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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 8th Aug 2012, 1:10 pm   #1
gezza123
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Default Taylor 61A

Hi guys,

I have got a Taylor 61A signal generator which seems to be working ok as you can see in the pictures.

The thing is that the manual tells me the power supply readings are as follows.
(A+B) = 6.3 volts AC, and is correct .

(C) = 300 volts and is not correct.

(D) = 175-200 volts DC and is not correct.

X Output is also correct.

Audio Output is also correct.

I tried to copy the circuit but it's in bad shape, so I have drawn a rough sketch of the power supply.
I have also disconnected the output of the transformer/rectifier to get my readings shown in the sketch. The scope pics are 1=5MHz. 2=10MHz. 3=40MHz. 4=FM.

The question is, why this should work so well at 25 volts HT? I have measured the transformer and find no shorts and the HT capacitor checks OK.

Thanks for your input.
Gezza
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Old 8th Aug 2012, 3:16 pm   #2
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Default Re: Taylor 61a

It looks as if your metal rectifier is knackered.

It's amazing how well these circuits were designed, such that they kept going with massive changes in HT supply.

Ron
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Old 8th Aug 2012, 8:19 pm   #3
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Default Re: Taylor 61A

Hi,

Ron, I have changed the rectifier and it's still the same output.

I will have a go at it tomorrow and take out the transformer and try a substitute in its place and then I will ring the transformer to test it.

As you said it's is amazing that it is still working, I will come back with any results.
Thanks for your input.

Gezza
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Old 8th Aug 2012, 9:58 pm   #4
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Default Re: Taylor 61A

Gezza

Doing the maths! when operating normally (300V at C and 200V at D) there would be 15mA flowing through the 7k resistor.

With your readings (C= 46V and D=25V) there is only 3mA flowing through the 7K resistor, so I would rule out any problem downstream of the 7K resistor!

That just leaves the rectifier (exchanged), transformer, and 20u capacitor.

(assuming the 7K resistor is good!)

I suspect the capacitor would be the most likely to fail, and easiest to check, either by substitution, or by measurement. How did you measure the capacitor? You would need to test it at 300 to 400V (certainly over 46V) to ensure no high voltage breakdown effects!

Regards

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Old 9th Aug 2012, 1:55 pm   #5
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Default Re: Taylor 61A

Thanks Peter for your information. It was the transformer and the 7K resistor is fine.

Here is an update:

I disconnected the transformer from the rectifier and connected up a 140 volt AC output transformer to the bridge and the DC was at 163 volts on (C) and the HT on (D) was 114 volts and all was ok. I also tried a new rectifier and still got 114 volts DC output.

It seems that the secondary of the transformer has gone faulty and producing only 45 volts, so has anyone got a transformer going spare? I think it needs to be (240 AC in) (200V out at 150 mA) and (6.3V - 0-6.3V 0.6 mA).

Here are some more pictures out of the detector of my AR-88D, the generator was set to 10MHz AM and there was a good audio output.

Thanks again,
Gezza
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Old 10th Aug 2012, 11:26 am   #6
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Default Re: Taylor 61A

Quote:
Originally Posted by gezza123 View Post
It seems that the secondary of the transformer has gone faulty and producing only 45 volts, so has anyone got a transformer going spare? I think it needs to be (240 AC in) (200V out at 150 mA) and (6.3V - 0-6.3V 0.6 mA).
6.3-0-6.3 is unusual, but 0.6mA sounds about a thousand times too small!

Does it run 12.6V heater valves?
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Old 10th Aug 2012, 3:19 pm   #7
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Default Re: Taylor 61A

Hi,Herald,
I made a booby.

It is two seperate circuits for the heaters, 6.3v for (V2 ECC85)+(V3 ECF80) should require at least 1 amp.
And 6.3v for the V1 12AT7 at 300 ma.

I have now managed to copy the circuit, so you can see that they use the
12AT7 with a seperate Htr.

I can't seem to find a transformer with this heater arrangment and was thinking if I would be able to use one 2Amp heater section without affecting the rest of the circuit, as they also use one off the heaters for 50 c/s X deflection voltage.
Thanks for your input.Gezza
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Old 10th Aug 2012, 6:45 pm   #8
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Default Re: Taylor 61A

That's an interesting circuit! They also seem to use the other heater section to do something with V1B grid, presumably to sweep (wobble) the output frequency in sync with the scope trace drive. Perhaps the two heater voltages have to be in opposite phase to put LF at the left end of the sweep? Maybe someone whose circuit analysis is less rusty than mine could illuminate here?

If you can find a TX with two separate 6.3V windings, they could always be connected in this fashion by using the start of one connected to the finish of the other as the centre tap.

If my guess is right and you could live with a reversed wobbulator display, I should think you could use just the one winding.

A 150mA HT secondary sounds rather generous- that valve set shouldn't take much more than about 60mA HT altogether.

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Old 11th Aug 2012, 9:57 am   #9
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Default Re: Taylor 61A

Hi, Chris,
Yes looks complex, here is how it works.

I have managed to scan all the manual now, but its 12meg so to big to go on here, so if anyone needs it give me a PM shout.

Thanks again for your input, Iwill now put out a request for a transformer.
Gezza
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Old 11th Aug 2012, 9:13 pm   #10
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Default Re: Taylor 61A

Thanks for that- I'm afraid I'm still no wiser as to exactly what V1A is up to, though 50Hz FM does get a mention.....

The transformer looks like a candidate for Ed Dinning, perhaps?

Or maybe you could find one of the RS "maka transformer" kits which consisted of a prewound primary on a split bobbin and a set of laminations and clamp. They came in various ratings and you could wind whatever secondaries you wanted within the space constraints onto them.
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Old 17th Aug 2012, 10:54 am   #11
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Default Re: Taylor 61A

Taylor 61A AM/FM signal generator.
May I suggest that it is simpler than you think. The reactor sweep coils (L1,L2) appear to require about 12V rms/30V pk through the phase shift network C47/VR6. So use two 6.3v windings in series and split the valve heater load between the windings. So almost any small transformer giving 200-250V/50ma and 6.3v 2a should be enough for the valves etc, and another 6.3 transformer in series to drive the Sweep. Even a 8 to12V bell transformer could be used. The sweep voltage is not particularly critical, current maybe 0.5A. These small valve transformers were available at one time for TV pre-amps, but seem to have disappeared.
I am estimating the voltages and the current loads from the details given in the manual, but can measure it on my model if you would like that done.
The feed to V1B grid is for the sweep blanking, an ingenious arrangement described in the manual. V1A is the FM/Sweep oscillator, and V2B the AM oscillator.
Interesting is the use of ECC85, which is effectively the equivalent of the ECC81/12AT7 with internal screen, as the AM/Xtal oscillators. This is done presumably as the internal screen reduces interaction between the Crystal marker section V2A and the AM/Marker oscillator V2B. I will experiment sometime on using the ECC81/12AT7 instead.
I see you have the manual, else PM for a copy by email. It comes to 4.8M. wme_bill m0wpn
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Old 17th Aug 2012, 3:28 pm   #12
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Default Re: Taylor 61A

Hi,WME bill.
Thank you for your input, and suggestions, (Your write up will be put in the service manual for further reference).

I did pay a lot for this unit before the transformer decided to throw its hand in, and would like to put it back as near as original if I can.

I am looking at getting a rewind but the cost at this present time is prohibited so it is on hold.

I have tried two transformers and it works ok but they are two big, so yes please I would like to take you up on your offer to have the right voltages and current measurements for the transformer if this is ok by you and you have got the time, as this may help the rewind engineer to have the correct volts/Amps outputs for the transformer.
Thank you again Gezza
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