it coincidence or a 7A13 I was looking on ebay, with missing knobs at a very good price, just got sold because of this thread ??

author: JoeO
author_rank: Frequent Contributor
timestamp: July 19, 2018, 11:15:16 am
content: It happens more times than you might imagine.I would never post a link to an item on ebay that I was looking at purchasing.