Mains Hum - Ekco A274

[cjwhoishe]

Hello all,
First time poster on this forum but I have been an avid follower of it for a while.
I have decided that when I retire (from the electronics industry) I intend to expand my small hobby of collecting old and interesting electronics equipment, and repair/refurbish what I have acquired.
The radio I have a problem with is one which was used daily by my mother (10 hours a day) back in its day but has been in store since 1985. It is an Ekco A274, a nice VHF only set.

The problem is that there is a significant amount of mains hum.
I have changed all the main suspect capacitors C31, C34, C35, C36, C37. (C36 & C37 did reform though)
'That' capacitor certainly needed changing.

What happens is that from immediate power on there is noticeable mains hum which increases as the valves warm up.
If I remove all of the valves and power up the unit there is still a hum from the speakers but not as loud as when initially turned on with the valves cold.
The unit was heavily used in its day and the mains transformer show signs of having got hot (see pics)
A couple of questions I have are:
1. Is it normal to have audible hum from the speakers of this model even before the valves have warmed up?
2. Can you get transformer faults that cause coupling between the mains transformer and the output transformer?

This is an initial cursory check of the unit to see if it is worth keeping and not relocate it to the spares pile. If the transformers are not the main fault I will follow up with repairing this unit, including changing the mains lead etc.

[paulsherwin]

The A274/A277 isn't a hummy set so you have a fault. The hum should be barely detectable with the volume at minimum and your ear against the speaker. There is minimal transformer to transformer coupling.

The first thing to do is work out how any current is flowing through the output transformer primary with the rectifier valve removed. Is there some sort of leakage around the valveholder?

[hsbhachu3d]

I don't know much about this set, but is your rectifier cap shot? I have ALWAYS put high value caps, much higher capacity than originals and they always fixed the hum issue. Usually a 500v 200uF or so usually does the trick. I'd check that first.

[cjwhoishe]

Thanks Paul,
I did not think they were hummy, but this one definitely is.
The key thing to me is that there is mains hum with the rectifier missing.
I have now isolated the primary and with all the valves missing there is still mains hum that I can hear from 4ft away (in a quiet area).
Hence my question about transformer mutual coupling.
(I know its from the speaker because if I unscrew the speaker isolator it also goes)
What do you think would be a suitable resistor to put across the primary to simulate the impedance of the valve/PSU etc.?

I have realised I did not post the worst picture of the mains transformer, this is attached and looks to me like it has definitely got hot and the insulation has melted/burnt.

[julie_m]

Originally Posted by hsbhachu3d

I don't know much about this set, but is your rectifier cap shot? I have ALWAYS put high value caps, much higher capacity than originals and they always fixed the hum issue. Usually a 500v 200uF or so usually does the trick. I'd check that first.

That's not always a good idea. Too much capacitance can impose stress on the rectifier valve; because the reservoir capacitor (the one nearest the rectifier) tries to draw a large current when a voltage is first applied to it; and the bigger the capacitor, the longer it takes to charge and hence the worse the wear and tear on the electrodes in the valve. If you go beyond 47µF with an EZ80, or 100µF with a UY85 (based on figures from National Valve Museum), you could be damaging the rectifier valve.

The smoothing capacitor (downstream of the resistor or choke) can be larger than the reservoir capacitor, but this will take longer to charge. And anyway, valve amplifiers are quite good at ignoring ripple and noise on the power supply rails -- they are not running so close to the rails as ICs.

If you want to experiment to see whether you can make a big improvement with a small number of microfarads, look to screen grid and cathode decoupling capacitors .....

[Station X]

If the output transformer primary winding is isolated, short circuit it and see if the hum disappears.

The valves are out, so you won't have the problem of the output valve's screen grid handling the anode current as well.

Is there a possibility the hum could be coming via the components wired from the output transformer secondary to the output valve's cathode?

[paulsherwin]

The symptoms described appear to be a classic case of transformer coupling, but it should affect all models if present, and it doesn't affect mine. Several forum members have A274s or A277s and none have mentioned problems with transformer coupling.

The mains transformer certainly looks a bit cooked, but that shouldn't matter if the windings haven't failed.

[jonnybear]

Have you looked at the EL84 valve base ? pin 3 is cathode pin 4 is heater if you have a leak between 3 & 4 this could be source of your problem, as there is a feedback route to the secondary side of output transformer, if you remove wires to pin 3 this will prove if that is where the hum is coming from. also check indicator lamps are intact.
John

[cjwhoishe]

Thanks all.
I thought I was loosing it not being able to work out how this hum was being introduced.
The DC voltage conditions when fully assembled are OK and the transformers run cool, which would indicate the transformer is not burnt out.
RF sensitivity is not the best, but that is another issue)

StationX
If I short the primary while it is isolated and the valves are all missing the hum from the speakers vanishes. I was going to attempt this by putting a load matching the valve impedance across the primary.
This clearly indicates that the primary is picking up the hum and coupling it to the secondary.

As I see it there are 2 options for how the hum is getting there without the circuit powered (rectifier missing):-
1. There is leakage (as pointed out) to the heater AC line somewhere.
2. There is magnetic coupling between the transformers. The 2 transformers are at 90 degrees to each other so the flux field from the mains transformer must have significantly changed to create the problem.

Leakage on the EL84 valve base between pins 3 & 4 could explain it, but if I remove the link between the junction of C35 and R23 and the output transformer secondary it is still there.
The transformer actually has more than 4 tags with the others being used as connection points for the tone control. I think I will disconnect these and see what happens.
At that point it is not far from being able to actually remove the output transformer and I can then try physically moving it around.

I was not sure if I had missed some bit of clever circuitry (around the -ve feedback via C35 and R23) to try and cancel out this hum when the unit was running.

[paulsherwin]

I agree that the best approach is to electrically isolate the transformer primary and see of the hum is still there. If it is, then as you say the flux pattern of the mains transformer must have changed because of heat stress. I've never encountered this before.

Changing the rest of the Mouldseals will probably sort out the sensitivity issue. There is one hidden away in the VHF tuner head which is easy to miss. These radios have sensitivity typical of domestic VHF valve radios of the era, not outstanding but perfectly adequate in most circumstances. The built in aerial is pretty useless though.

[cjwhoishe]

History of events today/this evening.

1. Completely isolated the primary of the transformer and also removed the wires to the tone circuitry, this did not get rid of the hum.
2. Measured the signal on the speaker with a scope, it was about 200mv P/P (which I equate to about 5mW into the speaker) but far from a sine wave.
3. Unbolted the mains transformer which has allowed it to lift and I've held it up with some spacers on longer bolts, like magic the hum went and it is perfectly acceptable (but the signal on the scope is still about 100mv P/P).

It is now all back together with only 5mm of spacers under the transformer and all is well.
I can't hear the hum on initial power up and I get the expected background noise level once the valves heat up.
Could the re-tightening of the mounting bolts have been the main influence?
From an electronics point of view all I need to do now is change the Hunts capacitors, as you say the one in the VHF module is well hidden and looks interesting/difficult to change.

I think that this has been a bit of a learning curve for me as maybe my expectations from the radio may have been too high.
This is the third valve based unit that I have repaired the first two being a pair of LEAK TL12 plus amplifiers (these are another story and I may create a thread in the vintage audio forum).
These are really quiet and I was somehow expecting this radio to be equally quiet.
I am also more familiar with transistor amplifiers and modern electronics designs.
I have been in the game for 40 years but transistors and DTL (diode transistor logic) were what I cut my teeth on, not a valve in sight.

[Nickthedentist]

Well done on your progress so far, what a strange problem!

As to the cap in the VHF front end, it's not very difficult at all, just be careful not to melt any nearby things with your iron!

See pics:
https://www.vintage-radio.net/forum/...25&postcount=2

Nick.

[paulsherwin]

These are good well designed radios but the expectations of 1950s domestic sets were very different from the modern hifi market. There will be much less top end than you are used to and there will be some hum, though it certainly shouldn't be obvious in normal use.

The A274/A277 makes a good first valve set - easy to work on and not complex, typical valve radio issues to sort out, and a very usable radio once working properly. The coverage can be tweaked above 100MHz to cover Classic FM. Even higher frequencies are possible but you risk losing R2 at the LF end if you shift the tuning range too much.

[cjwhoishe]

This radio was deliberately chosen for 2 reasons, I had one we used many years ago and it would be a good teaching aid for later taking on a Trough-Line 2 to go with the TL12 plus amplifiers that I have.

I don't think I was expecting anything like modern HiFi equipment. I was just expecting the noise level to be similar to the Leaks because they have EL84 output valves (albeit push-pull). I should have realised there would have been different design decisions made based on their relative costs.

I intend to change the hunts capacitors one at a time to see what impact each change has.
I feel I will get the most from it this way.

It is interesting that I have reduced the signal on the speaker by 50% (6dB) but now cannot hear anything.
Strange how your hearing is, or is the speaker not responding to this lower signal?

[Station X]

I wonder whether the original hum problem was down to bad earthing/grounding of the transformer core?

[Boater Sam]

Originally Posted by cjwhoishe

I intend to change the hunts capacitors one at a time to see what impact each change has.
I feel I will get the most from it this way.

How refreshing to have somebody do things the right way, wish they all did it like this.
More power to your iron!

[cjwhoishe]

Boater Sam
Thanks for the comment.
I've always found that you need to know how/why changing something fixes your issue. It helps because you can apply this knowledge to another completely different fault. Just changing everything doesn't help at all in knowing what was wrong.

Having said that I don't think I've fixed the poor sensitivity after changing all of the hunt capacitors, in fact I'm sure I haven't as my ramblings below will indicate.

I was progressively changing the caps/measuring them/powering up and seeing the effect from C27 backwards towards the front end and most had little effect, the AGC voltage across C29 was -1V if it was lucky. I then changed C20 Power decoupling for V3 and on power up it was perfect the AGC voltage was -20V and the magic eye showed the first sign of movement I had seen (75% full scale).

Looking at the circuit and position it made no sense that the effect was so dramatic, so I re-powered it up and it dropped back to being wrong. This though had shown me what the set was capable of.

Having changed all these capacitors I checked the tuning frequency using one of those FM re-transmitters you can use in cars. The dial was out by very little at both end of the scale and the magic eye showed about 50% when the unit was close to the set.

I still think there is something wrong and after looking at the circuit I have a couple of questions.
1. Is it normal to apply AGC to only 1 IF stage(the last one) could the first stage not get over-driven?
2. The input RF amplifier (V1A configured as grounded grid) does not look normal for an ECC85 application could this be my cause of problem in that the ECC85 is 'picky' about working in this mode?
I have ordered an ECC85 as it looks the most promising for a fix, put the radio together and am going to use it with the low sensitivity for a while to see if there are any other issues with it.
Back at work next week so available time will again be restricted.

[Herald1360]

ECC85 in grounded grid seems a reasonable way to go for low input z and voltage gain together with no stability issues.

In FM I don't think it matters if early stages get driven into clipping- its just called limiting which is useful to get rid of any AM on the carrier. It's only the frequency deviation that matters and that doesn't care about the waveform!

[cjwhoishe]

All,
The new ECC85 has arrived and been fitted. The result is now much better. I am probably now getting the best I will get out of this set.
The only bit left now is to tidy the case.
I have mixed views on this, to make it pristine and fully restored does not feel right to me. I think I prefer to see the wear and tear accumulated over the years, with it still being presentable.
I guess its similar to trends with classic cars, there is an upsurge of trying to keep them 'original' and not over restore them.
What is the general view here?

[Nickthedentist]

I agree. I try to preserve the original finish at all costs. It's amazing how smart (yet aged) these 1950s sets can look if you clean the finish with aerosol foaming cleanser to get rid of all the accumulated grime, touch-in scratches with coloured wax/felt-tips/woodstain then finish with normal household aerosol polish.