Repairing a Roberts R707-timeconsuming or what?

[Radiocruncher]

Originally Posted by wireless_john

Now its out I can see the capacitors are all axial with a sort of plastic boot over them. Haven't seen that before and assumed they were radial.

Well done in getting the module out John I bet it fought well!. The module I took out didn't have the plastic covers over the electrolytics. I did 2x LP1171 modules and one had the plastic covers and the other didn't. I assume they are there to stop them shorting against other components as they are tightly packed. Good luck with the overhaul.

Graham

[wireless_john]

I've attached a copy of the crib sheet I'll be using to replace the transistors and electrolytics on my LP1164. The transistor connections were copied from Grahams (Radiocruncher) post and I've added the capacitor locations and polarities. Also the transistor pinouts.

I actually do have AF124, AF125 and AF126 so I'll put those in at the appropriate places. Looking at the specs in my 1968 Mullard Data Book though there doesn't seem to be a lot of difference between them!

The photograph is of the LP1164 that I removed and you'll notice that I have managed to overheat the track under the tab on the right hand side and it's off the board. :-(

Children and grandchildren arriving today so a job for next week!!

John

[Radiocruncher]

Nice work John. You've taken it a step further again and looks nice and clear. I tend to put all AF127's in if im not reusing the old transistors. They have sorted the problem every time so far. Just had some more arrive so best get on with some more module repairs. Probably got about 10 modules to do in all!!. The track tab that has gone isn't connected to anything and just secures the can to the board so won't matter. A blob of hot glue will keep it in place. Just going to carry on with the recap on my R707. Still waiting on a couple values in the post.

Graham

[wireless_john]

So that's all the transistors and electrolytic changed and now about to begin the reconnection of the module and the switch unit.

Of the transistors that came out only the AF114 was faulty and that had shorts from emitter to screen and a high resistance from collector to screen. The other two had hfes around the 130 mark and seemed OK.

That probably explains why it was OK on VHF when it uses the AF115 and dead on the other bands.

John

[Radiocruncher]

I bet you're glad it's nearly over John.

Mines all back together and working great. Just had another one arrive today and it's very quiet on FM and no reception on SW/ LW or MW so it's another module out job I expect. It's the older version with the black push buttons.

Putting it back in is a bit quicker than taking it all apart. Just watch out when you solder the solid wires back that the switchboard is level with the main board. If it isn't your push buttons will be slanted. I know because I had to desolder mine and start again!

Good luck

Graham

[wireless_john]

Thanks for the tip Graham!

John

[flyingtech55]

Originally Posted by Radiocruncher

Tim. Interesting to hear that you repaired the modules for Roberts. If you ever come across that service sheet it would be interesting to see the modifications they did for the AF12x series as I thought they were electrically very similar.

Hi Graham

The company I worked for were a Roberts dealer. As previously mentioned, the original idea was that, in the event of a module failure, the module was to be renewed in its entirety. However, the supply of new modules appeared to dry up in fairly short order. ISTR that for a while you could send the modules back to Roberts for them to repair. This was neither a cheap or speedy option and they wouldn't accept them if they had previously been opened/messed about with.

They then issued an instruction sheet describing how the substitute AF12x transistors for the original AF11x devices. There is something at the back of my mind that you were also supposed to change the value of the emitter resistors from 100 ohms to 168 ohms or something like that. I have looked for a copy of the sheet but have never been able to find it. It was a very long time ago. I don't remember changing the resistors because as you know they are very small and the track is very, very fragile and easy to lift. If we could manage to renew the transistors without collateral damage we felt we had done well let alone trying to substitute resistors as well. I think it's probably easier now with temperature controlled soldering irons, desoldering braids and those big magnifying lense things on the Anglepoise type mounting. I was doing this back in the days of the X25 soldering iron and the spring type solder sucker with its associated recoil.

HTH

TimR

[Radiocruncher]

Originally Posted by flyingtech55

I don't remember changing the resistors because as you know they are very small and the track is very, very fragile and easy to lift. If we could manage to renew the transistors without collateral damage we felt we had done well let alone trying to substitute resistors as well. I think it's probably easier now with temperature controlled soldering irons, desoldering braids and those big magnifying lense things on the Anglepoise type mounting. I was doing this back in the days of the X25 soldering iron and the spring type solder sucker with its associated recoil.

Tim

Thanks a lot for the update. We are spoiled these days with modern gadgets so I understand how easy it was to lift a track on those boards back in the day. It wasn't that long ago that I was using the same methods. So far with the few different modules I've repaired I've had 100% success so I wont be messing with the resistors unless absolutely necessary.

Thanks

Graham

[Tim]

When removing these modules I CUT those wire straps( one obviously has to de-solder the resistors). Then when replacing the module I solder new thinner wires onto the module, and then solder them ONTO the existing stubs(already left long anticipating this) on the main board, leaving a small overlap as I have found it easier than wrestling the board into position. It also means one can fit the button bank in FIRST, and then connect the wire straps. Joining the wires in this manner is also advantageous as it aids future removal and also minimises damage to PCB track.
It's obviously important to make good notes or take photos, but it's fairly obvious which go where.

[Radiocruncher]

Tim. Great tip. I was wondering why Roberts went with solid uninsulated wires. I thought maybe to add rigidity to the switch board or maybe because it made the initial assembly easier. I will try your way on my next one which is sat here patiently waiting.

Thanks

Graham

[nebogipfel]

Graham, Just wanted to say I'm really enjoying your Roberts radio restoration videos. Looking forward to the next one and hopefully some Hackers next? Excellent stuff

[Radiocruncher]

John.

Thanks very much for the feedback. You never really know what people think unless they comment so it's appreciated. I have a Hacker Hunter as you may have seen that needs a new switched volume pot so that will be done soon hopefully. Plus many many more

Thanks

Graham

[wireless_john]

Gloom! Got the module back in, everything wired up, switched on, same problem! Only FM. About to go on holiday as well so it'll have to go on the back burner for a while.

Could have been worse I suppose, at least the FM is still working! I too enjoyed the videos - very useful to see how someone else does the same job. That is a good idea Tim. I might do that if I have to take the switch unit off again.

John

[Radiocruncher]

Originally Posted by wireless_john

Gloom! Got the module back in, everything wired up, switched on, same problem! Only FM

John

That's a shame as thought you had it sorted. At least you have tackled the hardest job of all so you can eliminate that from your fault finding. It's sometimes good to step away from it after major module surgery and come back with a fresh mind from holiday. Thanks for the kind comments re the YouTube videos. Glad you like them.

Have a good one.

Graham

[crackle]

By the way, I dont know if you have found this thread but there is a very nice circuit of the IF module, drawn by Ron Bryan on this thread. https://www.vintage-radio.net/forum/...ghlight=LP1164

Mike

[wireless_john]

Yes I have seen that Mike - very well done too and very useful. Thanks.

John

[wireless_john]

Well, I'm stumped with this now. If I wave a modulated 470kHz around on the switch board at the connection that goes to the 1st AM IF transformer in the module, I hear it in the speaker. (I have a Marconi Instruments 2022 sig gen BTW)

Since the first transistor in the module is the second FM IF amplifier, and it works on FM, I assume it must be OK.

I can't hear the local oscillator on any AM band and I can't work out why it's not running! Whatever it is, it's common to all three bands, SW, MW and LW.

Any ideas?

I should say with the same receiver I can hear the LO on a Hacker Hunter and a Skymaster, so it's not the radio I'm listening with! I'm listening on 1.379MHz and tuning around 909kHz where R5 is.

[Radiocruncher]

John

I did think it could have been the diode on the am out of the module but if your getting a signal through it then it could be okay. Simon Spiers has done a couple of videos that may help. It's certainly work check the connections on the ferrite antenna. The alignment could have been messed with if someone was attempting to try to make it work by twiddling the cores as I assume that the AF11x's in the module were bad. Have a look at these links

https://youtu.be/LmBO5B3bRU0 https://youtu.be/YqYW3C5MQlQ

Keep at it.

Graham

[wireless_john]

Thanks for that Graham. He's got some nice test gear there! I think the module alignment must be OK as it works on FM and I can hear 470kHz though it on the AM side.

After reading another thread on here I think I'm going to have to:

1. Take the module out again :-(
2. Check/replace C1, C2, R3 and R4. And anything else nearby like D1.
3. Check the transistor is actually OK and soldered in the right way round - just for my own peace of mind. I have got another AF125 if necessary.
4. Reassemble and cross fingers.

Away on holiday tomorrow so it will have to wait a while now.

John

[mark pirate]

1. Take the module out again :-(
2. Check/replace C1, C2, R3 and R4. And anything else nearby like D1.

It is not worth the effort of replacing the transistors without also replacing the caps, it is such a pain to have to remove the module again

I expect Roberts had a test jig for testing repaired modules, it would certainly be interesting to find out if they did repair them in house, or simply returned them to Mullard.

Mark