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Old 8th Jan 2018, 3:25 pm   #1
awc
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Default Regentone AHG3W Handygram Record Player - Hum problem

This 1955 player has no available circuit diagram as far as I can see and has been on this Forum before and where there are some photos of the chassis.

The problem is that although the player's hum level is barely negligible when the HT comes up, it progressively deteriorates over the next 5 - 10 minutes and eventually distortion sets in.

I have swapped out the three valves, UY41, UBC41 and UL41 to no avail and also changed the smoothing and reservoir caps as well as the coupling caps from the volume pot and the tone control.

I have put a meter on the grid of the UL41 and this shows that the voltage goes from negative to about 2 volts positive (when I switched off) relatively slowly at a steady pace with the hum level increasing at the same time.

The UL41 grid cap is a ceramic which should be .001 uf (reads just under this). The cathode resistor gets quite hot and has no bypass capacitor. The remaining resistors read somewhat on the high side but not massively so.

I guess something is changing value as it heats up, but what?

Any thoughts on where to look next appreciated.

Alex
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Old 8th Jan 2018, 3:43 pm   #2
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Default Re: Regentone AHG3W Handygram Record Player - Hum problem

Anything connected to pin 4 of the UL41 socket, Alex (thinking of DAC90As)?
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Old 8th Jan 2018, 4:09 pm   #3
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Default Re: Regentone AHG3W Handygram Record Player - Hum problem

I would have thought that the gridleak (680K?) on the UL41 could be O/C or the ceramic coupling cap (leaky?) to the UBC41 were likely suspects.
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Old 8th Jan 2018, 4:18 pm   #4
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Default Re: Regentone AHG3W Handygram Record Player - Hum problem

Pull the UL41, connect a 470 (ish) ohm 10 watt resistor across the UL41 valve holder heater sockets, then trouble shoot from there.

Lawrence.

Last edited by ms660; 8th Jan 2018 at 4:33 pm.
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Old 8th Jan 2018, 4:22 pm   #5
barrymagrec
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Default Re: Regentone AHG3W Handygram Record Player - Hum problem

45 volts at 100mA is 450 ohms...
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Old 8th Jan 2018, 4:24 pm   #6
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Default Re: Regentone AHG3W Handygram Record Player - Hum problem

Quote:
Originally Posted by barrymagrec View Post
45 volts at 100mA is 450 ohms...
Yes, sorry, forgot the zero, my apologies.

EDIT: Post#4 now corrected.

Lawrence.

Last edited by ms660; 8th Jan 2018 at 4:34 pm.
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Old 8th Jan 2018, 6:07 pm   #7
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Default Re: Regentone AHG3W Handygram Record Player - Hum problem

Many thanks for the prompt responses.

Nothing connected to pin 4 of the UL41, the gridleak reads about 980k so probably not the problem and have replaced the ceramic coupling cap, but all to no avail, just being mesmerised watching the voltage on the grid of UL41 rise inexorably as the minutes tick by.....

Lawrence, I am afraid my capabilities are on the practical side of things, so although I understand what you want me to do with the 470 ohm resistor, I haven't a clue what to do to "trouble shoot" as you put it.

Alex
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Old 8th Jan 2018, 6:15 pm   #8
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Default Re: Regentone AHG3W Handygram Record Player - Hum problem

It's going to be an internally leaking valve or a leaky valve base.
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Old 8th Jan 2018, 6:19 pm   #9
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Default Re: Regentone AHG3W Handygram Record Player - Hum problem

I'd be worried about a UL41 grid-leak that was more than 250K: the UL41 is renowned for suffering with unpredictable internal leakage effects and a higher-than-absolutely-necessary grid-leak can only exacerbate the effects of this leakage when [not if] it happens.

That you're seeing a +ve voltage on the UL41 grid at any time is bad news. I've fitted a 100K grid-leak [and 1Kv-rated coupling-capacitor] to my Eddystone 840A just to be on the safe side.

Old, leaky UL41s can be 'zapped' in various ways that can go some way to short-term dispelling the internally-conductive deposits that cause leakage, but the UL41 does seem to be a deeply-troublesome and unreliable valve. Even Philips/Mullard issued instructions as to how to substitute a UL84!
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Old 8th Jan 2018, 6:20 pm   #10
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Default Re: Regentone AHG3W Handygram Record Player - Hum problem

Quote:
Originally Posted by awc View Post
Nothing connected to pin 4 of the UL41, the gridleak reads about 980k so probably not the problem and have replaced the ceramic coupling cap, but all to no avail, just being mesmerised watching the voltage on the grid of UL41 rise inexorably as the minutes tick by...
If a 680 k resistor has gone to 980k it`s well shot, a carbon resistor that far out can wander about all over the place dependent on applied voltage, I`d replace it.
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Old 8th Jan 2018, 7:05 pm   #11
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Default Re: Regentone AHG3W Handygram Record Player - Hum problem

Quote:
Originally Posted by awc View Post
Lawrence, I am afraid my capabilities are on the practical side of things, so although I understand what you want me to do with the 470 ohm resistor, I haven't a clue what to do to "trouble shoot" as you put it.
Alex, what I suggested is practical, it would allow you to check for the presence of a +ve grid voltage with the valve removed and the power applied, that would make it easier to find the problem, especially if it's due to leakage from a voltage source via a component other than the grid coupling capacitor.

Lawrence.
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Old 9th Jan 2018, 11:12 am   #12
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Default Re: Regentone AHG3W Handygram Record Player - Hum problem

Thanks Lawrence for the explanation. OK, have changed the 680k grid leak resistor for one that actually read 680k, but no improvement. Changed the 220 ohm cathode resistor with same result. Changed UL41 for a new UL46, but ditto.

So, removed UL41 and wired 450ish ohm resistor across terminals 1 and 8 and read dc volts on grid capacitor valve tag. This showed a much lower reading of 5 mV but this did slowly increase and was about 8 mV by the time I switched off.

Does this point to the valveholder being the trouble? It does look a bit greasy, no doubt from the wax which has dripped on it from the mains transformer which is situated immediately above it.
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Old 9th Jan 2018, 11:41 am   #13
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Default Re: Regentone AHG3W Handygram Record Player - Hum problem

Make sure pin 4 isn't connected to anything.

Lawrence.
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Old 9th Jan 2018, 11:46 am   #14
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Default Re: Regentone AHG3W Handygram Record Player - Hum problem

See posts 2 and 7
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Old 9th Jan 2018, 12:04 pm   #15
awc
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Default Re: Regentone AHG3W Handygram Record Player - Hum problem

There is one other thing I would mention, insofar as I have connected an earth to the chassis and when doing some testing a few days ago, it tripped out a few times the portable 30mA RCD I always plug players undergoing work into, as getting to the consumer unit under the stairs is such a pain!

This stopped happening though for no apparent reason I could work out. The mains transformer is normally fixed separately from the chassis and is in fact sitting separately on the workbench at the moment.

Alex
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Old 9th Jan 2018, 12:43 pm   #16
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Default Re: Regentone AHG3W Handygram Record Player - Hum problem

Pin 3 of UL41 should also have nothing connected.
Is there an earth connection to the transformer?
Did the hum/rising g1 volts start before or after the 3 core earthed cable was fitted?
Monitor the cathode volts on the UL41 as it warms up, increasing would suggest that the valve or holder is defective BUT check the cathode resistor value both cold and hot before condemning either.
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Old 9th Jan 2018, 1:54 pm   #17
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Default Re: Regentone AHG3W Handygram Record Player - Hum problem

He's already replaced the cathode resistor.

With no components connected to the g1 pin of the valve base and no valve plugged in with heater line bypassed with 450 ohm resistor as before, there should be absolutely no volts whatsoever on that pin. Any voltage indicates a leaking/tracking valve base. Try giving it a thorough cleaning, but replacement may be the only reliable cure - it's a very common problem.
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Old 9th Jan 2018, 3:06 pm   #18
awc
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Default Re: Regentone AHG3W Handygram Record Player - Hum problem

I can confirm no connection to pin 3. The secondary is bottom connected to the chassis. I've tried disconnecting the earth lead from the chassis but all to no avail, so it does look as if as a last resort I'll have to change the valve holder.

I'll report back when I've done this with the outcome.

Thanks again for everyone's help.

Alex
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Old 9th Jan 2018, 3:14 pm   #19
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Default Re: Regentone AHG3W Handygram Record Player - Hum problem

So still a small voltage on the g1 pin with everything removed from it - yes?
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Old 9th Jan 2018, 3:29 pm   #20
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Default Re: Regentone AHG3W Handygram Record Player - Hum problem

If the answer to the above is yes and you've still got mV on the g1 pin 6 base connection even after a good cleaning, try giving it a good scratching with a sharp object all around the pin, particularly between this and the adjacent pin five which is where the leakage is likely to be coming from.
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