Another troublesome R707

[agardiner]

Hi all,

OK, so I have a particularly troublesome Roberts R707 on the bench. Here is the work so far....

Set came in with no apparent life. Confirmed amp was working. Cleaned all switches etc, and replaced some electrolytics which were reading high esr. No change of course, as the IF module was bound to have issues! Quick check on the supply rail to the IF module showed it was the culprit, as the voltage was low at 6.1V. Removed the module, recapped it and 'blasted' the AF transistors.

Reassembled; IF voltage now at 8.4V. FM working perfectly. However, AM is all but dead. Sometimes it does just about work, with a very low level coming from the module. Alignment does not seem to be the issue. However, more often than not there is no AM operation at all. I suspect the IF module is still the culprit, but FM is perfect, so it clearly is fine at 10.7MHz.

Thoughts please? Has anyone every produced a schematic for the module?

Many thanks,
Adrian.

[telstar]

Originally Posted by agardiner

Hi all,

OK, so I have a particularly troublesome Roberts R707 on the bench. Here is the work so far....

Set came in with no apparent life. Confirmed amp was working. Cleaned all switches etc, and replaced some electrolytics which were reading high esr. No change of course, as the IF module was bound to have issues! Quick check on the supply rail to the IF module showed it was the culprit, as the voltage was low at 6.1V. Removed the module, recapped it and 'blasted' the AF transistors.

Reassembled; IF voltage now at 8.4V. FM working perfectly. However, AM is all but dead. Sometimes it does just about work, with a very low level coming from the module. Alignment does not seem to be the issue. However, more often than not there is no AM operation at all. I suspect the IF module is still the culprit, but FM is perfect, so it clearly is fine at 10.7MHz.

Thoughts please? Has anyone every produced a schematic for the module?

Many thanks,
Adrian.

Hi Adrian. I have also a Roberts R707 that refuses to work on all bands. The amplifier does work, and I have replaced the transistors in the module. Why not google the module number and see if someone has done a plan of the module

[ronbryan]

Adrian

Here is a link to the LP1164/1 circuit diagram and component layout drawn in 2008, which seems a long time ago now.

https://www.vintage-radio.net/forum/...ad.php?t=30889

Ron

[paulsherwin]

Check the AM diode. (Yes, it's inside the module )

If it's dead you may want to change the FM discriminator diodes too before they go bad.

[Squipper]

It's still entirely possible that the module transistors may be faulty, even after zapping, and with FM seeming to work OK. The AM mixer/osc may not be oscillating on AM but still passing enough 10.7MHz signal to give acceptable FM reception. The AM oscillator transistor (in the module) also serves as the 1st FM IF amplifier and is capable of passing enough current to work in the IF mode but not as an oscillator.

You could also have a dry joint, or a faulty coupling capacitor, or any of a multitude of other problems, it's hard to diagnose properly over the interwebs. It seems it's perhaps best to replace those AF115/AF116 in the module, maybe for AF127 if you have - or can get?.

[Philips210]

Just a small tip when working on these sets. If I'm having problems with the IF unit I usually connect it to the main board with jumper wires until I'm sure it is fit to go back. It can save a lot of wasted time and diappointment.

I'd be inclined to replace the transistors with AF12x types for reliability.

Regards,
Symon.

[G6Tanuki]

I never bother 'zapping' these transistors; there's a risk that the zapping process will also zap one or more of the thin wires that connect the lead-outs to the semiconductor 'die' or otherwise cause some overcurrent-trauma to the P-N-P junction's insides, and it's well established that zapped tin-whiskers grow back in a totally unhelpful way, and somehow will conspire to cause a failure at some maximally-inconvenient future date.

For a long-term fix [so you know you won't ever have whisker problems again] fitting new transistors is a good use of your remaining hours in this planet. Don't bother though with "New-old-stock" AF11x transistors; the whiskers are a metallurgical thing [rather than something that happens in-use] and they grow just as fast in stored-but-unused parts.

[agardiner]

Thanks all, I think the consensus is that the IF module is the cause. I will have to remove it again, so will replace the transistors, and the detector diodes for good measure. Will report back when I have had the chance to do so.

Cheers,

Adrian.

[camallison]

I have some AF125s if those will do the job. Just message me if you want them, and how many. I bought them from the late Mr Birkett, and don't need them.

Colin Allison

[agardiner]

Thanks Colin, I will send a PM.

[agardiner]

OK; finally had a chance to fit the AF125's that Colin kindly supplied. All bands working properly again.

Odd that the AF116's that they replaced worked fine as FM IF transistors, but clearly no oscillator action on AM. Still, another set fixed. Thanks all and particular thanks to Colin.

[G6Tanuki]

Good to hear of another R707 being brought back from the dead; in full working order and fed from a supply that can deliver plenty of current without sagging they really do sound good when you turn the volume up.

But I'm sure you will agree that the 'inside the case' engineering by Roberts, with the profusion of un-insulated hand-soldered wires seemingly installed at random between the switch-module and the RF/IF chassis - is truly horrible.

Japanese manufacturers did it so much better in the same timeframe, with flat ribbon-cables and lock-down end-terminations.

Really does show up the reason why UK radio-manufacturing crashed-and-burned.

[Paul_RK]

Originally Posted by G6Tanuki

But I'm sure you will agree that the 'inside the case' engineering by Roberts, with the profusion of un-insulated hand-soldered wires seemingly installed at random between the switch-module and the RF/IF chassis - is truly horrible.

Japanese manufacturers did it so much better in the same timeframe, with flat ribbon-cables and lock-down end-terminations.

Really does show up the reason why UK radio-manufacturing crashed-and-burned.

Does it? Hacker construction was altogether more elegant and accessible, but not for much longer as they indeed crashed and burned: admittedly this Japanese example is a little older, a National RF-100Y from the mid-'sixties, but I don't think the uninviting rats'-nest style of its interior was generally much changed in Japanese production by the time of the R707's release in 1969.

Paul

[cathoderay57]

I'm not a fan of zapping the transistors in the IF module, although clearly it will sometimes yield success. The AF12* types are still available from Langrex. I got 3 AF124 for my Roberts R600 for about a tenner because I figured that if I was going to the trouble of removing the module and desoldering the AF114 transistors, replacement with new types would probably be a more long-lasting solution. As you have discovered, repairing the module twice is a pain. Jerry

[agardiner]

Yes Jerry. Personally I am a fan of zapping AF117's etc in sets, and have never had a return. However, the IF module is horrible to work with so I will always replace them in there in future!