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Old 11th Jun 2019, 1:55 pm   #1
Martin G7MRV
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Default No LW/MW receive - Sony ST-S311

Not particularly vintage I'm afraid, but I've been asked to look at a Sony ST-S311 receiver with no LW/MW receive.

I have the service manual but I'm unfamiliar with this model so need time to study it. I'm just wondering if anyone has worked on these before and knows of any stock faults?

The FM side works so I'm looking at the AM front end/IF circuits.

Any hints would be most welcome.

Martin
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Old 11th Jun 2019, 4:14 pm   #2
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Default Re: No LW/MW receive - Sony ST-S311

I can't see a detector diode in the AM section of the pcb on the web image, normally this would be a first "go to" fault. There is what looks like an 18 pin chip in this section so the detector diode may be a part of that chip.
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Old 11th Jun 2019, 5:47 pm   #3
Martin G7MRV
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Default Re: No LW/MW receive - Sony ST-S311

No, it looks like its all done in the LA1245 IC. Thats one of my two chief suspects! Im hoping it will actually turn out to be the band switching transistors, but knowing the mash-up cobbled together monstrosity of an antenna this has been used with, and the lack of any static bleed resistors on the antenna input, I think I could be looking for one of those chips!
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Old 11th Jun 2019, 7:31 pm   #4
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Default Re: No LW/MW receive - Sony ST-S311

Several eBay sellers, including Littlediode, offer this Sanyo chip reasonably cheaply.
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Old 11th Jun 2019, 9:10 pm   #5
Martin G7MRV
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Default Re: No LW/MW receive - Sony ST-S311

Just a case of deciding if it is that chip!

Well, done some preliminary checks and the voltages on the pins of it are wildly different to those marked on the circuit diagram! According to the schematic, most pins have just a tiny DC voltage on them, not more than a volt or two in most cases - im measuring 5 or more volts on most pins!

My only trouble is that im looking at fixing this as a favour to my work colleagues, in order not to have to send it away for repair which will mean its out of use for weeks, so really the company should obtain the parts, but of course none of the 'preferred' suppliers stock them, so they wont buy it for me! So I want to be as sure as possible that its the right part at fault before I spend my own money on the companies behalf!
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Old 11th Jun 2019, 9:32 pm   #6
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Default Re: No LW/MW receive - Sony ST-S311

We used to get round the "preferred supplier" blockage in development by using a tame preferred supplier who were quite happy to quote for anything. They just added 25% handling charge to whatever they paid for the parts! Our purchasing dept may or may not have twigged what was going on (being a part of a multinational the actual purchasing was done in India so quite possibly not....)? We got some very esoteric electronic components for some development jobs from Newbury Welding
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Old 11th Jun 2019, 9:41 pm   #7
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Default Re: No LW/MW receive - Sony ST-S311

Quote:
Originally Posted by Martin G7MRV View Post

im measuring 5 or more volts on most pins!
If you have not already done so it might be worth a quick squint around to make sure you have not lost a 0V line / track in that area, starting at the actual 0V pin on the IC, obviously.

If even that pin is not at 0V with respect to chassis, then obviously you will be on to something.

(edit - great minds think alike - see also Biggles)
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Old 11th Jun 2019, 9:41 pm   #8
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Default Re: No LW/MW receive - Sony ST-S311

Might be worth checking if the 0 volts connection is ok as if this was open circuit you may find all the ic pins at the supply rail voltage, depending where your meter negative probe was referenced to.
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Old 11th Jun 2019, 9:47 pm   #9
Martin G7MRV
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Default Re: No LW/MW receive - Sony ST-S311

Can anyone offer any thoughts as to whether the LA1245 IC failing might have taken out any other parts?

The diagram shows there are quite a few switching transistors used to select the signal line into the chip from the front end filters, but not much else that looks sensitive to me?
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Old 11th Jun 2019, 9:48 pm   #10
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Default Re: No LW/MW receive - Sony ST-S311

Quote:
Originally Posted by Biggles View Post
Might be worth checking if the 0 volts connection is ok as if this was open circuit you may find all the ic pins at the supply rail voltage, depending where your meter negative probe was referenced to.
Alan.
First check against chassis was zero, but i'll check them all over again shortly.
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Old 11th Jun 2019, 10:21 pm   #11
Martin G7MRV
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Default Re: No LW/MW receive - Sony ST-S311

0v line looks ok. Supplies look ok. Band switching looks fine.

There is no reception on either LW or MW, which I think means I can probably rule out the band switching itself, as i'd expect to only lose one band. FM works fine, so I can rule out all that side and the PLL, plus the post IF and control sides. It does seem to just leave the LA1245!

Ive ordered one from Littlediode. I might be out of pocket by a fiver, but, knowing how this receiver has been used, and the fact it performed flawlessly for years and only failed after a bizarre lash up of around 150ft of 50 ohm coax and a bodged CB antenna were put on as a supposed stop-gap for monitoring a particular service, plus the fact that there is no protection on the antenna terminals, its definitely chief suspect!
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Old 11th Jun 2019, 10:54 pm   #12
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Default Re: No LW/MW receive - Sony ST-S311

Looks like you might be right. Worth changing the chip to eliminate it. The old rule where the likelihood of the component being faulty is inversely proportional to the number of pins could come into play if you are unlucky! I had a Sony radio a few years ago which went faulty and that appeared to be the chip. I never investigated the fault further, so it could have been something else but everything was on one chip so it seemed likely. Maybe worth checking for leaky electrolytic caps if the chip change doesn't do the job.
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Old 12th Jun 2019, 10:52 am   #13
Mike. Watterson
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Default Re: No LW/MW receive - Sony ST-S311

Has it got a ceramic filter for AM / 455kHz? I've seen them suffer mechanical damage.
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Old 12th Jun 2019, 10:55 am   #14
Martin G7MRV
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Default Re: No LW/MW receive - Sony ST-S311

Yes there is, although I cant think how it could get damaged physically - the set has been mounted in a 19inch rack for donkeys years!
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Old 12th Jun 2019, 11:21 am   #15
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Default Re: No LW/MW receive - Sony ST-S311

Or just failed?Seem to remember changing them years ago in tv,s.
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Old 12th Jun 2019, 11:29 am   #16
Martin G7MRV
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Default Re: No LW/MW receive - Sony ST-S311

My initial thought was that static build up, or, the cumulative effect of the high RF fields present on our site (due to a rather large TV tower...) might have damaged the front end, but looking at it, it seems the antenna is effectively grounded through the coils of the input filters. I had thought of adding a 100k bleed resistor when ive repaired it, but it seems its not necessary.

All I can really do for now though is wait for the IC to arrive
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Old 12th Jun 2019, 11:33 am   #17
Martin G7MRV
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Default Re: No LW/MW receive - Sony ST-S311

Quote:
Originally Posted by HamishBoxer View Post
Or just failed?Seem to remember changing them years ago in tv,s.
always a possibility, although would a filter fail suddenly? I'd have thought it would have degraded over time?
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Old 12th Jun 2019, 11:42 am   #18
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Default Re: No LW/MW receive - Sony ST-S311

I had a 1980s Pioneer tuner where AM had failed due to a shorted cap inside a toko if can. The coil and integral cap were a series tuned cct and the shorted cap messed up chip biasing. Luckily, with a bit of track cutting, an external cap did the job.

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Old 12th Jun 2019, 12:38 pm   #19
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Default Re: No LW/MW receive - Sony ST-S311

I really cannot remember why I changed these back in my tv servicing days,though they would have failed suddenly then.I am pretty sure.

They used to be pretty cheap from what I remember.
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Old 12th Jun 2019, 7:50 pm   #20
Martin G7MRV
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Default Re: No LW/MW receive - Sony ST-S311

Well ive had time this evening to set up for more than just a few voltage checks!

The first thing to note, is that the voltages marked on the schematic around the chips - all relate to the receiver being in FM mode! So, I cant go by any of that for what the LA1245 should measure when active!

Ive got a 1233kHz, 1kHz at 70% AM signal being fed into the antenna connection. Ive been able to follow this with my 'scope through the front end filters and to the input of the IC. Likewise, I can trace a signal at the pins for the IF filter, the IF oscillator, and on the output pin, where I have what looks like a pretty distorted 1kHz signal.

I can 'scope that detected signal as far as the input pin of the LA3401 IC, and I believe that chip is working because its the FM subsystem, and FM works.

But, I have to give it a signal stronger than about -35dBm before I start seeing these traces. They are however definitely the injected signal, as they disappear if I tune away.

Anyone know what a typical MW or LW signal level would be at the receive antenna?

Im guessing it would be a heck of a lot weaker than -35dBm! (thats strong even as an off-air feed for a transmitter!)

So, my suspicion is still with the LA1245 IC at present
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