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Old 12th Jan 2021, 4:58 am   #1
Bob_KA1GT
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Default Help with FRG-7700 tuning problem needed

My FRG-7700 has been sitting unused for a couple of years. It was fine the last time I used it. When I turned it back on today it would not tune. The display counter does not change and the frequency does not change.

I checked the VFO output with a scope. Looks like it tunes from around 2.5 to 3.6MHZ and the output is around 0.5v peak to peak. So I suspect the VFO is fine and the frequency display is probably fine. The inside of the radio is very clean with no sign of corrosion. I re-seated all the multi-pin connectors, but that mad no difference.

What I see is as follows, where xxx indicates rapidly fluctuation digits

At 0-5 Mhz it displays 3.xxx at 0, 5.xxx at 1 and 6.038 at 2,3,4and 5 MHz
At 6-12 MHz the display is 0. xxx
At 13-20 MHz the display is 21.345
At 21-29 MHz the display is 30.554

The radio is working because it picks up an AM station at 6.038 MHz!

I'm not really up to speed on what's going on inside the 7700 but I think the VFO output is mixed with various PLL oscillators to generate the desired Rx frequency. Do the values displayed above (21.345, 30.554) give any clue as to what's going on? Are there any known "weak points" in the PLL circuitry that should be looked at first? Is there a PLL unit troubleshooting guide? I've looked on the web but not found anything very useful yet.

Any hints/tips from those familiar with the radio would be much appreciated.
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Old 12th Jan 2021, 6:25 am   #2
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Default Re: Help with FRG-7700 tuning problem needed

I don't know anything specifically about the FRG7700, but there has been a general problem with Japanese made amateur radio equipment of that era onwards.

PLL loop bandwidths are generally not very high, so VCO microphonics are a problem that the manufacturers reduced by squirting a viscous rubbery stuff over the PCBs in the VCO areas. Typically a radio switches between three VCOs to cover the 0-30MHz band.

What happens after a couple of decades or so is that the rubbery gunge changes, hardening and corroding components. The ceramic trimmer capacitors in the VCOs are especially susceptible to damage from it. It corrodes the metallised 'plates' off of the ceramic substrates.

If this is your problem, then the cure amounts to carefully picking the muck off of the PCB and fitting new trimmers and then readjusting the VCOs so that the tuning voltage lies within specified bounds as you tune across that VCOs range.

This is a common problem, and for equipment with the stuff on their boards it is a case of 'when' not 'if'

David
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Old 12th Jan 2021, 12:00 pm   #3
eaglework
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Default Re: Help with FRG-7700 tuning problem needed

Hi Bob,
I have one of these that went deaf a while ago and I'm still working through that problem.
Lots of useful information here http://foxtango.org/frg7700/FRoG-7700.htm that may be of assistance.
Jon
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Old 12th Jan 2021, 12:22 pm   #4
James Duncan
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Default Re: Help with FRG-7700 tuning problem needed

I had a 7700 that had that fault, Could not find the reason immediately but by accident it was left switched on for a couple of days and big surprise it then worked normally.
Radio was then out of use for about a year and same fault observed, once again left it on for a couple of days and it worked perfect. Radio was put into use and fault has never returned?
very strange
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Old 12th Jan 2021, 1:04 pm   #5
Jon_G4MDC
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Default Re: Help with FRG-7700 tuning problem needed

A fault affecting all bands probably isn't related to a VCO as there are 4 different ones.

Checking that the 6.4MHz reference crystal is going using a different RX would be a good place to start? It should be possible to hear it when a "sniffer" probe is placed nearby.

This should help too..

https://www.radioamatore.info/attach...ice_manual.pdf
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Old 12th Jan 2021, 11:26 pm   #6
Bob_KA1GT
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Default Re: Help with FRG-7700 tuning problem needed

Thanks for the suggestions and pointers. Up to this point, unfortunately the problem has not miraculously gone away

I checked the VFO is working OK. I also checked the 6.4MHz crystal oscillator is working and the 47.6 MHz crystal oscillator is working. I don't know if their levels are correct, but they are all oscillating at their designated frequencies.

The band switch does appear to be selecting the four different VCOs since the display changes for the four different band segments, but I don't know if it's providing the correct "divide by N" signals to the phase detector chip. The schematics aren't easy to read (small and not always the best quality) and finding the various components on the board isn't easy either. I can find points on the schematic I have yet to identify on the board!

The 4 VCOs are in a screened box. You can remove the top, but the components are all covered in wax, so it's hard to get in there and look at what's going on, but the output should be accessible. That's the next step, though their output is supposed to be mixed with 47.6 MHz (which is there) and the output sent to the display. The display appears to be working which makes me think the outputs of the VCO isn't right.

I'm still a little suspicious of the band switch. I don't trust mechanical rotary switches, especially ones which may be 40 years old. Though it seems to be switching the VCOs, it does more than that. It doesn't look easy to remove, or even get inside to spray switch cleaner. Might have to drill a hold in the switch housing to get some cleaner in there.

I'm still open to ideas from anyone who has worked on the 7700 and seen similar issues.
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Old 13th Jan 2021, 2:07 pm   #7
mickm3for
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Default Re: Help with FRG-7700 tuning problem needed

Hi its a well known fault with pll vcos covered in wax? the wax absorbes moisture and upsets the vcos i have in the passed used hot air /hairdryer to temp. overcome the problem perm fix remove wax but hot air proves fault is in the vcos this is why when left on for days it drove the water? out. I dont know if their is any indication ie dots on display if pll is unlocked on that rig if i can find out will post Mick
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Old 13th Jan 2021, 2:11 pm   #8
Jon_G4MDC
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Default Re: Help with FRG-7700 tuning problem needed

While that is a known problem it would be wrong to assume that is the reason on 4 VCOs.

The next best thing to do is to measure DC Voltage at TP03 on the PLL Unit. This is the tuning control Voltage for all VCOs. What is happening there will be a clue to the problem. Does it tune smoothly in step with VFO tuning? Is it stuck high or low? etc.

If the behaviour of TP03 is normal then this is probably a problem with the 47.6MHz Osc, the mixer or the counter.

Last edited by Jon_G4MDC; 13th Jan 2021 at 2:29 pm.
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Old 13th Jan 2021, 2:12 pm   #9
mickm3for
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Default Re: Help with FRG-7700 tuning problem needed

Hi their is a page with setup, checking of pll on fox tango web site
http://foxtango.org/frg7700/FRoG-7700.htm
gives voltages to check if pll is locked hope this helps Mick
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Old 13th Jan 2021, 6:21 pm   #10
Bob_KA1GT
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Default Re: Help with FRG-7700 tuning problem needed

NOTE: After writing the following and checking back with the FRG-7700, it now appears to be working again. It's been on now for maybe 48 hours, so the trick of leaving it on for long enough seems to have "fixed" the problem.

I'm not sure now whether it was working when I took the measurements.

The voltage at TP3 varies smoothly with the VFO. It can be anything from <2v to >8v depending on the band selected and the VFO setting. The range on any one band is around 1v or so. I believe this is the voltage being supplies to each of the VCOs by the MB-8718. This suggests the VFO is fine and the MB-8718 is fine. I have yet to find a datasheet for the MB-8718 or found a reference to what the voltage range on TP03 should be

The 47.6 MHz oscillator itself appears to be working OK, I don't think the counter has a problem. The radio isn't tuning when the VFO changes. One of the band switch positions seems to put the displayed frequency around 6MHz and U have heard a SW broadcast station there which was not affected by tuning.


Now it's fixed itself I can't do anything else to find out what the problem was of course. I suppose I could leave it off for another year and see if the problem re-occurs. It's sort of sounding like it might be the wax in the VCO unit. I can't be 100% sure it was still faulty when I made the measurements at TP03 since I wasn't watching the display and didn't have the audio turned up.

Anyway thanks for all the help and advice. Next time it happens I'll know better where to start and I'll post here again with an update (and probably a request for help!).

73, Bob
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Old 14th Jan 2021, 12:01 pm   #11
Jon_G4MDC
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Default Re: Help with FRG-7700 tuning problem needed

Well it's a happy result that it is working again!

Your description of the TP03 behaviour makes me think the PLL was not to blame too.

I think it's much more likely to have been that 47.6MHz mixing was not happening for some reason.

That would leave the counter nothing to count and no conversion between first and second IF.
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Old 14th Jan 2021, 4:49 pm   #12
James Duncan
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Default Re: Help with FRG-7700 tuning problem needed

Well done, it is moisture ingress that causes this common fault, you just got rid of the moisture by leaving the radio switched on
I have known a radio to be set on a radiator for a couple of days have the sam effect
cheers
MM0HDW

Quote:
Originally Posted by Bob_KA1GT View Post
NOTE: After writing the following and checking back with the FRG-7700, it now appears to be working again. It's been on now for maybe 48 hours, so the trick of leaving it on for long enough seems to have "fixed" the problem.

I'm not sure now whether it was working when I took the measurements.

The voltage at TP3 varies smoothly with the VFO. It can be anything from <2v to >8v depending on the band selected and the VFO setting. The range on any one band is around 1v or so. I believe this is the voltage being supplies to each of the VCOs by the MB-8718. This suggests the VFO is fine and the MB-8718 is fine. I have yet to find a datasheet for the MB-8718 or found a reference to what the voltage range on TP03 should be

The 47.6 MHz oscillator itself appears to be working OK, I don't think the counter has a problem. The radio isn't tuning when the VFO changes. One of the band switch positions seems to put the displayed frequency around 6MHz and U have heard a SW broadcast station there which was not affected by tuning.


Now it's fixed itself I can't do anything else to find out what the problem was of course. I suppose I could leave it off for another year and see if the problem re-occurs. It's sort of sounding like it might be the wax in the VCO unit. I can't be 100% sure it was still faulty when I made the measurements at TP03 since I wasn't watching the display and didn't have the audio turned up.

Anyway thanks for all the help and advice. Next time it happens I'll know better where to start and I'll post here again with an update (and probably a request for help!).

73, Bob
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Old 14th Jan 2021, 8:06 pm   #13
Bob_KA1GT
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Default Re: Help with FRG-7700 tuning problem needed

Well, I'm glad it's working, but sorry it's not actually "fixed". One day It will happen again, and when it does I'm now better equipped to track down the actual issue. I know the 47.6 MHz oscillator has been working all the time, but there could have been a bad buffer stage or mixer. Now it's all working I can take some additional measurement so I know what to expect when looking at things like the VCO and mixer outputs.

Slightly off topic, but does anyone know why the bridge rectifier gets hot enough to fry an egg on! With a small heat sink attached it's still at over 90C which seems excessive. Lots of people report this, so it appears to be "normal" for this radio, but it seems odd it should get so hot when the whole radio is supposed to draw <40W total. So that's maybe 4A or less and the bridge is, I believe, rated at 8A. That's hard to verify though as I can't find the official spec shoot on it. I haven't measured that, but I'm temped to replace the bridge rectifier with a larger unit (15 or 25A) and a larger heat sink. It just doesn't seen right it should get that hot, even though many bridge rectifiers are rated for operation at a case temperature of 100C.
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