[ICOM] power supply trouble with ICOM IC-R70 Receiver

[John Hagle]

You may be right about that ground screw. Thanks. Someone previously 
bridge soldered the hexagonal standoff post where that mounting screw 
is. The screw was snug, not tight. Will tinker some more. Thanks.

But I still wonder about that 60Hz ripple frequency.

jrh


Clif Holland wrote:
> Look around and tighten the ground screw.  They didn't hardwire the 
> ground. In their infinite wisdom the ground for the power supply is 
> thru a screw that comes loose and causes all sorts of problems.
>
> Clif Holland KA5IPF
> www.avvid.com
>
>
>
>
> ----- Original Message ----- From: "John Hagle" <jhagle at madbbs.com>
> To: "ICOM Reflector" <icom at mailman.qth.net>
> Sent: Wednesday, November 01, 2006 9:28 PM
> Subject: [ICOM] power supply trouble with ICOM IC-R70 Receiver
>
>
>> I am having some trouble with an old Icom IC-R70 receiver. From time 
>> to time it develops a loud hum and the S-Meter light goes dim. The 
>> problem is probably in the power supply.
>>
>> The power supply is conventional, non switching. It uses a power 
>> transformer tapped for both 110vac and 220vac feeding a bridge 
>> rectifier, filter capacitor (47,000mfd) and a three transistor 
>> regulator circuit including what appears to be current limiting 
>> circuit followed by some additional RF bypass caps and a small 
>> electrolytic. Pretty standard stuff.
>>
>> However, the condition is intermittent. It is not sensitive to a bump 
>> or to thermal changes. It occurs at completely random intervals, hot 
>> or cold.
>>
>> At first I thought "electrolytic" but now I am not so sure. When in 
>> the working condition there are several volts of saw-tooth ripple at 
>> the output of the bridge. However, the ripple is at 60Hz rather than 
>> the 120Hz I would expect. Again, this is when the radio is working. 
>> And when working, the raw B+ measures about 15 volts.
>>
>> If a single diode in the bridge rectifier is open, then the bridge 
>> would function as a half wave rectifier and the ripple would be at 
>> the AC line frequency of 60Hz. If a second diode were to open, then 
>> the bridge could still function as a half wave rectifier.
>>
>> With the hum condition in the radio, the raw B+ measures only about 
>> 7.5 volts. And I believe that the saw tooth ripple voltage is still 
>> 60Hz. (I didn't get the scope probe on it in time).
>>
>> Are these bridge diode packages known to be intermittent like this in 
>> older Icom receivers.
>>
>> Or should I be looking elsewhere.
>>
>> thanks in advance
>>
>> de N2JH
>> ex AA2GV es WA2SXH
>>
>>
>>
>>
>>
>> ----
>> 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/
>
>