Hacker RP38 Hunter - UK Vintage Radio Repair and Restoration Discussion Forum

jshearer · 11th Feb 2005, 3:30 pm

I am trying to sort out a Hunter. the reported fault was batteries need replacing every 2 weeks. The current on Minimum volume is 20 ma and steady.
As the volume is increased the current naturally rises but at about 50ma it starts to intermittently short to 0. The output of the radio is not affected as the shorts hardly last but are clearly seen on the meter.

Sounds like an electrolytic capacitor to me but before I start on them has anyone got any ideas

Thanks
JUlian

Paul_RK · 11th Feb 2005, 7:53 pm

Hi Julian.

Hacker sets are prone to quite a few problems with electrolytics - an Autocrat I was given (working perfectly, but two owners had tired of its high battery consumption) turned out to be drawing around 70mA of quiescent current because of a leaky one. With this intermittent short, though, I'm thinking the cause most likely mechanical in nature - cabinet vibration at high volume levels putting something more or less directly across the 18 volt supply line. So I think I'd first try seeing if it still happens with an external loudspeaker connected to the set.

Looking at the circuit without very much time available just now, practically the only component in a position to produce anything like a dead short on the supply is C2, a 160uF capacitor on the amplifier board. Whether vibration ever results in an intermittently shorting capacitor I couldn't say: but tapping it with a finger whilst the set's at low volume might reveal something.

Paul

ukcol · 12th Feb 2005, 12:18 am

Quote:

Originally Posted by jshearer

The current on Minimum volume is 20 ma and steady.
As the volume is increased the current naturally rises but at about 50ma it starts to intermittently short to 0.

Hi Julian,

Cut the link between the collector of TR4 (which is one of the output transtors AC176) and the 18volt rail, and measure the current. If a short circuit current is then observed I would replace both output transistors TR4 and TR5 (AC176 and AC128) and then set up the DC balance and quiesent current. A large current here can be caused by a fault anywhere in the audio amplifier as it is DC coupled throughout, but my money is on the output transistors.

To set up, turn the volume to zero, adjust RV1 for half the battery voltage at the junction of R11 and R12, then adjust RV2 for a quiesent current of 3mA (measured at that cut link).

jshearer · 13th Feb 2005, 11:37 am

Colin and Paul
Thanks for the info very useful even though I got the explanation the wrong way round you seem to know what I meant.

It is the faulty component that goes down the current goes to OL on the meter.

I removed C2 and checked it on a capacitance meter and was OK but with the current being measured at the link and tapping the output transistors breakdown was noticed. I am trying to source the transistors.

Do you know of any equivalents

Thanks
Julian

ukcol · 13th Feb 2005, 11:49 am

Hi Julian

CPC have them in stock. Go to this link and use their search box.

http://cpc.farnell.com/jsp/home/homepage.jsp

Regards
Colin

Tim · 14th Feb 2005, 1:28 am

I did an RP73 recently, which was drawing about 60mA. This was due to faulty output transistors, in this case AC188/187.
All ok once these were replaced, with supply current at about 20mA.
AC128/127 will work as a temporary measure, just to prove a hunch!!.

11th Feb 2005, 3:30 pm	#1
jshearer Retired Dormant Member Join Date: Aug 2004 Posts: 9	Hacker RP38 Hunter I am trying to sort out a Hunter. the reported fault was batteries need replacing every 2 weeks. The current on Minimum volume is 20 ma and steady. As the volume is increased the current naturally rises but at about 50ma it starts to intermittently short to 0. The output of the radio is not affected as the shorts hardly last but are clearly seen on the meter. Sounds like an electrolytic capacitor to me but before I start on them has anyone got any ideas Thanks JUlian

11th Feb 2005, 7:53 pm	#2
Paul_RK Dekatron Join Date: Nov 2003 Location: Fakenham, Norfolk, UK. Posts: 4,259	Re: Hacker RP38 Hunter Hi Julian. Hacker sets are prone to quite a few problems with electrolytics - an Autocrat I was given (working perfectly, but two owners had tired of its high battery consumption) turned out to be drawing around 70mA of quiescent current because of a leaky one. With this intermittent short, though, I'm thinking the cause most likely mechanical in nature - cabinet vibration at high volume levels putting something more or less directly across the 18 volt supply line. So I think I'd first try seeing if it still happens with an external loudspeaker connected to the set. Looking at the circuit without very much time available just now, practically the only component in a position to produce anything like a dead short on the supply is C2, a 160uF capacitor on the amplifier board. Whether vibration ever results in an intermittently shorting capacitor I couldn't say: but tapping it with a finger whilst the set's at low volume might reveal something. Paul

13th Feb 2005, 11:37 am	#4
jshearer Retired Dormant Member Join Date: Aug 2004 Posts: 9	Re: Hacker RP38 Hunter Colin and Paul Thanks for the info very useful even though I got the explanation the wrong way round you seem to know what I meant. It is the faulty component that goes down the current goes to OL on the meter. I removed C2 and checked it on a capacitance meter and was OK but with the current being measured at the link and tapping the output transistors breakdown was noticed. I am trying to source the transistors. Do you know of any equivalents Thanks Julian

13th Feb 2005, 11:49 am	#5
ukcol Rest in Peace Join Date: May 2003 Location: Harlaxton, Lincolnshire, UK. Posts: 3,944	Re: Hacker RP38 Hunter Hi Julian CPC have them in stock. Go to this link and use their search box. http://cpc.farnell.com/jsp/home/homepage.jsp Regards Colin

14th Feb 2005, 1:28 am	#6
Tim Dekatron Join Date: Jan 2003 Location: Bradford on Avon, Wiltshire, UK. Posts: 3,310	Re: Hacker RP38 Hunter I did an RP73 recently, which was drawing about 60mA. This was due to faulty output transistors, in this case AC188/187. All ok once these were replaced, with supply current at about 20mA. AC128/127 will work as a temporary measure, just to prove a hunch!!.