Run down batteries in a Roberts R707..

[stevehertz]

I was listening to - not a fringe - but not a strong station on FM yesterday on my Roberts R707. I was a tad disappointed with the reception, although it's never great where I live (BBC Radio Stoke). As I'd never changed the batteries since I put new ones in the set when I got it, it got me to thinking that maybe they were low and causing a problem. Checking the 6V batteries they both read 5.6V off load. Today, on R2 the set sounds absolutely fine, so I put the slightly poor reception from BBC Radio Stoke yesterday down to the normal problems of it not being a particularly good station for me to receive and the generally 'wet' atmosphere. So, no problems with the set's performance as far as I'm concerned. But, back to those battery voltages, what (low) voltage have you found that the R707 will tolerate?

[Herald1360]

Batteries are generally regarded as flat when they're down to about 1V per cell, so I would expect any reasonable design to operate at least down to that level. Available output power will be down about 3dB but otherwise the set should be fine.

My Hackers are quite happy on 12V so your Roberts ought to be OK on 8V.

[Sideband]

The off-load voltage is irrelevant. It's the on-load voltage you should measure. My guess is that if they are only reading 5.6V off-load, the batteries are flat and they will be much less on load. You only need to switch the set on with the volume at zero to measure the battery voltage. Less than about 5V on-load and I suspect cross-over distortion, dependent on volume level will be quite high.

[Nicklyons2]

IMHO these sets were fairly heavy on batteries; post 3 is correct, it is on load you should measure and anything lower than 4.5V per battery with any sort of volume will be hopeless on these sets. As a point of interest, an old Ever Ready brochure told me that cells were regarded as fully exhausted and spent at 0.9V/cell (zinc-carbon tech). Some ancient transistor sets will play ball at this, but many later particularly VHF sets are very poor below 1.1V/cell; which is probably why many sets aren't particularly good with rechargeables (unless designed for them).

[Nuvistor]

It's the internal resistance of the battery that starts to rise as the battery runs down. Using a sensitive voltmeter will not show the correct voltage. Some meters have a battery test but I have only seen them for 9v and 1.5 volts, these present a larger load than the normal meter, they cannot perform as good a test as you get get by using the equipment the battery is used in though.
http://data.energizer.com/PDFs/BatteryIR.pdf
Frank

[broadgage]

As others have said, the best test is to measure the battery voltage on load, either in the set or by imposing a dummy load whilst testing.

If this is not convenient, then the off load voltage can give a very crude indication of battery condition. Even a small decline from the nominal voltage indicates a discharged battery.
I would regard less than 5.8 volts off load or less than 4 volts on load as being in likely need of replacement, for a 6 volt disposable battery.

[julie_m]

Yes, just probe the battery contacts while the radio is switched on and turned up to your usual listening volume. That will show you the exact voltage it is trying to deliver to the set under real-life conditions.

It's the on-load voltage that falls; the cells' internal resistance increases as the chemicals become used up, and there are fewer charged particles available to carry current.

[stevehertz]

Thanks everyone. On load they read 5.18 and 5.26V, so from people say, they're ready for replacement. Saying that, the radio is working 'fine' on a strong station at low/medium volume, so until I actually hear a truly noticeable decline in performance I may just run them down until that point is reached. Hell, my pocket money is only 50p per week! It is, after all, a portable transistor radio, not my hifi, albeit my favourite portable - and that includes all my Hackers. These 707s are the bees knees..

[Nuvistor]

One complaint we had when FM started to become more available on portable radios in the late 60's early 70's was that using FM ran the batteries down more quickly and was inefficient and expensive.
Bit like the complaints about DAB battery consumption today.
Frank

[paulsherwin]

We have been having some odd weather over the last few days and you may have been getting some co-channel interference.

In my experience, the FM sensitivity of Roberts sets of that era isn't outstanding, though usually perfectly adequate.

[stevehertz]

Good point Paul. Given that the 707 - in my opinion at least - is/was head and shoulders above most other Roberts' sets, would your poorish FM sensitivity comment still apply?

[paulsherwin]

I wouldn't say poor, just not especially good, though that is true of most domestic sets of that time. The R707 still uses a Ge IF strip, though the FM front end is Si. Typical sensitivity did improve in the 80s and 90s, with the switch to Si and then ICs.

R707s are well regarded because of their sound quality rather than their great sensitivity, but this is still adequate for a domestic set, even a top of the range model.

[stevehertz]

Gotcha.

[Sideband]

Ever thought of building a mains power supply for it? Lots of designs on the web using cheap regulators.

[stevehertz]

Not at all, no. I've got plenty of mains sets and hifi receivers dotted around the house. I don't actually mind buying batteries, money was not a driving force or motivation for the thread, but like I said, I'm not going to run out and buy new batteries until the set actually bleats about it. I 'rotate' my sets anyway, it'll probably be a Hacker Sovereign 2 I'll be listening to next week.

[alanworland]

Just been listening to Clare Teals Big Band and Jazz programme on Radio 2 on my 707, love it, sounds good on the set and out of interest I measured battery voltage at good volume, both 5.75v
Batteries been in the set for a couple of years with occasional use but holding up well I thought.

Alan

[Nickthedentist]

Don't forget the crude but very useful cell goodness test, which is to simply apply a multimeter set to the 10A range across it for literally a second and note the peak current supplied, which should be at least 5A for most alkaline cells in good condition. Compare with a new cell of a similar type to "calibrate" your meter.