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Old 30th Nov 2021, 11:54 am   #21
Freya
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

Could you take a picture of the waveform from the right hand socket, would be interesting to see how they compare.
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Old 30th Nov 2021, 2:09 pm   #22
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

Audio Output from Taylor 65B
As you can see, it's a nice clean waveform.
The unmodulated RF is similarly clean.
It's just the mixture of the two that's off!
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Old 30th Nov 2021, 3:27 pm   #23
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

Quote:
Originally Posted by Radio Wrangler View Post
I agree with Stephen, that there must be something wrong with Howard's sig gen. There are reasons why that design isn't wonderful at the best of times, but his example is a lot worse than I'd expect.
The 500pF grid coupling capacitor will allow the 400Hz to settle back to 0V so I don't think you are wrong. The problem is the output does not appear to be AC coupled which would then show the modulation as symmetric. Has the output capacitor been replaced as only a little leakage would look like this on a 10M scope probe.

It would be useful to compare the modulated HT of the two SG's.
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Old 30th Nov 2021, 3:34 pm   #24
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

Think its hidden in the shielded output connector block, been quite some time since I was inside mine.
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Old 30th Nov 2021, 5:45 pm   #25
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

Yes, I replaced the 0.01uF output capacitor.
In fact, I replaced all the caps except for C5 (500pF) & C6 (2nF).

Quote:
It would be useful to compare the modulated HT of the two SG's.
Where exactly should I be looking - V1 Anode?
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Old 30th Nov 2021, 5:50 pm   #26
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

Quote:
Originally Posted by HowardB View Post
Instead of a symmetrical amplitude modulated wave, only the bottom half of the wave is modulated. The top half is flat.
The Windsor/Taylor 65c has a 1000 pF capacitor between the slider of the fine attenuator (VR2 in the Taylor 65b) and the top of the switched attenuator (R9 in the Taylor 65b). With the attenuator resistance, this capacitor acts as a high pass filter that makes the waveform appear more symmetrical by removing the 400 Hz component.

You could try connecting a resistor (say about 2 kΩ) between the RF Output and ground so that C7 acts as a high pass filter. This might make the waveform look more symmetrical but is unlikely to make any difference to a receiver under test because the circuits are tuned to attenuate frequencies other than the one to which it is being tuned.

The photographs show examples of the modulated waveforms on my Windsor 65c. There is a big variation depending on the frequency with the amplitude decreasing at higher frequencies.

The upper channel is the slider of the fine attenuator and the lower channel is the RF output with the attenuators at maximum. Both channels were set to 0.2 V/div. The last photo shows both inputs at GND so that the negative bias mentioned in post #12 can be judged in the other photographs.

David
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Last edited by Neutrino; 30th Nov 2021 at 5:59 pm. Reason: Typo
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Old 30th Nov 2021, 6:24 pm   #27
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

Can you not try a resistive load for the modulator (you may need to use a coupling cap to not start to short any HT) that way you should see the AF wave and see how it behaves loaded when compared to actually modulating the oscillator - at least that way you can confirm that the AF stage/modulator and the Modulation transformer are OK.
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Old 1st Dec 2021, 7:19 pm   #28
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

Thanks David (Neutrino) for the filter idea.
As suggested, I added the resistor at the output and the wave shape is greatly improved as shown below...
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Old 1st Dec 2021, 7:31 pm   #29
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

If you changed VR2 that should measure 2K but actually measures 6k8 it would also remedy the situation ?
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Old 1st Dec 2021, 9:59 pm   #30
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

Changing VR2 won't fix it but it should not be a problem in normal use. As already said, these are basic signal generators for radio alignment, a professional SG would have a known fixed output impedance but here it is all over the place.
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Old 2nd Dec 2021, 10:38 pm   #31
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

Just possibly, you made a slightly rash judgement on the emission capability of rectifier V1 6X5 in post #8?

Have you changed it with a known good valve?

Try temporarily plonking (technical term) two 1N4004, 7, etc, across the anode to cathode of V1; you can leave the valve in circuit.

Conditions apply.

Pls report back on this specific test.
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Old 2nd Dec 2021, 11:03 pm   #32
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

Thanks Radio1950 - I'll hold fire on the silicon diodes right now as I should have a new V1 6X5 to try by Saturday. Let's see if that changes anything. I also hope to get all the valves tested this weekend. I'll let you know how it goes.

Freya - VR2 was quite hard to reach so I may have misread the value at 6K. I'll double-check it tomorrow and report back.
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Old 3rd Dec 2021, 3:48 am   #33
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

Meanwhile, if you get bored, or if postie is late ...

Brutal option follows!

You could just -

. lift pos end of C4 C5
. probe cathode of V1 6X5 with CRO, using resistive divider if necessary
. DVM cathode V1 to earth
. warm up 2 mins
. observe CRO to see full wave rectified AC with symmetrical humps.

If not, it is probably low emission of one half of V1, or crook pwr transf or crook wiring joint.

. power down, restore C4 C5
. warm up 2 mins

. short junction of R1 and mod transformer to earth momentarily, with your choice of 100R or (insulated) screwdriver.

. measure volts V1 cathode to earth, including resistance of 100R or screwdriver.

. do maths to get mA DC from V1.
. the DVM will read approx average DC volts
. compare with V1 spec sheet.
. expect 35 mA or so

If low current, V1 is suspect or crook wiring.


By the way, can you (hand on heart) confirm the power transf pri tap is set for your local mains voltage, and that valves have a measured 6.3 V on heaters please?
.

Last edited by Radio1950; 3rd Dec 2021 at 3:49 am. Reason: I am in two minds as to whether to cut it all
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Old 3rd Dec 2021, 4:17 pm   #34
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

Hi 1950,

I disconnected the smoothing caps as suggested.
(I assume that's what you meant, they are C1 C2, or C5 C6, depending on which cct diagram).
The photos below show the input (anode) and output (cathode) waveforms.
So it appears that we have half-wave rectification!
Just checked the wiring on V1 and the 2 anodes are connected together.
Is this wrong?
The diagram below which I think comes from the Taylor Instruction Manual shows the anodes to be separate and connected across the transformer secondary.

By the way, the transformer primary is set for 240v.
V2 heater is 6.7v ac.
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Old 3rd Dec 2021, 4:44 pm   #35
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

If it helps, on mine the anodes are both connected to separate tags on the transformer.
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Old 3rd Dec 2021, 7:18 pm   #36
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

My Windsor 65c has the anodes connected together as shown in the diagram in the link. I suppose that there is a small cost saving on the transformer.
https://www.vintage-radio.net/forum/...2&postcount=11

My Windsor 65c is accurate to within 1% up to 8 MHZ and within about 2% up to 80 MHz which I think is remarkably good for and instrument of this age. It has a well shielded attenuator so that the RF output can be adjusted to a low level and a filter on the mains supply to prevent R.F. leaking though the mains cable. This is what was needed in a radio service department to align AM receivers.

I agree with post #13. Donít worry about the waveform if your Taylor 65B produces an R.F. output reasonably close to the correct frequency.

David
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Old 3rd Dec 2021, 10:50 pm   #37
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

Thanks Stephen & Neutrino.

I can't imagine that the rectification (half or full) matters that much, as long as the HT is at the right voltage and current, and the ripple is low.

Anyhow, the new rectifier arrived today and it seems to have improved the output - see screenshot below. This is probably as good as it gets, so let's call it a day.

A big thank you to everyone that contributed to the discussion.

Howard
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Old 3rd Dec 2021, 11:37 pm   #38
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

One last optional request please, no screwdrivers involved.

Can you set freq to 1 MHz, MOD ON, and probe the oscillator anode with 10X.

Click!

Just curious.
.
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Old 4th Dec 2021, 4:01 pm   #39
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

Hi 1950,

Sorry, but I can't do the 1 Mhz test at the moment. I'm currently investigating the power supply section and it's all in bits at the moment.

The mains transformer seems to have developed a fault at some point. The centre tap is open-circuit. As a workaround, someone has re-wired this section.

The anodes of V1 have been joined together and are fed by one end of the transformer. The other end of the transformer has been grounded. The net result of this is that V1 is now acting as a half-wave instead of a full-wave rectifier. I thought this wasn't a problem, but on second thoughts, it might be.

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?

By the way, the valves have been tested and are all good with emissions of 80-90%

Does anyone have a suitable replacement transformer?
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Old 4th Dec 2021, 5:12 pm   #40
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Default Re: Taylor 65B Sig Generator: Dodgy Waveform

Re: Open circuit center tap....Screen connection ?:

https://www.vintage-radio.net/forum/...2&d=1416775292

https://www.vintage-radio.net/forum/...d.php?t=111474

Lawrence.

Last edited by ms660; 4th Dec 2021 at 5:31 pm. Reason: link added and re-write
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