Marconi 559 hum

[PaulR]

Thanks all - just a quick reply from work this morning (don't tell anyone!).

PJL - I wondered about a valve fault but the ripple remained on the HT line after I had pulled it so that doesn't seem as hopeful.

Peter - the old field coil is sitting on the bench away from the main chassis so I don't think it can be being picked up from the mains transformer.

I will be very cheesed off if it turns out to be the mains transformer as I have done lots of work on the set and it has come up very well.

Paul

[Billy T]

Originally Posted by Herald1360

It sounds as though your problem may be caused by the lack of hum cancellation caused by the substitution of the loudspeaker effectively removing the old speaker field from the output valve audio circuit.

With reference to the circuit posted, the speaker field coil as originally configured would have induced unwanted hum into the speaker coil, which was then cancelled by L19, the hum-bucking coil. Because the speaker is now a PM type, the humbucking coil is surplus to requirements and since it has no hum to buck, depending on the circuit confiuration it could now induce hum. I have seen two variations on this theme.

In the first, the humbucking coil was a winding on the speaker coil itself, so replacement with a PM speaker caused no problems.

In the other, the coil was external to the speaker and was part of the field coil. This appears to be the case for your set, and if the old field coil has been retained, the bucking coil has no hum to buck, but may still be connected. I am, of course, referring to the circuit you posted, which I understand is not the correct one for your set, however the same basic principles apply so it is quite possible that this coil is the source of your hum problem.

If it is still in circuit, disconnect it and also remove the resistor R25 if it is present, then connect the speaker directly to the secondary of the sound output transformer.

The reason why it is 50Hz not 100Hz and is present immediately on power up would be because the source is direct from the transformer. In theory there is no reference to ground on that circuit to enable a voltage to develop, but there are a number of components on the secondary of the transformer that could provide coupling and there is also the possibility of interwinding leakage.

I could be barking up the wrong tree here, because I don't have the correct circuit, but the principles are clear: 50Hz hum with the rectifier out means magnetic, capacitive, or direct coupling of that 50Hz current into the speaker transformer secondary or primary.

Checking the primary is simple, just disconnect one end of the winding and if the hum goes away, look on that side of the transformer for the culprit(s).

However, if somebody posts the correct circuit and it is materially different, I might have to revise my opinion.

Cheers

Billy

[PaulR]

Thank you, Billy

It turned out that the circuit correct, except that the valve types are different.

The hum does in fact disappear from the speaker when the output valve is removed but returns as it heats up when replaced even with no HT present. The only explanation I could think of is that the few volts ac on the HT line are acting as HT and pulling some output from the valve. I wonder, therefore, if it is not due to the speaker/field coil setup.

I think it might be useful to list the situation so far.

* All paper and electrolytic capacitors replaced

* Set had been "got at" and connections to original electrolytic can cut. Can re-stuffed and wired up according to the Trader sheet.

* Original speaker replaced by PM one. Original field coil still in place to act as smoothing choke.

* Set works well except for 50Hz hum at speaker.

* 50 Hz smooth AC ripple on HT line measured at "far" end of smoothing resistor as 4v peak to peak on oscilloscope. Comes on before rectifier warms up.

* Ripple remains after rectifier pulled.

* Hum disappears from speaker if output valve removed but remains on HT line.

* Hum returns to speaker if output valve replaced but only when valve warms up even weith rectifier pulled and no HT.

I will check the mains transformer as suggested by Colin this evening othe jobs permitting.

Many thanks all

Paul

[PaulR]

If I disconnect the anodes it has no effect on the ripple, but when I diconnect the HT line from the transformer the ripple stops. Bad news?

Paul

[frsimen]

Paul

Is the chassis connected to mains earth? If not, leakage current flowing to earth through the 'scope leads may be the source of the ripple on the display. Either earth the chassis via the mains plug or, if the 'scope has an earth terminal, you could try connecting a short length of thick cable from that to the radio's chassis and see if the ripple has disappeared. Probably best to do the insulation checks already described first though.

If it is earthed, you may have an earth loop - try leaving the earth lead from the scope probe disconnected.

Paula

[PJL]

Originally Posted by PaulR

PJL - I wondered about a valve fault but the ripple remained on the HT line after I had pulled it so that doesn't seem as hopeful.Paul

We could easily be chasing the wrong thing here as we are operating it without HT.

I am still interested in the output valve. Ripple on the HT will not do anything unless it has a path through the transformer primary. The implication is the valve is conducting. Maybe it is acting as a diode although I find it hard to beleive it conducts with just 2V AC on the anode? Try measuring the DC volts on the anode when it hums.

[PaulR]

The chassis is earthed and the ripple is apparent on the oscilloscope without the probe earth connected.

I have just somehow managed to blow up my multiranging DMM by connecting across the HT on the output valve anode. The fuse didnt blow so it must have been a fault in the meter - ah well!! Anyway using my old analogue meter there is minus a volt or so DC on the output valve anode and 5v AC. The hum is definately there from the speaker only when the valve warms up with no rectifier and disappears altogether with the o/p valve pulled. I will try to take some photographs of the oscilloscope trace with and without the rectifier tomorrow as swmbo is getting restless now, particularly as I managed to trip the mains when my meter probe slipped and plunged the house into darkness!!

Paul

[PJL]

Well that answers that then. The valve is rectifying the AC.

When you take out the output valve do you still have the AC volts on the HT?

[Billy T]

Well, given that some car radio valves for 12 volt DC 'HT' are simply selected normal valves and work fine on the much lower HT voltage, conduction with 2 volts is not impossible. Just out of interest, I'd put a temporary 100 ohm cathode resistor on V5 and check what voltage is developed, or simply open the cathode connection and put a milliamp meter in series and see what current is flowing.

This is a real brain-teaser, and to solve it I think you will have to find where that AC on the HT line is coming from when the rectifier is out.. Is there any ac on the centertap of the HT winding for the rectifier anodes? Imbalance there might poke 5 volts AC up through C29 to the HT rail, in fact it could be more than 5 volts at that point. Technically there is no reference point to ground when the rectifier valve is out but maybe there could be leakage to the core.

You may not even be looking for a fault as such , it could be an historic transformer winding error that put the centertap a few turns off to one side and the radio may always have had that hum. Alternatively, it may not be a fault at all and might be a normal condition that was never in evidence when the set used an EM speaker, perhaps because of lower efficiency or simply, less sensitivity.

However, we should also remember that as the set was originally designed, the speaker would not have been working until the HT was established, and may also have had a very restricted low frequency response, so you are actually hearing something that might have even escaped the set's designers and manufacturing QC because it could not have manifested in the set's original configuration.

I think you have narrowed it down quite closely with the valve in/valve out scenario, so really you are looking for reasons why a few volts of AC appear on an HT rail that should be showing nothing at all, and is solidly bypassed to ground by the HT filter caps. It may be a condition, not a fault and if so, a filter cap from center-tap to ground may cure it.

I wish you luck!

Cheers

Billy

[PaulR]

Before I carried out Billy's suggestion I decided to have a poke around the mains transformer and the resistance between the connections to the rectifier heater is ok, but the centre tap to each end reads around 60K. Would the disconnection of the centre tap lead to the problems encountered?

PJL - Yes the AC remains when the output valve is pulled.

Paul

[PaulR]

Sorry, please ignore the last post - I must have still been asleep!!

The resistance between the ends of the HT winding is 600 ohms and the resistance from the centre tap to each end is around 300 ohms but, yes, the 4v AC is there on the centre tap. If I disconnect everything from the centre tap there is a 40v AC on it according the the oscilloscope

Paul

[PJL]

Paul,

4V on 1600ohms gives us a current of 2.5mA. This doesn't sound great but for all I know it might be 'normal' as it is not just leakage but capacitance too. You could try disconnecting the ht winding from the rectifier socket and measuring again.

[Herald1360]

At 'scope input impedance levels, even a bit of capacitive coupling between windings could give a few volts, but maybe there is a leakage problem in the transformer. I thought I asked the other day whether the 4V ac on the HT might be the rectifier heater voltage but I must have misposted as I can't see the mail anywhere.

Can you check with a megger or similar for any leakage resistance problems between transformer windings or the windings and frame? If you don't have a megger, have you got another radio with an isolated HT supply you could use for a spot of testing with about 10K and a milliameter in series?

I am, I must admit, rather confused about exactly how the radio is configured PSU wise. Is it still wired exactly as per the trader sheet circuit except that the EF speaker has been replaced by a PM one? If so, I would suggest disconnecting L19 altogether and feeding the speaker directly from the O/P TX. I would then increase the value of C26 to clean up the ripple on the bias supply to the O/P valve which will no longer be doing anything useful. Then see how it sounds.......

[PaulR]

Thank you, both.

I have a variable 400v power supply so I think that the next step might be to apply some voltage between various windings on the transformer to see whether there is any leakage. Before I do, should I be looking for no DC leakage at all and what maximum voltage should I apply?

The power supply is wired up exactly as per the Trader sheet. There were various non original electrolytics dotted about but the solder connections were so poor that most of them fell off when I moved them.

Paul

[PaulR]

I should have said, by the way that the AC remains when the HT leads fron the transformer are disconnected from the rectifier base.

[Herald1360]

I'd check between the rectifier heater winding and chassis with the winding disconnected from everything else using about 300V (don't forget the series resistor just in case!). Similarly for the HT secondary.

I still reckon, though, that the problem arises from unbalancing all the neat humbucking arrangements by the change of speaker.

[PJL]

I am inclined to agree with Herald1360. A bit of ripple is not normally a problem with beam tetrode outputs as they are essentially constant current devices.

As the 2.5mA is a small fraction of the sets consumption I would ignore it for now (though stick a 3A fuse in the plug).

Try sticking tacking a 0.1 or even higher between the output grid and earth.

[Billy T]

Originally Posted by Herald1360

I still reckon, though, that the problem arises from unbalancing all the neat humbucking arrangements by the change of speaker.

I agree in principle, except that the actual humbucking is very simple, being just L19 and R25 and there is no requirement for humbucking with a PM speaker. There is the additional complication of being able to hear a hum immediately upon switch-on, something that the designer would never have heard because the speaker would not have any worthwhile field until the rectifier warmed up and HT was established. For all we know they may also have had a response roll-off engineered in to minimise the audibility of residual 50 Hz hum with that circuit.

In my personal experience, having the choke in the negative side of the HT supply is a less common configuration, but having said that, I have a 1948 NZ built local design set (with PM speaker) that has just that configuration, and a similar bias resistor chain. This really threw me the first time I looked at it because I couldn't fathom why the filter choke did not have HT on it. Unfortunately we don't have the luxury of Trader Sheets and for many of our radios (apart from international brands) circuits are rare as well. This may be a case where reverting to an EM speaker may be helpful, or some judicious modifiications made to the circuit.

However, with the rectifier and output valve both out of circuit, C28 is the only (apparent) return path for any ripple on the HT line that may then be present on the primary of the SOPT, so if it hums with the sound output and rectifier valves out, disconnect one end of C28 and see what happens. It is pretty unlikely to be the culprit as it will be something like a 10n or 4n7.

If it still hums, it is time to look seriously at L19 because it would be a pretty safe bet that it is coupling 50Hz into the speaker, but also inducing voltage back into the soptx primary, thus modulating the HT line. In fact you could try just disconnecting L19/R25 from the circuit along with R25 as it is not needed with a PM speaker. I think it may possibly be the secondary of the soptx that is coupling 50Hz into the primary, thus superimposing it on the HT line, not the other way around.

Cheers

Billy

[Herald1360]

The humbucking isn't just L19, there's a fair amount on the bias supply too, which is why I suggest increasing C26 considerably as well as disconnecting L19.

[PaulR]

Right - first of all thank you all for the help and suggestions.

L19 appears to be with the field coil as previously suggested. I have disconnected it and shorted its terminals. Increasing C26 by .47 reduces the 50Hz rumble considerably. I tried 1uf there as well but it didnt do any more so I will permanently put a .47 across it. I think it is probably as good as we will get now.

I have some cosmetic work to do on the frame that holds the tuning dial etc then reassemble it all. I have taken before and after photographs and will write up that "Success Story".

Once again grateful thanks for everyone's help. I have learned a lot!

Paul