Taylor Model 68A

[Sinewave]

Hello.

So, I picked this Taylor up a few weeks ago. The power lead had been chopped off, so I didn't really expect anything much.

After a few careful checks I powered it up and have discovered that it does work. It works reasonably however on one range, 110mhz - 240mhz range, it doesn't always give an output. Sometimes some winding up and down the dial gets a reading for a moment, as does switching the modulation, but nothing appears to be quite repeatable.

I'm just going to check over the switch contacts, but if anyone has any other suggestions in the meantime, then please do feel free to offer some advice on this restoration project

[HamishBoxer]

Yes,I would certainly clean the controls and switches with De -Oxit. Also possibly the valve bases (pins).

[Sinewave]

I've just done the switches and gave it another go.

It's behaving similarly. When going up the range, it'll shoot to over 200mhz and then up to 340mhz, which is beyond the range of this set. Then after a moment of more twiddling it runs through the range as it should.

I had wondered if it was the HF Oscillator valve.

I'll give the valve bases a good clean out. I do't have De-Oxit, I use Servisol Super 10, but when I run out I'll be trying out some De-Oxit as it gets a good name.

[Sinewave]

Ok, done the valve bases. Similar behaviour, however frequencies now reading low on all ranges. I wondering whether to expect a valve. I've no idea at the moment if I have any 12AT7's.

[Bazz4CQJ]

I've owned one of these for quite some time and it has always been a very reliable basic workshop sig gen and quite useful. You seem to be doing the obvious checks in terms of fault-finding, so I cannot really offer any specific comment on the problem your unit has.

I vaguely recall that the user/service manual is "back to front" in terms of the written description of the two oscillator circuits which are used to cover the 8 frequency ranges. I think the only work I've ever done on mine was to replace the black, 'Mouldseal' Hunts caps, which I think are in the modulator section and have a poor reputation for longevity, and I added one or two additional RF decoupling caps to help minimise RF leakage down the mains lead. Good luck!

B

[Sinewave]

Yes it's a lovely little unit. My one certainly has seen better days. I saved it from the scrap yard. The chassis exterior is rusty, the mains lead was chopped off, so perhaps there was a problem someone else couldn't find.

I should check out and perhaps replace some of those Hunts capacitors as you say they're not known for longevity and didn't get their reputation for being time bombs for no reason!

[Freya]

They sometimes need a new smoother, also check any suspect caps on the grids of the two valves. They are quite accurate still without any calibration but do need at least 10 mins warmup.

[David G4EBT]

It uses a metal rectifier, which may well not be giving sufficient DC output, in which case the HT voltages on the anodes etc may well be low. Unfortunately, the circuit doesn't show any voltages, but that may be evident from the markings, if any, on the rectifier. I'd expect the DC Voltage on the output to be a bit higher than the AC Voltage into it. If the DC output is low, you'd need to replace it either with a bridge rectifier - easiest and neatest, or four silicon diodes.

Looking at the circuit, it has a couple of filter caps across the mains input - one from Line to E, the other from Neutral to Earth. Nowadays, they'd by Class Y caps. It won't stop the generator from working, but if they're waxy paper or mouldseal, it might be wise to replace them, or for now, to at least snip them out.

You don't say if you have the manual - I doubt you'll make much headway without it, just by guesswork and speculation.

You'll find it here if you don't already have it:

http://www.g8glz.net/downloads/Taylo...ser_manual.pdf

Unfortunately, the resistor and capacitor values aren't shown in the manual, but you'll be able to check them visually in the generator, measure them and compare them with what their markings show.

Are you measuring the output waveform on a scope, or just with the output power meter?

Hope that helps a bit.

[Bazz4CQJ]

Some notes I made some time ago show the main HT line at~110V.

I have the plain model 68, but the 68A has the nice additional feature of a RF level control and a meter to use with that. I did once try to modify mine to the 68A configuration; the level control simply adjusts the HT. The first attempt didn't work, but I might have another go at that sometime.

B

[Sinewave]

Thanks for the replies so far, I'll try and take another look tonight and report back to you with my findings.

In response to David, I've been checking for basic functionality with a frequency counter, but haven't checked the actual output level as of yet, but I'll put a scope on it tonight and see what that is.

[Andrew2]

Could the frequency jumping and unpredictability be due to a dirty tuning cap, especially the rotor contact springs? The fact that it begins to settle down after a few twiddles might indicate so.

[Sinewave]

Originally Posted by Andrew2

Could the frequency jumping and unpredictability be due to a dirty tuning cap, especially the rotor contact springs? The fact that it begins to settle down after a few twiddles might indicate so.

I thought that, but then it only does that on the one range. However I did give the tuning wafers a good inspection and clean with contact cleaner and air and it remained to behave the same.

[Bazz4CQJ]

How well do you know the behaviour of your DFC? I'm guessing that, particularly on the VHF range, the output could change a lot from high to low, and my experience of the budget DFC's which I've had is that too little or too much input and they can give weird readings?

B

[Sinewave]

Originally Posted by Bazz4CQJ

How well do you know the behaviour of your DFC? I'm guessing that, particularly on the VHF range, the output could change a lot from high to low, and my experience of the budget DFC's which I've had is that too little or too much input and they can give weird readings?

B

It's a Racal-Dana, usually very reliable.

I'm just having another look, as David has pointed out already, the schematics give no values, so just having a look at what I can check for now. Will test the output of the rectifier shortly.

[Sinewave]

I'm getting 320VDC on the rectified output before the smoothing. This is far higher than the expected peak value of the input to the rectifer.

[Sinewave]

Oddly though, right now, it's behaving just as it should. It's been switched on for quite some time as I've been probing around and scoping up.

Rectified output on the scope looks quite normal.

[Sinewave]

That 320VDC looks to be correct actually. I may have made a mistake when probing the input to the rectifier earlier as I've just checked the input which is 280VAC.

I may have had a poor connection earlier to begin with as I'd at first came up with a much lower voltage.

I've not done much more tonight, so I'm still none the wiser on the occasional issue on the higher range, but will need to move into RF output stages and get some figures down on the performance of the unit.

[Sinewave]

Ran it again last night, it behaved itself generally. Perhaps some poking around, de-soldering and re-soldering sorted one or two things out?

Back to it this afternoon. Peak to peak output on the scope is measuring 580mV at 100Khz.

If it's not misbehaving there's not much else to do I guess...

[Sinewave]

Originally Posted by Bazz4CQJ

Some notes I made some time ago show the main HT line at~110V.

I have the plain model 68, but the 68A has the nice additional feature of a RF level control and a meter to use with that. I did once try to modify mine to the 68A configuration; the level control simply adjusts the HT. The first attempt didn't work, but I might have another go at that sometime.

B

I measured at the R18/R20/C26 junction get 127VDC.

[Bazz4CQJ]

Yep, 127 sounds better than 320. As mentioned above, the one thing I did try to do was to do a home conversion to make nine like a 68A model with the meter and level control. I managed to find a meter which I thought was suitable and fitted nicely behind the small window on the top RHS, but, for some reason, it didn't work out. I may give it another try during the winter.

B