Tektronix 535A / 1A1 calibration chicken or egg problem - Page 1
Forum section: electronics / repair

author: Martin Hodge
author_rank: Unknown
timestamp: January 16, 2020, 04:20:41 am
content: I have just finished restoring a Tektronix 535 oscilloscope and I'm wanting to start the calibration procedure. The problem I have is that my only vertical plugin is a type 1A1 that itself was restored and in need of calibration. The 1A1 had a nearly dead nuvistor on the output amp and one jfet with rusted off leads on the ch2 input amp. After replacing the nuvistor and installing a new matched pair of Tek Jfets on ch2, the plugin is providing a sane waveform.The chicken and egg problem is that the calibration procedure for the 535 calls for a calibrated type K or L vert plugin, and the procedure for the 1A1 calls for a calibrated 547 scope. The 1A1 seems happy in the 535, but where to start calibration? Am I overthinking this?

author: Martin.M
author_rank: Frequent Contributor
timestamp: January 16, 2020, 08:18:21 am
content: Quote from: Martin Hodge on January 16, 2020, 04:20:41 am457 don`t exist, you mean a 547 what is a 50MHz classic Tek.You can use any trustful 50MHz scope, but take care for high voltages at the plug from the plugin, the +350V is also connected there a a lot of "modern scopes" will be killed by get a connection to it.When you have only the 535A and you will not buy another 500`s scope you can live with adjust directly the plugin.If there are other 500`s at home, they need swapp the plugins allover, means the vertical sensitivity of the slot is to calibrate.This sensitivity is in all Tek with the large slot the same to make possible the swapping of plugins between scopes.Exceptions: all 560 Series Scopes bec. the plugins of them have inside the complete vert. deflection amplifier, they are to adjust for their own scope frame.Martin

author: Martin Hodge
author_rank: Unknown
timestamp: January 17, 2020, 01:23:51 am
content: 457 was a typo, corrected.I do have a 547 that the 1a1 will go into eventually. But it is next in line to refurb. For now I will use my TDS360 with 10x probes to watch the differential signal on pins 1 and 3 of the plugin socket while the 1a1 is in the 535A. Feeding the 1a1 with signals from my AWG2021 I should be able to dial in the 1a1 which I'll then use to cal the 535A. Sound good?

author: David Hess
author_rank: Super Contributor
timestamp: January 20, 2020, 01:33:40 am
content: If you only have one 500 series mainframe, then there is no need for standardization between mainframes since you will not be moving plug-ins between them.For amplitude calibration and standardization, it is possible to use a good AC voltmeter attached to the vertical signal interface between the oscilloscope and plug-in to calibrate and standardize both with a high quality low frequency square wave applied to the plug-in input. Offhand I do not know what the signal level is for the 500 series but it is out there somewhere.A DC voltmeter connected to the same spot can calibrate zero or offset.Transient response standardization without the standardizer plug-in will be challenging.

author: Martin Hodge
author_rank: Unknown
timestamp: January 23, 2020, 05:04:07 pm
content: According to the 1M1 test insert manual, the 500 series plug-in interface is simply 100mV per division on a differential pair, both with a +67.5 Volt bias. These signals go directly into the grids of a matched pair of 12BY7's. So simply probing this interface with a trusted scope while everything is plugged in and running should be all that's necessary. There is only one gain adjustment in the 535A vertical amp. I'm not touching the delay line and it's 29 variable caps.Knowing this I've been able to ascertain that the 1A1 is still sick. So I have a Type 53/54K on the way which is a much simpler, single channel plug-in that will live in the 535A. The 1A1 will wait for the 547.