Elusive hum on Stella-ST239U

[runradio762]

Hello,

I wondered if anyone could please offer advice on a problem I have been unable to solve.

I have a 1958 Stella ST239U Bakelite vintage valve radio. I have coaxed it back to life but not entirely successfully. Whilst I would expect some mains ‘hum’ the amount is excessive. Sound quality, selectivity and sensitivity are all excellent whilst the main voltages are as per cct diagram. Listening to music the hum is drowned out but listening to speech it is very noticeable between words or during silent periods.

This is what I have done so far.

Replaced all the electrolytic capacitors.
Replaced C66 (was short cct) and C51 (was out of tolerance).
From another old set replaced valves UCH81,UF89,UABC80,UL84 & UY85, I have not replaced the UF80 rf and fm osc valves.

Other than go through every remaining capacitor and resistor I am not sure what to do next? I would be very grateful for any pointers form experienced restorers.

Thanking you kindly,

Rob Wynne

[Techman]

C66 is across the mains input and is often short circuit - most sets run without it and if you do replace it, you should always use a special 'X' type for safety and NOT a standard capacitor.

C51 is just tone correction, but can go leaky and cause problems, or burn out that resistor.

The big question is whether the hum varies with volume control setting, in other words if you turn the volume right down to nothing does the hum remain at the same level? If it does, it's likely that C1 or C2 (or both) are lacking capacitance. If the hum goes with minimum volume, then it's something else causing the problem.

You should look at C49 before using the set anymore, as leakage in this capacitor (known as 'that' capacitor) can and will cause a lot of damage. I say 'look' rather than replace on sight, as some of these Stella sets use a reliable ceramic type in this position, so check that there's no significant positive voltage on the grid of the valve that it feeds via R26 - post a picture of it.

[yestertech]

As Techman says, the pertinent question is whether the hum is present at all volume levels or varies with the volume setting. If the former, then C1/2 would be suspect, but you've changed these.
If the latter then does it vary depending when tuning into/from a station ?

Some other points to consider depending on the above:

The UL84 is not the most reliable valve, but you've checked by substitution.
If the set is constructed with a PCB, then leakage around the UL84 valve holder can cause this.
There is hum cancellation circuitry around the output transformer - is the transformer original and is R1 present and the correct value ?
Is the mains connection the 'right way 'round' that is, is mains neutral definitely tied to chassis. Some sets are 'fussy' about this ! ( for safety, this is essential anyway )

Andy

[runradio762]

Dear Tecman and Andy,

Thank you both for taking the time to read and reply to my request, the hum is ever present and amplifies with volume increase and does not vary when tuning into and out of a station. I will have a look at your suggestions this evening.

Kind regards,

Rob

[Sideband]

Check voltages around the UL84, particularly around the control grid. If you are not sure about any of the readings obtained, post them here.

[runradio762]

Gentlemen,

Following your advice this evening I completed the following:
1. Checked mains polarity and it is correct.
2. R1 is within tolerance 1047 Ohms
3. C49 ‘that capacitor’ is an old black waxed ‘Philips’ type, it was reading 104nf and not 22nf as stated, I replaced it with metalized polyester equivalents.
4. R26 is within tolerance 1027 Ohms
5. V6 UL84 G1 P2 reads 30.5V G2 P9 reads 186V.

Changing C49 has resulted in slightly less hum, and I am wondering if I am being too fussy or unrealistic for a 63 year old set. The O/P transformer appears to be original, although it has been fed into and additional audio transformer for and external speaker (it came loose in transit causing havoc, I have unsoldered and remove it).
I made a short video of the radio with volume changes and tuning in and out of stations, in which the 'hum' can be heard, unfortunately it is mp4 and will not attach. If you are happy to forward me an email address or Whatsapp number I can send it.


Once again thank you for your time patience and advice.
Rob

[Herald1360]

No way should there be 30 odd volts on UL84 g1! Either the replacement C49 is leaky, the UL84 socket is tracking, or the UL84 itself is toast, killed by the old C49!

[Station X]

Disconnect the new C49 from UL84 pin 2 and check whether the voltage on pin 2 drops to zero.

If C49 is leaky you'll be able to measure a voltage between its disconnected end and chassis.

[yestertech]

A 'swap' of valves was mentioned in post 1 and they were from another set, so it's possible given the notoriety of UL84s that the replacement is as bad or worse than the original. If C49 checks OK then either both UL84s are toast or there is tracking going on around the valveholder. ( 30V is lot of tracking !! )

Andy

[PJL]

Did you put the original UL84 back as it would be a good idea to try the swap again as C49 was almost certainly leaky? Those black Philips caps are the same as wax capacitors and should always be replaced.

[telstar]

I was informed by radio collectors in Holland that the bitumen coated capacitors never lasted very long.
Cheers
Mike

[runradio762]

Thank you all for your comments and help. I tried yet another ‘that capacitor’ and voltage grid voltage now down to less than 5v dc. So, I decided to buy a new UL84, resulting in slightly less hum and better sound quality. I have not ‘cured the hum’ but have had lots of fun playing with the old set!