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Old 2nd Jul 2018, 7:30 pm   #1
FiveBobRepairs
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Default Roberts R707

Picked up the R707 today. First impressions are good.
The knobs and the perspex are very grubby, the wood ends need sanded and oiled, and the grill has multiple pox marks and a couple of dents. But from previous experience with R505s and the R700, all of these are easily rectified, I know the routine (thanks to reading these forums!).

Handle strap is mucky but stitching perfect, and aerial 95% straight, good enough for me. All knobs & buttons intact and complete with chrome tops. And no tears in the vinyl bits. Not even any decoratorís paint spots, which is a first for me.

Main disappointment is itís actually working, sounding excellent on all wavebands. So no challenge at all. Until that is, I decide to hunt down the AF11*s as a preventative measure.

Last edited by AC/HL; 2nd Jul 2018 at 10:52 pm. Reason: Thread split
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Old 2nd Jul 2018, 8:24 pm   #2
Philips210
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Default Re: Roberts R700 disassembly

Hi

FiveBobRepairs.

If the R707 is a late model and has the LP1164/1 02 IF module then it will have AF12x transistors fitted, see here: https://www.vintage-radio.net/forum/...d.php?t=141075 post#5

Regards,
Symon.
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Old 2nd Jul 2018, 9:38 pm   #3
FiveBobRepairs
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Default Re: Roberts R700 disassembly

The ďLP1164/1Ē marking does have a smudge under it, but I donít think the smudge is Ď02í, canít make it out.

Unlike the photo in that thread which has a printed label, mine just has the module number stamped in fading ink on the metal. From the fact it is working so nicely, I am betting it has AF12x.

I havenít begun to dismantle the radio yet. Once the insides are outside, maybe I can prise the lid of the module without desoldering it, just to see what it contains?
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Old 2nd Jul 2018, 9:50 pm   #4
slidertogrid
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Default Re: Roberts R700 disassembly

Sorry but you can't get the top off. The PCB in the module is soldered to the can and then the bottom is clipped on. the module is fitted to the PCB of the set with the bottom "lid" down so you have to remove the module to get the lid off.
Then to remove the board from the module you have to unsolder the two tags from the can.
If it works I wouldn't worry.
Intermittent operation cured by tapping the main panel isn't always the transistors in the module on more than one occasion I have found dry joints on the wavechange panel.
Rich
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Old 2nd Jul 2018, 10:53 pm   #5
FiveBobRepairs
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Default Re: Roberts R707

Thanks for that info, and description of module construction.

I like to think Iíd have had the sense to examine things, and either not try at all, or to stop prising at the module before damage was done. But better to know up front that prising it just wonít work.

I donít intend to worry about it too much, as long as it isnít broke.
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Old 3rd Jul 2018, 4:52 pm   #6
David G4EBT
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Default Re: Roberts R707

There's a tendency where Roberts are concerned, to focus on the Mullard units (where fitted) or the AFxxx transistors as know culprits. However, as far as the R505 and R700 are concerned, electrolytic capacitors pose a considerable risk of causing damage. I was given a non-working R505 a few months ago by a neighbour who was about to bin it. On switch on, it came on briefly for about 2 seconds, then nothing. On a metered power supply it was evident that there was a short, and TR11 & TR12 (AC187/Ac188) were getting very hot. However, on testing them, they'd survived.

The fault was due to C40 - 680uF - connected to the emitters of TR11 & TR12, it having gone short circuit, registering as a 0.53 Ohm resistor. I replaced that, which restored the set to working order, but given that C41 & C42 (150uF & 470uF) could also put a dead short across the battery, I replaced those too as a precaution.

Let's not forget that these sets are 50 years old, so to not replace those caps as a matter of course is just inviting trouble - if not now, then later. As to the R707, if C42 (250uF - modern equivalent 220uF) goes short circuit, it will place a dead short across the two 6V batteries in series, which could have interesting consequences. Likewise, if C41 goes short circuit, it will short the emitters of TR6 & TR7 to ground, which will make them feel very unhappy.

For the small amount of work and expense involved, I'd change the electrolytic caps on sight, but at the very least, check their capacitance, leakage, & ESR.

I've attached relevant snippets of the circuits of the R707 and R505 to illustrate what I mean.

Also the readout of C40 from the R505, which shows that it is no longer a capacitor - it had morphed into a 0.53 Ohm resistor.

The R505 service data states that a scope should be connected across the speaker, a 1KHz sine wave injected between S1e and chassis, then R36 adjusted to show a symmetrical waveform at the onset of clipping. Not a bit of use to anyone who has neither a signal generator or scope. I have both, but rarely use them, though out of curiosity I did check the R505 and found it to be in need of adjustment as illustrated in pics 5 & 6 - before and after adjustment.

I guess the moral of that is that if we have test gear, for the small amount of effort involved, why not use it?

Hope that's of interest.
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Old 3rd Jul 2018, 11:43 pm   #7
FiveBobRepairs
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Default Re: Roberts R707

Valid points, David.

Per safety concerns, a 996, or even a PP9 battery, would contain enough energy to do something awful in worst case circumstances. I should clarify though, I do not sell these items on, or even allow them to be used without my own supervision.

Interestingly, one of my R505s behaved similarly to your description. Once other (trivial, think ‘switch cleaner’) faults were rectified it worked nicely for a few seconds, then audio went all wobbly, and the AC187/188 overheated, emitting actual smoke. On checking, the (adjustable) dc bias conditions were miles off, so transistors going into runaway. Simply resetting the bias and quiescent current per service manual restored normal operation, sounded good again, and AC187/188 stayed cool. I assumed the bias presets had simply been ‘tinkered with’, but given your experience, regardless of the fact it is working nicely, I may check the electrolytic cap conditions in that set sometime soon.

I do not have oscilloscope or even a professional signal generator. But I tend to believe in a transistor audio circuit, as long as bias levels and quiescent currents are in spec, and everything works well and sounds good, there is not that much a ‘scope can add?
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Old 9th Jul 2018, 6:40 pm   #8
FiveBobRepairs
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Default Re: Roberts R707

The plot thickens re my R707. I could swear it was working perfectly on all bands when I got it but a few days later, FM still fine, AM very faint and just audible at full volume.

In a fair and just world, this would have been caused by muck in the switches. No such luck, no matter how I tried to prove it was the case, the switches were not to blame. So after checking the obvious coupling components from IF module output to audio input, out came the LP 1164/1 module. Sure enough, second stage AF114 has dead short, case to collector. Amazing really that FM still worked, I guess the fact it has an extra stage (serving as mixer/oscillator on AM) allowed it to make up for the bad one?

Anyroads, all out with no observed damage. Given the nightmare of accessing the module’s innards I am going to not just replace the AF11* with AF12* but also renew the handful of small electrolytics, have some decent branded specimens on order.

Grateful as always for the various helpful threads that I have been pointed at, or dug out myself, on these forums. This one in particular... where would I be without it...?

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

Next comes the challenge of reasembly. Lets hope all these photos I took of the proliferation of bare and coloured wiring, from main pcb to switch assembly, were sharp enough....
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Old 26th Jul 2018, 8:38 pm   #9
FiveBobRepairs
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Default Re: Roberts R707

All done now, but what an ordeal. Can’t believe I said I was initially disappointed that it didn’t seem much of a challenge, talk about tempting fate!?

As stated earlier, it transpired I was mistaken in my ininitial appraisal am was deadly silent. It wasn’t dirt in the switches and so the LP1164 module had to come out. Second stage AF114 was shorted, so I replaced all three AF11* with AF12*, also renewed the electrolytics.

On reassembly... fm kind of worked but am was still not working. Symptoms had changed though, I now had hissing on am, but absence of local oscillator (which was working before first intervention). So out came the module again. Turned out the AF126 I’d used for first stage (am mixer/oscillator) was a dud with open circuit emitter and I’d stupidly failed to test it.

Reassembled again with a carefully tested AF124. Worked nicely at first impression, but on closer inspection, not quite right. FM AFC was ‘pulling’ towards the high frequencies, and generally unstable with AFC enabled at anything above 100MHz. So out came the module yet again. One of the ratio detector diodes was virtually short circuit in both directions. Replaced both.

Finally, after fixing all of above, it works nicely on all bands. And I think I can consider myself something of an expert in R707 module removal. Trying not to think too hard about how much money I spent in desoldering braid. For absence of doubt I can confirm module removal is possible without removing the switch pcb, but you probably need to be well practised, I managed it at the final attempt.

I did also replace every single electrolytic in the end. Considering the total effort, it seemed a shame to compromise.
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Old 31st Jul 2018, 11:47 pm   #10
Philips210
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Default Re: Roberts R707

Well done for your perserverance FiveBobRepairs! The R707 is horrible to work on, not as bad as a R700 though, but worth repairing all the same. They are not really in the same league as Hacker sets for build quality and serviceability.
Regarding the R707 IF module, I use some short, temporary, jumper wires to test the module away from the the main board. This saves the wasted effort of refitting an untested IF unit. The same idea also applies to the Mullard IF unit used in the Roberts R600.
As you probably know, the tin whisker problem is not only confined to the AF11x transistors but also applies to the germanium audio output transistors such as AC128, AC188 etc. I had such a problem with the AC188 in my last R707 that I repaired. It had a low reading between emitter and the case of the transistor. The normal junctions were otherwise OK.

Regards
Symon.
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Old 1st Aug 2018, 5:49 pm   #11
FiveBobRepairs
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Default Re: Roberts R707

Good point about the tin whiskers in AC128 etc. The AF11*s seem to better known for whiskers but from a quick search of these forums and elsewhere confirms you are of course right, the others grow them too. I have never witnessed it myself in AC128 etc, but my experience is very limited, restricted to just a few restorations. Maybe Iíll have a prod around my collection, should be easy enough to just check for unexpected connections to the cans, without any dismantling.

Just prior to the R707 I fixed up an R700, three AF11* were shorted, I replaced the lot. I am actually undecided about which one was harder. The R707 was tough in my case, but then the IF module did have to come out three times, which maybe coloured my opinion. Only by the third time around, had I worked out how to remove it without removing the switch pcb.

Have to say I am in no doubt about the sound quality, compared to R700. R700 is good, and maybe looks more modern (though itís not), but R707 is better sounding to my ear. In fact quite splendid, by any measure.
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Old 1st Aug 2018, 5:51 pm   #12
ms660
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Default Re: Roberts R707

Conductive whiskers aren't restricted to transistors either.

Lawrence.
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Old 4th Aug 2018, 10:42 pm   #13
Philips210
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Default Re: Roberts R707

Hi

I seem to recall a problem with a carbon track potentiometer that suffered from a low resistance path from the track to the body of the pot. A replacement cured the fault but I didn't investigate the actual problem with the faulty pot. In hindsight, this could well have been due to tin whisker growth. The pot did look a little corroded and had a whitish appearance to the body. It had probably been in a high humidity environment.

Regards
Symon.
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