Roberts R707; no AM.

[MikeBeith]

I have an R707 which is fine on VHF but dead on all AM - LW,MW & SW.

Is the AM oscillator within the LP1164? Or should I be looking for a switch fault? I've read the threads on the AFxxx so I plan to change those at the same time if I have to open the module. I also note the switch problem threads but mine doesn't seem to be intermittent. When I operate the push buttons to self clean them nothing happens to suggest anything.

Also would some kind soul advise me please on removing the front panel - the knobs seem awful tight!

Mike

[G0PKH - Pete]

Hi.

At this stage I would look for a switch fault. Since VHF is fine, I would not tend to blame the IF module at this stage.

I have had a similar problem on one of mine, and it turned out to be a broken connection on the double sided switch board.

[Brian R Pateman]

Pete's right. Switches are the main cause of problems of this type in R707s, I have one waiting on the to do shelf as I write..

Check all the connections to the switch board and re-solder all the pins anyway, even if they look OK.

Then a squirt of Servisol and exercise the switches with enthusiasm.

Regard opening up the LP1164 very much as a last resort.

Regards,

[MikeBeith]

Hi Pete,

Thanks for the reply. I assume that I have to take the front panel off etc to get at the switch mountings - I suppose I could do a continuity check on all the pin connections and their destinations - asuming of course i can identify the switch numbering against the circuit. I'm puzzled why all three AM bands should be out.

Any tips on removing the dial/front panel?

Mike

[MikeBeith]

Hi Brian and thanks. I want to leave that module as alone as I can!!!!

I read all the posts........nuff said.

Mike

[Darren-UK]

Originally Posted by MikeBeith

Hi Pete,

Thanks for the reply. I assume that I have to take the front panel off etc to get at the switch mountings - I suppose I could do a continuity check on all the pin connections and their destinations - asuming of course i can identify the switch numbering against the circuit. I'm puzzled why all three AM bands should be out.

Any tips on removing the dial/front panel?

Mike

No.

Turn the carrying handle round to the front of the radio, clear of the tuning scale.

Open back of radio, remove batteries and release the two chassis-securing screws located on the underside of cabinet. Release the third screw which secures the telescopic aerial. Lift out the battery retaining bar.

The chassis can now be removed through the top of the cabinet, complete with scale, knobs, buttons etc. You can remove the chassis enough to clean and examine the wiring, switches, PCB tracks etc.

In doing this, note that the chassis cannot be totally removed due to various wiring; you will see what needs disconnecting in order to effect a total chassis removal ( speaker, external aerial socket - if fitted, and so on )

[Mike Phelan]

Originally Posted by MikeBeith

Hi Pete,
[...] I'm puzzled why all three AM bands should be out.

Mike

Possibly because the FM switch needs cleaning?

[G0PKH - Pete]

Certainly the problem I had was with the print leading from the FM switch.
It provides power to the AM tuner module.

It was a bad joint on the underside of the board, needed a wire link installing IIRC. It was some time ago now.

[MikeBeith]

Hi Gentlemen,

I've now done a full investigation of the switch assembly and definitely reckon it's is something within the LP1164.

I checked the whole of the switch matrix in conjunction with the circuit diagramand perfect continuity to all other switches, components and the dreaded module. I then did an 'expected voltage' check. Power to the LP module is there all the time as expected (AM/FM) via pin 23. Of most interest were the LP 1164 pins 12, 13, 15 & 17. Assuming an inductively coupled oscillator (too many connections for a Colpitts) the feedback winding in the oscillator collector circuit with pin 17 providing the volts and 13 providing the return .Only 0.2V on pin 17. Same on the appropriate pin of the osc. coil T2. The LP1164 pin 15 is the padder connection.

So it looks like something duff in the module - groan, groan. Anyone tried to remove it without taking the switch out?

Also, knobs seem hellish tight - are they supposed to just pull off? Don't want to damage the plastic dial.

Mike

[Tim]

Hi Mike. I think you will probably need to unsolder at least some of the switch connections. ISTR it can be moved out of the way, but take notes ,photos etc. Set aside a whole day!
The knobs are supposed to pull off, and I too have one that has extremely tight knobs. You could try threading a hankie or similar underneath and giving a good heave. They may respond well to some GENTLE heat from a hair dryer. It doesn't matter if the brights come off, they are easy enough to re-glue.

Good luck.
Tim

[MikeBeith]

Hi Tim,

I plan to give it a try when I have some more time. I bought some more solder wick this afternoon!

Knobs - I suppose it could be the other way round and have them too loose and cause a nuisance!

Mike

[Mike Phelan]

Sometimes knobs are difficult to remove because of a film of rust on the spindles; I always smear Vaseline on these when I reassemble, and often take the knob clips and any other small metal parts and chuck them in a jar of IPA and cooking oil while the rest of the job is done.

[MikeBeith]

Good idea, Mike

Mike

[Andy Day]

Hi Mike, to confirm others opinions, mine had a dodgy set of switches but responded to poor mans Servisol, a squirt of lighter fuel! And the knobs do push on, but have an incredibly tight spring around the boss. Normally the boss breaks off and you have to glue it back on. The Mullard tuner units which have brass shafts seem worst.

[Roger Rabbit]

Try giving the module a few sharp taps with a screwdriver with the set switched to am. Any noise or crackles will indicate one or more of the AF's has grown a beard. If you're really lucky it may spring into life (but it won't last long). Its a tedious job to change em, but worth it!

[MikeBeith]

Hi RR

I'll try the tapping when I get a few hours tomorrow. At least it may confirm the IF module is the problem.

Mike

[Mersey_Swimmer]

It's still possible for you to have a dud AF11x in the module, with FM OK. The AM mixer/osc may not be oscillating on AM but still passing enough 10.7MHz signal to give acceptable FM reception.

The other problem could be a dry joint, or a faulty coupling capacitor. I had one with the little plate capacitor from the mixer's emitter (if I remember correctly it comes out on pin 15) physically broken. Despite its miniscule size the value is 10n (0.01uF). I use those tiny little multilayer ceramic jobbies as replacements.

Should you have to open the module, whatever else you do I'd strongly recommend you replace all the little blue electrolytics as a matter of course. the originals will have almost certainly dried out. I always fit tantalum beads for reliability.

Pete

[MikeBeith]

Well, I tried tapping the LP1164 module this morning and the AM bands still did not work and as a bonus FM disappeared. At least it now looks as if the module is duff although I will check switch continuity once more before the inevitable module removal.

Mike

[MikeBeith]

I'm listening to the repaired R707 as I type this and thanks to all those that responded to my earlier thread. The symptoms were 'working on FM but not on any AM band'. Among the suggestions was 'a quick tap on the Mullard Module LP1164' - the am osc/am.fm IF circuitry. This certainly narrowed down the fault finding - it was now dead on FM as well!

After a search for all the threads relative to this module I cleared the work bench and unsoldered all the connections on the left and right hand sides of the wavechange switch pcb (after making notes/photos etc of connections) I then was able to reach under the switch assembly with the soldering iron and solder wick and detach the module pins from the pcb.

Inside the module I checked the AF115 and the AF116 transistors - sure enough two of them showed shorts between either emitter,base, collector and case. I removed them (3) and replaced them with AF127s. I read somewhere that the screen connection is not needed so I cut this lead from each transistor. After careful replacement of the module and reconnection of the switch pcb connections on power up I was rewarded with that lovely rich quality sound of a Roberts.

So if you need to get into the IF module it's relatively easy (but needs a lot of time) and you can do it without removing the switch, front panel etc etc.
Incidentally it's easy to check out the switch and its pcb by a detailed continuity check against the circuit. The am oscillator transistor (in the module) also serves as the 1st FM IF amplifier and it is capable of passing enough current to work in the IF mode but not as an oscillator

The sound is superb and the repair is certainly worthwhile.

Mike

[Andy Day]

Wel done Mike, it is worthwhile isn't it? I go to sleep most nights listening to Classic fm on my R707, it does have a lovely tone, on a par with my Hacker RP25A.