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Old 30th Sep 2020, 11:28 pm   #41
Sinewave
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Default Re: Bush AC41

Yes with volume right up, hear the tap through the speaker as I tap a screwdriver on the chassis.
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Old 6th Oct 2020, 9:32 pm   #42
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Default Re: Bush AC41

So I've made some measurements.

V1, ECH42
  • Pin 2, 220v anode
  • Pin 3, 135v anode of triode section
  • Pin 5, 80v grid 2 grid 4

V2, EBF80
  • Pin 1, 90v
  • Pin 6, 230v

V3, EL41
  • Pin 2, 210v anode
  • Pin 5, 220v screen grid
  • Pin 7, 5.5v cathode

Just the odd pop through the speaker when prodding around at the meoment, no other sign of life as of yet....
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Old 7th Oct 2020, 6:29 am   #43
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Default Re: Bush AC41

Try playing something through the pickup (or volume control - same thing). Just ensure the radio is on the gram setting if it has one.
If you still hear nothing you've isolated the fault to the output phase.

I suppose you've checked the output transformer?

Gabriel
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Old 7th Oct 2020, 8:51 am   #44
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Default Re: Bush AC41

According to my circuit diagram, the gram input is via triode section of V1. So when you touch the gram input you should get a decent buzz, not just a “click”.
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Old 7th Oct 2020, 11:26 am   #45
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Default Re: Bush AC41

Oh yes, I've just seen the circuit diagram. I'd say try going in at the volume control then, as Jerry suggested. If that works but not the pickup, you've isolated the problem to v1 and a couple of affiliated capacitors.

If it doesn't, you'll be looking at a problem from v3 onwards. It will be a process of exclusion.

May be worth trying an external speaker too
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Old 7th Oct 2020, 12:34 pm   #46
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Default Re: Bush AC41

I'll take another look at that, but when I touched the gram input with the radio set to gram, I don't recall getting that familiar buzz/hum sound. I did get it when probing around for voltages somewhere else though. I'll take another look.
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Old 7th Oct 2020, 4:46 pm   #47
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Default Re: Bush AC41

I've been probing aroung a bit more today and I suspect there may be a proplem in the combined on/off volume switch. If I jiggle it about, it's almost bursting into a bit more life with some scratchy sounds from the speaker. I'll take the power direct to the transformer and see if I have another volume pot to eliminate this.
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Old 7th Oct 2020, 5:27 pm   #48
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Default Re: Bush AC41

Do you have any switch cleaner ?, if so try squirting some in the volume control, this may overcome the problem in the short term. It would also be advisable to clean the wavechange /gram selector switch, and to operate several times, they can tarnish over the years if not used and cause problems.
I note from the service data that you should have 3.3 volts on the cathode of V1, pin 7, when in the gram position.
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Old 7th Oct 2020, 6:18 pm   #49
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Default Re: Bush AC41

I did pop another volume control in and gave the wave/gram switch a shot of Super 10 and lots of clicks. Since the change of pot, it's more consistent with the hum/buzz that you'd expect from an input stage being prodded and pops when being tapped with a screwdriver. I get 2.7v on V1 pin 7.
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Old 7th Oct 2020, 6:26 pm   #50
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Default Re: Bush AC41

Sounds like you got some life there, have you got a sig gen, so that you could test the IF amp. Also can you scope to see if osc. Is running?
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Old 7th Oct 2020, 6:37 pm   #51
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Default Re: Bush AC41

That's something I can do, yes.
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Old 9th Oct 2020, 5:13 pm   #52
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Default Re: Bush AC41

I hope to get some lab time later, so will have a prod around again and see what I can find.
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Old 27th Oct 2020, 10:39 am   #53
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Default Re: Bush AC41

So, an update.

In brief, I made some checks, injecting 470khz with modulation tone into the set, via V2 pin 2 and then later via V1 pin 6. Tone was heard fine, though at this point it was quiet through V1.

Various voltage checks on all valves suggests things are ok.

Some general fettling, reseating valves, replacing wax caps, making checks, general poking around, some stations were coming through, though very weak.

I then reflowed all solder joints and tightened up a lose grounding point I found, then the radio appeared stronger with the tone through V1

Next I had the switch wafers apart and have given them a good clean with a fibreglass pen and some 600 grit paper, then cleaned out any debris and gave it a good squirt with some switch cleaner/lubricant.

Now the radio is a bit stronger, but the stations I can pick up sound like listening to a radio on SSB!

I'm reasonably happy that the local oscillator is working and the IFTs are working, so just wondering what I could look at next.
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Old 27th Oct 2020, 10:45 pm   #54
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Default Re: Bush AC41

Clearly the AF amplifier is working after a fashion but to resolve whether the distorted sound and low output is an AF, IF, or RF problem should be the next move. When you applied the tone modulated signal from the Sig gen did it sound clear? Ideally try inputting some music to the gram input from an audio source such as an iPod or phone. Try also measuring the AGC voltage from the EBF80 AGC diode at zero signal and strong signal. There should be a significant difference. It's rare but possible that one or other of the EBF80 diodes has failed causing low/ distorted sound or insufficient AGC. Other than that the set probably would benefit from a realignment. Cheers, Jerry
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Old 29th Oct 2020, 10:30 pm   #55
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Default Re: Bush AC41

Yes I suspect you're right. Probably the next thing I should be checking.
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Old 16th Nov 2020, 9:18 pm   #56
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Default Re: Bush AC41

I'm looking at the alignment now, considering the work that's taken place so far.

I've managed to adjust L13 and L14 for maximum output, but I'm finding that adjustment of L6 and L7 isn't making any difference. But the modulation tone is heard so the circuit must be working I presume?

Edit: Output needed attenuating when moving from V2 to V1.
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Last edited by Sinewave; 16th Nov 2020 at 9:33 pm.
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Old 16th Nov 2020, 10:22 pm   #57
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Default Re: Bush AC41

Must I build a dummy aerial to carry on with the rest?
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Old 16th Nov 2020, 11:42 pm   #58
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Default Re: Bush AC41

LW is fine, but MW, I can't get the mod tone for the 500m mark anywhere near it, it's audiable just after the 200m mark even though the generator is set to the correct frequency. I can follow it around as I adjust the L9 and L4, but it soon drops off. Just wonder if there are any suggestions at this stage.

Before doing the RF section, aligning the IF made a huge difference.
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Old 17th Nov 2020, 11:00 am   #59
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Default Re: Bush AC41

Quote:
Originally Posted by Sinewave View Post
Must I build a dummy aerial to carry on with the rest?
On some older signal generators such as the Avo All-Wave a dummy aerial was provided for the purpose of improved impedance matching to receiver circuits. Two are provided on the All-Wave; one for medium frequencies and one for high. IMHO you don't need to use one for domestic receiver alignment even though Trader Sheet #1186 for the Bush AC41 calls for its use for RF alignment. It would do no harm to include an isolating capacitor (0.1uF or lower) between the "live" probe of the sig gen and the receiver aerial socket. The most important thing is you must turn down the sig gen attenuator output to the lowest level at which a tone is audible in order to defeat receiver AGC action, which will otherwise mask changes to output level as you try to adjust alignment. If L6 and L7 still refuse to peak then I would be suspicious of C4 and C7, although these are usually silver mica types that rarely drift or fail. For the MW RF alignment problem it sounds like the MW padder capacitor C10 might have drifted so check that and try again. Cheers, Jerry
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Old 17th Nov 2020, 2:34 pm   #60
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Default Re: Bush AC41

If the receiver's manufacturer calls for the signal generator signal to be via a dummy antenna then it's done for a reason, it's so the impedance as presented is the same as what was presented in the factory when the receiver was aligned.

Sometimes peaking without a dummy antenna straight from the signal generator will be ok and sometimes peaking to the antenna at the installation will be ok but then again sometimes it won't, it all depends on the coupling method and the coupling factor used in the receiver between the antenna input and the tuned circuit circuit.

I personally would align via a dummy antenna, it's not as if the skill needed to make one is high or the cost to make one is high.

Lawrence.
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