Roberts R700 disassembly

[FiveBobRepairs]

I'm attempting to overhaul an R700. Basically working, but a few intermittent AF11*s that I will replace with AF12*. I might replace a few electrolytics too. But I am struggling with a few aspects of the process of disassembly...

Tuning scale

On previous Roberts radios I have encountered, the top panel came off easily once I'd removed anything that was holding it (sockets/aerial/side trims etc). This one seems to be stuck hard to the metal, maybe with double sided tape. It does begin to lift if I gently apply force, but I'm worried that the adhesive will tear off some of the print. I may not need to remove it but if I do, is there some technique I need to be aware of?

FM Module

This has an AF114 that seems to be OK, but I'd like to replace it anyway. So far as I can see it is held in place with soldered pins, presumably attached to the tuning capacitor. I've already learnt that Roberts PCB print does not tolerate any abuse, so I am worried that I might wreck the PCB during removal, if I end up tugging against the solder. Again... any tricks worth knowing?

AM Module

This is definitely intermittent, works for a while if you tap it the right way, my suspicion falls on the AF117s within it. For removal, I'm inclined to simply snip the pins securing it to the main PCB, and replace with new wire on reassembly. I reckon that should minimise trauma to the PCB, rather than trying to unsolder all in one go. Good idea or bad?

[FiveBobRepairs]

Update, FM tuner module extracted intact. Maybe I was lucky. All AF114's on FM side now replaced with AF124, working nicely. The AM side remains dodgy but that can wait, and anyway I’ll need to buy more replacements for the remaining transistors.

It has responded well to cosmetic restoration, the dents in the grill are gone, wood ends nicely oiled, vinyl bits cleaned up prrfectly and no immaculate. I’ll try and find time to post a pic or two at some point after final reassembly.

Intrestingly, when I came to desolder the 1st IF transistor, I noticed existing flux stains and slight scarring to the pcb.... this wasn’t the original transistor... it’d been done before. Upon closeer inspection, that transistor also had different markings from the others. I have reason to believe this radio was last used in ‘78, so this is just more evidence (if it were needed) that AF11* failures were already happening in 1970s.

I’d still be grateful for any advice on removing the top perspex tuning scale, which appears to be stuck fast. The volume control could do with a proper deep clean, and I need to remove the scale first, to unscrew the pot. Suggestions welcome...?

[OldTechFan96]

I restored an R700 in January 2017. They are great sets and worth restoring. I think that they were Roberts first set with FM.

To defeat the double sided tape between the tuning scale and the chassis you could dribble some isopropyl alcohol between the join. After waiting for a bit apply gentle force to see if the perspex lifts off. You could use WD40 if you don't have any alcohol.

It is always worth removing the scale to access the dial cord, pulleys and the tuning capacitor screws. Also, a surprising amount of dust can make its way behind the tuning scale!

I think that you are right about removing the FM RF board. Four pins connecting the board to tuning capacitor. Desoldering these pins should give you enough wiggle room to work on the board. Trying to desolder these pins can be a bit tricky as there is not too much room to work. You should be fine as long as you have a good solder sucker. TR1 (RF amp) will be worth testing. It was an AF178 in mine.

I would desolder the AM module as if it were a normal capacitor or resistor. Don't worry too much about lifting a pad or trace. They usually lift when they are exposed to high heat for too long.

I hope this helps!

[simpsons]

An alternative to using a chemical to losen the adhesive is to use dental floss to "saw" through the joint. Cleaning the surface by rubbing the residual material off.

This is the process used in removing 3m self adhesive pads from plastic surfaces in particular motor cars.

Chris

[FiveBobRepairs]

Many thanks, OldTechFan96 & simpsons.

I already posted an update 4 or 5 hours ago, but as I am a newbie my posts still need to be deferred for admin approval. All that you say is helpful and relevant, but please allow for 'time shifts' and overlaps in what may appear to be my response. Of course, I'm not sure where in the sequence this reply will appear either!

I didn't test TR1 as I already knew it was working OK. The little PCB module is now back in place with a new AF124 replacing the old AF114, so I'll probably leave it alone unless and until it acts up again. You are right, desoldering the four pins easier than I expected, a combination of sucker and wick, but I'd rather not do it more often than necessary.

Thanks for advice per the IPA for perspex panel removal. I have been meaning to buy some neat IPA any roads for other purposes, maybe I'll let it wait until I get hold of some.

I also have some dental floss, worth a try too. In fact, wish I'd known a while ago, when trying to remove a bluetooth gadget stuck to my car's dashboard by a previous owner.

Not on my list of questions, but I was relieved how easily the (slightly dented) speaker grill came free. No IPA or anything else needed, basically the glue was just very weak. After getting it off the box and laying it face down, then judiciously tapping from the other side with the back of a teaspoon, the dents came out a treat. No idea if that's the usual method, but it worked for me.

[Philips210]

Hi

Good to see you've made some progress on the R700. They're a nice set when working but they are a labour of love to repair. The tuning scale is held on with double-sided tape strips which can be tricky to remove. It's all too easy to force the issue and damage the printed legend. It's good advice from OldTechFan96 to use IPA to help soften the adhesive.

One of the worst points about the R700 is the way the components are fitted to the PCB. After insertion, the component leadouts are squashed flat then soldered. This makes it difficult to remove the component. It's then necessary to cut the leadout on the component side of the board then unsolder the stub. This makes it a more involved process than what is normally the case.

In the AM IF module, which is a fiddly job to remove from the main PCB, I replaced the AF117s with AF127s and changed all the electrolytic capacitors and any carbon composition resistors that were out of tolerance.

My FM tuner had a tin whisker problem with the AF114 which I replaced with a AF124.
There are a few notes in a thread I started on my R700 back in February 2015: https://www.vintage-radio.net/forum/...d.php?t=113608

Hope this is helpful.

Regards
Symon.

[FiveBobRepairs]

Symon,

I already found your thread before joining the forums, and before starting the work. It was extremely helpful, as was so much else on these forums.

Per electrolytics, I can’t help but think how rubbishy modern components are. Circa 10 years ago, a PC of mine died just outside 1 year warranty. Turned out supply voltages were all way out of spec. On inspection, every single electrolytic in the PSU was bulging and distorted.

In the case above, I replaced all caps with branded Panasonics of a higher temp rating and 10 years on, no more failures. All the same, it leaves me with low confidence in modern caps, vs 50 years ago. If the 50 year old ones are still OK, I probably trust them to see another 10 years more than I’d trust new ones to last 10 years.

[Philips210]

Hi FiveBobRepairs.

Glad my notes were of some help.
The electrolytics in my R700 were in quite a state showing outward signs of leakage and corrosion. The problem with a number of these is excessive leakage current which in some cases is going to help drain the batteries prematurely. As the R700 is in my view one of the worst Roberts radios for servicing, worse than the R600 and R707 by a fair margin, I decided to replace the lot so that I didn't have to go back in there to replace any others that became faulty.
Likewise with the AM IF module it pays to change the lot.

For switch mode power supplies and computer repairs I also like to use decent electros where possible such as Rubycon and Panasonic. Anything half reasonable from a reputable source should be OK for radio repairs. For my R700 I used CPC's "Multicomp" brand radial electrolytics which are just fine.

Regards
Symon

[Tim]

I have found that when removing these modules that have connections to the board, it's better to cut the solid un-insulated connecting wires near the module. Then the module can be removed.
Install new stubs of wire( easier now because you only have a short piece to remove and you don't have to bend and wrestle to get 3 or four pieces through little holes.) just long enough to overlap the "sticking up" lengths.
They can be laid alongside for 3/8" (10mm) or so and easily soldered with minimum disruption and reduced risk of damage to the PCB/s. Any work in the future will also be aided, as the module will be easier to remove.

[FiveBobRepairs]

Tim,

Yes that is pretty much what I was thinking, for the AM module at least.

Doesn’t really apply to the FM tuner module in R700, as that is really just a standalone module with a lift off cover, soldered through blind holes in the middle of the module’s own pcb, attaching it to the totally inaccessible terminals of the tuning capacitor underneath. For that, I had to bite the bullet and carefully and fully desolder these terminals, which actually went well.

I’m actually away on travels at the moment, but will tackle the AM module when I get back. I will of course post an update, too.

Many thanks.

[FiveBobRepairs]

Thanks for all help & advice.

R700 project is now complete, or so I hope. All spruced up, fully working, sounding good and, to my biassed opinion at least, looking the bees knees.

Some feedback, on points discussed...


Tin Whiskers

Three transistors had shorts to case, being:

TR4, AF114. FM second IF.
TR6, AF117, AM mixer/oscillator
TR7, AF117, AM 1st IF, inside the module.

These, and all other AF11x, were replaced with AF12x.

Module removal

Both the FM and AM modules came out without too much struggle. I did not cut the AM module's mounting pins in the end, I just carefully desoldered, no apparent damage to PCB. On closer inspection, it occurred to me that a problem with cutting them is that they are actual physical pins, with a larger head at the tip, which bears against the reverse of the module PCB and holds the module down. I theorised that replacing them with wire links would not have given the same mechanical support, and might have tended to simply pull the tracks off the PCB over time.

All I did to the modules was AF11x substitution. I left all else alone apart from check a few electrolytics for leakage, they were fine, so remain in place. I know I may regret not doing more, I'll take that chance.

Perspex top plate removal

This was a near disaster. Turned out to be stuck down with a heavy wedge of glue right down each edge, rather than the sticky tape I had assumed. IPA didn't really help. Leaving it in last week's 30C sunshine helped, but not enough. Eventually a combination of IPA, hot sun, and dental floss got it off. But the glue had also torn away at the perspex, removing areas of print from the reverse. Lots of pinholes, and some gaping clear patches, that should have been black or white.

On removing the old adhesive from the steel frame, the only thing that worked was some Maplin "Label remover" solvent. When tried same on an inconspicuous corner of the perspex however, it simply dissolved the print as well as the glue. So the glue is still there on back of perspex, though very gently wiping with IPA made it a lot less tacky, albeit with even more damage to the print. But hopefully it won't stick itself on again quite so fiercely, in case I ever want to remove it again.

I have also lined the contact areas on the metal frame with PVC tape, so if the perspex does stick itself down again, the tape if not the glue should come loose for future removal.

And for the awful looking scarring to the print, amazingly, by dabbing tiny amounts of black/white enamel paint over the damage on the printed side, the end result is almost perfect, you really would never notice it. I was lucky that the scars were all in easy places to paint in, ie solid black or solid white. If any lettering had been damaged, not sure what I could have done.

Other faults

Just one. On reassembly all sounded good, but I had quite loud background hiss. It was same at all levels of volume control and unaffected by wave change buttons, so attention focussed on front end of audio. Tapping C63, 47nF polyester, part of the tone control network, made the hiss come and go. Wiggling same capacitor also made it come and ago, until about the third wiggle one of the leads fell off completely. This may have been an own goal as it is mounted across the pins of the base control pot, which I had desoldered and removed to gain access for the transistor replacements. I might have overheated it. Anyway, an easy fix, new capacitor, no more hiss.

Just one tip - fat fingers, small objects

I'm new to these forums, and feel that any tips I can offer will be like teaching grandmother to suck eggs, but I'll chance it anyway...

...When replacing the AF11x transistors with AF12x, a problem is that the replacements have quite short leads, and need to be lowered into a tight, deep, gap between the IF cans. I could have used pliers or tweezers, but still couldn't get much of a grip. I found the ideal tool was a 2" piece of plastic tubing , cut from some stuff that I had in the garage (like car windscreen wash tube). With help of a small wedge of paper to tighten the grip a smidgen , the tubing slipped over the AF12x cans with just enough grip, allowing easy manipulation, and freely pulled off once soldered in place.

And now for something different

My fame now spreading within family thanks to the R700, I have been offered an R707 by a relative. I have only had a quick glance at a photo, it looks a bit ropey, and in worse shape than the R700. But obviously I have accepted. I guess this is how habits form?

[Philips210]

Hi FiveBobRepairs.

Well done for getting your R700 going, they're a labour of love to repair. Whilst I like the R700 in general, it's a truly awful set to repair.
I had similar problems with print coming off the back of the tuning scale due to difficulty in removing it. I also used some Humbrol type enamel paints for the repairs which turned out quite well. The whole idea of sticking the scale down is poor. Hacker also used a stick down scale on many of their radios and you can often see the printed legend along the edges has deteriorated. I think that's due to decomposition of the foam type strip which possibly attacks the print. Later Roberts sets, including the R707, used two 'U' channel pieces to hold the scale to the chassis, altogether a much better idea.

That's a good tip using some tubing for inserting the AF12x transistors which would seem easier to guide in (finger and thumb) the transistor than fiddling with pliers.

The R707 has a reputation for not being the easiest to service but I would say they're considerably easier to repair than a R700. There's none of that squashed component leadout nonsense to deal with on the R707, so that makes component replacement easier.
I have 3 R700s and 3 R707s and about 5 thousand other sets to look at some time.

Regards,
Symon.

[slidertogrid]

A very interesting read! A Roberts that's more awkward to repair than a R707? I didn't think such a thing existed!

When you come to do the R707 let me know if you need any parts as I have a scrapper in the loft with if I remember rightly a missing speaker and a badly broken cabinet.

I have repaired a few modules on R707s and found they are a bit of a pig to remove as the wavechange switch panel is in the way.
Removing a few of the link wires though gives just enough room to desolder the module and straighten the wires to ease it out. Refitting is a bit fiddley as you have to get all the wires from the module through the holes in the main PCB at once before the module will fit back into place.

Good luck with it!
Rich.

[FiveBobRepairs]

Thanks for the offer of R707 bits.

Must admit, I was prepared for the R707 being possibly worse than the R700. Glad to hear it my not be the case and in particular, at least the top plate may not be glued!

My only other experience I have is three R505s, which were a breeze in comparison to the R700.

Mind you, the R700 switch assembly also had to come away, just for access some of the FM IF transistors. I decided to take copious notes and photos, then left most of the wires attached as I pulled it to one side as required, checking for fractured wires on reassembly.

I will start a new thread for the R707 when I get my hands on it, if there is anything interesting to say about it.