Frg7700 display issues - UK Vintage Radio Repair and Restoration Discussion Forum

kernowcam · 31st Jan 2020, 9:59 am

I have just decided to have a look at my frg7700 display problem.
One segment seems to be on I think all the time. Same one on each digit. ie the 6 looks like an 8 . I believe it may be a faulty chip , oki msm5524. So any ideas? And does anyone have a spare chip please? Quite rare.

Jon_G4MDC · 31st Jan 2020, 10:02 am

I have known leaky diodes to cause that sort of thing on a Kenwood.
If there are any used around it, it's well worth checking them before blaming a rare and hard to get driver IC. 2x 1N4148 fixed it for me!

kernowcam · 31st Jan 2020, 12:36 pm

Thanks jon I will lookout diagram

orbanp1 · 31st Jan 2020, 1:30 pm

Here is an interesting thread on that display chip:
https://www.hamradioforum.net/archiv...hp/t-2631.html
Also interesting is the Japanese web-page that thread is pointing to, where they started to reverse engineer the counter/display chip.
You need to run it through the translator though.

Regards, Peter

Jon_G4MDC · 31st Jan 2020, 4:23 pm

D24, D25 in the attached snippet were to blame for my TS130V doing strange things in the display.

PETERg0rsq · 3rd Feb 2020, 2:26 pm

If it is just one segment on each digit, then it is very likely the chip that is faulty (MSM5524)

Unfortunatly this is virtualy unobtainable, and replacement with an alternative very difficult, as it does all the clock functions also!

However do check the board for dry joints/bad connections, and check resistor network RB01.

It could be the display, but unlikely, but worth measuring the volts from the driver chip.

If I recall correctly the display runs about -100Vdc and the driver chip sinks the volts to turn on the segment.

If it was one digit, then it would likely be a driver transistor

SiriusHardware · 3rd Feb 2020, 4:47 pm

Kernow, how do you feel about ordering from China?

https://www.alibaba.com/product-deta...666429832.html

(If you search more carefully on Alibaba there is at least one seller selling them in ones, but it is not clear whether they are the DIP version or the flat-pack SMD version).

SiriusHardware · 3rd Feb 2020, 7:32 pm

I've noticed that there is a LED output version of this chip, the MSM5523RS. If it is generally pin compatible (I have not checked) you could either:-

Remove the VFD display and build / fit an equivalent LED display directly driven by the '23 IC,

or,

Fit the '23 IC, isolate the LED drive outputs from the existing display and introduce extra buffer / driver circuitry as required to allow the LED driver chip to drive the original display. This second scheme would have the merit of separating the actual display driving components from the complex, hard to get controller chip so if any of the drivers failed again you would just replace those. It would also keep the radio / display looking original.

31st Jan 2020, 9:59 am	#1
kernowcam Pentode Join Date: Dec 2011 Location: Camelford, Cornwall, UK. Posts: 188	Frg7700 display issues I have just decided to have a look at my frg7700 display problem. One segment seems to be on I think all the time. Same one on each digit. ie the 6 looks like an 8 . I believe it may be a faulty chip , oki msm5524. So any ideas? And does anyone have a spare chip please? Quite rare.

31st Jan 2020, 10:02 am	#2
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: Frg7700 display issues I have known leaky diodes to cause that sort of thing on a Kenwood. If there are any used around it, it's well worth checking them before blaming a rare and hard to get driver IC. 2x 1N4148 fixed it for me!

31st Jan 2020, 12:36 pm	#3
kernowcam Pentode Join Date: Dec 2011 Location: Camelford, Cornwall, UK. Posts: 188	Re: Frg7700 display issues Thanks jon I will lookout diagram

31st Jan 2020, 1:30 pm	#4
orbanp1 Heptode Join Date: Nov 2009 Location: London, Ontario, Canada Posts: 675	Re: Frg7700 display issues Here is an interesting thread on that display chip: https://www.hamradioforum.net/archiv...hp/t-2631.html Also interesting is the Japanese web-page that thread is pointing to, where they started to reverse engineer the counter/display chip. You need to run it through the translator though. Regards, Peter

31st Jan 2020, 4:23 pm	#5
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: Frg7700 display issues D24, D25 in the attached snippet were to blame for my TS130V doing strange things in the display. Attached Thumbnails

3rd Feb 2020, 2:26 pm	#6
PETERg0rsq Heptode Join Date: Jun 2009 Location: St Helens, Lancashire, UK. Posts: 641	Re: Frg7700 display issues If it is just one segment on each digit, then it is very likely the chip that is faulty (MSM5524) Unfortunatly this is virtualy unobtainable, and replacement with an alternative very difficult, as it does all the clock functions also! However do check the board for dry joints/bad connections, and check resistor network RB01. It could be the display, but unlikely, but worth measuring the volts from the driver chip. If I recall correctly the display runs about -100Vdc and the driver chip sinks the volts to turn on the segment. If it was one digit, then it would likely be a driver transistor __________________ SPECIALIST.....Knows everything about nothing EXPERT..........Knows nothing about everything

3rd Feb 2020, 4:47 pm	#7
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,546	Re: Frg7700 display issues Kernow, how do you feel about ordering from China? https://www.alibaba.com/product-deta...666429832.html (If you search more carefully on Alibaba there is at least one seller selling them in ones, but it is not clear whether they are the DIP version or the flat-pack SMD version). Last edited by SiriusHardware; 3rd Feb 2020 at 4:54 pm.

3rd Feb 2020, 7:32 pm	#8
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,546	Re: Frg7700 display issues I've noticed that there is a LED output version of this chip, the MSM5523RS. If it is generally pin compatible (I have not checked) you could either:- Remove the VFD display and build / fit an equivalent LED display directly driven by the '23 IC, or, Fit the '23 IC, isolate the LED drive outputs from the existing display and introduce extra buffer / driver circuitry as required to allow the LED driver chip to drive the original display. This second scheme would have the merit of separating the actual display driving components from the complex, hard to get controller chip so if any of the drivers failed again you would just replace those. It would also keep the radio / display looking original.