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Old 22nd Aug 2014, 5:47 pm   #1
sexton_mallard
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Default Ferguson 384U (Finesse)

I was fortunate to be given this radio along with some other items which will feature on this forum. After the usual safety checks and slow 'wake up' via a lamp limiter, I am rewarded with very good FM reception. The tuning is sharp and not 'lumpy' like the older pre-stereo era FM equipped radios in my collection.

The AM tuning gang and drive cord is not moving and all the AM bands appear to be dead - I suspect the switching which I will investigate when I pull the chassis from the cabinet. The mains electrolytic looks original and is dated 1959. Along with a few of the usual expected black Hunts Moldseals, I see a few small transparent plastic caps which appear to be transparent polystyrene? I take it these were appearing in the late 1950s as a search of a few under chassis shots of other 384U sets shows these same caps with the shiny metal foil visible inside. From experience, how reliable are these seemingly modern caps? I will check them as normal. Another question I have is has anyone successfully removed and replaced the plywood baffle from the cabinet as I may attempt to replace the speaker cloth which is damaged?
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Old 22nd Aug 2014, 9:16 pm   #2
Phil G4SPZ
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Default Re: Ferguson 384U (Finesse)

I have had occasional problems with polystyrene capacitors becoming intermittent, but they aren't usually considered to be candidates for replacement on sight.
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Old 23rd Aug 2014, 5:57 pm   #3
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Default Re: Ferguson 384U (Finesse)

Those polystyrene caps are usually found to be ok, but in a set with FM reception and no MW I'd suspect them to be leaky. Do you have a capacitance tester to hand?
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Old 19th Apr 2020, 9:49 pm   #4
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Default Re: Ferguson 384U (Finesse)

Thread reopened by request.
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Old 19th Apr 2020, 11:06 pm   #5
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Default Re: Ferguson 384U (Finesse)

This set jumped to the front of the roundtoit queue as my dad expressed interest in it for daily use. The chassis was duly removed from the cabinet and the AM drive cord was found to be snapped as the AM tuning gang was seized solid. This was freed off with some 3-in-1 as the lubricant (probably a grease, has set solid). The FM drive cord which was still intact were also freed off and works well but the FM tuner jams about 2/3rds of the way to the 100Mc/s end of the scale.

For initial tests a few of the Hunts caps were replaced so I could run the set without too much risk, the grid voltage on the UL84 dropped from 2-3v to under 100 milivolts. There is a pronounced 50 c/s hum but not severe enough to distort audio at normal volumes. FM reception is pretty good and once the piano switch contacts where cleaned with a bit of Servisol on cotton buds and other jiggery-pokery, the LW and MW came in pretty well. The bar aerial coils have broken loose and need sticking again to get best reception.

The set draws 100 milliamps on an Avo 8 across the pull out voltage setting socket. The smoothing/reservoir can feels warm to the touch but it is close to the UL84 and probably has been baked in its lifetime. The metal rectifier also runs very hot, the part of the chassis acting as a heatsink is piping hot directly behind it. I may bypass it with a silicon diode and wirewound resistor in due course. Voltages around the UL84 seem close to spec with 14v on the cathode, 180v on the anode.

The set runs for up to 6 seconds without mains power which suggest the reservoir cap is good but possibly the smoothing cap in the 3 way can may be suspect. The UL84 was tested in another radio and worked fine suggesting no major internal leaks to cause a hum. Now to carry on with tracking down that hum.
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Last edited by sexton_mallard; 19th Apr 2020 at 11:12 pm. Reason: Fix typos. UL84 incorrectly written as EL84 in places.
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Old 1st Jun 2020, 11:37 pm   #6
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Default Re: Ferguson 384U (Finesse)

Progress has been good with replacement of all the Hunts caps and a couple of resistors that were over 2x high in value. The dial string was reused as I did not have any suitable replacement. The break was knotted and sealed with a dot of super glue and does not cause a problem when this small lump goes around the pulleys. The last job on the chassis to do is investigate the smoothing/reservoir can. The set performs well on FM and MW/LW with plenty of volume.

The UL84 grid cap was replaced but the grid voltage slowly creeps up from a few millivolts to about 250 millivolts after about 10 minutes. I guess the valve must have a internal leak. Once the smoothing can has been sorted I will run the set for longer keeping an eye on the voltage. Is this fairly typical/acceptable for a UL84 which looks original? I don't have a substitute with me at the moment, the radio I borrowed a UL84 from is currently not accessible.
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Old 2nd Jun 2020, 11:06 am   #7
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Default Re: Ferguson 384U (Finesse)

The low anode volts but expected cathode volts on the UL84 plus the hot rectifier suggest that the rectifier is going high resistance. They could also suggest, since the UL84 IK is about right, that there is a problem with the valve and internal leakage. That could explain extra hum, too. Does the hum come up as the grid volts rise? Disappear if the grid is shorted to chassis?

Maybe the dying UL84 has hastened the demise of the rectifier? They seem to be failing in a sort of lethal embrace. At least the output transformer will have benefited from the rectifier failings!
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Old 3rd Jun 2020, 11:41 pm   #8
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Default Re: Ferguson 384U (Finesse)

Quote:
Originally Posted by Herald1360 View Post
The low anode volts but expected cathode volts on the UL84 plus the hot rectifier suggest that the rectifier is going high resistance. They could also suggest, since the UL84 IK is about right, that there is a problem with the valve and internal leakage. That could explain extra hum, too. Does the hum come up as the grid volts rise? Disappear if the grid is shorted to chassis?

Maybe the dying UL84 has hastened the demise of the rectifier? They seem to be failing in a sort of lethal embrace. At least the output transformer will have benefited from the rectifier failings!

Thanks for the suggestions. The hum is more or less continuous and does not vary as the grid voltage rises. Shorting the grid to ground kills the hum down to a very faint hum. Does this suggest the hum is being injected via the previous stage to the grid and that HT smoothing is OK and the valve is not a source of the hum?
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Last edited by sexton_mallard; 3rd Jun 2020 at 11:43 pm. Reason: make question more accurate
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Old 4th Jun 2020, 12:55 pm   #9
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Default Re: Ferguson 384U (Finesse)

The hum could be from the previous stage- try disconnecting the coupling capacitor. If it's still there, it's probably the UL84 itself or possibly its socket.
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Old 6th Jun 2020, 12:05 am   #10
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Default Re: Ferguson 384U (Finesse)

Armed with the knowledge that the hum could possibly be coming down the signal chain to the grid, I decided to start at the valve and work back. I snipped the grid resistor R27 (Trader 1383) by a tag strip. After switch on, no difference... the snipped lead ends of the resistor had sprung back and remade a contact. I then carefully pushed the gap open with a plastic rod...very loud hum. I opened and closed the gap about 2-3 times with loud popping from the speaker and the hum went...turned the volume up and a lovely clean FM station played hum free!

Could the loud pops caused an internal valve short to be cleared? Could I have 'bopped' the grid in effect?! I will resolder the resistor back or replace it as it has crept up from 4.7K to about 6K when measured. I will report back if the grid voltage creeps up again as before.

I think I will retire the contact cooled selenium rectifier with a silicon diode and resistor on a tag strip and get the HT back up to full spec with some wirewound resistor trial and error.
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Last edited by sexton_mallard; 6th Jun 2020 at 12:07 am. Reason: Typos and layout
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Old 6th Jun 2020, 12:18 am   #11
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Default Re: Ferguson 384U (Finesse)

Leaving the grid open circuit is not a good idea as the anode current will run wild. The fix is likely to be short lived as the cause is typically deposits on the inside of the glass. I have had some success using a gas hob spark igniter which is likely to clear a larger area of the conductive deposits, however, a replacement valve would be safer.
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Old 6th Jun 2020, 12:33 am   #12
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Default Re: Ferguson 384U (Finesse)

Quote:
Originally Posted by PJL View Post
Leaving the grid open circuit is not a good idea as the anode current will run wild. The fix is likely to be short lived as the cause is typically deposits on the inside of the glass. I have had some success using a gas hob spark igniter which is likely to clear a larger area of the conductive deposits, however, a replacement valve would be safer.
Thanks PJL. I will bear this in mind in the future. I will run the set and see if the hum returns and obtain a replacement UL84. Oddly I did not notice a prominent hum when I tested the UL84 in another set (a Ferranti U1032). Unfortunately that set is on the other side of the Irish sea at the moment so I can't swap valves again.

The valve does have that high hour baked look with a brownish tint and the getter darkened. Generally if the valve seem to work fine I disregard the appearance. I do remember the UL84 in the Ferranti looked newer and there was a slight drop in volume only when the valve from this set was tried in the Ferranti.
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Old 6th Jun 2020, 10:46 am   #13
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Default Re: Ferguson 384U (Finesse)

Hmmm.... I did suggest disconnecting the coupling capacitor not the grid resistor (leak or stopper). It does sound like a plausible diagnosis has resulted though

The 4k7 resistor on the grid will be the stopper and a drift in its value per se to 6k won't have much effect but it wouldn't hurt to replace it and the 470k grid leak. Use a metal film type or at least a good quality carbon film job. Dropping the grid leak to 330k would also help a bit with the tired UL84 at the expense of a slight volume loss.
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Old 7th Jun 2020, 2:21 am   #14
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Default Re: Ferguson 384U (Finesse)

I just repaired a radio with a similar arrangement not too long ago. It also had some hum at volume at 0. If I shorted the grid of the output valve (UCL82 in my case), the hum got almost away (but was still more than it should have been). The cause was the old 2x50uF mains reservoir that failed and also got leaky.

The story goes: the set had a bad 2x50uF mains reservoir (one section measured 0.1uF, the other 10uF), which I decided not to disconnect, but just parallel it with the two 47uF capacitors for the time of the repair. I noticed after a while that the 2x50uF capacitor got a bit warm after a while, but was hard to decide if it gets heated by the valve nearby or by pulling too much current.

For the test, I disconnected one of the bad sections (the one that measured less than 0.1uF) and the hum completely went away. My theory is that, either there was too much current being pulled through the output TX by the leaky cap, or there was some AC ripple that was coupled inside the 2x50uF can, since most likely one of the capacitors internally lost the ground connection.

I also did one more test: I pulled the rectifier valve out (to break the heater string), and used a solid state diode in its place. The set drew about 5W or 25mA at 220V and the mains filter got warm after 10-20 minutes. Anode voltage also only climbed to around 265V.
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Old 7th Jun 2020, 8:47 am   #15
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Default Re: Ferguson 384U (Finesse)

Having experienced the explosion of an old smoothing electrolytic and the consequent wasting of a lot of time cleaning up the resulting mess, I never leave the old component connected electrically. Leave it in situ by all means for appearance’s sake above chassis, but isolate the positive terminals below chassis. A short piece of tag strip two tags long can often be used to create a new isolated anchor point.
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Old 7th Jun 2020, 9:02 pm   #16
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Default Re: Ferguson 384U (Finesse)

It was more or less a test, I knew the capacitor would need to be removed. But it was interesting to see that an almost open capacitor (in terms of capacitance) would cause the radio to hum
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Old 7th Jun 2020, 10:19 pm   #17
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Default Re: Ferguson 384U (Finesse)

Measuring capacitance often doesn’t tell you much about the condition of an electrolytic capacitor, the other key values being leakage resistance (effectively a resistor in parallel with the capacitor) and ESR or effective series resistance. Leakage really needs to be measured at or near the working voltage, hence an old-fashioned Megger on its 250 volt range is a really useful bit of test gear. Measuring ESR is a bit more complicated, but cheap ESR meters are available. The point is that if a reservoir/smoother is anything less than perfect - a common situation in vintage sets - it will need replacing, now or in the future. Best to replace it now, and sleep nights!

One thing I’ve found is that leaky electrolytics often exhibit higher capacitance than the marked value.
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Old 8th Jun 2020, 10:41 pm   #18
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Default Re: Ferguson 384U (Finesse)

The unintended abuse of the UL84 has resulted to minimum hum but the grid is back up to +2 volts so the set will not be used until a replacement arrives. The stopper resistor was replaced and the grid leak resistor which has gone up to 550K will be replaced with a 330K as suggested. I confirmed the grid leak was in the valve by pulling out the valve and bridging the heater pin sockets with a few wirewound resistors paralleled to give about 90 ohms to complete the heater chain. The voltage on the grid socket was virtually nil to 20 millivolts.
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Old 8th Aug 2020, 11:11 am   #19
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Default Re: Ferguson 384U (Finesse)

A NOS UL84 turned up and the selenium rectifier was put out to pasture with a silicon diode and 30 Ohm 7w wirewound resistor to bring the HT on the anode of the UL84 to about 210-215v. The hum?

Hummmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm!

I had no choice but to isolate the mains electrolytic can, a small 3-in-1 Plessey can on top of the chassis next to the UL84.

I really did not want to dig this out as there is a cramped nest of wires and the output transformer underneath. Lack of space and room under the chassis to allow the wirewound resistor to radiate heat safely rules out placing the 3 new electrolytic caps beneath the chassis so a re-stuff is my only practical option. The Plessey can is slightly too small so I may restuff a taller can that came out of a Grundig set after checking for clearances in the case.

With the can out of the chassis disconnected, It appears the 32-40-40 can has shorted internally with a 40-32mfd internal short so to give the effect of a 72mfd. There was no short to the case/earth/ground. The can overall is not leaky as it held a charge well. (One test with a solid state rectifier was to flick the power on and off after 1 sec before the valves heated up and conducted, not very accurate but the can discharged slowly from about 280v over 10 15 minutes on all terminals). I know two were strapped with a resistor so the short would not reveal itself until the can was removed. My component tester showed 77mfd on two sections, and my multimeters showed a dead short across two of the the terminals.
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Last edited by sexton_mallard; 8th Aug 2020 at 11:24 am. Reason: Correct errors.
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