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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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26th Jul 2015, 4:48 pm | #21 |
Dekatron
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Re: Ekco A22 Advice
I am probably going to make a fool of myself (again) but if you connect a siggen to the input of an IFT (or other tuned transformer) and a 'scope to the output I would have expected a peak in output amplitude at resonance. After all, that's what the IFT does, doesn't it, transfer signals on the IF frequency while holding back all others.
So why would you see a dip on this test? |
26th Jul 2015, 7:34 pm | #22 |
Dekatron
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Re: Ekco A22 Advice
You're right of course Tony - you would indeed see a sharp peak at resonance - not a dip. That's what comes of typing a message on my I-pad balanced on my knee with one eye on the TV.
So for the avoidance of doubt, the sig gen coupled to the primary of the IFT with the 'scope coupled to the secondary will show peak amplitude on the scope at the resonant frequency. Hence, if the IFT is intended for 465 kHz, and the sig gen is set to that frequency, the cores can be adjusted to peak the IFT. In this case - with the 220 pF cap rather than 100pF as indicated on the maker's data, sweeping the sig gen across either side of 465 kHz will indicated if the IFT is more or less on frequency. Apologies for causing any confusion. (Not very useful information to anyone who has neither a 'scope nor a signal generator of course).
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26th Jul 2015, 7:48 pm | #23 |
Dekatron
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Re: Ekco A22 Advice
Ah, OK... So I've not gone totally senile yet...
Incidentally, I've seen a cheap alignment osciallator type of sig-gen (meaning one with marginally-useful calibrations (!)) which had a little magic eye (looked like an EM81) with the input from a front panel socket. You used that as an indicator on the stage you were aligning and I guess tuned for minimum separation of the pattern. Saved needing a 'scope. I would hope the reason you had one eye on the TV was that you were repairing same . Once when asked 'What do you watch on TV', I replied without thinking 'The final anode voltage'. |
27th Jul 2015, 8:30 am | #24 | |
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Re: Ekco A22 Advice
Quote:
I have tried testing both 465kHz and 110kHz IFT's using this method with the IFT's in and out of circuit. It doesn't work, at least it doesn't work for me. I found a YouTube video where someone else tried it and it didn't work for them either. The resonant frequency is way off what it should be. The 'scope probe loads up the IFT and detunes it. Acting on advice received in these forums I connected 10K resistors in series with both windings. It still didn't work. This test lead me a merry dance looking for non-existent faults. Eventually I reinstalled the IFT's in the set and tried conventional alignment with an AF output meter. This worked fine. Perhaps I was doing something wrong, but I don't know what it was.
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27th Jul 2015, 9:24 am | #25 |
Dekatron
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Re: Ekco A22 Advice
IF transformers usually have a reasonably high L/C ratio a small change in C can shift the resonant frequency a reasonable distance, eg: L = 1.17mh, C = 100pf, Fr = approx 465 Khz, add Ca and Cs say 15pf then Fr becomes approx 433 Khz, if I've got the maths right.
7 to 10pf valve anode capacitance (Ca) is not unusual, that plus 5pf stray capacitance (Cs) from the wiring/valve socket etc and it soon mounts up. The method normally employed is to get the stages up and running, fully mesh the tuning gang on MW and inject RF from the sig gen on the mixer grid then manually sweep the generator, from that you can usually tell if the IF's are in the ball park. As to the 220pf capacitors instead of the 100pf capacitors specified in the manual, it might be that the IF transformer specs were changed during production to give a wider bandwidth and thus better "fidelity"......maybe. Lawrence. |
27th Jul 2015, 12:14 pm | #26 |
Pentode
Join Date: Jul 2008
Location: Nottingham, UK.
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Re: Ekco A22 Advice
Thanks for all your thoughts. I think I'm just going to have to accept that it has been working perfectly well with the 220pF cap in place so I'm going to leave it there, along with the slightly melted 100pF cap on the other side. Fingers crossed it will still work when re-assembled...
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2nd Dec 2017, 8:30 am | #27 |
Diode
Join Date: Apr 2015
Location: Middelburg, Holland
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Ekco A22 questions.
Hello,
I see the use of different speakers, and some of them have blue section on dial and some have black section. Does anyone know anything about this? Greetings, Peter. |
3rd Dec 2017, 9:17 pm | #28 |
Dekatron
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Re: Ekco A22 Advice
This is an old thread, but I didn't see it first time around.
I wonder what finally happened as regards to the repair/restoration of this remarkably well preserved example of this set? My advice at the time would have been to do the absolute minimum to this set. Seeing as it seems that it was already capable of working to a reasonable standard with all its original parts, it would have been a case of studying the circuit diagram and identifying any critical capacitors whose failure would cause damage or unreliability and replace only those. I would have checked circuit voltages and replaced any very out of value resistors. I wouldn't have touched any of the wiring unless it was where wires were either twisted or bound together, or touching in some other way. You can't compare the problems and dangers with old rubber house mains wiring with the wiring in a radio set like this one. Remember, many sets are wired in places with bare, un-insulated wire, so there's nothing written in stone that says that the chassis wiring has to have insulation if it's fixed point to point. If you're worried about wires touching, then just move them apart. I think you need to be very experienced in radio repair/restoration before taking on a rare and valuable set like this one, particularly one in such remarkable and original condition, and particularly with regards to completely rewiring it, otherwise it can soon end in tears. |
3rd Dec 2017, 9:42 pm | #29 | |
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Re: Ekco A22 questions.
Quote:
https://www.vintage-radio.net/forum/...ad.php?t=47965 Hope that's of interest.
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17th Apr 2018, 5:33 pm | #30 | |
Pentode
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Re: Ekco A22 Advice
Quote:
Incidentally, most of the wiring was in a terrible state, with the rubber resembling something like plaster of Paris - completely missing in some areas. In retrospect, firing it up even on the lamp limiter seems rash - I'm lucky to have got away with it. |
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5th May 2018, 7:50 pm | #31 |
Hexode
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Location: Blackpool, Lancashire, UK.
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Re: Ekco A22 Advice
If you haven't already restored this lovely set. Can I just make a point about the 'EBL31' Valve.
When you remove this valve, be very careful when removing the 'Top Cap' connector and certainly do not do any twisting action to remove it.... These valves in particular, have a the habit of their Top Cap coming off with the connector. The heat from these valves seems to cause the Top Cap bonding cement to fail over time. The earlier type 'EBL 1' with its larger Top Cap didn't seem to suffer the same problems so often. P.S. They are not interchangeable, 'EBL 1' has a 'side contact' base and not an Octal base like the 'EBL31'.
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5th May 2018, 8:50 pm | #32 |
Dekatron
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Re: Ekco A22 Advice
All the EBL31 valves sold by UK valve dealers, though marked as EBL31s are converted EBL1s with an octal base fitted over the side-contact base and the 3/8" top cap left in place. Useless for A22s as the modified valves are much too tall to allow the back of the A22 to fit back on.
I have an A22 and my son has two, so I've converted several N.O.S EBL1s by cutting off the lower part of the side contact base, leaving the upper part in place so as not to disturb the metalising, then fitting an octal base of the correct height in place and wiring it up. I've also replaced the 3/8 EBL1 top cap with the correct EBL31 1/4" top cap. It's quite challenging, and calls for some care, but original EBL31s have been rare for decades,so really, it's often the only option. I've covered that in threads on the forum and in the BVWS forum.
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