UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Test Gear and Workshop Equipment

Notices

Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

Closed Thread
 
Thread Tools
Old 12th Apr 2018, 3:28 pm   #1
twizzle
Triode
 
Join Date: Feb 2017
Location: Bradford, West Yorkshire, UK.
Posts: 12
Smile HP oscilloscopes - 1715 / 1725a / 1740

To all who may have a repair interest concerning HP oscilloscopes - 1715 / 1725a / 1740 and perhaps others that have a connection to this family.

I recently purchased an HP1725a scope that was sold as working but actually had some significant problems that in some cases started off as minor but progressed from there. I was very grateful to find posts of people’s experiences repairing these units which although didn’t help me fully (a few red herrings unfortunately) did give me some pointers which certainly in some areas lead me to the solution...albeit via an interesting journey.

Therefore, I’m putting up my solutions so that maybe someone else can benefit in the future.

1. Calibrator output - Not -3v @ 1KHz, more a couple of volts negative at no frequency at all that disappeared to nothing at all after a week or so. Simple and straight forward, U1 on A14 board, RCA CA3046.

2. Start and stop markers on Main Intensified sweep. This started as only one marker available on the stop control and nothing on the start control. I’d seen something on another post that mentioned bad connections on the back of these controls and controls in that area and sure enough I jiggled a couple of connections and this fix lasted a week before it went off again. More wire jiggling with a view to getting the soldering iron out and working again. Went off the 3rd time a day or so later and wire jiggling was no longer the fix. This problem then morphed into something more bizarre where the stop marker kind of worked but didn’t move smoothly along the waveform but jumped from peak to peak as the control was rotated. Something I’d read (actually plenty I’d read) made me suspect one or two of the transistors on the board (A18) controlling this function but despite really seeming duff in circuit turned out as usual to be totally fine. I put the board back in and powered back on. Flash, bang and a puff of smoke and my heart sank fearing I’d made a mistake removing and re-installing the transistors. However, the cause was actually R64 10ohms on the -15v rail that had exploded. Replaced it and ALL problems fixed. Clearly it had been on its way out for a while.

3. The 4 slowest speeds of the main timebase were not working and although I never use anything in this range it was an irritation. It’s a longish story but after some very careful and close inspection I found by chance that the –ve lead from capacitor C17 100uf (board A11) had corroded and broken off. It was still soldered into the PCB and stood upright and bent just as if it were part of the cap but it wasn’t. You could hardly slide a piece of paper between both parts but they were not joined. Frustratingly when I tested this part it was absolutely fine and in spec but the pressure of my probe must have pressed the lead to the body so I assumed it was OK. Doh! I think sums that up. Replaced and all perfect.

4. Delayed sweep problem. This was an absolute headache and perhaps I was not on the ball soon enough but in my defence I was distracted by many other posts on this fault that lead me astray. The fault has been described elsewhere on the forum and there is also a photo attached that shows exactly what I had. Although on my scope it started as one thing where the delayed function WAS working but degenerated into something totally unusable, again within a week of owning the scope. If using a square wave there would be 2 intensely bright dots on the screen left with the delayed portion of the signal to the right. With a sine wave the 2 dots would be a very bright vertical line for obvious reasons but again with the delayed waveform to the right. After a week or so there was just the bright line or dots and nothing else. This has been referred to as the ‘Dreaded Delayed Sweep’ problem. I could write a lot about this but all anyone needs to know is that the culprit was (yet another) RCA CA3046 chip fault. U7 on the main A8 board. It was actual Q3 within the chip that outputs the Delayed Gate. Clue there I reckon... That’s two of those rascals in one machine so if I ever see another that’ll be one of the first things that gets pulled out and tested!

Sorry it’s all a bit of a long read but if it helps fix your problem then you’ll hardly care I hope.

Best regards and thanks to all who can be bothered to post their experiences, tips and tricks. For me, it’s very much appreciated.

Kevin
twizzle is offline  
Old 12th Apr 2018, 6:17 pm   #2
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,801
Default Re: HP oscilloscopes - 1715 / 1725a / 1740

Useful!

Thanks
David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 12th Apr 2018, 7:23 pm   #3
turretslug
Dekatron
 
turretslug's Avatar
 
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,385
Default Re: HP oscilloscopes - 1715 / 1725a / 1740

Thanks for posting- there must be quite a few of these venerable, complex but still eminently useful 'scopes around. I wonder if the CA3046 problems are symptomatic of encapsulation problems, given the likely age and that they'll have been around far longer now than the manufacturer would have forseen. Perhaps a worrying trend for the future regarding older semiconductors of the lower-cost plastic encapsulation era- viz. the Lockfit problem- it may be that valves are the long-term survivors!
turretslug is offline  
Old 13th Apr 2018, 3:38 am   #4
Electrical Rod
Retired Dormant Member
 
Join Date: Mar 2018
Location: Hahira, Georgia, USA.
Posts: 32
Default Re: HP oscilloscopes - 1715 / 1725a / 1740

Makes me wonder, I have seen plastic crumple in short order, but I have several valves reaching 100 years old, still good. I`m glad I kept all my old radio and test equipment tubes, etc.. Looks like buy and or replace new stuff world.
Rodney
Electrical Rod is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 1:26 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.