Hacker RP10, o/p stage rectifiers. - UK Vintage Radio Repair and Restoration Discussion Forum

radiogammon · 17th Jan 2006, 8:52 pm

I have just acquired an old Hacker Herald RP10 which I would like to bring back to life. The first problem which I noticed when the back was removed is that it uses a small Westalite metal rectifier in the output stage and that the plates on this have separated and come adrift. In the circuit (attached), it is shown as MR1 and MR2. I have not seen this before in an output stage and would like some advice on a suitable replacement. Presumably I could use two diodes, but what type? Also, what is the function of this in the circuit? Any advice would be appreciated.

ukcol · 17th Jan 2006, 9:25 pm

MR1 and MR2 are connected across each quiescent current set up pot for the output stage. The quiescent currents have to be accurately adjusted to 3mA, as there is no separate DC balance pot. The purpose of the diodes is to stabilise the quiescent current for temperature changes. As temperature increases, so quiescent current increases, but as temperature increases the voltage drop across the two diodes decreases. This in turn reduces the quiescent current providing a degree of stabilisation.

I do not know the normal forward voltage drop of these little metal rectifiers (but no doubt someone will) but for a germanium diode it is of the order of 0.2 volts and 0.6 volts for silicon; and in both cases this voltage drops with increase in temperature (which is what we want).

You could measure the drop across the good diode and then chose silicon or germanium for a replacement diode as appropriate. When you measure the voltage drop, disconnect the two links in the output stage to avoid damage to the output transistors.

When you have found a suitable replacement you will have to set up the quiescent current correctly and accurately as per the manual. If you have not got a manual, a copy is available from Paul Stenning.

17th Jan 2006, 8:52 pm	#1
radiogammon Retired Dormant Member Join Date: Dec 2002 Location: County Down, Northern Ireland, UK. Posts: 275	Hacker RP10, o/p stage rectifiers. I have just acquired an old Hacker Herald RP10 which I would like to bring back to life. The first problem which I noticed when the back was removed is that it uses a small Westalite metal rectifier in the output stage and that the plates on this have separated and come adrift. In the circuit (attached), it is shown as MR1 and MR2. I have not seen this before in an output stage and would like some advice on a suitable replacement. Presumably I could use two diodes, but what type? Also, what is the function of this in the circuit? Any advice would be appreciated. Attached Thumbnails

17th Jan 2006, 9:25 pm	#2
ukcol Rest in Peace Join Date: May 2003 Location: Harlaxton, Lincolnshire, UK. Posts: 3,944	Re: Hacker Herald RP10 MR1 and MR2 are connected across each quiescent current set up pot for the output stage. The quiescent currents have to be accurately adjusted to 3mA, as there is no separate DC balance pot. The purpose of the diodes is to stabilise the quiescent current for temperature changes. As temperature increases, so quiescent current increases, but as temperature increases the voltage drop across the two diodes decreases. This in turn reduces the quiescent current providing a degree of stabilisation. I do not know the normal forward voltage drop of these little metal rectifiers (but no doubt someone will) but for a germanium diode it is of the order of 0.2 volts and 0.6 volts for silicon; and in both cases this voltage drops with increase in temperature (which is what we want). You could measure the drop across the good diode and then chose silicon or germanium for a replacement diode as appropriate. When you measure the voltage drop, disconnect the two links in the output stage to avoid damage to the output transistors. When you have found a suitable replacement you will have to set up the quiescent current correctly and accurately as per the manual. If you have not got a manual, a copy is available from Paul Stenning.