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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 19th Jan 2020, 2:14 pm   #1
dave_n_t
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Default Unusual intermittent fault cured

A small Japanese receiver, caseless, was in a box of 'assorted wrecks' I bought from a Radiophile auction some time ago. Before dismantling it, I thought I'd see if it was still functional. A few basics (battery, speaker, Deoxit on the volume control) allowed it to show signs of life (after the electrolytics had re-formed). But there was an annoying intermittent 'dying' effect, apparently caused by any slight movement. Obviously a loose connexion - battery snaps, vol control, solder joint gone off, etc.

A lot of searching, prodding, resoldering produced nothing. And then I noticed that the ferrite rod was expecially sensitive; yet its litz wires were soldered nicely in place with plenty of wiggle room. Then the penny dropped - the winding was resting on the volume control, and a sharp edge on the latter must be cutting thru' the litz insulation (see photo). A bit of judiciously-place insulating tape solved it.


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Old 19th Jan 2020, 2:52 pm   #2
merlinmaxwell
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Default Re: Unusual intermittent cured

I love fixes like this, I store the info in my head so when designing something I don't fall into the same trap.
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Old 19th Jan 2020, 5:37 pm   #3
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Default Re: Unusual intermittent fault cured

Tut! Tut! Not like you to miss that Dave! (I'll have to pop 'round and supervise you a bit more often!!)
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Old 19th Jan 2020, 6:06 pm   #4
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Default Re: Unusual intermittent fault cured

'like'.
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Old 19th Jan 2020, 7:36 pm   #5
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Default Re: Unusual intermittent fault cured

This is spooky - about a year ago there was yet another thread about replacing Ge devices with Si and I looked for a scrap simple set to take some measurements, and in a box of 'assorted wrecks' there was the PCB from an unknown Japanese pocket portable with no case which I had found years ago. An early 6-transistor 'AM-only' set with transformer-coupled push-pull audio output for speaker or earphone. Exactly like yours Dave. The only indication of brand or model was the legend SRV-62 on the tuning knob.

I'd not done any repairs for so long that I needed to ramp up my practice, starting on something very simple, like this. Applied 9V and clicked the volume control 'on' - dead short. Fair enough, I was expecting some leakage on those 60-year old electrolytics and maybe the O-P transistors so I started pulling them out. Still dead short. The supply line goes only to an electrolytic, the oscillator coil, and some resistors, so I pulled out the coil/transformer and checked for shorts to the can. None. Beginning to despair I put an ohmmeter across the supply line, it was virtually O/C - until the vol ctrl switch was clicked 'on'.

'How did that happen' you might ask (well I did) so by cutting tracks in the supply line it turns out that the switch shorts to the track earthy end when on. The bits went in a box for another day. Let's compare our two boards
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Old 19th Jan 2020, 11:59 pm   #6
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Default Re: Unusual intermittent fault cured

Quote:
Originally Posted by Tractionist View Post
Tut! Tut! Not like you to miss that Dave! (I'll have to pop 'round and supervise you a bit more often!!)

You're most welcome Tractionist! The teapot's always full (even though the biscuit tin may not be).
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Old 20th Jan 2020, 12:04 am   #7
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Default Re: Unusual intermittent fault cured

Quote:
Originally Posted by Julesomega View Post
Let's compare our two boards

As you say, uncanny - but I think that they're actually different models (but with a certain 'family likeness )


In mine:


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there's a gap between the tuning cap and the first IFT; in yours there isn't. But both fail in having the tuning coil almost/actually resting on the volume control & on-off switch.
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Old 20th Jan 2020, 11:13 am   #8
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Default Re: Unusual intermittent fault cured

Good one.
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Old 20th Jan 2020, 12:07 pm   #9
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Default Re: Unusual intermittent fault cured

Well done for finding the fault, they are sometimes far from obvious!
My most perplexing fault was a bad pcb track whick looked OK but measured several K ohms.. upon close inspection it had a spot of mild discolouration - something attacked the copper - of course, a simple wire link fixed it.
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