11-04-2012, 12:55 PM
Hi all,
Right - definitely a shortage of transistors on this site, so about time I started posting some of my recent jobs...
Many of you will know the Roberts R707. It was the flagship model for many years - this example is from 1973, so roughly mid-way in the run. Thanks are due to Skywave for finding me this set
This example is slightly unusual because it has a "chunky", heavily grained Rexine covering, and slightly lighter teak side panels. Normally, the Rexine is much smoother, and the sides are a shade or two darker. Having now seen examples of both types in the flesh, I prefer the "chunky" version shown here...
Some background to the R707:
These sets are highly regarded for their sound quality, and special mention should be made of the audio sections. This set uses an active Baxendal tone control circuit, which is relatively rare in transistor radios. Also, there is a ganged volume control, meaning that as the volume is turned down, any noise contribution from the preamplifier stage is reduced along with the signal. Arguably overkill, but interesting nonetheless. One of the sections has a "loudness tap" - not shown in the service data. All in all, the volume control is pretty unique, and if you have a faulty one, a scrap set is your only source.
The active devices are the usual Lockit types (BC148, BC149, BC158), with AC187/188 for the output - the VBE multiplier being an OC71. All pretty conventional circuitry, and pretty reliable. The set runs from 12V (two large lantern batteries or an external supply), and develops approximately 1.5 watt into a 4 ohm loudspeaker - a 4 by 7 inch Rola - relatively small for the size of case...
The RF and IF stages are not so nice. As you probably know, these sets use the dreaded Mullard modules. The FM front end is an LP1179/2, and actually, this is OK as it used silicon transistors. But, the LP1164 contains 3 AF115s, which are notorious for the dreaded tin whiskers.
But worse, the mechanical design of these sets is appalling! I don't mind taking the module apart and "treating" the AF115s, but to get to it, you have to move the PCB carrying the switches out of the way. For the uninitiated, this is a total nightmare - the switchboard is joined to the main PCB by a maze of bare wires around all 3 sides, and unsoldering all these wires is the only way to move the board. But, being Roberts, there are no silkscreen markings to help you relocate these wires, and the service data doesn't help much either. You need to take lots of close-up pictures before you embark on this, and then you need another set to compare to, because inevitably there will be a couple that aren't clear. Oh, and Roberts used special PCBs with weak glue that lifts with the slightest application of heat, and special solder that doesn't work very well. So you need to be very careful when working on these!
The cabinet:
A quick check showed that the set appeared to work, albeit with some intermittency, so I put the chassis to one side to work on the case. Just 3 screws release the chassis, then you have to remove the DC input connector, the loudspeaker and the battery carrier. Before I do anything, I give the set a good clean with foam cleanser and scrub the Rexine and grill with a nail brush to make sure there are no unpleasant surprises.
These sets are rather challenging to restore well. So much depends on what you have as a starting point, and there's a degree of luck involved as well. The main problem is the grille, which is pressed/expanded aluminium. It's soft, so picks up scratches and dents really easily.
You might think that the dents can only happen in the area of the loudspeaker cut-out, so it's a simple matter to push the dents out from inside. Not so! Roberts inexplicably routed out a recess either side of the loudspeaker aperture, and this is invariably where the dents gather. OK, so we need to remove the grille and hammer it flat. Yes, but the grille is glued on at the factory. Whether you can remove it in one piece without making it worse is a matter of luck - and in this case, luck was on my side.
Step one is to find a suitable piece of wood, and cut it to size - then you can place the grille onto it. The edges of the grille are folded down, which is why the timber needs to be the right size. To get the dents out, a combination of rolling and hitting is required - a wall-paper roller is a good tool for this. Of course, it will never be perfect because the material has stretched on impact, but it can be reasonable with patience.
The flattened grille needs to be glued back in place otherwise it won't be flat. For this, I used some incredibly strong double-sided tape - I was worried that any glue might seep through the holes in the grille. Of course, it will never be 100% flat in the area of the loudspeaker. I did contemplate installing a couple of thin strips of birch ply to give the grille something to sit against, but my woodwork tools are currently buried under old radios. Maybe next time...
Before glueing back on, the side cheeks need rubbing down. Remove the handle and DC input trim, and then unhook the grey piping and fold it down out of the way. I start with 240 grit unless it's really bad - luckily, these sets were oiled from new, so there's no varnish or lacquer to remove first. After steaming out any dents and dings, 240 is followed by 320, which is followed by Danish Oil. I hear cries from the back row - but this is a 1970s transistor that was originally oiled - French polish would be wholly inappropriate
After 3 coats, the cabinet is given another clean with foam cleaner to remove all the dust (and evidence of a previous battery leak). Then the cleaned up handle and badge can be reapplied. The rear panel needed attention; these are thin hardboard covered in Rexine and with a layer of thick black card glued to the inside - this invariably needs glueing back in place. Finally, the trim around the DC input connector can be cleaned with metal polish before being put back in place.
The chassis:
Remove the knobs, the strips at the top and bottom, then the scale. You have to unsolder connections to the sockets first, which is a pain. Then the scale can be cleaned, which means several rub-downs with metal polish to remove the scratches. I learned a while back to leave the sockets attached to the panel, otherwise the panel will crack when you lever off the securing star-washers. Compared to Hacker, these panels are really thin, and pick up scratches easily. Inevitably. the metal polish leaves residues around the sockets, so you'll need to take the time to remove these with a soft cloth or brush.
The tuning had some "dead" spots caused by worn teeth, so I carefully swapped the gears from a scrap set. That was easier said than done, but well worth it. Obviously, lubricate everything that moves while it's all apart. And check that the rubber grommets between tuner and chassis are in good condition, otherwise the tuning spindle leans to one side...
Paying closer attention to the chassis, it was working well on FM and SW, but dead on MW and LW. The oscillator wasn't. I cleaned the switch bank several times, but no joy, so decided to dive into the module. I discovered that someone had been here before me, and had cut the PCB tracks to the screen connections of the AF115s. However, after a while, the tin whiskers reach more than one electrode, and cause all sorts of strange behaviour. So I blasted them away and reinstalled them, carefully bridging the cut tracks with fine TCW. No, I didn't convert to silicon on this occasion...
When it came to reinstalling the connections between the main PCB and the switchboard, I decided to start again. This resulted in a much neater job than normal, and in many ways was easier than the usual trick of bending the board out of the way once enough of the wires had been unsoldered. And thankfully, the module worked perfectly - clearly the leakage caused by the whiskers in the first IF transistor were enough to stop it oscillating at lower frequencies...
After a line-up (audio and RF, IF was fine), the chassis was reassembled. But the loudspeaker, battery tray and side brackets were damaged by a previous battery leak - time to raid my spares supply. Luckily, I have a fair few of these sets - shame they're all scappers really... I also took the time to neatly twist all the wires between the chassis and loudspeaker, DC input socket and battery tray. It's worth making sure the connections at the input socket are neat, otherwise the battery will foul them...
Next, give the knobs a bath, and polish the metal bits. I went through my stocks of scrap R707s to find the best set - these are vulnerable designs that pick up scratches or dents easily, and the "brights" frequently fall off.
The final job was to replace the aerial - these are fragile and always kinked or broken. Some people can fix them; I can't! Luckily, I discovered 2 brand new aerials in a BBC local radio workshop, and it's been some years since we last used these sets in anger!
Performance:
These aren't bad sets. Quite sensitive on FM, and tune to 104MHz, unlike many sets of the period. The AFC action isn't very good, and the switch is back to front - press to switch AFC off! Not bad on AM either, although not as good as a Hacker. Strangely, when you switch to LW, a 100K resistor is added in series with the audio feed, so it's quieter than the other bands. No-doubt there was a good reason for this...
The sound is pretty good, I have to concede. Not as full-bodied as a Hacker, and it's brighter when you turn the treble up. From the diagram, it looks like they might not have fully implemented the FM de-emphasis (must measure it), but with the treble full up, the treble is ear-splitting. Some people like that, of course. The bass is reasonable, but boomy with the bass at full. It's a small loudspeaker, and there is a lot of bass boost available, so the amplifier can run out of steam if you're not careful. But, of all the Roberts sets I've got, it's probably the best of the bunch, and a welcome addition to my "restored" pile
I didn't take any in-progress pictures, but will add some to this thread with the next set I tackle (father-in-law wants one). For now, here are the "after" pictures. Enjoy
Thanks for reading,
Mark
Right - definitely a shortage of transistors on this site, so about time I started posting some of my recent jobs...
Many of you will know the Roberts R707. It was the flagship model for many years - this example is from 1973, so roughly mid-way in the run. Thanks are due to Skywave for finding me this set
This example is slightly unusual because it has a "chunky", heavily grained Rexine covering, and slightly lighter teak side panels. Normally, the Rexine is much smoother, and the sides are a shade or two darker. Having now seen examples of both types in the flesh, I prefer the "chunky" version shown here...
Some background to the R707:
These sets are highly regarded for their sound quality, and special mention should be made of the audio sections. This set uses an active Baxendal tone control circuit, which is relatively rare in transistor radios. Also, there is a ganged volume control, meaning that as the volume is turned down, any noise contribution from the preamplifier stage is reduced along with the signal. Arguably overkill, but interesting nonetheless. One of the sections has a "loudness tap" - not shown in the service data. All in all, the volume control is pretty unique, and if you have a faulty one, a scrap set is your only source.
The active devices are the usual Lockit types (BC148, BC149, BC158), with AC187/188 for the output - the VBE multiplier being an OC71. All pretty conventional circuitry, and pretty reliable. The set runs from 12V (two large lantern batteries or an external supply), and develops approximately 1.5 watt into a 4 ohm loudspeaker - a 4 by 7 inch Rola - relatively small for the size of case...
The RF and IF stages are not so nice. As you probably know, these sets use the dreaded Mullard modules. The FM front end is an LP1179/2, and actually, this is OK as it used silicon transistors. But, the LP1164 contains 3 AF115s, which are notorious for the dreaded tin whiskers.
But worse, the mechanical design of these sets is appalling! I don't mind taking the module apart and "treating" the AF115s, but to get to it, you have to move the PCB carrying the switches out of the way. For the uninitiated, this is a total nightmare - the switchboard is joined to the main PCB by a maze of bare wires around all 3 sides, and unsoldering all these wires is the only way to move the board. But, being Roberts, there are no silkscreen markings to help you relocate these wires, and the service data doesn't help much either. You need to take lots of close-up pictures before you embark on this, and then you need another set to compare to, because inevitably there will be a couple that aren't clear. Oh, and Roberts used special PCBs with weak glue that lifts with the slightest application of heat, and special solder that doesn't work very well. So you need to be very careful when working on these!
The cabinet:
A quick check showed that the set appeared to work, albeit with some intermittency, so I put the chassis to one side to work on the case. Just 3 screws release the chassis, then you have to remove the DC input connector, the loudspeaker and the battery carrier. Before I do anything, I give the set a good clean with foam cleanser and scrub the Rexine and grill with a nail brush to make sure there are no unpleasant surprises.
These sets are rather challenging to restore well. So much depends on what you have as a starting point, and there's a degree of luck involved as well. The main problem is the grille, which is pressed/expanded aluminium. It's soft, so picks up scratches and dents really easily.
You might think that the dents can only happen in the area of the loudspeaker cut-out, so it's a simple matter to push the dents out from inside. Not so! Roberts inexplicably routed out a recess either side of the loudspeaker aperture, and this is invariably where the dents gather. OK, so we need to remove the grille and hammer it flat. Yes, but the grille is glued on at the factory. Whether you can remove it in one piece without making it worse is a matter of luck - and in this case, luck was on my side.
Step one is to find a suitable piece of wood, and cut it to size - then you can place the grille onto it. The edges of the grille are folded down, which is why the timber needs to be the right size. To get the dents out, a combination of rolling and hitting is required - a wall-paper roller is a good tool for this. Of course, it will never be perfect because the material has stretched on impact, but it can be reasonable with patience.
The flattened grille needs to be glued back in place otherwise it won't be flat. For this, I used some incredibly strong double-sided tape - I was worried that any glue might seep through the holes in the grille. Of course, it will never be 100% flat in the area of the loudspeaker. I did contemplate installing a couple of thin strips of birch ply to give the grille something to sit against, but my woodwork tools are currently buried under old radios. Maybe next time...
Before glueing back on, the side cheeks need rubbing down. Remove the handle and DC input trim, and then unhook the grey piping and fold it down out of the way. I start with 240 grit unless it's really bad - luckily, these sets were oiled from new, so there's no varnish or lacquer to remove first. After steaming out any dents and dings, 240 is followed by 320, which is followed by Danish Oil. I hear cries from the back row - but this is a 1970s transistor that was originally oiled - French polish would be wholly inappropriate
After 3 coats, the cabinet is given another clean with foam cleaner to remove all the dust (and evidence of a previous battery leak). Then the cleaned up handle and badge can be reapplied. The rear panel needed attention; these are thin hardboard covered in Rexine and with a layer of thick black card glued to the inside - this invariably needs glueing back in place. Finally, the trim around the DC input connector can be cleaned with metal polish before being put back in place.
The chassis:
Remove the knobs, the strips at the top and bottom, then the scale. You have to unsolder connections to the sockets first, which is a pain. Then the scale can be cleaned, which means several rub-downs with metal polish to remove the scratches. I learned a while back to leave the sockets attached to the panel, otherwise the panel will crack when you lever off the securing star-washers. Compared to Hacker, these panels are really thin, and pick up scratches easily. Inevitably. the metal polish leaves residues around the sockets, so you'll need to take the time to remove these with a soft cloth or brush.
The tuning had some "dead" spots caused by worn teeth, so I carefully swapped the gears from a scrap set. That was easier said than done, but well worth it. Obviously, lubricate everything that moves while it's all apart. And check that the rubber grommets between tuner and chassis are in good condition, otherwise the tuning spindle leans to one side...
Paying closer attention to the chassis, it was working well on FM and SW, but dead on MW and LW. The oscillator wasn't. I cleaned the switch bank several times, but no joy, so decided to dive into the module. I discovered that someone had been here before me, and had cut the PCB tracks to the screen connections of the AF115s. However, after a while, the tin whiskers reach more than one electrode, and cause all sorts of strange behaviour. So I blasted them away and reinstalled them, carefully bridging the cut tracks with fine TCW. No, I didn't convert to silicon on this occasion...
When it came to reinstalling the connections between the main PCB and the switchboard, I decided to start again. This resulted in a much neater job than normal, and in many ways was easier than the usual trick of bending the board out of the way once enough of the wires had been unsoldered. And thankfully, the module worked perfectly - clearly the leakage caused by the whiskers in the first IF transistor were enough to stop it oscillating at lower frequencies...
After a line-up (audio and RF, IF was fine), the chassis was reassembled. But the loudspeaker, battery tray and side brackets were damaged by a previous battery leak - time to raid my spares supply. Luckily, I have a fair few of these sets - shame they're all scappers really... I also took the time to neatly twist all the wires between the chassis and loudspeaker, DC input socket and battery tray. It's worth making sure the connections at the input socket are neat, otherwise the battery will foul them...
Next, give the knobs a bath, and polish the metal bits. I went through my stocks of scrap R707s to find the best set - these are vulnerable designs that pick up scratches or dents easily, and the "brights" frequently fall off.
The final job was to replace the aerial - these are fragile and always kinked or broken. Some people can fix them; I can't! Luckily, I discovered 2 brand new aerials in a BBC local radio workshop, and it's been some years since we last used these sets in anger!
Performance:
These aren't bad sets. Quite sensitive on FM, and tune to 104MHz, unlike many sets of the period. The AFC action isn't very good, and the switch is back to front - press to switch AFC off! Not bad on AM either, although not as good as a Hacker. Strangely, when you switch to LW, a 100K resistor is added in series with the audio feed, so it's quieter than the other bands. No-doubt there was a good reason for this...
The sound is pretty good, I have to concede. Not as full-bodied as a Hacker, and it's brighter when you turn the treble up. From the diagram, it looks like they might not have fully implemented the FM de-emphasis (must measure it), but with the treble full up, the treble is ear-splitting. Some people like that, of course. The bass is reasonable, but boomy with the bass at full. It's a small loudspeaker, and there is a lot of bass boost available, so the amplifier can run out of steam if you're not careful. But, of all the Roberts sets I've got, it's probably the best of the bunch, and a welcome addition to my "restored" pile
I didn't take any in-progress pictures, but will add some to this thread with the next set I tackle (father-in-law wants one). For now, here are the "after" pictures. Enjoy
Thanks for reading,
Mark
