[ICOM] IC775DSP display backlight problem

[Martin Sole]

Hi again,

I haven't had a chance to look at the display problem for a day or so 
but I am beginning to suspect the transformer as I stated before.

Your earlier message mentioned that the display itself is 120 volt AC. I 
wonder if this needs to be at a specific frequency or if I could use 120 
volts at 50 Hz in order to prove the display element?

Martin, HS0ZED


Anders Janis SM4RNA wrote:
> The only problems with the backlight so far is that my and two other 
> radion suffer from dry joint / bad soldering. In two cases it is the 
> back of the display itself, the long row of with very thin wires and 
> quite large holes in the PCB. When this problems appear the sub-VFO 
> digits start to flicker. In my radio it was the transformer that I had 
> to re-solder as the backlight it self started to flicker.
> 
> Will be intresting to herar from you when it works again.
> 
> de Anders SM4RNA
> 
> 
> ----- Original Message ----- From: "Martin Sole" <msole at loxinfo.co.th>
> To: "ICOM Reflector" <icom at mailman.qth.net>; <sm4rna at telia.com>
> Sent: Sunday, June 18, 2006 10:19 AM
> Subject: Re: [ICOM] IC775DSP display backlight problem
> 
> 
>> 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
>>>> ----
>>>> 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/
>>>
>>> ----
>>> 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/
>>>
>