Hacker RP72 Sovereign III

[Crackle]

Hi
After having finished servicing the last 6 Hacker radios, I am now embarking on a few more Hacker radios belonging to my friend. This time there are 3 Sovereign III's in the batch, and a couple more Sovereign II's
The first radio, a RP72, needed a lot of work to get the switches to work reliably, but after a while I discovered the main cause of the problems with switching to LW was a weak joint on the LW aerial trimmer which had broken and was intermittent.
I have done a RF realignment of all bands so as to get Gold in on MW and Classic FM on FM and R4 aligned with its mark on LW.
Something strange is happening though, and I wonder if anyone else has had this on their a RP72. If you tune to LW R4 and then switch to MW there is a fainter R4 signal at exactly the same spot on the dial. Both LW and MW are working very well in their own right, I can receive the Irish RTE station on LW and I can also get Gold on MW which is also fairly difficult around here on a portable radio.

This RP72 radio does not have a built in mains power supply, but it does have an oval hole in the back where a mains plug would fit. I notice both the other radios, which have a mains supply are RP72MB's.
Was the RP72 never supplied with a mains adaptor?

Thanks
Mike

[Terry]

I have done a RF realignment of all bands so as to get ...  R4 aligned with its mark on LW.
Something strange is happening though, and I wonder if anyone else has had this on their a RP72. If you tune to LW R4 and then switch to MW there is a fainter R4 signal at exactly the same spot on the dial. 

Mike. as well as I can make out from a hazy picture on the net (Sovereign III), the R4 mark on Long Wave seems to coincide with 416m on Medium Wave.
In which case you are receiving the R4 Crystal Palace transmitter - 720kHz/416m 750W.
The low power would account for the faint signal.
By the way, is the R4 LW mark at 200kHz/1500km or the current 198kHz/1515m?

[Crackle]

Hi Terry
Thanks for your reply. I did not realise there is a R4 on MW at 416m, that would explain it then. I did think for a while that it may be some leakage through the switch cleaner, but no.

cheers
Mike

[Mark Hennessy]

This RP72 radio does not have a built in mains power supply, but it does have an oval hole in the back where a mains plug would fit. I notice both the other radios, which have a mains supply are RP72MB's.
Was the RP72 never supplied with a mains adaptor?

The "MB" stands for Mains/Battery, so the earlier set wouldn't have had the built in PSU.

However, some non-MB sets do have the opening for one. Indeed, some have the opening, but a modified plastic trim is fitted that actually blocks it. It seems that there was a "transition" from the old to the new set.

Of course, back panels do get swapped around by repairers/restorers...

The non-MB versions have a 12V DC input socket.

Nice sets. Technically very competent. But somehow, they don't quite have the same appeal for me as the earlier Sovereign II does. All that extruded aluminium would have been very expensive to tool up for, so it's a shame the screen-printing is not so robust.

Something to check for - early sets can break out into motor-boating, and the cure is to add an earth wire from the ground of the tone control PCB to the earth point on the volume control. You'll find that the later sets will have that done already, but there's a chance the earlier one might not.

And to keep the tradition alive, the new power amplifier connectors can be as unreliable as the older ones

[Crackle]

This radio had a horrible intermittent low frequency howl, probably motor boating as you said. It seemed to be caused by the earth/signal braid on the 4 pin plug into the amp board not making good contact. I will have a check to see if the mod you mentioned is fitted.
Thanks for the tip.
Mike

[Crackle]

Actually, come to think of it, I had a similar problem with a Sovereign II. The bass got unstable when the volume or bass control were turned up. This was cured by cleaning and resetting the contacts of the plug into the AF amp board.

Mike

[Crackle]

Something to check for - early sets can break out into motor-boating, and the cure is to add an earth wire from the ground of the tone control PCB to the earth point on the volume control. You'll find that the later sets will have that done already, but there's a chance the earlier one might not.

And to keep the tradition alive, the new power amplifier connectors can be as unreliable as the older ones

When I checked to see if there was an earth strap going to the volume control, there was not one there, and so I added one as sugested.

   

The plug and socket had very bad connections and switch cleaner did not cure it. The symptoms were a VERY loud low frequency howl, I believed caused by a loose connection to the signal screen.
Each connector was carefully removed from the plug and reshaped, then the retaining clip was raised again with a sharp blade before inserting back in the plug body. I agree this is a poor connector, should have been gold plated or even silver plated rather than bare metal may have been better. It was still possible to cause it to howl when wriggled even after cleaning and reshaping the contacts, and adding the earth strap, but it was OK if left alone.

I found a common fault on a RP72 and a RP72MB, The FM was intermittent. It was very difficult to locate the cause, even the force of spraying freezer spray anywhere on the board caused it to go intermittent. But I eventually traced the problem to a broken solder joint on L3 on the FM tuner module.

   

This may be caused by movement between the circuit board and the tuning cap, so I made sure the 3 screws securing the board to the tuning capacitor were nice and tight.
I also put plenty of solder above and below the wire to the coil to hold it firm.

   

The LW fault on the first RP72 was caused by a broken / dry joint on the LW aerial trimmer capacitor. It was shy of solder causing a weak joint and the capacitor had somehow lifted from the board.

   

There were a few other solder joints looking like this so I went round them all and re-flowed with fresh solder.
The last job on the RP72 was to refit the negative snap connector back onto the battery holder where it had been pulled off. I used a large washer on either side of the nylon and bolted the spring contact back into place. It is important to use a countersunk type screw on the outside and cut the surplus thread on the inside of the spring, or else the spring will not compress enough.

       

Mike

[Mark Hennessy]

Nice work

The dry joint on that trimmer cap is a beautiful example of how they can go. Though I don't think I've seen one quite that impressive on a Hacker before. Over the years, I've noted a lot of variation in the quality of soldering on Hacker PCBs - I guess it depends on who they paid to do the "stuffing", as I'm sure it wasn't done in their factory. Certainly not by this time. Though, if anyone knows differently...

Some earlier boards were hand-soldered, but most were wave-soldered, like yours. Often, the flux wasn't cleaned off, and it ends up looking pretty grotty after all this time.

I don't recall seeing that tuner fault before, but it's definitely worth bearing in mind for future repairs. It has reminded me of a similar fault on an FM tuner, where a resistor lead had been cropped too short before soldering, and it had broken away from the solder. From above, it looked fine. From below, it looked OK - not "dry" - but the lack of excess wire poking out of the solder joint was a strong clue

[Mark Hennessy]

Oh - forgot to say earlier: the earth wire mod is "official", as it's mentioned in the service manual...

[Crackle]

I don't recall seeing that tuner fault before, but it's definitely worth bearing in mind for future repairs. It has reminded me of a similar fault on an FM tuner, where a resistor lead had been cropped too short before soldering, and it had broken away from the solder. From above, it looked fine. From below, it looked OK - not "dry" - but the lack of excess wire poking out of the solder joint was a strong clue

Yes, I think worth noting, the joint is obviously weak by virtue it is just a simple butt joint, and the screws securing the PCB had become slightly loose, as well as the screw securing the plastic cover.

Mike