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Old 17th Aug 2012, 11:47 pm   #81
ronbryan
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Default Re: Advance LF Generator Type SG66

Dave

You might be able to sharpen up the +ve edges by bridging R56 with a ceramic capacitor - try something like 0.1uF, but as the cathode follower can only source current, I don't think the -ve edges will be much different.

It's also possible that C30, being electrolytic, is rather a poor capacitor at 50kHz and above and could benefit by being bridged with a 0.1uF ceramic cap.

Ron
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Old 18th Aug 2012, 5:13 pm   #82
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Hello Ron,

I tried bridging R56 and C30, it made no difference at all. My thinking was then that the problem may be further on.

First I checked at the top end of the Zener diodes, no problem here with a rise time of 0.5us we had a 32V PtP (2 x 15V Zeners) square wave. At the pot wiper was the poor waveform in all positions of travel except when at the top end to give 30V output.

Removing the wiring to the attenuator and the meter circuit cured the problem, we had an acceptable square wave throughout the travel of the pot.

A check on the attenuator switch said that was fine with an overall resistance of 10K and switching as expected between the output terminal and ground.
It was reconnected to the pot. Turning the output waveform down using the attenuator switch results in the bottom of the waveform becoming distorted (Images 1 and 2)
Using the pot to reduce the output results in rounding of and an increase in the rise and fall times. (Image 3 at full output, image 4 at 10V, Image 5 at 2.5V)

Reconnecting the wire to the meter circuit results in further rounding of and a greater rise and fall time. The other end of the wire is not connected. Connecting the meter circuit end of the wire makes it far worse. (Image 6 wire reconnected to the pot only and Image 7 also terminated at the meter circuit).

The main culprits for the problem seem to be the wiring from the attenuator switch to the output terminals and from the pot to the meter circuit, they are at opposite ends of the chassis. The metering circuit seems to be the main culprit.

Images 5, 6 and 7 on next post (#83).

Dave
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Old 18th Aug 2012, 5:14 pm   #83
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Images 5. 6 and 7

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Old 18th Aug 2012, 11:25 pm   #84
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Default Re: Advance LF Generator Type SG66

Dave

Can you post the meter circuit. It's not in the snippet you provided.

Whatever the circuit stray capacitances are, they will be charged and discharged via the impedance the 20k pot is presenting to those capacitances, which as you have observed will cause slower rise and fall times.

Have you tried adjusting RV9 to try to make the mark to space ratio nearer to 1:1?

Ron
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Old 19th Aug 2012, 4:57 pm   #85
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Default Re: Advance LF Generator Type SG66

Hi Ron,

Sorry missed that circuit out, it's here now with non relevant parts removed for clarity.

Yes I did all the adjustment including the mark space ratio.

Update now follows:

This is now the result I have. Refer to the included circuit for the metering.
S3 is the Sine/Sine-Square switch now shown in the Square Wave position.

I found that the problem was being caused by C37 across RV14, I removed C37 and reconnected the wiring to RV14. RV14 was reset to give a meter indication 30V at 1KHz with an output of 30V at the terminals.

from 5Hz up to 20KHz we have a good square wave with a rise and fall time of about 0.5uS. It remains good right up to 125KHz but from 20KHz up the voltage gradually drops by 5V as indicated by the meter. However at the output terminals the voltage remains constant at 30V PtP. This 5V drop is being caused by my removal of C37. I have now fitted a trimmer for C37 and am now able to adjust for a 30V reading on the meter with minimal impact on the square wave.

If the output voltage is reduced using the Adjust Volts pot (RV10) the first thing to happen is that the top and bottom are rounded off. At about 22V the top starts to round upwards and the bottom starts to go wavy. By the time we reach 5V the top and bottom are well distorted. At 1V both have become slightly worse.
This happens at higher frequencies from about 10KHz up to 125KHz. The lower frequencies are OK.

Setting RV10 to an output of 30V and using the attenuator switch to reduce the output produces much the same effect giving rise to a square wave with poor top and bottom also with large overshot from 3V to 100mV at the higher frequencies as before.

In all cases at 300mV and 100mV the top and bottom become dirty.

I can not see any way that I can further improve the square wave output, as the Adjust Volts (RV10) and the attenuator switch now seem to be the sole cause of the square wave problems.
Quite how Advance can quote the spec as they do in the manual is beyond my comprehension.

Quote:
Square Wave Output impedance varies up to 5K with level setting. Output level 0 to 30V pk.pk. Rise and fall times 0.75uS maximum. (Sine wave output available at same time with some distortion)
End Quote.

The reference to sine wave distortion would be more correctly described as considerable from 15V PtP upwards with the exception of bands 1 and 2.

Dave
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Old 20th Aug 2012, 11:38 am   #86
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Default Re: Advance LF Generator Type SG66

Advance SG66. Glad to hear you have now got it working.
About the square wave, you do not say what results are if you disconnect the meter circuit, ie the wire from RV10 slider to RV14/C37.
I would expect the cathode follower output to drive the zener diode choppers and the output attenuator with no difficulty. It could be that the output capacitor C30 (100+60uf) has gone high impedance at high frequencies, so attenuating the output under the load of the attenuator resistors. As suggested by ronbryan. The voltage either side of C30 may well be the same, but the current available to flow through the attenuator is restricted.
I assume you have checked the cathode follower valve V4A, and that it can give the peak current required to drive the load.
The meter circuit should provide little loading unless the smoothing capacitor C35 or the germanium diodes (MR5-8) are failing. Investigate that after confirming the output itself is OK. RV14/C37 are only used to set the meter calibration to the 30v or so output voltage from the zener diodes and as confirmed by your scope. wme-bill
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Old 20th Aug 2012, 4:41 pm   #87
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Hi WME-bill,

Disconnection of the wiring from RV10 To the attenuator switch and RV14/C37 is all covered in posts #82 and #85.
C30 seems to be fine as the waveform at the top end of the Zener diodes remains OK.
V4 has been checked by substitution with no change.
My findings with RV14 and C37 are detailed in my previous post #85
C35 and MR5 to MR8 I have not checked, I don,t think I have anything to replace the diodes with though. Parts list says type CG46/H, don't know of these and a net search provides nothing relevant. They look something like the MS1H.

Dave
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Old 20th Aug 2012, 9:17 pm   #88
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Default Re: Advance LF Generator Type SG66

SG66 square wave troubles.
The waveform at the top of the zener diodes was checked at high frequency I am sure.
CG46 are early BTH/AEI point contact germanium diodes. GEX44, OA79, OA81, OA91, GD4, AA119 or almost any Germanium detector diode will do. If you have to replace one of the four in the bridge, I suppose to do a perfect job you should replace all four with a similar type, but unless the backward resistance is very different, it will not make a visible difference. The similar meter circuit in the J2B oscillator uses OA70, and the later SG70 uses 1S44 silicon. Incidentally, the SG70 is virtually identical to your SG66, looks like a repackaged model for marketing purposes only. There have been no changes around the square wave generator and attenuator, so it seems unlikely to be a regular occurring design fault. wme_bill
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Old 21st Aug 2012, 11:07 pm   #89
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My latest findings maybe proving that the problems are not caused by the meter circuit. However I don't know what I can put it down to except maybe the wiring. The wire from the attenuator to the output terminal, the wire to the output ground terminal and the wire from RV10 to RV14/C37 are all in a small loom running across the front plate and past the sine wave output terminals.

21/08/2012

With the meter circuit totally disconnected, points 7 & 8 lifted and wire from RV10 to RV14/C37 lifted.

Checked at the top of the clipper diode MR15 the waveform shows as good regardless of the setting of RV10 and the attenuator switch.

At the output terminals
At full output of 30V PtP and at the frequency of 125Khz down to 11KHz the waveform has rise/fall times of 0.5uS, that is the last point I am able to measure these times. The waveform remains good and square down to 5Hz.

Using RV10 to control the output level
Starting at 125KHz and reducing the output by 10V PtP results in rounding of the corners (lower fall edge and upper top edge). Reducing the frequency appears not to make the rounding any worse, although it becomes impossible to view at the time base period gets longer. The wave form looks to remain constant right to 5Hz.

Reducing the output further to 10V PtP and at 125KHz to about 10KHz results in further rounding of the edges and some slight sloping of the top and bottom. From 10KHz down to 5Hz looks to be OK.

Reducing the output to 0.5V Ptp at a frequency of 5Hz and up to 50Hz looks ok, at 50Hz some overshoot starts to come in, this increases with frequency up to 8KHz where the top and bottom start to slope of. It then continues to do so up to 125KHz where it is no longer a reasonable square wave.

A further reduction in amplitude results in more top and bottom distortion (Image_a). As the frequency is reduced the top and bottom distortion move towards the start of the trace, (Image_b and Image_c)


Using RV10 to control the output level
With RV10 set to 30V PtP.
Reducing the output to 10V PtP causes corner rounding and some bottom distortion (Image_d) from 125KHz to 50KHz where the top starts to slope (Image_e).
A further reduction in frequency results in overshots (Image_f)

NOTE:
The overshoots could be due to the probe as it can be adjusted out by the trimmer but is not then correct when viewing the call waveform on the scope.

Further reductions in output amplitude produce similar results as above right down to 100mV PtP output.

Reconnecting RV10 to RV14/C37 produces some overshoots to appear (Image_g at settings of 3V, 1V, 300mV and 100mV PtP at the higher frequencies.

As these checks have been carried out with the metering circuit completely disconnected and appear to be much the same as when the meter circuit is connected I can’t see that the metering components are causing the problems. Save maybe a very small percentage if any.

I will however check them to be absolutely sure.

Dave.
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Old 21st Aug 2012, 11:08 pm   #90
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Default Re: Advance LF Generator Type SG66

Balance of Images

Dave
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Old 28th Aug 2012, 9:45 pm   #91
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More tests and the results:

28/08/2012

I decided to check out the attenuators using a Farmell LFP1 although this is not perfect on square wave, particularly at 1MHz. However for this exercise I only need up to 125KHz. Incidentally the sine wave is good across the frequency range.
I can only get a maximum of 15V PtP from the Farnell, half of the normal SG66 output.

First I disconnected the input to RV10 from C30. The meter circuit was also disconnected at points 7, 8 & 13 on the meter board, and S3 set to Sine & Square.

I fitted the Farnell with an adapter and a BNC tee piece, connecting one end of the tee piece to the scope channel 1 via a probe.
I then connected the output of the tee piece to the top of RV10 and ground via a screened cable.
Channel 2 of the scope was connected to the SG66 square wave output terminals via another probe. Both probes were set to x 1 as the square wave was better at x 1 that at x 10.
The SG66 was not powered up.

Using RV10 to control the output amplitude:

At 100Hz we had a good square wave from the Farnel and with RV10 at maximum a good square wave at the output terminals of the SG66.
Reducing the output with RV10 we had a good square wave right down to zero.

At 1KHz again we had a good input waveform from the Farnell and a good output waveform with RV10 at full output.
Reducing the output the waveform remained good.

At 10KHz the output from the Farnell has slight overshoot at the top leading edge and slight bending of the lower leading edge.
This results in the output waveform having a slight bend and overshoot at the top leading edge and rounding of the lower leading edge. (Image_1a)

In all cases of the traces shown in an image the top trace is the input from the Farnell and the lower trace the output at the SG66 terminals unless otherwise stated.

Reducing RV10 first caused the overshoot to disappear and the edge to round of slightly. Further reduction caused the top and bottom edges to slope of slightly. Further reduction causing the sloping to disappear and a very small overshoot to return. However it was nothing really dramatic.

At 100KHz The output from the Farnell has a very distinct overshoot and a slight upward slant at the bottom.
At the SG66 output terminals the overshoot is present much the same as the input waveform. Also the top is now slanting slightly upwards. The bottom now has a distinct slant downwards (Image_2a)
Reducing the output amplitude with RV10 results in first the overshoot vanishing then severe rounding of the waveform (Image_3a). Continuing to reduce the output amplitude results in the rounding being reduced and an overshoot returning (Image_4a). The output waveform is now at 50mV.


Using S2C (Attenuator) to control the output amplitude:


At 100Hz, 1KHz and 10KHz the results are the same as using RV10.
At 100KHz the results are the same as for using RV10 except that at switch positions 0db, -10db and -20db the top and bottom overshoot of the output waveform is far more severe (Image_5a).
The distortion is again at the higher frequencies from about 10KHz upwards to 100KHz and upwards.
This shows there is distortion of the square wave taking place with only RV10, the attenuator switch S2C, the 2 diodes MR1 & MR2, RV14 and C37 to S3B and the associated wiring in circuit between these components. Note the wiper of S3B is lifted at point 8 on the meter board.
I have also now noticed that some 50% of the overshoot shown on the output waveform from the Farnell is being caused by being connected to the SG66.

Powering up the SG66 causes no additional distortion save some HF pickup showing on the top and bottom of the waveform. This is only present with the SG66 on its higher frequencies from about 200Hz upwards.

I now need to reconnect the metering circuit and run through the tests again.

Dave
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Old 29th Aug 2012, 11:48 am   #92
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29/08/2012

I have now reinstated the metering circuit. RV10 is set to full output as is the attenuator switch S2C with the Farnell generator set to 15V PtP @ 1KHz.
The meter shows the correct output of 15V PtP.
We have a good input and output square wave, they look to be pretty much identical.

So to the tests.

At 100Hz & 1KHz both input and output waveforms are good. Using RV10 to reduce the output amplitude produces no distortion, as does using the attenuator switch.

At 10KHz this is identical to the test result given in my previous posting when using RV10 or the attenuator switch.

At 100KHz at 15V PtP the input and output waveforms are the same as in the previous posting. Again the result of using RV10 or the attenuator switch to reduce the output are identical to that of my previous posting.

Again powering up the SG66 does not seem to introduce any additional distortion.

On face value it seems that the metering circuit does not introduce any additional distortion into the output waveform. I would from this result assume that the 4 diodes MR5 to MR8 and the smoothing capacitor C35 are in good order.

Quite what to do now I don’t know. The distortion we have with the square wave, at the higher frequencies, must be coming from somewhere, but where is the question.

Dave
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Old 10th Jan 2015, 5:15 pm   #93
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Default Re: Advance LF Generator Type SG66

Dave, I just went over my SG66 which iI bought recently.

It does use an ECC88 in the oscillator.

Mine had some issues
1) Unstable output
2) You could hear the oscillator 1-5kHz range
3) Needed calibration
4) Fan noisy (originally not even working)

So, manual is free on the web...

Actions:

1) Amplifier : replaced coupling and decoupling caps .47. 1.0, .15, .15 although old ones tested OK for capacitance when out!

2) Replaced ECC88 with a Siemens one ... microphony gone.

3) Oiled fan

4) The IMPORTANT BIT FOR LAST:
Setting the oscillator for minimum distortion: RV6

The manual says either measure 1mA AC through thermistor (no way!) or voltage drop across its series resistor. Point is, it NEEDS 1mA to flow.
Since resistor is listed as 1.8k it says look for 1.8V rms across it.
BUT the resistor is NOT 1.8k, it is 1.5K so we need voltage to be 1.5V
IN FACT, this is SO CRITICAL one needs to measure resistance of R26, mine was 1.54K

WHERE IS IT I hear you ask. Under the oscillator board! BUT you can access from above on the FRONT FACE of the oscillator board, to the left and right of the thermistor, those naked connecting hard wires...

ADJUSTING RV6: if the voltage drops below the target 1.54V true rms on mine the oscillator goes out. (cathode of oscillator). As soon as it goes at 1.6V perfect. If I push up to 1.8V rubbish comes out!

I think this may be where you went wrong. Is it?

5) Calibration
This was subsequently a breeze ....
The only issue was the meter calibration at 100kHz via C34. Just outside its range ... because R65 is not 15K but low at 11.7K. I could not be bothered to replace ... 100kHz is measuring 3% higher!

It is totally stable now ... everywhere, although it does take 20 min (the manual say wait 30min!).

I hope this helps.

Kyriacos
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Old 12th Jan 2015, 12:24 pm   #94
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Default Re: Advance LF Generator Type SG66

Hello Kyriacos,

Thanks for adding your information to the thread, it may well help somebody to set up one of these units in the future.

As for my unit it was not that straight forward as the oscillator circuit was not that as shown in the manual, nor was it built the same.
The manual talked about the oscillator printed circuit board, which did not exist in mine. It was made up on an aluminium plate with tag strips down 2 sides and mounted on pillars to the main chassis.
I can only think that somebody had modified it for some reason in the units previous life.

Yes I think you made a wise choice when you avoided measuring at the thermistor as when setting mine I managed to turn the thermistor into a fuse. That slowed progress for a long while.

Eventually I found another Advance generator in scrap condition, this supplied a replacement thermistor. I was then able, eventually by careful adjustment, to get the oscillator to start on each range when switched, also to be able to go through the full frequency range without any noticeable adverse effects. I was satisfied with the results.

The problem I had left was with the square wave distorting, as in the previous pages of this thread.
I was unable to find a cure for this and so it remains to this day.

The generator has only been used once and that was not for audio reasons but as a 50Hz AC supply to set up a VVM, the SG66 will produce an output in excess of 100V if not terminated.

As for using it for audio testing that wont happen as I don’t have anything to do with amplifiers or such. Can’t remember why I bought it now but I did a lot of hours of work on it. Its probably time to move it on.

Regards Dave

P.S. I have added 2 pictures of the oscillator unit.
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Old 12th Jan 2015, 3:31 pm   #95
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Default Re: Advance LF Generator Type SG66

That board looks more old Tektronix than anything Advance I ever saw. I once had an SG62, it looked mint condition, but its output fluctuated all the time. I eventually tracd it to a dry joint on one of the valve socket's connectors. Not obvious to the eye. It made me think that it had given the fault, then got put on a shelf until disposed of years later for me to buy. The best type of useage senario!
Les.
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Old 12th Jan 2015, 8:30 pm   #96
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Default Re: Advance LF Generator Type SG66

Hi David

My SG66 looks exactly like yours.

Yes some "odd" references to the boards but I just took it to be their "language"!

See, under the board the resistor is also 1.5K in yours, so the voltage you are looking for is 1.5v or just over across it. But you can measure it from above with the "board" in its place!

I will see what my square output is and report later!

K
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Old 12th Jan 2015, 9:16 pm   #97
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Hi Kyriacos,

Are you saying that your SG66 has the oscillator panel built the same as my SG66 and is not on a PCB.

Yes the resistor is 1K5 but the circuit is not as the one in the manual, it is quite different.
If you like I can post it here as I did draw it out.

That would be interesting to see your results for the square wave output.

Dave
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Old 12th Jan 2015, 11:17 pm   #98
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Hi Dave

Yes, the oscillator panel is exactly the same as mine.

I did not bother to compare the details of the schematic with my machine, because I got the schematic from the internet, it is not original WITH my machine.

My AVO VCM 163 valve tester has differences between machine and schematics regarding capacitor values, e.g. listed part 68uf, on machine 60uf (on 3 machines from friends).

I have by now decided to believe the machine 90% and the schematic 10% when there are differences!! Only exception is valves as these are easily replaced by users over the years, so this can get a bit more tricky ...

My square output is very good between 100Hz and 90kHz. Above 90kHz the horizontal line becomes wavy, and the edges round.
Below 100Hz, the horizontal level remains straight but at an angle which gets steeper the lower I go.

The p-p voltage is very accurate throughout.

Well the square wave gets better in the high frequency range when I increase the voltage.
I need to test those amplifier valves ..... better pull my AVO out ... It is boxed up somewhere

Will post when I do that.

K
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Old 13th Jan 2015, 2:45 am   #99
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Default Re: Advance LF Generator Type SG66

The waveforms at the upper ans lower ends of the ranges are about the same as they are on my Heathkit set that goes from 1hz to 150khz.
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Old 16th Jan 2015, 9:08 pm   #100
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Default Re: Advance LF Generator Type SG66

Hi Kyriacos,

Your square wave output seems to have the same problem as mine.
I am not able to test the output valves as I have the AVO 2 panel tester.

Interested to know the results you come up with.

Regards Dave.
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