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Old 20th Apr 2024, 3:34 pm   #1
jazzman
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Default FRG 7700 bandpass filters

Hi guys, I must state from the off that I'm not that technical , I just get a buzz out of getting stuff to work with my limited knowledge.

So the issue is this, as I switch through the bands, signals drop like a brick after 7Mhz.
I also noticed that disconnecting and reconnecting the antenna up to the 7 Mhz produced a significant rise in signals and background static. After 7Mhz there was very little effect in the reconnecting of the antenna.
Poking around I discovered that putting a test meter lead onto the common rail of the diodes D02 through to D12 brought the signals pouring in, so much so I was listening to sideband loud and clear on 17Mtrs....just on a 24 inch piece of wire!
So to cut a long story short I put a jumper lead between the antenna in and
D12, the result is I'm receiving strong signal right through MW all the way up to 28Mhz sideband.
At first I though maybe the switching diodes either end of the various filters and taken some serious volts with the exception of the 7Mhz filter and down.
Doing a few voltage readings on each filter shows a drop when the band switch matches the filter being tested.
So that's where I'm at...
Any thoughts or ideas would be appreciated.

Regards,
Mike.
c
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Old 20th Apr 2024, 4:16 pm   #2
Andrew2
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Default Re: FRG 7700 bandpass filters

Hmmm. I don't know the exact input arrangements for the 7700, but I do know that the inductors are prone to going o/c after many years, especially if the receiver has been stored in damp conditions. Probably the dreaded 'green rot'(?) where the wire is soldered to the pin. Back in the late 80's a friend had a 7700 with exactly this problem.
I'm assuming that the filters are selected by diodes which perhaps get their switching voltage via the coils?
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Old 20th Apr 2024, 4:50 pm   #3
jazzman
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Default Re: FRG 7700 bandpass filters

Hi Andy, thank you for the swift reply.
I'll have a detailed look at the inductors etc with a loupe.
Although the signals are good and strong now, it bugs me because it's not how the receiver should be, that is with a jumper wire bypassing the filters. I like things to be original.
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Old 20th Apr 2024, 6:10 pm   #4
Mr 1936
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Default Re: FRG 7700 bandpass filters

Hi jazzman

As you are quoting component references, I assume you have a service manual. I have only been able to find a poor scanned copy online, but here goes.

As you say, the six RF bandpass filters (Octave Ranges) are selected by a diode each end, and these do indeed rely on a DC bias path through the inductors to function. However, I think you are saying that these diode bias voltages are changing as expected.

I am just wondering if the fault lies upstream of this, e.g. in the lowpass (rejects above about 30 MHz) filter which is in circuit above 2 MHz

This receiver uses upconversion to a high first IF (48.055 MHz) on all bands, but I cannot find anything that switches or changes operating mode above and below a 7 MHz threshold.
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Old 20th Apr 2024, 6:34 pm   #5
jazzman
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Default Re: FRG 7700 bandpass filters

Thanks guys for the replies and info therein.
I do indeed have a service manual but only a pdf version which is very poor to read.
I'll certainly have a very close look at the inductors etc for corrosion and oc.
It had occurred to me in the early hours last night, that it could of course be upstream of the filters, since the circuitry between the filters and the band switch has little to do with signal processing as such, but merely to organise which filter goes with each band. The poor pdf and my lack of knowledge makes it difficult to read and understand what is going on in this upstream area.

It's worth mentioning that the voltage changes I observed were taken on the line that comes from the the nearest controlling transistor and not on the diode side, so although there was a voltage change it's possible the diodes are not seeing it buy virtue of faulty inductors.

I'm sorry if my descriptions are lame, it's just that I have a passing knowledge without any depth.
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Old 21st Apr 2024, 2:45 pm   #6
vosperd
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Default Re: FRG 7700 bandpass filters

Hi
I have a copy of the original Instruction manual but it doesn't include a circuit diagram.
I can eventually scan the lot but if there are a few pages you need straight away I can get them to you.
Don m5aky
Just found the circuit diagram, although it's an original manual the previous owner separated the pages and put them in palsic folders!

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