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Old 3rd Sep 2014, 8:51 pm   #1
Andrewausfa
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Default Telequipment D65 Intermittent fault

I picked up this 1976 era 15 MHz 'scope at a rally recently as it semed in excellent condition, especially after a little cleaning and has several features my more modern 'scopes don't have. However, it has a really annoying fault that I wonder if anyone else may have experience of.

The issue is intermittent triggering. When powered the triggered sweep may decide to a) start halfway across the screen, b) in the correct position and then fade or c) work as advertised. Or not at all. Hmmm. When it does work all functions seem to work correctly.

With zero experience of working on 'scopes, my first thought was to carefully clean all the switching contacts where I could and to press or twist all the knobs and buttons lots of times. I also carefully removed and reseated the transistors bar the FETs. I noticed one electrolytic (C421) was bulging and had leaked so replaced that, switched back on expecting an instant fix but nope, it was the same.

Having got really annoyed with it a few days ago I gave the Trigger Level knob a good flick with my finger and that brought the sweep to life. A-ha! I thought, dodgy connection. I've had a prod and tap around the sweep/trigger board (pic below) components with no luck but however I found that by exerting a slight pressure on the board around where the brown variable resistor is, the 'scope works perfectly. Not keen on removing the board but would opinion suggest this might be a dry joint, cracked track or another known fault?

Thanks - Andrew
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Old 3rd Sep 2014, 9:19 pm   #2
MotorBikeLes
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Default Re: Telequipment D65 Intermittent fault

Nice little scopes, but they have a strange triggering circuit setup procedure. If you can, take great care to get it spot on, and it should reward you.
However, I suspect the intermittancy is due to board design. Double sided with carp print through links. Look at all the little round through links. You probably won't see anything wrong, but get a hot iron and make every one "bubble" a bit, then just cool. Of course, if you have a good suction desolder station, you could suck each one dry, then appy a minimum of fresh solder. I can almost guarantee a successful result.
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Old 3rd Sep 2014, 10:11 pm   #3
Andrewausfa
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Default Re: Telequipment D65 Intermittent fault

Les, you beauty! I've 're-flowed' those little holes, switched on and the sweep started straight away. It's never done that before. Switched off, waited a few minutes, powered on and it still sweeps. Fingers-crossed your tip has worked.

Many thanks

Andrew
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Old 5th Sep 2014, 7:28 pm   #4
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Default Re: Telequipment D65 Intermittent fault

I don't think I have ever been called a "Beauty" before!
I have come across this sort of problem many times. The problem is that simply applying an iron and more solder is OK until next time. Maybe days, maybe years.
One technique I developed (probably not suitable for the TQ) especially on Grundig tuning modules with thro' components (not tuners) was to apply my HOT iron to one side, and let it bubble madly until there was a "burst, but not filled up again" bubble event. I then turned the board over, ensured a good joint with a little fresh solder, then back to the first side. Here I would apply JUST ENOUGH solder to ONE SIDE ONLY of the resistor or whatever. Do not resolder the full circle. Guaranteed never to fail again. I once started work at a place where they had loads of such modules (teletext if I remember). Not ITT, but obviously ITT design and chips. The top engineer had found that he could resolder them on Frioday night, but by Monday morning, they were generally not working again. I suggested I could fix them, and quickly did three of them. After about a week with no problems, I was given about 20 more to fix, and a load of sets were sent on their way. (We used to repair customer returns for a warehouse style operation).
When I see that type of construction, if anything is wrong, I just blitz the board. Usually works.
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