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Old 5th Dec 2021, 1:56 am   #41
Radio1950
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

Gday HB

I don't have a Taylor 65B.
I am only an innocent antipodean bystander.

In my post #33, I was attempting to test the emission capability of rectifier V1 with Volts across R1 to ground.
And the CRO test would show up other rectification issues.
All the above assumed full wave operation as per circuit.

In your last post you have measured the total current drawn by V2 and V3 in normal service, which is different.


So.... aware of your posts to date, and the circuit variations which appear to be bubbling up, you are now in a situation where you may not know if your beast is ex factory, or modified later, to make the best of a faulty transformer?
I would think that it is unlikely that, the factory would install a full wave type transformer and only use half of it, even though later models may have used half wave.

We cannot see the beast, and you will have to tell us what you think has happened to your specific unit.

If you get 190 volts at V1 cathode, that should be enough, provided hum on the HT is not a problem.

If you are considering actually using this unit, as distinct from a nostalgic resto, then you may have to put up with it as it is.
You will have some hum, probably, mod distortion probably, and some free FM, probably.
But for rough tests on BC receivers, it may be just useable.


If it were me, and if I was going to use it, and one side of the power trans is crook, I would use a bridge rectifier across the good winding, and plonk in some extra filter capacitors.
This is to reduce hum, and a higher anode voltage may make V3 operate in a more linear fashion as modulated oscillator.

May have to raise R1 up to 7K5 or even 10K.

I still think that optimisation of the mod is possible without being too quixotic.

Does that modulated RF distortion reduce as you reduce the RF level?
I wonder if the grid circuit is loaded too much by the RF level control and ladder attenuator?
Taking the RF output from the oscillator grid circuit is perhaps not the best design idea.
But it works.

Good luck.
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Old 5th Dec 2021, 12:26 pm   #42
Neutrino
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

Quote:
Originally Posted by HowardB View Post
What's the ac voltage across the anodes on your one?

I measured 194v at V1 cathode and 75 volts across R1 (194 - 119) which gives 7.5 mA.
How does this compare to your set?
I measured:
  • 161/160V a.c. at the rectifier anodes.
  • 194/187V d.c. and 6/7 V a.c.at the rectifier cathode and one end of the 10 kΩ resistor between the electrolytic capacitors.
  • 137/117V d.c. and 0.4 V a.c. at the other end of the 10 kΩ resistor.
This is about 57/70 V across the 10 kΩ resistor which gives 5.7/7.0 mA. This increases to 63/73 V at the lowest frequency setting.

The first reading is switched to INT. MOD. The second reading is switched to EXT. MOD. C.W.

A photograph of the mains transformer secondary connections is below for you to compare. I think that the black wire is the screen and is connected to ground. One of the two pink wires is connected to ground and other goes to the anodes of the rectifier (My anodes are connected together). One of the top two wires with the red sleeve is connected to ground and the other is connected to the rectifier pin 7 for the heaters.

The electrolytic capacitors would be prime suspects if mains hum is a problem. However, you stated in post #1 that you have replaced the usual caps.

As can be seen in the photographs, I replaced the electrolytic capacitors and waxed paper capacitors except the one connected across the heater supply. Itís awkward to get at and at 6.3 V a little bit of leakage is not going to matter. I did not change the flat rectangular waxed mica capacitors. These are unlikely to be defective and any change would alter the calibration.

David
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Old 5th Dec 2021, 6:41 pm   #43
HowardB
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

Thanks 1950 - I took your advice and tried replacing V1 with a bridge rectifier.
The first photo shows the waveform at R1 with smoothing caps removed. Full-wave rectification.
213v and 133v each end of R1, so 8mA current.

The next photo shows the improved output;
top trace: 1 MHz modulated,
bottom trace: internal modulation.
The output is the best I've seen so far. Pretty happy with this.

I tried increasing the smoothing caps but didn't make a lot of difference to the modulated output.
The bridge is just tacked in at the moment.
I'll tidy it all up and probably leave it this way.
It should be fairly easy to remove the bridge and revert to the original if required.
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Old 6th Dec 2021, 12:17 pm   #44
HowardB
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

Hi Neutrino,

Thanks for taking those measurements. They are quite similar to mine. Not surprising as our sets seem to be wired in the same way.

Seeing that our sets are the same, it's probably less likely that they were rewired as a result of a fault and more likely that they actually left the factory that way.
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Old 22nd Dec 2021, 12:48 pm   #45
HowardB
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

Just to wrap this up, I'm adding details of the rectifier mod...

I replaced the 6X5 with a silicon bridge rectifier to get full-wave rectification.
The bridge is 1000v, 2A.
It has been mounted in an 8 pin socket - see photo.

Code:
Socket   Bridge
Pin1:      AC   
Pin2:      -
Pin3:      AC
Pin8:      +
The mod calls for just one wiring change:
One end of the mains transformer secondary was originally tied to ground.
This has now been moved to pin 1 of the valve base.

This modification gives 213v, 133v at the ends of R1.
So 80v across R1.
R1 measured 11.4K
Current through R1 = 7mA.

The mod can easily be reversed if required to put the set back to its original state:
Move transformer wire back to ground, unplug the 8 pin socket, insert the 6X6.

Howard
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