(SOLVED) Tektronix 2215 Y-axis fault present on CH1 and CH2 - Page 1
Forum section: electronics / repair

author: Potential Resistor
author_rank: Contributor
timestamp: June 23, 2025, 08:27:12 pm
content: Hello, first of all thanks for having a look at my post, and any assistance would be appreciated.I have recently saved, from the recycling skip, this Tektronix oscilloscope.My aim was to restore it to a clean working scope.So far, I have cleaned it inside and out, polished some deep scratches out of the plastic screen, replaced the Schaffner branded mains socket with an new equivalent module, replaced the Rifa capacitors, an X-class and some others, some of the electrolytic capacitors, the focus resistors, checked that the CRT filament transformer had been modified, confirmed all the voltage rails were present and within specification. All good so far.However, I was just about to check the calibration and discovered an issue with the Y-axis which is affecting both CH1 and CH2 in exactly the same way (See attached photographs showing what happens to the waveform as it is positioned up and down the y axis).In addition, I have attached to this post (see below) what I believe to be the relevant parts of the Tektronix Schematic diagram with the voltages I have measured at the various points around the circuit. I took the measurements after the scope had been running for quite some time and with the front panel settings according to the Tekronix manual.All in all the scope looks great now and with the exception of this issue is nice and stable with potentially many years of use left in it. I'd really appreciate some help with this as I feel think I'm about 80% there with it, but having difficulty on the final straight. I did see another scope with what looked like a similar issue, that involved the 2 * (BFR96) transistors (Q386 & Q376).....I have the necessary tools and enough skill to follow instructions should anyone be able to help resolve this problem.Thanks again,

author: ondraN
author_rank: Regular Contributor
timestamp: June 24, 2025, 07:47:38 am
content: I think the transistor Q376 is faulty because its UBE voltage is around 2V, which is definitely not right. But it could also be caused by the transistor Q350 or some resistor around it. R351, R350.

author: Potential Resistor
author_rank: Contributor
timestamp: June 24, 2025, 07:56:33 pm
content: Hi ondraN, many thanks for your input ;-)Your reply got me motivated and I've done a bit more testing this evening.Also, having checked the resistors you mentioned R350 & R351 (which showed nothing out of the ordinary, 220 ohms and 210 ohms respectively, which were the same values as the opposite side of the circuit too), I too am led to the transistors:Q350 (PNP - part number - SPS8236)Q377 (NPN - part number - SOO6075)Q376 (NPN - part number - BFR96)I'm going to have another play now and I'll update later.

author: Potential Resistor
author_rank: Contributor
timestamp: June 28, 2025, 11:20:20 pm
content: Back again with an update.I tried some freeze-it spray on the aforementioned transistors, whilst observing the CRT display with a waveform in the clearly dodgy zone. I didn't observe anything that would narrow it down for me.Next step was to swap around the Q376 (BFR96) and the Q386 (BFR96), that change flipped the issue on its head.So I definitely need a new pair of BFR96 transistors.Found some online, be back when fitted.

author: Kwakerman
author_rank: Regular Contributor
timestamp: June 29, 2025, 12:51:49 am
content: Fairly common issue so order a couple for spares, also worth checking the 4 big resistors in that area, they can go out of tolerance. The PCB is normally discoloured in that area on high hours scopes.

author: Potential Resistor
author_rank: Contributor
timestamp: June 29, 2025, 07:00:41 pm
content: @Kwakerman. Thanks for that information.So. I've looked through my resistors and have nothing to replace those 4 x (340R 1% 0.5 watt metal film resistors).Can anyone recommend a suitable (available and reasonably priced) resistor? I don't seem to be able to find a 340R, only 330R.

author: Kwakerman
author_rank: Regular Contributor
timestamp: June 29, 2025, 09:21:48 pm
content: Are they actually out of tolerance? If not then they can stay but just keep that in the back of your memory if you get a fault later.

author: Kwakerman
author_rank: Regular Contributor
timestamp: June 29, 2025, 09:24:57 pm
content: Mouser do themhttps://www.mouser.co.uk/ProductDetail/YAGEO/MFR50SFTE52-340R?qs=UFD7vfw3J8oD76X%2FPdoZtA%3D%3D

author: Kwakerman
author_rank: Regular Contributor
timestamp: June 29, 2025, 09:28:34 pm
content: And Farnellhttps://uk.farnell.com/vishay/mrs25000c3400fct00/res-340r-1-600mw-axial-metal-film/dp/9467408

author: Potential Resistor
author_rank: Contributor
timestamp: June 29, 2025, 10:10:49 pm
content: @Kwakerman. Many thanks again for all the information.However, I've just been digging around some more in my parts bins and found a 10 pack of new/old RS brand 680R 0.5w metal film resistors.Is there any reason why it wouldn't be feasible to put 2 of these in parallel for each of the original resistors to get the 340R but with greater power handling?If I remember rightly the originals measured around 313 Ohms (in circuit and cold) so they are out of spec but not burnt and no discolouration on pcb. The scope in general doesn't look like it has had a hard life.

author: Kwakerman
author_rank: Regular Contributor
timestamp: June 30, 2025, 07:33:33 am
content: Don't see why it wouldn't work, its not a particularly high frequency or precision circuit. That said, you will probably find the originals are fine. You could always lift one end of each resistor for an accurate reading, parallel paths will be the likely cause of the lower than anticipated readings. Resistors usually go high rather than low.

author: ondraN
author_rank: Regular Contributor
timestamp: June 30, 2025, 07:38:40 am
content: IMHO, if these resistors are not brownish and the PCB underneath them is normal and their value is fine, I wouldn't change them.

author: Kwakerman
author_rank: Regular Contributor
timestamp: June 30, 2025, 08:22:56 am
content: Agreed

author: Potential Resistor
author_rank: Contributor
timestamp: June 30, 2025, 07:32:54 pm
content: Well I took both of your advice: @Kwakerman, @ondraN.I lifted a leg and found the resistor actually measured 339 Ohms, so the PCB and the resistors look fine. Thus, they are staying :-)Looking forward to receiveing the BFR96 x 2 (Plus 2 spares), shouldn't be long now.I'll follow-up when they've been installed.Cheers.

author: Potential Resistor
author_rank: Contributor
timestamp: July 02, 2025, 09:18:53 pm
content: Well it's happy days here :-)I received the 4 x BFR96 transistors (Phillips brand), put a new pair in (which I have to say was a fiddly affair, not keen on the package style, even when I shaped them from the back side first). Hey presto, the display is behaving itself.I've warmed him/her they/them up and am just working my way through the calibration stages.I'll let you know how I get on.

author: Potential Resistor
author_rank: Contributor
timestamp: July 03, 2025, 06:52:34 pm
content: Thanks to those who contributed to my efforts, I'll see if I can pay-it-forward on here sometime / somehow.Attached is a photograph of the "near enough for farm work" refurbished, saved from the landfill, Tektronix 2215 showing off a 1Vpp ramp @ 1Khz on CH1 (0.5 volts per division setting), and a 2Vpp sine @ 1Khz on CH2 (1 volt per division setting).The BFR96 transistors I removed measured (on a basic tester 3V and 0.1mA for hFE test) hFE 262 on one and a hFE 152 on the other.A diode test on the hFE 262 transistor measured 742mV accross the base and emitter and 666mV accross the colector and emitter (nothing registered on leakage test I performed)A diode test on the hFE 152 transistor measured 1.25V accross the base and emitter (after falling from a couple of hundred millivolts higher) and then did the reverse when I swapped polarity, so was definitely misbehaving. Across the collector and emitter measured 667mV (again nothing registered on the leakage test I performed).Thanks again, I learned a lot whilst going through this process. I might just get my broken Kikasui COS6100 out now and see if I can fix that.

author: Kwakerman
author_rank: Regular Contributor
timestamp: July 05, 2025, 07:03:52 am
content: Congrats and good work.