HP1715A oscilloscope repair problem - Page 1
Forum section: electronics / repair

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: June 02, 2017, 03:21:14 pm
content: I am currently working on a HP1715A faulty oscilloscope and I am hoping to get some confirmation whether the issues I am having is down to a faulty 'substrate assembly' A1 on the Y pre-amp board, part number 5081-3028.This oscilloscope was working in as much as it powers up and all LT and HT rails are working OK. Both input channel FETs were blown and I have replacements. However, the major known problem was that the trace was off the screen. Carefully working back from the Y amp, I have determined that the Y amp board is OK. For example, if both inputs are shorted then the trace centers correctly and voltages are equal on the Y plates. So tracing back to the pre-amp board, we have different voltages at the two DIFF OUT pins on the A1 IC. We have 0v on the CH A and CH B inputs. We have correct voltages at both the Position pots with the wiper swinging from -15v through to +15v as expected and both resistors between wiper and IC pin are OK. CH A and CH B select voltages appear to be correct. However, the CH B Position control seems to do nothing. The CH A position control has minimal impact. When CHOP or ALT are selected, only one trace is visible. CH A BAL and CH B BAL have nowhere near enough adjustment range to center the trace although the voltages do vary when the control is rotated. Adjusting the CH A Position control seems to also move the trace when CH B is selected.With the base of the two output driver transistors disconnected from the two DIFF OUT pins of the IC, the trace will center and the Y plate voltages are equal. With the IC removed from the board, all voltages for each channel are pretty much equal or as expected.There are some minor voltage variations on the supply as the 4 x 2.7ohm resistors supplying the +/-15v to parts of the board are a little high at around 3.1ohm. The zener diode regulating V1 may be faltering since I am getting 6.5v instead of 6.19v. I don't think this is enough to explain the problems that are being presented and as far as I can tell, everything else is OK, although the timebase sweep generator is behaving strangely.The question is whether these symptoms are consistent with a faulty A1 chip and whether there is anyone out there that has a HP part number 5081-3028 or a complete A3 (Y pre-amp) board or even a faulty scope for parts that I could buy?I have been directed to Sphere in Canada who appear to have the part, but it is quite expensive especially with the shipping cost. Still there is a part around the other side of the world that could possibly fix this scope and this will have to be my last port of call should other options fail, but at this point I don't know what other problems there may be down the line , hence hoping for some further guidance and re-assurance.

author: oldway
author_rank: Super Contributor
timestamp: June 02, 2017, 08:11:58 pm
content: Indeed, it seems that A1 is faulty.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: June 03, 2017, 12:06:51 pm
content: Yes, that's the board in question. Here is a photo of mine. I have highlighted the components that are a little suspect. Also included is a photo of the chip. One thing I didn't point out above is that the chip runs very hot, which I would not have expected in the pre-amp stage.

author: oldway
author_rank: Super Contributor
timestamp: June 03, 2017, 01:29:37 pm
content: With this kind of oscilloscope, it is better to have 2, one that works and one to recover parts.It's not the same thing as a Tektronix 465 ! You find easily all parts you need for a 465.....These HP oscilloscopes are very rare.I have a working HP1725A and the HP1715A serves me for parts.Many parts are the same.For my HP1725A, I already had to recover another hybrid circuit, the final amplifier vertical U2.The HP1715A was working but it seems to have a bad CRT....focus is right at the lower side of the screen but very bad at the upper side.I never investigated this problem because my intention was to use it as a donor for my HP1725A.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: June 03, 2017, 02:22:04 pm
content: Well with one of these HP scopes with all of their hybrid parts I can understand why that advice is often given. I have set up an eBay search to keep an eye for one to come up.