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Old 10th Feb 2020, 5:42 pm   #1
dazzlevision
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Default AM IFT with integral ceramic filter

A long shot, but does anyone have this TOKO unit? It’s from a Sharp CD-302E midi system and us used with a (Sanyo) LA1265S AM/FM IF amp/detector IC.

The ceramic filter is electrically leaky and stops the tuner working on AM and FM.
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Last edited by dazzlevision; 10th Feb 2020 at 5:59 pm.
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Old 10th Feb 2020, 11:55 pm   #2
Restoration73
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Default Re: AM IFT with integral ceramic filter

Not a fault that is common. If the IFT is OK you could replace the filter with a
Murata CFM455D which should fit inside the original can.
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Old 11th Feb 2020, 1:19 am   #3
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Default Re: AM IFT with integral ceramic filter

Could the bonded-on filter pins be cropped off close and a replacement fitted on the print side of the PCB? It looks as if the IFT and ceramic filter are "standard" components bonded into a plastic shell for production convenience/substitution inconvenience. If it was me, I might try to slice the old filter off the IFT with a craft knife, but also if it was me it might end in bloodshed and bad language...
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Old 11th Feb 2020, 8:21 am   #4
dazzlevision
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Default Re: AM IFT with integral ceramic filter

Thanks for your responses.

The service manual specifies an AM IF alignment frequency of 450kHz.

It does look like the ceramic filter can be removed from the white plastic outer housing, so Iíll do this and check itís markings. Then, Iíll have to find a source of supply, as I donít have anything suitable in my ceramic filters/resonators drawer (but plenty of 10.7MHz ones!).
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Old 11th Feb 2020, 4:34 pm   #5
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Default Re: AM IFT with integral ceramic filter

I unsoldered the unit and removed the ceramic filter part. Unfortunately, it had no cover, so no type number info.

Anyway, I measured the resistance between each outer pin and the central (ground) pin. I was surprised to find it “infinity”. So, I refitted it and the tuner worked correctly for a few minutes before the fault returned - sigh!

I found what appears to be a very similar Toko part on Ebay and will report back when its been fitted. I may still need just the 455kHz three terminal ceramic filter.....
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Old 11th Feb 2020, 6:55 pm   #6
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Default Re: AM IFT with integral ceramic filter

There have been sporadic reports that the presence of DC-to-ground voltage at the filter-to-leadout bonds can encourage metallic migration across the element, leading to a leakage phenomenon- I've now taken to including a film capacitor in series with these filters when using them in the hope that the supposed many giga-ohm range isolation will help protect them. The cynic in me says that the OEMs are aware of the migration problem but feel that it is such a slowly developing thing that "not my problem by then" applies and they save a capacitor as a result. To be fair, only a small percentage of tuners/radios will have avoided going to landfill by that time.
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Old 11th Feb 2020, 11:03 pm   #7
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Default Re: AM IFT with integral ceramic filter

Quote:
Originally Posted by dazzlevision View Post
The service manual specifies an AM IF alignment frequency of 450kHz.
Once Region 1 broadcasters adopted the multiples-of-9 channel spacing plan, it was realised that 450kHz could become a world-wide standard IF, being a common multiple of both 9 and 10, and so many MW/LW tuners from the Far East used 450kHz filters rather than the 455kHz, 459kHz, 468kHz etc. options that had previously been adopted for different markets.
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Old 15th Feb 2020, 7:47 am   #8
dazzlevision
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Default Re: AM IFT with integral ceramic filter

Well, although I now have obtained a very similar TOKO IFT/ceramic filter unit, it is unfortunately a 455kHz type.

So, before I try this, I added a miniature 22nF film capacitor between the 2.7k resistor and the output side of the ceramic filter, to block any dc current through the ceramic filter.

After I did this, the crackling was no longer present and the tuner works on Long, Medium and VHF bands.

However, on Long Wave Radio 4, there is a continuous low level background (heterodyne?) whistle, which is also present on the only other two (weak) LW stations that I can receive at my location. I don’t think the whistle was present before the crackling fault appeared, so it looks like the passband of the filter must have changed...?

I have just remembered that I have a scrap Aiwa music system in my loft and, looking at the circuit, it has an AM IF of 450kHz and uses a similar IC and IFT/ceramic filter unit to the Sharp system, so I will remove this and fit it in the Sharp unit, to see if it then works correctly on MW and LW.
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Old 17th Feb 2020, 11:45 am   #9
dazzlevision
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Default Re: AM IFT with integral ceramic filter

Quote:
Originally Posted by turretslug View Post
There have been sporadic reports that the presence of DC-to-ground voltage at the filter-to-leadout bonds can encourage metallic migration across the element, leading to a leakage phenomenon- I've now taken to including a film capacitor in series with these filters when using them in the hope that the supposed many giga-ohm range isolation will help protect them. The cynic in me says that the OEMs are aware of the migration problem but feel that it is such a slowly developing thing that "not my problem by then" applies and they save a capacitor as a result. To be fair, only a small percentage of tuners/radios will have avoided going to landfill by that time.
Remark from a TOKO data sheet on their 10.7MHz ceramic filters:

“If our ceramic filter is impressed DC for long time, rarely the defect may cause.
Please connect to an IF amplifier after a DC cut condenser is inserted in the output side of the product beforehand for safety design.
And please consider so that DC isn’t impressed directly between GND terminal and Output terminal.”

Despite the poor translation from Japanese to English, I think the meaning is clear.

Well, I have removed a very similar TOKO 450kHz IFT/ceramic filter part from an Aiwa unit, which uses the same LA1265 IC, and fitted it into the Sharp unit (with the dc blocking capacitor I had fitted removed). It worked and there was no whistle - initially, but it has returned intermittently.

The intermittent nature suggests an external cause to me. So, I have checked to see if the whistle was interference from nearby lighting/electronics, using a few other radios, but they don’t exhibit this effect. These other radio use ferrite rods on AM and have analogue tuners (no local oscillator by frequency synthesis system), whereas the Sharp system has an AM loop aerial, rather than ferrite rod and uses LO frequency synthesis tuning.

I’m a bit puzzled as to what to investigate next!

Perhaps the mods might think it best to now move this thread from "wanted" to the "vintage radios" section?
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Old 17th Feb 2020, 12:21 pm   #10
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Default Re: AM IFT with integral ceramic filter

The break-down of these filters when connected across DC has been a known problem for years, one of the major Ham radio brands had the same problem.

As you have found, the fix is to put a capacitor in series to isolate it from DC.

You may have to adjust the value of the cap as it will interact with the rest of the chain and change the bandpass characteristics.
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Old 17th Feb 2020, 1:59 pm   #11
dazzlevision
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Default Re: AM IFT with integral ceramic filter

OK, I think I've established what is happening now.

I made a slight adjustment to the IFT core (the one in the same outer plastic case as the ceramic filter) but this was spot on, even with the added dc blocking capacitor.

I then put a transistor radio right next to the Sharp system, both tuned to 198kHz (Radio 4 LW). Both exhibited the background whistle. When I powered down the Sharp, the transistor radio whistle stopped.

So, it is interference radiating from the Sharp system. I then thought it is most likely to be a micro controller clock (or the like) causing it. I touched my finger on the Tuner Controller SMD IC on the front display/controls PCB and the whistle frequency changed.

So, I am now looking for a fault or a cure for this......or was it something I didn't reassemble correctly when I initially dismantled the Sharp to get at the tuner PCB (and replace some blown LCD backlight filament lamps on the front panel PCB...?? I shall try to find out!

I have been sent on a wild goose chase, thinking that the ceramic filter leakage and dc blocking capacitor addition was responsible for the whistle.
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Old 17th Feb 2020, 5:26 pm   #12
dazzlevision
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Default Re: AM IFT with integral ceramic filter

Despite lacking any scope probes that are really suitable for SMD work, I did manage to attach it to the two clock pins of the tuning controller IC (snippet of circuit attached - CL1 & CL2). The clock frequency is around 160kHz and given that the whistle disappears when the probe is connected (capacitive loading), I reckon it is actually higher than 160kHz, so perilously close to 198kHz.

I’ve also established that the relative orientation of the unit and the loop antenna, relative to the direction of the Droitwich transmitter, does affect the whistle.

I’m confident all the grounding screws that fix the PCBs to the “chassis” are tight and all lead dressing is as originally found.

So, maybe I’ll try reducing the tuning controller’s clock frequency a little - but hope it doesn't affect its correct operation. On second thoughts, maybe I should first test the Radio 4 LW reception with the metal outer case fitted - but it’s plastic!
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Last edited by dazzlevision; 17th Feb 2020 at 5:35 pm.
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Old 18th Feb 2020, 8:58 am   #13
dazzlevision
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Default Re: AM IFT with integral ceramic filter

I found another AM loop aerial with long leads and when its a couple of feet away, it works without any background whistle. So I think I'll use this, rather than the Sharp one that clips onto the back cover of the unit (which is made from hardboard).

I don't have any SMD rework tools, so I'll leave the tuning controller IC's clock frequency determining components "as is" (R213 82k and C202 33pF, which is not on the circuit extract, but goes from pin 57 to 0V.)
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