"Advance" J2 generator: coils?

[Skywave]

This AF generator is undergoing a routine overhaul. This particular specimen has what appears to be a pair of tapped coils*, connected in series, and then fitted in parallel to the input of the 600 ohm / 20dB attenuator.
*(They might actually be wire-wound resistors - but I doubt it.)

Is anyone here familiar with these coils in the J2 model? I have the cct. diagram, but these two items do not feature on it. They are clearly factory-fitted items; read: they are meant to be there.

So, why are they there? HF compensation? The O/P level, with or without the 20 dB attenuator in circuit, is not markedly affected, freq.-response wise, if they are included - or omitted - from the cct. I cannot see that they are needed at all - but there must be some valid reason for their later embodiment.

Thanks.

Al. / Skywave.

[AlanBeckett]

Al,
Picture of mine attached.
Where?
Alan

[Skywave]

Mine is different to yours: I have the J2; yours looks like the J1.

The J2 has a 600-ohm O/P level volt-meter on the front panel. In my unit, the 20 dB attenuator (R25 / R26 / R27) is mounted on a paxolin group board to the right of the mains transformer (reference your photo.); between this attenuator and the front panel are the two coils(?), mounted on the aforesaid paxolin board. But see later down this page . . . .

I'm starting to have a few second thoughts about these 'coils': they may be wire-wound precision resistors after all. Each one has 4 terminals: only 3 of each 'coil' are used: in each case, there is a Y internal connection: 'Coil' 1 is 150 / 150 / 850 ohms; 'coil' 2 is 150 / 150 / 1050 ohms. They are interconnected in a very strange series-parallel connection, resulting in a final resistance of 672 ohms that is shunted across the unswitched input to the 20dB attenuator. The O/P meter is also connected to this point (monitors the input voltage to this attenuator only - the 5 ohm O/P is not monitored). The puzzle with all this thinking is why are these 'coils' there? Something to do with the meter fitment in the J2 variant? (Which also does not appear on the 'universal cct. diag.' of the J1, J2, J1/NA . . . etc., as produced by 'Advance' - but they do make a mention of the meter . . . but not the coils . . ).

But there are more clues. On the top side of the chassis I can see a clear pencil line that has been made as a guide for drilling the two holes in the chassis to mount that paxolin board: 4BA hardware. It seems unlikely that this item would have left the factory with this pencil line so clearly visible. However, the wiring to these 'coils' looks original - unlike the wiring to the meter. So perhaps what I'm looking at is not an 'Advance' design change at all! (Oh, and yes, since I know just where I obtained this item from, albeit it many years ago - a certain Government Department not a million miles from here - that could well explain quite a lot! )

Now upon test, the 600 ohm O/P level is correct by the front panel meter (RMS volts) - and corresponds using a 'scope and a VTVM - when feeding a 600 ohm load. Moreover, the 20 dB attenuator does indeed produce an accurate 20 dB attenuation, and all this at all frequencies. And the waveform looks remarkably pure and 'fits' the dial calibration very well too. These observations / measurements are with the 'coils' disconnected. Connecting the 'coils' make no substantial changes to all or any of that - just a slight reduction in the input level to the attenuator (as you would expect) - which is easily compensated for by slightly turning up the 'level' control.

So I'm tempted to simply remove these 'coils', tidy the rest of the unit up, do a final re-cal./ adjust-on-test . . . and call it a day. This Item may be old and look a bit tired [that I can relate to!] - but it does provide a stable, clean & healthy O/P (1 watt into 600 ohms; 500mW into 5 ohms) over the range 15 Hz to 50 KHz; a useful piece of test kit.

And that's the full story to date; but there is clearly some 'history' to this Item.

Al.

[Skywave]

A picture - or in this case, four - says a thousand words . . . .

The last pic. clearly shows the pencil line.

Al.

[AlanBeckett]

Al,
Ah, so.
I'll fish out my J1 and J2 and take more pictures.
Alan

[AlanBeckett]

Originally Posted by Skywave

Mine is different to yours: I have the J2; yours looks like the J1.

I have to say young Al that I'm disappointed that you think I can't tell my J1 from my J2

However, I think I know why they are different - mine are the later versions in the 'squarer' case. As far as I can tell these correspond to the schematics we have. So, perhaps there was some 'Value Engineering' after your earlier versions, or possibley you have a Special.

What you need is some photos of the internals of the earlier version, like yours.

Alan

[Skywave]

Originally Posted by AlanBeckett

I have to say young Al that I'm disappointed that you think I can't tell my J1 from my J2
Alan

Yes, I know! . . . And thanks for the pics.

I get a bit confused with these 'Advance' generators; J1 or J2; later or earlier types; variations for Europe, USA and the home / U.K market. (Not to even mention those that have been 'modified' ). And I'm not sufficiently acquainted (yet!) with the underneath-the-chassis views of all of them to enable me to authoritatively state, upon an eye-ball inspection . . .

"Ah-Ha! Now that is a J-x" . . . or whatever.

But it's these very aspects - plus the detective work, as in my example - that, for me, makes collecting & renovating Test & Measurement kit so satisfying.

Al.

[Skywave]

14 Sept. 2010: update.

So there is an early and a late version of the J2; main difference (probably the only difference) is the appearance of the front panel & associated knobs, etc. In what follows, I'll refer to these two variants as the Mark 1 and the Mark 2.

Since the above posts, I've been fortunate enough to see the insides of a later Mark 1 - and, lo and behold, the components fitted - and their layout - are identical to the Mark 2. And that's when things start to get interesting. Mine is a very early version: serial number 457 and the colour of the front panel is olive green, not grey, as is the standard for the Mark 1. There are one or two other more subtle differences that have also come to light - some that confuse the issue even more! Hmm.

O.K.; decision time. In essence, and on test, this generator meets its published spec.: that's a good start. So, as per my earlier post, I am now going to make the necessary changes so that it becomes a 'true' Mark1 (except for changing the colour of the front panel! ). This is not going to be a lot of work and also will not, per se, be some strange modification. Then I can use it - as the manufacturer intended. And then I can move on - to the next repair job.

Al. / Skywave

[Skywave]

Project completed - however it was necessary to substantially increase the uF value of the HT rail filter cap. in order to remove residual hum amplitude modulation. It was also necessay to strip, clean & re-assemble the O/P level pot.: 25 k-ohm log. carbon with SP on/off switch: a suitable replacement was not available.

The removed 'coils' have been retained, appropriately labelled as to their origin & stored. (One day, some relevant fact might just arise . . . . now where did I put those 'coils' ? . . .)

On test, freq. calibration is good; 20 dB attenuator is within 1 dB all freqs.; O/P level meter 'tells the truth': RMS volts (600 ohm output); signal looks very pure & clean on the 'scope. (I don't have the necessary kit to accurately measure distortion - yet ).

Time to move on to the next job . . .

Al.

[Skywave]

I have no further input to make to this Post. I now leave it in the hands of the Moderating Team to decide if it should now be closed.

Al. (20-09-2010)

[Skywave]

Moderators: this Thread can now be closed.

Al.