Hacker RP25

[Crackle]

Hi
I have been given a collection of Hacker Sovereign II's to repair by someone who bought one from me a little while ago,  6 Sovereigns II's in total.

On one of them, I think an early model as it has 3 x AF117 transistors, the AM is dead, so I intend to replace the 3 suspect transistors with AF125's.
I also found the wiper was broken off RV28, although FM was working fine.  I have found a replacement 4.7k trimmer and have some notes on setting the adjustment,    

but I dont have a wobulator. Is there an easy way to set the new control?

Thanks
Mike

[Mark Hennessy]

In practice, I've never found the adjustment to be all that critical, surprising as that sounds.

From a look at the manuals for the RP38 Hunter, RP71 Harrier, and the RP76 Black Knight, that preset is described as "AM rejection", which makes a lot more sense than the details given in the RP25/37 manual. Some manuals are even more vague than the RP25 - some make no mention at all of it, while others begin FM alignment instructions by suggesting stating that it should be put in the centre position at the start, but no further mention is made - presumably on the assumption that everyone should know that it is for AM rejection?

Later models (e.g. Aviemore, Sovereign IV/RPC1, Consort) did away with it completely.

To adjust, inject 10.7MHz with AM modulation, then adjust for minimum output. No wobulator needed.

I've found that the FM IF alignment on these sets is usually quite good providing no-one has messed with them, and if I have a go at tweaking, the improvement is usually marginal. The same cannot be said for RF alignment, which will be miles out on both AM and FM. Start by aligning the pointer with the right hand edge of the openings on the scale. The scale markings are rather precise, but with patience, you should be able to get it millimetre-perfect, which is intellectually satisfying

Having said that, a full IF lineup is usually very worthwhile on the Mayflower sets. Just goes to show...

I don't have a wobulator - instead I sweep my Marconi TF2008 RF generator. I have an old Wavetek 162 function generator to do that, and also drive the X input of the 'scope. What's nice about the Wavetek is that it allows me to stop the sweep and with the rotation of a pot, I can swing the output over the whole 360 degrees of the waveform - this allows me to put the spot on the centre of the 'scope screen and set the RF generator to exactly 10.7Mz, and by swinging left or right, I can see how many kHz per horizontal division I have before setting the sweep stimulus going. This saves having to think about markers, etc.

I only use the Marconi because I have it. If not, there are plenty of other general-purpose generators that can be swept either side of 10.7MHz - you don't need a dedicated RF generator for this. Apologies if I'm stating the obvious, but with a bit of lateral thought, there's usually a way to work around a lack of specific gear.

Hope this helps,

Mark

[Crackle]

In practice, I've never found the adjustment to be all that critical, surprising as that sounds.

That is probably why it sounded very good with the wiper missing and just acting as a 5k resistor in circuit.

Your method sounds easy, I will give it a go this afternoon.

I replaced the three AF117's (one AF117 in the FM front end had already been changed for an AF127) and it is working very well on AM. I have done an AM RF alignment, it did need peaking, and it is receiving very well now. FM is working very well so I am not going to alter that other than adjust VR28.
After cleaning the controls and testing for crackles etc I put it all back in the case and then found the treble was very intermittent, after removing the control to give it a full clean I discovered the problem was just a dry joint. Where the 10R resistor loops through the end terminal solder tag there was no solder wetting the tag, it was just relying on the resistor wire feeding through the solder tag hole.

These sets are very well made, I like the way the holes in the printed circuit have little hollow rivets for the transistor leads to go through, a nice touch.
I think the owner is going to be very happy with it, he bought this one new in the 60's. He has been buying them ever since and has one in every room.

Thanks
Mike

[Crackle]

Hi Mark
Thanks for your advice and useful information.
I have now finished this radio, just 4 more to go. I tried the adjustment with a 10.7 MHz am signal and I agree I could not find a null point where the noise was quietest. So I changed the trimmer for a 10k one, which made no difference but at least it was easier to adjust. (the 4.7k one was a multi turn pot with about 3 turns, it was the only one I had)
The only effect turning the pot had was, it went from quiet to loud.
I did find though that with the telescopic aerial fully retracted and tuned to a weak station, I could find a best point of clear reception with adjusting the pot about 1/3 way from the end stop. So I left it like this.
I have not seen a Hacker RP25 with this type of handle, it clips in like a Roberts, was it original?

Here are a couple of photos.

Thanks
Mike

[Mark Hennessy]

Hi Mike,

That's good news

Yes, that handle is original. The earlier RP25s came with that. As did the later RP18s (the original Sovereign). Then it changed to the type with the Rexine strip, but actually there were two types of those - one that pushed down like this one, and the later type that was secured by the screws. Confusing when you don't know! The later type requires you to slide off the chrome covers, and these are perfectly rectangular. Earlier ones had a distinctly tapered appearance, and that's how you can tell at a glance whether the handle pushes off or needs to be unscrewed.

It's worth saying that although the blue Philips/Mullard capacitors are generally very reliable, I note a couple of other types - probably DALY? These are known for leaking heavily. Of the two I can see, one is the decoupler for the AM IF panel - heavy leakage can bring down the HT and cause weak signals (and if the ESR goes high, it can cause whistles and squeaks as you tune across the band - and ultimately can stop LW working). I'd change this on sight, but that's obviously a personal decision. The other one looks like the output coupling capacitor. If it leaks, it'll increase the quiescent current, but hopefully won't do much harm beyond this. If it falls in value (or rises in ESR) then the audio quality will be affected. Again, I'd replace it on-spec, but that's just me

I was confused by the earlier comment regarding the treble control. That 10Ω resistor is just a protection resistor for the headphone socket, and a bad earth there should only affect the headphone itself. I suspect that was a (happy!) coincidence. One thing to watch for with these sets - I've had a loss of bass before, where the bass control has had virtually no effect on the audio. I traced it to the bass control itself. With the bass turned to maximum, there was less than 1kΩ of resistance to chassis - there should be 50kΩ or thereabouts. I took the pot out and could see nothing wrong, and it righted itself as a result of being disturbed. I strongly suspect a tin whisker!

So, I'm wondering if a similar thing was at play here? I can't immediately see how, but as I say, I can't see it being caused by that resistor...

I completely agree with your comments about the build quality. Those people who don't understand need to get one to restore. And if they are still unconvinced, they should try fixing a Roberts R707! Or, just about any other competing transistor radio from the same era.

Good luck with the rest - I'm sure it'll be good news all round before the week is out

Mark

[Crackle]

Hi Mark
Yes the 10r resistor is just for the earphone socket, but the wire from that resistor loops through the end tag on the treble pot and connects to the centre tag. It was soldered onto the centre tag but there was no solder on the end tag, I thought this could be the cause of it being intermittant.
Thinking again about it, you are correct, this would not in itself cause it to be intermittent as it would still work as a variable resistor with just one end connected to the orange wire and the black wire on the centre tag.
Anyway it is working without crackles now.

Thanks
Mike

p.s.
Ill have a look at those other caps you mention. cheers

[Mark Hennessy]

That's interesting - one to lodge away for the future...

As you might already know, the wiring to the loudspeaker and headphone socket changed at some point, and as a result, the 10Ω resistor was moved to the negative tag of the loudspeaker, and 3 wires (rather than 2) went to and fro between there and the headphone socket. This means that the headphone ground now came from the amplifier output connections rather than the chassis - which seems like a good move to me.

[Crackle]

The RP25 I repaired yesterday was the one I had sold him. It had been working perfectly for the 2 or 3 years that I had it, but failed, completely dead, a couple of months after he bought it.
The problem turned out to be the on/off switch, it was still making a nice loud click but was not connecting. The 2 moving contacts on this DPST (Double Pole Single Throw) switched pot are pins held in a moulding which swings back and forth actuated by a spring connected to another leaver moved by the spigot on the end of the volume control.
The end pin (contact) had broken away and the broken corner of the plastic had lodged in such a position so it stopped the contact arm from fully swinging over and hence no contact was making.
Prised out the broken bits and it was all working OK again, it is only wired as a single pole switch so just the one set of contacts is needed..

Mike

[Crackle]

It's worth saying that although the blue Philips/Mullard capacitors are generally very reliable, I note a couple of other types - probably DALY? These are known for leaking heavily. Of the two I can see, one is the decoupler for the AM IF panel - heavy leakage can bring down the HT and cause weak signals (and if the ESR goes high, it can cause whistles and squeaks as you tune across the band - and ultimately can stop LW working). I'd change this on sight, but that's obviously a personal decision. The other one looks like the output coupling capacitor. If it leaks, it'll increase the quiescent current, but hopefully won't do much harm beyond this. If it falls in value (or rises in ESR) then the audio quality will be affected. Again, I'd replace it on-spec, but that's just me

The 3rd Sovereign II to repair was a Blue RP25A. The fault on this was LW not working, MW and FM were both OK.
Exactly as Mark noted above, the Daly electrolytic C21 350uF, the AM decoupling capacitor, was leaking badly it had an ESR of over 9 ohms. I changed it for a 470uF 25v one and LW worked immediately. This was the only Daly electrolytic in the radio. The brass leads broke off as I straightened them where they were bent over the coper track. The same sort of thing happens to those red and black Plessey caps with the yellow label, and they also tend to leak. So I have found in the past that they also generally need changing on sight.
Both LW and MW needed realigning, FM was OK so that has not been touched.
In this radio a couple of screws holding the chassis bits together had come out, these were replaced.
Someone had bypassed the earphone socket, so I rectified this and gave the socket a good spray with Servisol & clean and exercise with inserting a plug in it numerous times.
The tuning knob had a cracked split shaft bush under the spring clip and this has been glued with epoxy which is still curing.
Photo of the radio when the case is back together. Meanwhile here is a photo of the Daly capacitor.
Mike

[Crackle]

The next Hacker has a faulty jack socket. Has anyone tried to remove one of these, it looks difficult with the spring clip encircling the plastic body of the socket.
thanks
Mike