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Old 11th Feb 2009, 6:28 pm   #1
Pete_kaye
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Default Jonell radio problem.

I am working again on a Jonell radio which is very similar to a 1952 Ace A50/1 ie same valves but slightly different layout.The original thread has gone.
There is only one problem ,weak on LW but good on Mw and SW.It has no internal aerial and a shortish 8' aerial will bring in lots of stations but only a squeak on Lw with 2 strong MW stations around the 900m mark just audible.If I connect to an attic aerial of 40'+,then Lw comes through quite well(R4 ,RTI daytime +2 French) and the others very well.What is this indicating?
I have measured the voltages on the first 3 valve top caps and this altered the tuning slightly and dropped the volume on V1 and V2 only;the readings were -.4V (V1/2) on Mw and -2V on V1 and-0.4 on V2 (LW).On V3 it was always zero.
Touching with my finger just gave a buzz on all three.Can someone explain these results and what might be causing the poor LW.
It is all original underneath.I have located 3 banks of trimmers (6 in all) and 6 associated coils but not sure which is which.Minor adjustment to 2 trimmers will move the Lw stations around but not improve volume.
Am I right to think that a short aerial should have been enough originally?
ps I have connected chassis to mains Earth(AC onlyset)
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Old 11th Feb 2009, 6:51 pm   #2
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Default Re: Top caps ;can you explain them?

The LW problem is around V1. It is either the oscillator side or the aerial input side. Probably caps.

Going off the Ace circuit, on the Aerial side, change all the little caps. C1, C2, C3 and C4. On the oscillator side, C8 and C11. Clean the wavechange switch.

It may need a bigger antenna.

Try changing the AGC caps, especially C13. And if you haven't done already, C24.

Cheers,

Steve P,
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Old 11th Feb 2009, 7:13 pm   #3
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Default Re: Top caps ;can you explain them?

Hello
Not knowing what valve types you have I will explain to my best what you have.
Valve types 6K8, 6K7, 6Q7, ECH35, EF37, EBC33 ETC all have the Control Grid (G1) connected to the top caps touching them with your finger will induce audio pick up at the mains frequency so this is the hum you hear. On the first two valves if you check voltages on the top caps you are introducing stray capacitance from your meter & leads so effectively altering the tuning of the aerial coil and IF depending on the valve.
Some older valves and TV line output valves the anode is connected to the top cap.
Cheers.
Trevor
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Old 11th Feb 2009, 8:35 pm   #4
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Default Re: Top caps ;can you explain them?

If trimmer capacitors and coil slugs move the positions of stations on the dial then this indicates that they are associated with the local oscillator, which must be running. Another set of caps and coils are associated with the aerial circuit and should peak the signal. Adjusting in this manner on say Radio 4 on longwave should give an idea as to what sort of output is available, but isn't a substitute for proper alignment. A longer outdoor aerial will always improve matters.

Don't overlook the fact that some radios add additional inductance and capacitance to the MW tuned circuits to give LW, so these should be adjusted first.

Six trimmers and six coils on a three band set indicates a possible RF amp stage. Is the tuning capacitor a three gang type?

Break through of MW signals on LW at the correct place on the dial generally indicates that the local oscillator hasn't changed freqency due to dirty switch contacts or open circuit coils. That doesn't seem to be the case here though,
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Old 12th Feb 2009, 12:49 pm   #5
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Default Re: Top caps ;can you explain them?

Looking at the circuit for the ACE set I see there is a series coil capacitor across the aerial input which should be tuned to the IF frequency to stop breakthrough of any unwanted signals around the IF frequency 465 kc/s. Possibly this is out of adjustment and could be attenuating LW signals. Otherwise I would measure the LW coils to make sure they have not gone open circuit.
Other comments I would agree with though I would doubt that C4 is faulty as they usually used mica caps. C2 might be the culprit as it is only in circuit on LW.
Pat G3IKR
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Old 12th Feb 2009, 5:38 pm   #6
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Default Re: Top caps ;can you explain them?

Thanks for the helpful replies.I have attached a photo of the 6 coils and trimmers.I have identified the LW coils as the 2 on the left upper /lower,Mw in the middle ...Some fine tuning did not improve Lw but bought in some weak stations on MW(eg603Khz R4 from Eire).I still need the extra long aerial ;am I right in thinking this should not be necessary
I visited my locla library to see the Ace circuit which is Not identical but has same valve line up and chassis .However coils are different.Also mine does not have the IF aerial stopper as mentioned earlier in this thread.
ps the tuner is 2 gang.
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Old 12th Feb 2009, 6:39 pm   #7
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Default Re: Top caps ;can you explain them?

Have you got another Mixer/Oscillator valve you could try. It could be losing it at LW.

Lots of such circuits have caps on the mixer and oscillator side linked to the valve. If it has these, check them. Remember that the impedance of a capacitor is higher at lower frequencies. So any cap 'in the way' so to speak could cause this.

Cheers,

Steve P.
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Old 12th Feb 2009, 7:06 pm   #8
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Default Re: Top caps ;can you explain them?

I dont think it is the valve as it works OK on SW and MW. Still think problem is in the RF circuits on LW. The pic does`nt help much. Can you check that the local oscillator is working on LW? Either by listening for it on another Receiver (frequency will be 465 kcs higher than the signal or dial reading so on 198 kc/s it should be around 663 kc/s) or by checking the grid current by putting a 1 ma meter in the bottom end of the grid leak. No grid current no oscillation. (Alternative would be to measure the anode current and see if it changes when you short out the oscillator coil).
You certainly should not need much of an aerial to receive a reasonable signal on LW. Most sets using those sorts of valves will receive several stations on LW with only a few feet of wire. Indee I would expect it to be acceptable without any aerial.
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Old 16th Feb 2009, 5:28 pm   #9
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Default Re: Top caps ;can you explain them?

I have had the V1 6K8G valve tested ;it was fine.I also looked inside the oscillator coil in case there was a broken slug inside but there wasn't.I had an idea that there might have been an aerial inside the back originally so if anyone has a Mayfair or Ace 50/51 could they let me know.
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Old 16th Feb 2009, 5:38 pm   #10
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Default Re: Top caps ;can you explain them?

I would caution about checking a radio's performance by touching top caps. Some older valves have anode top caps!
John.
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Old 16th Feb 2009, 8:59 pm   #11
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Default Re: Top caps ;can you explain them?

Yes.. the 807 output valve comes to mind. You will usually find 400-700v on the top cap. Not a nice surprise.
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Old 16th Feb 2009, 9:55 pm   #12
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Default Re: Top caps ;can you explain them?

You also don't want to find the top cap of a telly line output valve. Especially a PL509. That will jump out and bite hard if you get too close.
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Old 17th Feb 2009, 7:00 am   #13
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Default Re: Top caps ;can you explain them?

Hence my 'name'. I can still hear the 50 year echo of an older colleague as I checked a radio with my finger as a signal generator. "That's alright...until you hit a top anode."
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Old 17th Feb 2009, 5:05 pm   #14
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Default Re: Top caps ;can you explain them?

Back to the topic of my Jonell radio then.I have tried the test suggested by Oldtimer and got confusing resultsWith Jonell set to 198 on the dial and a long aerial,I did not find a oscillation around 663Khz on my Superdrug Dm 906 with frequency display.but I did get 2 points of oscillation at 673 when jonell set turned to 1400 and 1590m(~212 and 188khz).Does this indicate that If alignment is out? I have not touched the IF cans as don't have a sig. gen.
I repeated test with Jonell on 909 on MW and a similar effect.Only oscillate at 2 points slightly away from tuned station.
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Old 17th Feb 2009, 5:35 pm   #15
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Default Re: Top caps ;can you explain them?

There's something wrong there.

673-212=461.
673-188=485.

Two different IFs.

Try tracking the local oscillator with your superdrug radio whilst tuning the Jonell across the LW band. You don't have to be actually receiving a station. You should see a constant difference between the tuning indicators of the two radios which will be the Jonell's IF.
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Old 17th Feb 2009, 6:41 pm   #16
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Default Re: Top caps ;can you explain them?

Valves have top caps for various reasons:

* Not enough pins on the base to accommodate the connections (eg D1 diode with 3-pin base)

* Top cap grids to minimise capacitance to other electrodes (eg 6K7 RF pentode)

* Top cap grids to keep remote from heater pins to minimise hum pick-up (eg EF37A low noise pentode)

* Top cap grid for maximum insulation (just glass) for really low leakage - some electrometer valves

* Top cap anode for high-voltage operation (eg PL504 TV line output valve)

* Top cap cathode, again for high voltage operation (eg PY800 TV booster diode)
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Old 17th Feb 2009, 9:16 pm   #17
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Default Re: Top caps ;can you explain them?

Quote:
Originally Posted by Station X View Post
There's something wrong there.

673-212=461.
673-188=485.

Two different IFs.

Try tracking the local oscillator with your superdrug radio whilst tuning the Jonell across the LW band. You don't have to be actually receiving a station. You should see a constant difference between the tuning indicators of the two radios which will be the Jonell's IF.
I wiil try this but if I am off a station then I won't know accurately what frequency the first set is tuned to.Reading from the dial is a bit rough and readyeven when the station alignment is good,like mine is.
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Old 23rd Feb 2009, 8:28 pm   #18
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Default Was 'Top Caps 'now Jonell radio problem.

Yes the tracking test seemed to work on LW but difficullt to say when the frequency is from the dial but the dead? point moved by roughly the same ammount (in Frequency terms).I now have to confess that I overlooked something in earlier statement.
1.The heaters all have exactly 5.3 Vac using a DMM not the listed 6.3 it says in all the books .Does this matter?
2.I have jammed the slug in the LW aerial coil well in after I realised that it did not really 'peak' just seemed to have a brighter area.(On LW only).The whole unit (6 coils)with wave change switch is not easy to get to.Is there a way to loosen it off and withdraw itand then turn it round to use the other slot?
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Old 23rd Feb 2009, 9:14 pm   #19
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Default Re: Top caps ;can you explain them?

Get a moving coil (or another meter) and measure again. It should be 6.3v. 5.3 is a bit low. Looking at the Ace circuit, the valve heaters are in parallel. Check the earthing and the voltage adjust on the Mains Input.

Try taking the valves out and measuring the voltage. One may be taking too much perhaps?

Leave the LW slug for now, but clean the contacts on the Wavechange switch.

Cheers,

Steve P.
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Old 26th Feb 2009, 5:28 pm   #20
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Default Re: Jonell radio problem.

I have retested heaters with a new battery in DMM and got 5.7V ac this time .With the V3 out I still got 5.7 and the bulbs out.. same.I will retest with ALL valves out soon.
I decided to backtrack and tested the output from the big condensor.Red (reservoir)258 V DC (16MF),Yellow234(16MF),Green170(8MF).Do these seem right?
Next ,I tested the anode voltages .Compared to the figures given in Trader Sheet 1054,they all were ~20 to 40 v down.So were the screen readings.Maybe the electrolytic is tired out and not giving enough volts .Would it still work well on MW with an aerial if this were so?
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