Roberts 'Revival' - reverse polarity burnourt sorted.

[David G4EBT]

Recently I posted a request for a circuit on the Roberts 'Revival'. It was a long shot, and didn't prove fruitful. However, in searching the forum I did come across a link to the actual fault, namely, the indvertent reversal of polarity destroying components, all for the lack of a penny diode at the factory to obviate this risk, on what is, after all a very expensive radio which still carries the Royal Warrant.

It was suggested in that earlier thread that the audio chip - a TDA7231, and an adjacent electroytic, had most likely been destroyed. I also noted a scorched resistor on which the colour bands had been burnt off, though it measured 100R. I dismantled another Roberts Revival and confirmed that 100R was the correct value.

I discovered to my horror that the TDA7231 - a very useful and capable audio chip - has been obsolete for some years, even though the Revival is still on the market so whether the circuit has been modified on current production, or whther Roberts still have a large inventory, I know not. Fact is, I couldn't find a UK supplier of this chip. However, I did manage to track one down from a supplier in the USA, ('Audio Lab of GA') at the moderst price of $2.65 USD, plus $5.44 for shipping - $8.09 total - about a fiver. It arrived in eight days, and luckily came in 'under the radar' so I didn't get caught with any outlandish customs charges.

While I was waiting for it to arrive, I removed the old IC and fitted an 8-pin DIL socket, replaced the 100R resistor, and a 100uF 16V cap which had a bulging top and melted plastic sleeve. (It's the larger of the two in the pic). When the IC arrived I plugged it in, and the set burst into life - on FM only, for some reason. I've no idea whether that was 'collateral damage' from the polarity reversal or a separate pre-existing fault - it's my son's radio and he has it permanently tuned to Radio 4 on VHF.

The first thing that struck me on opening up the set was how small the PCB is, with its 'minimalist' curcuitry and complete absence of any transistors or diodes. There were just two small unidentifiable components, about 1cm square and 3mm thick, which I assume were LSI integrated circuits, carrying out several functions - AM/FM tuned circuits, mixer, IF stages and detection. If you saw the board away from the radio, you'd be hard pressed to even guess that the PCB is from a radio - only the tuning cap gives the game away.

Although it has to be admitted that externally, the set is not unattractive and looks well made and finished, and indeed, sounds pleasing in use, internally it was a different story. I was shocked at the crudeness of the MDF cabinet and the SRBP volume control PCB, with its row of perforations where it was one of many, and had been snapped off a larger board, no doubt with many more. Functional, it's true, but certainly not pleasing to the eye.

I've attached a couple of pics - one of the part of the circuit board where the audio chip is, the other, showing the inside of the empty cabinet. Make of it what you will - this section is 'Success Stories' so I don't want to take the thread off-topic.

I have to say that it would be very easy to reverse the polarity when changing the battery if the radio is switched on at the time as it may well be with a run down battery. Just momentarily touching the battery with the clip reversed - easy to do, with dire results.

Hope the info might prove useful to anyone who comes across a duff cheapo Revival at a car boot sale or wherever. Chances are, that it's the same fault. Provided you can source a TDA7231, it could be a bargain! (Or a lemon?). Ride at your own rsk!

David

[TuningIndicator]

That speaker looks completely lost in that cabinet, sausages and the Blackwall tunnel spring to mind.
How does the sound quality compare to say, an R606?

[paulsherwin]

I have an RD-60 Revival DAB set which has a very similar cabinet. My speaker has a much larger magnet than David's but the speaker cone is the same size, and much smaller than the speaker in say an R200. There isn't a lot of bass there.

I don't really understand why manufacturers fit such small speakers when they don't need to, given the tiny cost saving and great reduction in fidelity. However, that subject is OT for this thread.

[mhennessy]

Good job, and you've reminded me that I have a Roberts R757 with exactly the same problem. In fact, from what I can see, it looks like the same chassis and loudspeaker... Cricklewood sell the TDA7231 for £1 each - sorry to tell you this after the event, but actually, buy the time you've added postage and VAT, it's much the same cost. But thanks for reminding me - I've just bought a handful

I generally add an "idiot diode" to any set I service with a DC connector. Cheap insurance.

Regarding the case, I do a fair bit of woodwork, and can tell you that while MDF machines well, it's very hygroscopic. I have no hard data to support this, but it seems to change size more readily and by a greater amount than the birch ply used by Roberts 50 years ago. Do you think they've sealed the MDF at all? If not, a liberal coating of something like Danish oil might be worth considering...

I'd be interested in a picture showing the whole PCB, if you have one - if nothing else, to see if it's the same as my R757. The FM IC should be a TA7358, an SIL chip (datasheet available in the Files section of the Roberts Yahoo group). But yes; staggering how easy it is to make a radio

Mark

[newlite4]

Here are some images from a set I worked on a few years ago, you may well be shocked by the simplicity of its mechanical construction.
Neil

[David G4EBT]

Thanks for the interest guys, and for the helpful feedback, especially about Cricklewood, who I do use from time to time and find excellent, but never thought to check with, so google doesn't always come up with the answers we're looking for!

As to the speaker, small though it is, I do have to admit that the sound is remarkably good, and the push button tone control is effective too - one position better for speech, the other for music. And in fairness, the outward appearance and finish of the cabinet looks good, and the dial is neat and uncluttered. The fact that it's been hugely popular, with customers in their thousands for several years shelling out £150 is testament to its success, and Roberts (or rather Glen Dimplex by then) astute design and marketing.

But frankly, I was appalled at the dismal quality of the interior and as a woodworker, woodturner, and woodturning competition judge I found it offensive to the eye. Maybe I expect too much, but the various pieces weren't accurately machined, as can be seen by the large gaps, and, were fixed together with puny staples. Sure, it does the job, but this is a firm who for decades, has held the Royal Warrant, and maximised the cachet and prestige that this brings for all it's worth.

What surprises me isn't so much that the cabinet is MDF (or as Roberts describe it HDF'), which is stable, and used extensively to good effect, but that the parts will have been cut on CNC machinery, so it doesn't cost any more to accurately machine the parts.

This section is 'Success Stories' and I don't want to take the thread off-topic and go down the track of 'whatever happened to Roberts?' This has already been well debated in the past on the forum, and anyone interested can find that debate at this link:

'Roberts - when did the rot start?'

https://www.vintage-radio.net/forum/...ad.php?t=15509

I've attached some pics as requested by Mark, which show the following:

*The chassis in place
*Top view of the chassis removed
*A close up (not a good pic) of the two SIL chips which presumably are the FM and AM sections of the front end and IF? (Apart from the audio IC, there are no other active components such as transistors or diodes).
*A close up of the cabinet interior - make of it what you will.

The two SIL chips each have nine pins and the following markings:

BA4424N
BA4236L

There is also a small (about 1CM square) three pin component which can be seen between the two coil cans with the pink core and yellow core, marked FFD468 TDK. Maybe an IF filter or whatever?

I haven't yet had chance to do a search on t'internet to see if I can find out more about those components. I didn't know that there's a Roberts Yahoo group Mark, so thanks for that info - I'll taken a gleg at that.

Hope that answers all queries, and that the pics are of interest.

David

[mhennessy]

Thanks for the pictures

Wonder why the PP9 is floating like that?

Yes, you've identified the AM ceramic filter, and the FM filter is the other side of the chip - light brown, 3 legs, should say 10.7 on it.

I've seen this chassis in at least 2 other sets - I guess they've recouped their R&D costs

Mark

[Paul_RK]

Originally Posted by mhennessy

Wonder why the PP9 is floating like that?

It's just standing up to have its photo taken Its working position is on the floor held in place by the retaining blocks on floor and side, while the block it appears to be resting on in David's photo maintains its distance from the front board of the cabinet.

As regards the loudspeaker, the larger R550 Revival, which sold at £130 to this one's £100, uses a much more substantial one, but to my ears replicates fairly thoroughly the somewhat thin, reedy sound of the R250. It fares a little better at high volume levels, but listening to either model at high volume isn't an experience I can readily imagine anyone choosing, so the R550's relative unpopularity and eventual demise seem reasonable enough. I doubt the chassis would sound very much better with an 8"x5" Goodmans...

Paul

[David G4EBT]

Originally Posted by mhennessy

Wonder why the PP9 is floating like that?

It wasn't levitation!

The set was wasn't stood up when I took the pic - it was lying face down on the window shelf in the daylight to get better lighting.

Incidentally, although I'm not bothered about the fact that this particular set doesn't work on AM, (either LW or MW), and I've no idea whether than was a pre-existing fault or caused by the polarity reversal, has anyone got any thoughts as to what may have caused that? EG, one of the SIL chips? (The audio stage works on AM, or at least there's background noise rather than total silence). I was just pleased that only the TDA7231, one resistor and one cap were damaged, and that the set has been returned to life on FM.

(If anyone wants a TDA7231 chip, Cricklewood have just six left at the time of writing).


David

[mhennessy]

Originally Posted by David G4EBT

(If anyone wants a TDA7231 chip, Cricklewood have just six left at the time of writing).

How can you tell - did you call them? I ordered more than that last night, so I hope they're able to fulfill the order

I've never seen a schematic, but the BA4424 is the FM front end, and the other is a combined IF amplifier for both AM and FM. So I would assume this combined IC is probably OK as normally ICs tend to fail completely (but note the use of weasel-words there ). Looking at the datasheet, it's possible there's a faulty electrolytic capacitor or perhaps the switches could need cleaning. Certainly, the datasheet should give you some ideas to investigate...

[David G4EBT]

Originally Posted by mhennessy

How can you tell - did you call them? I ordered more than that last night, so I hope they're able to fulfill the order

Yes - rather than order online, I rang them up to check that they definitely had stock, ordered six, and asked how many they had. He said 'you've taken half the remaining stock - there are six left'. I didn't want to be greedy and corner the market, but my son has three other Revivals and we have one, which my wife received as a present some years ago.

Thanks for the tips on the possible causes of lack of AM Mark - it sounds very much as though the SIL chips have escaped damage, given that they're multifunctional and the set works fine on FM. I'll check the ESR of the other el-caps, and give the wavechange switches a squirt to see if that helps. There's not a crackle or anything when the LW/MW switches are operated, but the quiet background hiss is present on the audio, and a poke with a 1kHz signal injector on the volume control indicates that the audio stage is working in the LW/MW positions.

It's a bit odd not being able to follow the signal path as one would with a conventional solid state radio. I must look up the IC datasheeet - that might give some clues.

My son had actually written the radio off - he said 'blue smoke had billowed out if it, so was it any good to me for bits before he chucked it away?". The smoke can only have come from the scorched resistor, which did look burnt to a cinder, but tested perfectly at 100R as it should be. Tough little blighters!

The TDA7231 looks to be a very handy audio IC - it's rather a pity that it's become obsolete, but I guess that more and more chips are becoming SMDs anyway, rather than DIL.

David

[David G4EBT]

I've found and downloaded the datasheets on the BA4236L and BA4424N, and if anyone is interested, they can be found at these link:

http://www.ic-on-line.cn/IOL/viewpdf/BA4236L_39789.htm

http://pdf1.alldatasheet.com/datashe...M/BA4424N.html

It's truly remarkable what functions these chips perform, which are:

BA4236L:

Combined AM/FM detector and IF (10.7MHz) amplifier. The FM component cosists of a diferential amplifier and double balanced quadrature detector. The AM component consists of an oscillator, a double balanced mixer, a IF amplifier, a deteector, and AGC.
I had thought that the chip has nine pins, but in fact it has 18 in two stagerred rows, known as a 'ZIP18 package.

The BA4424N:

FM/TV front end, featuring an RF amp, mixer, local oscillator, IF buffer amp, and AFC diode. (That chip is indeed in a 9-pin SIP package).

I suppose that with fast PC processors, huge hard drives, memory sticks, mobile phones with built in cameras, i-phones, Blackberries and the like, the prices of which have plumetted in inverse proportion to the rate of technological progress and performance, that we're becoming inured to each new development, taking it for granted.

What's remarkable to me is that the Roberts Revival has been around for more than a decade, so these chips are really quite old now, and no longer cutting edge. They must have been quite a breakthrough in their day though.

David

[davegsm82]

That 7231 chip looks quite nice, and if supplies become short I recon you could substitute an LM386 into its place with a bit of a rewire job.

General characteristics are similar, gain of 20 with open loop and similar voltage handling etc. I've used them before, they're available for pennies from RS too.

[paulsherwin]

these chips are really quite old now, and no longer cutting edge

Consumer grade radios started to move from discrete transistor designs around 1990 - quite a long time ago now.

[Hybrid tellies]

Veru interesting, my Revival 250 which I bought new some time back has been very reliable with excellant performance on FM. But the small speaker does let this set down especially when you compare it to the size of speaker used and the sound quality from the RIC1.

[michamoo]

I think this maybe of some help ? Sorry the extreme edges are missing.

[neon indicator]

FFD468 TDK is IF filter.

[ultrageoff]

Just to say that I found this thread whilst looking to fix a Roberts Radio R250 Revival that had been victim to a reverse polarity attack. I can't quite remember - because I actually broke the radio many years before I finally decided I wanted to get it working again - but I think the design boasts a fairly worrying combination of an eccentric choice of polarity for the DC power input and a lack of protection against reverse polarity voltages, even though just a touch of the battery terminals is also enough to blow the radio! Anyway, I did read online that replacing the TDA7231 was either a complete fix that restored the radio entirely, or a partial fix that would restore the FM operation (read here, above) - so I wasn't terribly surprised to find that replacing the TDA7231 fixed the FM but not the AM (or MW).

BUT. It turned out, for me at least, that all that was required was a burst of rapid button pushes to select FM, then MW, then AM, which cleaned the long-unused switches up and restored perfect operation. My advice, replace any obviously 'blown' capacitors and the TDA7231, then give it a go and press all the buttons very enthusiastically, several times. Worked for me.

[David G4EBT]

Thanks for spotting the post and responding to it.

That's an interesting observation - next time I'm at my son's home I'll give it a go, if only out of curiosity! I just assumed that the lack of AM was down to damage caused by the reverse polarity trauma. However, given that the set is permanently tuned to Radio 4 on FM and is never used on AM - or for that matter to any other VHF station, it could well be a switch problem due to lack of use.

I've just restored a wooden cased Roberts 'RM33' - unusual in that whereas most of those sets I've encountered have been comb-jointed mahogany, this one had mitred corners and was of very nice burr walnut. It too works only on FM so I stripped it down hoping to give the push button switches a blast of switch cleaner, but they're hermetically sealed and, oddly perhaps - are connected to the main PCB with one of those flexible plastic membranes with conductive strips, beloved of AVO and Sinclair, so I decided to leave well alone. I passed that on to my Son too, so I'll give that the 'rapid finger treatment' to see if I can wake up the AM functions.