Tektronix replacement diode - Page 1
Forum section: electronics / repair

author: dl6lr
author_rank: Frequent Contributor
timestamp: June 07, 2022, 08:49:28 am
content: Hi,trying to fix the horizontal deflection of a Tek1240, I stumbled across a diode CR452 in the horizontal deflection path on board A06. Tek SM describes the diode asSEMICOND DVC,DI:RECT,SI 1000V,3.0A,A399Awith manufacturer Motorola and Mfr. Part No. being SR3681K. The same diode is used for CR360, see schematics.The board shows Motorola diodes with code 1520 and maybe datecode 789.However I did not find a reference to that part number in the Motorola databook Q3/88 or "the internet".Suggestions for a replacement type? I suspect this diode being bad (unconfirmed yet) as on TP452 I observe spikes below GND potential on the falling edge of the flyback pulse.

author: mansaxel
author_rank: Super Contributor
timestamp: June 07, 2022, 09:01:22 am
content: SomeNSN jugglinggives an alternate supplier being Vishay, under stock numberRGP30M.Might be easier. Dunno if 1N4007 makes it as a bodge; it naively has the voltage rating, at least.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 07, 2022, 09:51:33 am
content: The transformer may be running at a higher frequency and than a faster diode (UF4007) may be needed. One may also need a comparable higher current version like UF5408 ).

author: dl6lr
author_rank: Frequent Contributor
timestamp: June 07, 2022, 10:17:44 am
content: Quote from: Kleinstein on June 07, 2022, 09:51:33 amYes, this will run at abt. 16kHz, usual flyback of a CRT. RGP30M is in my drawer but I did not rate them as very fast (reverse recovery 500ns), so I will order some UF5408. Thanks for the suggestion, looks like a universal high speed power diode. TP452 should look like attached image, but spikes below GND. Have to investigate if this is the diode. Measurements are tricky as the flyback transformer in close vicinity induces quite some noise to the probe.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 07, 2022, 10:54:51 am
content: It looks like the diode does not see a high reverse voltage and one should get away with a lower voltage (e.g. 400 V or 600 V type) diode. This could be a slight advantage, as the high votlage diodes like 1N4007 are sometimes PIN diodes and can show a little more forward recovery and thus a more negative voltage peak.

author: dl6lr
author_rank: Frequent Contributor
timestamp: June 07, 2022, 01:04:15 pm
content: Quote from: Kleinstein on June 07, 2022, 10:54:51 amYes, turn-on time is one of the parameters seldomly called out in datasheets. Jim Williams addressed it in his article "Switching regulators for poets".As far as I see, the diodes reverse recovery time is not very interesting, as it should not carry any current when the deflection transistor opens (I left it out of the schematic, it is an NPN with emitter grounded and collector to TP452) and the voltage rises at TP452.Have to rip the Tek apart again as I thought I traced the problem down to a faulty MC1391P. Found out it wasn't...

author: dl6lr
author_rank: Frequent Contributor
timestamp: June 14, 2022, 08:16:24 pm
content: Quote from: dl6lr on June 07, 2022, 01:04:15 pmFixed the original problem with the deflection, this was a "Z-capacitor" acting as a shunt regulator (and measuring fine with a Peak Atlas ESR70). Tektronix installed the elctrolytic backwards in the 90ies...Measured the diode to be OK with static measurement. The negative spikes were an artifact induced by the flyback transformer, so the diode is fine.Thanks for all the suggestions (now I have some UF5408 in the drawer)