Analog Scope Vertical Trace Issue - Page 1
Forum section: electronics / repair

author: bhelonious
author_rank: Newbie
timestamp: December 10, 2023, 10:46:03 am
content: Hello -I'm working on a BWD 824 35MHZ scope that I purchased from an estate. Not sure if this will be the only issue but it's the biggest problem at the moment. I've attached a few photos to show the behaviour (can't upload a video but hopefully you'll get the picture.....unintended Dad pun....) - hoping that someone will be able to point me in the right direction. It's not my only scope but I'd like to learn by trying to repair this one.Basically the waveforms distort as the vertical tracer is moved. I've tested all the DC power and it's all correct and/or within tolerances listed in the service manual, and I've started testing the main board and the vertical output board but so far I've not found anything terribly out of spec EXCEPT when monitoring the two connections to the vertical output amplifier the voltages do not change smoothly when turning the vertical trace knob from full CCW to full CW (which I assume they should). The problems are similar on both channels. (FYI I'm using a Fluke multimeter for the DC and a working Tektronix 2248 scope for the AC readings.)Thanks for your help and let me know what other info I can provide to assist.Cheers

author: Zenith
author_rank: Frequent Contributor
timestamp: December 11, 2023, 01:51:34 pm
content: It seems like a fault in the final vertical amplifier as it's unlikely that both preamplifiers and intermediate amplifiers have the same fault.I'd check for burned or damaged parts, check that transistors are behaving properly, but basically trace it through and see what was happening with another scope against what you'd expect to happen from the circuit diagram. Also check with the other scope that the signal going into the final vertical amp is what you'd expect with vertical shift applied.Also take out the final vertical amp board (I assume it's a separate board) and check with a magnifying glass for cracked lands and dodgy looking soldered joints.

author: ondraN
author_rank: Regular Contributor
timestamp: December 12, 2023, 08:43:15 am
content: I had a very similar problem on an old Kikusui oscilloscope. This was caused by a defect in the electrolytic capacitors in the output stage of the vertical amplifier. The capacitors had very reduced capacitance and high ESR. After replacing it with Low ESR it worked normally.

author: BlownUpCapacitor
author_rank: Frequent Contributor
timestamp: December 12, 2023, 04:27:03 pm
content: This waveform resembles AC noise from the power supply. I had a similar problem on my Tek 7613. The trace distorts when offset. It was the filter caps going bad that caused this. Check all voltage on your PSU, and use an analog volt meter to measure the AC ripple. I find most cheap DMMs, which most people have, don't pickup AC hum on a DC supply very well.

author: p.larner
author_rank: Frequent Contributor
timestamp: December 12, 2023, 04:47:05 pm
content: is that supposed to be showing its calibrator signal?,what are the probes connected too?

author: ardiesse
author_rank: Newbie
timestamp: December 12, 2023, 11:06:23 pm
content: Long-time lurker, first-time poster . . .This is the classic presentation of crossover distortion. There'll be a fault somewhere in the vertical amplifier, between the vertical position control and the deflection plates. In this condition, the input signal has to overcome the "turn-on" voltage of an amplifier stage before any output voltage is generated - see your middle photo with the small positive and negative "bumps".Check all the transistor base-emitter voltages. There'll be a transistor or two which is biased "off" under no-signal conditions, or even a complete failure.

author: bhelonious
author_rank: Newbie
timestamp: December 13, 2023, 06:04:47 am
content: Thanks everyone hope to get back to it tomorrow and will try all suggestions. Will report back - Appreciate it!

author: bhelonious
author_rank: Newbie
timestamp: December 13, 2023, 06:35:47 am
content: It was a sine wave from a function generator - looked normal when monitored at the faulty scope’s input with my other scope