Hacker Hunter RP38A - UK Vintage Radio Repair and Restoration Discussion Forum

vinrads · 5th Apr 2018, 8:42 pm

I have just down loaded the cct for this radio the only one listed is for the RP38 not RP38A, as it turns out the output stage has different value resistors to the one I have! This itself is not a problem, all I need is to confirm the the idle current is the same. Mid point is mid point.

Any help appreciated, Mick.

Philips210 · 5th Apr 2018, 8:54 pm

Hi Mick

The Hacker Hunter RP38A doesn't seem to be covered in Paul's service information DVD. I checked in Radio & Television Servicing overall index and it is listed in the 1973-74 book. I have this but can't access it right now but will be able to check tomorrow.

Regards
Symon.

Nuvistor · 5th Apr 2018, 10:07 pm

The Hacker Yahoo groups have the RP38A manual.

vinrads · 6th Apr 2018, 8:58 am

Thank's for the replies the 73/74 book just gives the circuit changes between the early model RP 38.
The problem I have is identifying the driver T3 it looks like an AC 128 but the only markings are T2 on the case of the transistor, which seems a bit strange I was wondering if this is a selected transistor for some reason . just noticed the early one has five transistors in the output stage , and the later one RP38A has six .Mick.

Philips210 · 6th Apr 2018, 9:15 am

Hi Mick.

Regarding the oddly designated "T2" transistor have a look at some interesting coverage by Forum member Mark Hennessy on his website. On this link scroll down to The mysterious "T2". It's about half way down the page. https://www.markhennessy.co.uk/artic...ransistors.htm

Regards
Symon.

mhennessy · 6th Apr 2018, 11:01 am

Thanks for the link, Symon

It's still a bit of a puzzle, but as time passes, I'm becoming more comfortable with my hypothesis about the "T2" and "T3". But as always, any further information is welcome.

To the circuit, the RP38A uses an amplifier which is very, very similar to the A205 used in the Sovereign II RP25, Herald RP35, VHF Herald RP37, etc (the earlier RP38 used a simpler circuit, as it didn't need to provide separate bass and treble controls). This circuit was also seen in later models. Of course there were minor variations in component values and transistor type, but if you learn how this one works, you've covered a lot of Hacker sets. The Roberts R606 is pretty much a direct copy of this one too...

Rather than being a driver, T3 is the VBE multiplier used to bias the output stage. It leads a quiet life and is generally very reliable - I don't recall ever having to change one. It will be susceptible to tin whiskers, of course, just like T5 and T6.

Some people might describe T4 as the "driver", and that's understandable given text books at the time. Today, we'd call it the VAS (voltage amplifier stage) as it takes the signal in the form of a current from the collector of T2 and turns it into the voltage that ultimately appears at the output. As it happens, this circuit - like most transistor radios - doesn't have drivers. If the output stage had Darlington pairs or similar at the output, then the transistors that drive the output pair would be referred to as "drivers". Not needed for 1-2 watt transistor radios, but record players and hi-fi amps use them.

Assuming the set is working, the idle current measured at the link is 3mA and the mid-point is given as 8.85V with an 18V supply. In practice, it's not terribly critical, and "just under 9V" is good enough.

The usual problems with these amplifiers are noise, caused by T1 or T2, or various carbon-comp resistors in that area. Got the t-shirt

vinrads · 6th Apr 2018, 4:58 pm

Some very useful info there thank you , as you say the resistors are nearly all way out of spec, so I replaced them all , I have the set working with a bit of rustling noise , I need to replace the lockfit transistors , the Idle current and mid point set ok but there is a problem when re connecting the signal to the amp , I get a loud howling noise not motor boating more like a poss feedback , I can cure this by reducing the input capacitor C1 by half . any chance of a schematic of the output stage for the RP38A , Thank's Mick.

Philips210 · 6th Apr 2018, 5:18 pm

Hi

I had a look in the 1973-74 issue of R&TV Servicing book. The RP38A apparently uses a very similar audio board to the RP25 'Sovereign' Mk.II. This was then referenced to the 1970-71 issue. I can scan this if required.

Regards
Symon

mhennessy · 6th Apr 2018, 5:22 pm

The A205 schematic is on the page on my website that you kindly linked to earlier

(Click to enlarge it for easier viewing).

Mick, motorboating is usually caused by the 5 pin connector - check it carefully, especially the earth and HT lines. Don't forget the associated solder joints underneath the PCB. Also, the power switch in the potentiometer can go high-resistance and cause this fault.

Philips210 · 6th Apr 2018, 5:41 pm

Quote:

Originally Posted by mhennessy

The A205 schematic is on the page on my website that you kindly linked to earlier

Thanks for that Mark, it will save me scanning the article.

Interesting to note the problem with the 5-pin connector on the audio board. Is this due to corrosion/oxidisation or lack of contact pressure in the plug/socket connection?

Regards
Symon

Philips210 · 6th Apr 2018, 5:54 pm

Hi

Slightly OT, but in relation to the oddly designated "T2" transistor. The Roberts R707 also has a similarly designated transistor in its audio stages, this time there's a "T7" which I believe is equivalent to a OC71. ISTR the Roberts R600 has the same part as well.

Regards
Symon.

vinrads · 12th Apr 2018, 9:44 am

Further to my previous post regarding the howling noise when re connecting the amp to the radio,I found that by reducing the 0.1uf input, by half cured the problem !but I needed to find out what was causing the problem , so I cleaned the five pin plug and socket , re soldered all the plug/ socket / earth connections shorted out the on off switch.

Switched on just the same, very strange!this is at min volume , The cure was to fit an extra earth lead in the plug and solder it to the chassis on the radio in parallel with the screen lead , don't ask ! but it cured it even at full bass no problems , Mick.

5th Apr 2018, 8:42 pm	#1
vinrads Dekatron Join Date: Feb 2009 Location: Chesterfield, Derbyshire, UK. Posts: 3,766	Hacker Hunter RP38A I have just down loaded the cct for this radio the only one listed is for the RP38 not RP38A, as it turns out the output stage has different value resistors to the one I have! This itself is not a problem, all I need is to confirm the the idle current is the same. Mid point is mid point. Any help appreciated, Mick.

5th Apr 2018, 8:54 pm	#2
Philips210 Nonode Join Date: May 2007 Location: Redruth, Cornwall, UK. Posts: 2,580	Re: Hacker Hunter RP38 a Hi Mick The Hacker Hunter RP38A doesn't seem to be covered in Paul's service information DVD. I checked in Radio & Television Servicing overall index and it is listed in the 1973-74 book. I have this but can't access it right now but will be able to check tomorrow. Regards Symon.

5th Apr 2018, 10:07 pm	#3
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Hacker Hunter RP38 a The Hacker Yahoo groups have the RP38A manual. __________________ Frank

6th Apr 2018, 8:58 am	#4
vinrads Dekatron Join Date: Feb 2009 Location: Chesterfield, Derbyshire, UK. Posts: 3,766	Re: Hacker Hunter RP38 a Thank's for the replies the 73/74 book just gives the circuit changes between the early model RP 38. The problem I have is identifying the driver T3 it looks like an AC 128 but the only markings are T2 on the case of the transistor, which seems a bit strange I was wondering if this is a selected transistor for some reason . just noticed the early one has five transistors in the output stage , and the later one RP38A has six .Mick. Last edited by vinrads; 6th Apr 2018 at 9:06 am.

6th Apr 2018, 9:15 am	#5
Philips210 Nonode Join Date: May 2007 Location: Redruth, Cornwall, UK. Posts: 2,580	Re: Hacker Hunter RP38A Hi Mick. Regarding the oddly designated "T2" transistor have a look at some interesting coverage by Forum member Mark Hennessy on his website. On this link scroll down to The mysterious "T2". It's about half way down the page. https://www.markhennessy.co.uk/artic...ransistors.htm Regards Symon. Last edited by Philips210; 6th Apr 2018 at 9:22 am.

6th Apr 2018, 11:01 am	#6
mhennessy Dekatron Join Date: Jul 2007 Location: Evesham, Worcestershire, UK. Posts: 4,244	Re: Hacker Hunter RP38A Thanks for the link, Symon It's still a bit of a puzzle, but as time passes, I'm becoming more comfortable with my hypothesis about the "T2" and "T3". But as always, any further information is welcome. To the circuit, the RP38A uses an amplifier which is very, very similar to the A205 used in the Sovereign II RP25, Herald RP35, VHF Herald RP37, etc (the earlier RP38 used a simpler circuit, as it didn't need to provide separate bass and treble controls). This circuit was also seen in later models. Of course there were minor variations in component values and transistor type, but if you learn how this one works, you've covered a lot of Hacker sets. The Roberts R606 is pretty much a direct copy of this one too... Rather than being a driver, T3 is the VBE multiplier used to bias the output stage. It leads a quiet life and is generally very reliable - I don't recall ever having to change one. It will be susceptible to tin whiskers, of course, just like T5 and T6. Some people might describe T4 as the "driver", and that's understandable given text books at the time. Today, we'd call it the VAS (voltage amplifier stage) as it takes the signal in the form of a current from the collector of T2 and turns it into the voltage that ultimately appears at the output. As it happens, this circuit - like most transistor radios - doesn't have drivers. If the output stage had Darlington pairs or similar at the output, then the transistors that drive the output pair would be referred to as "drivers". Not needed for 1-2 watt transistor radios, but record players and hi-fi amps use them. Assuming the set is working, the idle current measured at the link is 3mA and the mid-point is given as 8.85V with an 18V supply. In practice, it's not terribly critical, and "just under 9V" is good enough. The usual problems with these amplifiers are noise, caused by T1 or T2, or various carbon-comp resistors in that area. Got the t-shirt

6th Apr 2018, 4:58 pm	#7
vinrads Dekatron Join Date: Feb 2009 Location: Chesterfield, Derbyshire, UK. Posts: 3,766	Re: Hacker Hunter RP38A Some very useful info there thank you , as you say the resistors are nearly all way out of spec, so I replaced them all , I have the set working with a bit of rustling noise , I need to replace the lockfit transistors , the Idle current and mid point set ok but there is a problem when re connecting the signal to the amp , I get a loud howling noise not motor boating more like a poss feedback , I can cure this by reducing the input capacitor C1 by half . any chance of a schematic of the output stage for the RP38A , Thank's Mick.

6th Apr 2018, 5:18 pm	#8
Philips210 Nonode Join Date: May 2007 Location: Redruth, Cornwall, UK. Posts: 2,580	Re: Hacker Hunter RP38A Hi I had a look in the 1973-74 issue of R&TV Servicing book. The RP38A apparently uses a very similar audio board to the RP25 'Sovereign' Mk.II. This was then referenced to the 1970-71 issue. I can scan this if required. Regards Symon

6th Apr 2018, 5:22 pm	#9
mhennessy Dekatron Join Date: Jul 2007 Location: Evesham, Worcestershire, UK. Posts: 4,244	Re: Hacker Hunter RP38A The A205 schematic is on the page on my website that you kindly linked to earlier (Click to enlarge it for easier viewing). Mick, motorboating is usually caused by the 5 pin connector - check it carefully, especially the earth and HT lines. Don't forget the associated solder joints underneath the PCB. Also, the power switch in the potentiometer can go high-resistance and cause this fault.

6th Apr 2018, 5:54 pm	#11
Philips210 Nonode Join Date: May 2007 Location: Redruth, Cornwall, UK. Posts: 2,580	Re: Hacker Hunter RP38A Hi Slightly OT, but in relation to the oddly designated "T2" transistor. The Roberts R707 also has a similarly designated transistor in its audio stages, this time there's a "T7" which I believe is equivalent to a OC71. ISTR the Roberts R600 has the same part as well. Regards Symon.

12th Apr 2018, 9:44 am	#12
vinrads Dekatron Join Date: Feb 2009 Location: Chesterfield, Derbyshire, UK. Posts: 3,766	Re: Hacker Hunter RP38A Further to my previous post regarding the howling noise when re connecting the amp to the radio,I found that by reducing the 0.1uf input, by half cured the problem !but I needed to find out what was causing the problem , so I cleaned the five pin plug and socket , re soldered all the plug/ socket / earth connections shorted out the on off switch. Switched on just the same, very strange!this is at min volume , The cure was to fit an extra earth lead in the plug and solder it to the chassis on the radio in parallel with the screen lead , don't ask ! but it cured it even at full bass no problems , Mick.