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

[k0sd at juno.com]

John,
   A very common problem.   Also check, and replace the big electrolytic
on the board right behind the speaker.   I've done a few dozen of them,
that fixes it 95 per cent of the time.  They dry out, and loose their
capacitance.   The small 24 volt control PS with the computer type cap
had the same problem, also the big ones in the 500 w and 1000 w 28v
transmitter PA power supplies (depended on your system configuration).

73's de Stephen, K0SD in FL

***************************
On Wed, 1 Nov 2006 21:32:47 -0600 "Clif Holland" <avvidclif at wildblue.net>
writes:
> 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/
> 
>