Roberts 727 mystery resistor and supply problem

[WaveyDipole]

I recently purchased this Roberts 727 from a member for use in the workshop. Cosmetically it is very clean with only slight wear and tear. It is also very nice sounding but it did have a problem with hum even with the volume turned all the way down. However it does work and I can get all of the stations that I like to listen to.

I had the chassis out of the case earlier this afternoon and having had a look at the DC power line with an oscilloscope, I was horrified to found 4v of ripple under load on a roughly 15v supply.

I replaced a couple of electrolytic smoothing capacitors C48 (C59 on the circuit diag) and C54, (one on the PSU board and the other on the main board) which took the ripple down to about half a volt and the hum seemed greatly reduced. There was still a faint bit of background hum, but realized it was not coming out of the speaker and when I picked up the PSU off the desk and it was gone. That problem is therefore solved.

I did find a rather odd thing though. On the PSU, there was a 1ohm (quarter watt?) resistor placed between one of the transformer secondary outputs and the junction between the AC leg and 2x ceramic caps of the rectifier. The thing is, that there was solid track running between those two points so the resistor was in parallel with the track and appears to have served no useful purpose. The solder was dull so it had been done some time ago, although it looks like it may have been added in by someone. Since with the resistor removed there is continuity between the two points and there are no resistors drawn on the PSU section of the circuit diagram I find this very strange....

I am also still concerned with anomalies with the voltages at various points.

For example, doesripple of 0.5v p2p still seems high? I know its an unregulated supply, so maybe this is normal? I replaced the two 470uF caps as well without any further effect, but I'm now wondering whether to put a 9v regulator in at the PSU board.

Which brings me on to the zenner, which appears to regulate power to the ICs. The PSU output is around 15v, which is much higher than the 9v indicated at pin 2 of the TBA1904 IC. No problem for the TBA1904 since it can run of a max of 20v. However, I expected 8.2v at the zenner, but when running off a 9v regulated DC supply (in place of batteries) I get much lower than that: 7.06v on AM and 5.70v on FM. This does rise to 8.16v on AM and 8.11v on FM when being run from mains. There is a 100ohm dropper resistor for the AM susbsystem IC which hopefully means that the supply is not exceeding the 8v maximum, but there is none for the FM subsystem which has a max operating voltage of 9v.

I am wondering whether to put a 9v regulator in at the PSU, although I'm also thinking that regulation is presumably the purpose of the zenner, so why such a voltage drop at the zenner when operating at 9v and such a variation when running off the unregulated mains? Does this sound like a faulty zenner?

[Chris55000]

Hi!

This almost certainly was a production-line mod, authorised by someone in the Roberts factory because the set you have was found to have a rough buzz and hum out of acceptable limits caused by the mains-transformer being wound out of tolerance, causing one rectifier-diode to receive a substantially greater a.c. input than the other, which would result in a distorted sawtooth-ripple across the reservoir cap (see my sketch), and a very obtrusive rough audible buzz/hum in the speaker!

Only slight snag was - the bod in the factory didn't complete this unofficial mod correctly, leaving a bit of the original tracking left to short out the voltage-equalising resistor!

You have one of two solutions:-

a) Use a drill-bit to enlarge the hole made in the PCB for this mod sufficiently to remove the unwanted bit of tracking or burn the bit of copper off with the tip of an iron (Carefully!), OR;

b) Replace the original mains-transformer with a better-quality component, which should give equal O/P voltages across the ends of it's secondary winding and remove the added resistor!

Regarding the voltage enigma you have, again this was a case of trying to make a silk-purse out of a sow's ear!

Roberts, having discovered (presumably early on in the design stage!) that the inexpensive mains PSU they'd worked out gave much more than the 9.5 - 10.5 V they'd expected on mains-operation, decided to (try!) and compensate for the high O/P voltage and ripple of the inexpensive PSU by interposing the 8.2V zener you mentioned in the '1904's supply pin!

The zener you've got is NOT faulty - it's just that on battery operation, there's not voltage there for it to do the job it was intended to do - the voltages you mention for battery operation are probably all you can expect when the IC is working correctly!

By all means use a 9V regulator on your mains-supply circuit - a 7805 TO-220 type regulator on a suitable heatsink with a 4.3V zener in it's common lead, cathode to the I.C. common pin, will do the job nicely - decouple the reg. with a 10u 16V tantalum in parallel aith a good-quality 10n disc ceramic to keep the reg. quiet!

Chris Williams

['LIVEWIRE?']

Could not a 7808 or 7809 8 or 9v regulator be used? I'm sure they are, or were made in 8 & 9v versions, so as long as the unregulated i/p voltage was high enough to allow the regulator to oprate properly, this would be a simpler solution than a 5v stabiliser with 4.3v diode.

[WaveyDipole]

Chris, thank you for you kind reply and insight. Its difficult to tell when that resistor was added but it could well have been a incorrectly implemented post production mod at the factory. It does have some gungy adhesive-type stuff on it which is also found in other parts of the radio.

Thank you also for your sketches and the regulator mod diagram. This radio does differ from your circuit sketch though as the secondary winding has no center tap. The TX output under load is just over 12vAC. The radio seems to draw about 30mA on AM and about 40mA on FM with low but comfortably to listen volume. At full blast on FM this rises to about 200mA. That the transformer is wound out of tolerance and providing a higher than expected output voltage is of course a very likely explanation. However although in the case of a center tap a low value resistor to balance out the ripple would make sense, but without a center tap, a 1ohm resistor would appear to make very little difference as a voltage dropper?

One other thing I noticed is that the part numbers on the PSU board are incorrect when compared to the circuit diagram, whereas the part numbers on the main PCB do match. It has just occurred to me - although it still does not explain that resistor - that maybe a PSU board from another model has been used, either in error at the factor, or replaced by someone else during the radio's lifetime?

Incidentally, I have now tested the zenner diode by lifting one leg off the PCB. It does indeed seem to be fine. I get about 8v across it with a 10v input at 10mA, rising to about 8.1v at 20mA.

[ms660]

Could it be a fusible?

Lawrence.

[WaveyDipole]

Lawrence, you might be onto something. The colour code is a little unusual, being brown-black-gold-gold-white - at least I think they are gold but they looks more like bronze which is not a valid colour. I didn't give it that much thought last night, but if my research today is correct then the first two bands would be the value - i.e. 10, the third band is the multiplier (10 to -1), hence 1ohm. The fourth gold band would indicate a tolerance of 5% and the white band indicates its a fusible type.

There is actually a marking on the circuit for a thermal fuse - TF 130C, but that's on the primary side, although there is no such device present on the PSU PCB. The tracks go directly from mains socket to transformer primary, although the device could be hidden in the socket assembly or under the transformer winding insulation as is sometimes the case. This particular resistor was on the secondary side and in that this configuration in the event would not have don't its job as a fuse!

It seems that the sensible thing to do might be to cut the track and re-instate the fusible resistor.

[WaveyDipole]

PS, I think I now understand the voltages I am getting, at least with a 9vDC input. The zenner and 220ohm dropper combine to drop and regulate the voltage such that voltage at the power input pins of the ICs is about correct. The LA1186N comes into play only when FM is selected at which point voltage at the input pin is around 5v which is correct. When switched to AM it drops to 0v. The voltage at the power input of the KA2247 is 5.7v on FM - which is about correct - but 7.06v when switched to AM, so, in fact, the power supply to neither chip exceeds the maximum operating voltage. The only remaining problem is that the input DC voltage when operating from mains supply is considerably higher and pushes the input voltage to one of the chips over the max of 8v. A simple 7809 regulator arrangement (or 7805 + 4.3v zenner as described by Chris) should prevent this. I am also curious Chris, why you recommend a 7805 + 4.3v zenner, rather than a 7809 with possibly a low value resistor at the ground pin to raise the voltage a little?

[Chris55000]

Hi!

7809 Regulators aren't as common or as easy to come by whereas even Maplin'd be able to supply a 7805 over the counter, either singly or as part of one of their daft "Kits" - the other reason is that these regulators have a minimum headroom (O/P to I/P voltage difference tn order to regulate efficiently) and I've found some makes can oscillate badly if the input voltage is near the minimum, about 11.8V to 12.5V for the 7809 type!

A 7805 with the usual type of 4.3V 500mW zener would give an o/p of about 9.2 to 9.3V, more or less exactly equivalent to a fresh set of batteries, which is what I had in mind as well in suggesting it!

It wasn't easily to make out all the colour-bands of the resistor in your pic, but I do know the type described by Member WaveyDipole - there were loads of 'em in Vestel & Beko telly chassis!

Could someone point me in the direction of the circuit diagram so I can study it and the relavent device data-sheets in order to assist the OP a bit more please?

Chris Williams

[Chris55000]

PS!

One other thing I noticed is that the part numbers on the PSU board are incorrect when compared to the circuit diagram, whereas the part numbers on the main PCB do match. It has just occurred to me - although it still does not explain that resistor - that maybe a PSU board from another model has been used, either in error at the factor, or replaced by someone else during the radio's lifetime?

Almost certainly the factory ran out of supplies of the original PCB that matched your diagram and had to substitute what they had available from another model, and I suspect further that supplies of the correct transformer for the design could well have been short, so this mod described and pictured in your first query had been hastily authorised in the factory to try and sort matters!

By the way it was virtually standard practice (for more years than I care to remember!) to use the full-wave two diode rectifier with a centre-tapped transformer for the mains psu in virtually all inexpensive pieces of mains/battery portable equipment of this type, hence my sketching such - mentally substitute what is present in my sketch with the actual rectifier you've got - it'll be half-wave or bridge if the two-diode circuit's not in use!

(It's barely possible that an agent (or more likely) one of these bulk minimum-wage warranty-return Repair Centres could have handled the set during the Warranty Period and had an alternative PCB supplied but I think this is very unlikely - almost certainly it was done in the factory as I described above!)

[WaveyDipole]

Chris, thanks for the further information. I agree that the resistor code was difficult to read in the photo, especially with that gunge on it.

I found a circuit diagram here in this thread:

https://www.vintage-radio.net/forum/....php?t=98690#3

The radio does indeed have a rectifier bridge rather than two diodes. I guess this model bucks the trend! In any case, I will simply adapt your regulator suggestion to the circuit.

Incidentally, I have also just noticed my mistake in the title of this thread. My radio is an R727, not R737. It would be appreciated if mods could please correct the title? Thanks.

[Chris55000]

AhI

That's why I couldn't find any info on either the "R737" or "TBA1904" - that was down to typos - the TDA1904 I had no trouble looking up!

Chris Williams

[Station X]

Thread title changed as this radio is actually a 727.

[WaveyDipole]

Chris,

I put your idea into practice and built a small 'regulator board' which I was able to solder into the two points where the power leads originally were connected and then soldered the power leads to the regulator board. I used a bit of vero bord so its not particularly pretty, but I just had enough space to fit it in. The heatsink I used was something I pulled out of a spares draw. I managed to get a single 4.3v zenner from Maplin but it seems to be OC and it didn't seem to do any voltage limitation when I tested it. Its a shame they didn't have two, as at 9p I would have gotten a spare. Since the output from the PSU is 14.5v, there was still plenty of overhead to get a reliable 9v output from a 7809 - also purchased from Maplin at the same time for the pricely sum of 20p. Instead of the zener I used a 100ohm resistor. This gave me 9.28v which is close enough although perhaps not as precise as using the zener. I set up the same capacitor arrangement as per your diagram.

I haven't re-assembled eveything into the case yet as I'm doing a small bit of work on it to tidy up the wooden ends. All the electronic bits are still on the bench and I'm using a spare speaker, but everything seems to work just fine and there appears to be little heat dissipating from the regulator. Hopefully the arrangement will contribute nicely to the longevity of the ICs.

Thanks for your contribution, and for the other comments as well.

[Chris55000]

Hi!

Very surprised your local Maplin had a 7809 and especially that cheap - even eBay hadn't got one that cheap when I looked!

The regulator you've built'll do nicely for powering your R727 and I'm pleased to have been of assistance!

Chris Williams

[WaveyDipole]

The radio has now been re-assembled and had half an hour of so of 'soak testing'. What I did notice is that on MW, when I put my hand near to the antenna (not extended), the signal faded considerably. I have to be careful where my hand is while tuning it. If I actually touched the antenna, it was considerably amplified. Not sure if this is normal?

The other minor issue was that the tuning drifted considerably during the first few minutes and I had to make at least a couple of adjustments to the tuning to get the station back, although it eventually seemed to stabilize.

It seems to pick up on FM quite well, even without the antenna extended.

I was also surprised about Maplin. Usually they are more expensive on components, although they do list them online for 29p.

[mhennessy]

I've just stumbled across this thread...

I'm not wishing to take away from the really neat job that you've done here, but I'm confused as to why you've ended up doing this.

Ignoring the mechanical buzz, the initial hum was caused by a faulty capacitor. Once the ripple was down to ~0.5V, that was "job done".

There's no reason to be concerned about the high voltage from the unregulated PSU. As you've established, the power amp IC is fine with it, and has good PSRR so won't hum. The zener (D5) is there to ensure the other chips are happy when the unit is running from mains, and is open-circuit when running on batteries. This is no problem as the ICs aren't terribly fussy about their operating voltage providing it's within a certain range - they were designed to work in battery operated sets after all.

If in doubt, just look to the brochure, where the power output is given as 1W on battery, and 2W on mains!

In this case, the additional regulator will do no harm beyond restricting the maximum output power to around a watt, which is only a 3dB difference anyway - so by all means stick with it. But it's not required...

The real mystery is that resistor.

I think Lawrence was right earlier when he suggested a fusible resistor. Hard to be sure without taking mine apart, but I think that it is in series with the secondary - or at least it would be if the assembler had remembered actually cut the PCB track

Actually, there's a chance that the hole was drilled by the PCB manufacturer as part of the normal production, and was intended to cut the track (but was a bit off for whatever reason). Re-tooling a PCB has upfront costs, but a minor modification to just the "drill file" might have been much more economical. Alternatively, perhaps it was manually drilled before the PCB was sent for "stuffing"? It's really hard to say without seeing it in the flesh, but it looks like there's flux in the hole, and it's too neat to have been done by hand after assembly IMHO. My R727 is a little hard to access at the moment, but I'll see if I can dig it out later.

But certainly, you can well imagine that being a useful modification. The thermal fuse shown on the schematic is buried within the mains transformer, and without that fusible resistor, a short on the secondary side would cause the mains transformer to run hot and open up the thermal fuse (which is non-resettable and non-replaceable - i.e., a new transformer is required). The fusible resistor might open up first. Obviously, when plugged in but switched off, the PSU is running, so the more protection the better

Again, really nice job and no criticism of that intended - just wanted to give some reassurance that the original designers did know what they were doing

[WaveyDipole]

My concern was that when running from the mains I was measuring over 8 volts on the DC input of an IC that is designed to run at a max of 8v. I appreciate that this is not too significant. There is probably a safety margin and evidently it has ran like this for some time so would likely have been OK, but the set is now a few years old and I was rather uneasy about that IC being pushed beyond its max.

It might still be possible to supply the TDA1904 separately to get the maximum output power on mains.

I haven't refitted the resistor, but in view of your comments perhaps I should.

[mhennessy]

Hi again,

I've just found my set.

Now I can see the PCB in the flesh, I can see that I was right about the resistor being in series with the secondary. However, I don't have the resistor, nor do I have the hole in the PCB. My set is 15076 - presumably yours is later?

The voltages are very similar to yours - on (but min volume) gives 14.4V (ripple is 350mV pk-pk), with 8.06V across the zener. The PSU voltage rises to 16V when switched off - lucky that 10uF capacitor is rated at 25V

All that is on FM.

There is no hum from the loudspeaker (but a fair bit of white noise), nor any mechanical noise from the transformer.

Moving to a bench PSU set to 9V DC, there's 5.37V, across the zener; rising to 7.3V on AM.

From the PSU's meter, I see the quiescent current is 19 and 28mA on AM and FM respectively.

Looking at the datasheet for the KA2247, I'm quite happy that 8.06V (or 8.2V in your case) is fine for that IC. Yes, the absolute maximum is 9V, but operation up to 8V is recommended. Datasheets are usually conservative about these ratings - after all, what could be in there to going to suddenly go wild at 9.00001V Similar comments apply to the LA1168.

If you were really worried, an easier (and invisible/sympathetic) approach would be to change the zener to 7.5V.

So, no need for owners of these sets to panic

Hope this helps,

Mark

[WaveyDipole]

Mark, Thank you for taking the time an trouble to report on your set and for the re-assurance. Mine is 21431 so it does appear to be somewhat later than yours.

[WaveyDipole]

Just for the record, I have re-instated the fusible resistor, having first cut the track in the appropriate place. While at it, I've realized that my 'mod' was incorrectly implemented so have removed it. The radio is now back to its factory condition, except that the intended fusible resistor mod is now correctly implemented. I believe this might be a good time to hide under a rock....