[ICOM] IC775DSP display backlight problem

[Martin Sole]

Anders,

Many thanks for the reply. If the display backlight is indeed 120 volts 
AC then this explains the significant power supply circuit used to feed 
it. I will try to produce a simple 120 volt AC feed at low current to 
feed it with and prove that it is working before proceeding further with 
the supply problem. As you noted the transformer may be shorted but I 
have no way to verify that so it may be that I have to order a new 
transformer and a couple of transistors directly from Icom and replace all.

It would be nice to know if others have experienced complete backlight 
failure of this type before.

Thanks again for the information and any other comments pertaining to 
this problem would be greatly received.

On a slightly unrelated note this is the first Icom other than a 706 
that I have played with in a long time and it does seem to be a very 
nice radio. I have long avoided Icom's for their, to my ears, harsh 
sound, but this one is very smooth sounding.

Martin, HS0ZED


Anders Janis SM4RNA wrote:
> My supplier says that it is driven by 120V AC. In low light mode the Q19 
> activates the voltage devider R200, R201 and R202 and then the voltage 
> is lower.
> 
> Have you disconected the wire to the light itself?
> 
> My guess is thet the transforer is shorted or that there are any other 
> flashover on the HV side.
> 
> /Anders
> 
> ----- Original Message ----- From: "Martin Sole" <msole at loxinfo.co.th>
> To: <icom at mailman.qth.net>
> Sent: Saturday, June 17, 2006 10:12 AM
> Subject: [ICOM] IC775DSP display backlight problem
> 
> 
>> Dear Icom friends,
>>
>> I have a problem with an IC775DSP display that I would appreciate some 
>> assistance with. The owner noticed the backlight flicker a couple of 
>> times then go out. This was accompanied by a burning smell and some 
>> smoke. Investigation shows that both Q17 & Q18 in the backlight driver 
>> circuit have disintegrated.
>>
>> I would first like to know what kind of voltage the backlight is meant 
>> to be driven with? The schematic shows it to be simply an LED driven 
>> from a rather complicated AC source. I therefore assume that the 
>> backlight needs more than just a few volts at a few mA.
>>
>> My second question concerns the voltage source itself. This appears to 
>> be some sort of oscillator driven from the 14 volt line but I am 
>> unfamiliar with the nature of its design. I did replace the damaged 
>> transistors with similar items albeit 100 volt rated rather than 120 
>> volt rated but they immediately self destructed so I assume I need to 
>> look elsewhere for the cause.
>>
>>
>> I can email scans of the circuit area if anybody would like to see 
>> before passing comment.
>>
>> Thanks in advance for any help.
>>
>> Martin HS0ZED
>> ----
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>> Icom Users Net: Sundays, 1700Z, 14.316 MHz
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> 
> ----
> Your Moderator: Dick Flanagan K7VC, icom-owner at mailman.qth.net
> Icom Users Net: Sundays, 1700Z, 14.316 MHz
> Icom FAQ: http://www.qsl.net/icom/
>