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Advance B4A signal generator.

 
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Joined: Wed Jul 27, 2011 7:13 pm
Location: Croydon

Advance B4A signal generator.

Post by sideband » Sat Sep 17, 2011 1:08 am

I've had this generator for about 25 years. Like most of the bits I've aquired over the years, this came from a Philips chuck-out and it ended up in the car. Originaly it worked intermittently and investigation found it was a broken pin on the rectifier valveholder. I had lots of new octal valveholders so I just changed it. The smoothing capacitor was way past it's best so that was changed at the same time. Back in those days, I hadn't heard of restuffing so I just neatly mounted a tagstrip in place of the old can and fitted two 15uF 350V caps. It's been faultless ever since and has been a reliable workhorse....until yesterday that is!

I was aligning a Ferguson radio and all started well, in fact I completed the job before I noticed a problem. There is a meter on the front that is used to set the modulation level and the carrier (CW) levels depending on the position of the front selector slide switch. I noticed that the modulation was indicating 60% and I normaly set it to the recommended 30%. Turning the modulation control back had little effect on the meter and at zero, was still indicating around 30%. The modulation level was varying correctly from 0 to 100%, confirmed by injecting a signal into the just aligned radio. So the meter was incorrect. Switching the slide switch over to CW and the meter indicated correctly. So it was whatever was driving the meter in the 'mod' mode that appeared to be the cause rather than the meter itself.

This evening I decided to investigate the cause and I thought it would be instructive to include the diagnosis and repair here. A circuit is essential with something like this and I had obtained one some time ago.

Having removed the dozen or so screws around the front, I removed the casing. To the uninitiated, the inside of one of these can look a bit daunting. However it's nicely laid out with the power supply and 6X5GT reccy at the top and the modulator circuit and 6SN7GT below. There is a large screened box on the left containing all the RF stuff and an ECC91 which can be left well alone in this case. Turning it upside down revealed all the components in the modulator circuit. This part is not screened in a box as it's really only an audio oscillator running at 400Hz but it is separated from the power supply by screening plates.

Looking at the circuit, there is a coupling capacitor from the cathode of the 6SN7, through a filter network and a diode which then ends up at the 'MOD-CW' switch. As the meter read OK on the CW position, it seemed I only needed to consider the coupling capacitor and filter network from the modulator. There is no component layout shown on the information I had so it was a question of working back from the meter and identifying components along the way.

Having done this, I switched on the generator, confirmed the fault was still present and reached for the testmeter prods. The coupling capacitor to the filter network is a 0.5uF at 250V, one of those metal-clad jobs that seem to go on forever. The cathode of the 6SN7 is at 110V and this was confirmed by the testmeter. Moving the test prod to the other end of the capacitor that goes to the filter showed 18.7V. That's not good and following the circuit through the filter, diode and on to the meter showed that it ended up at the meter as a 25.5mV DC offset. That does'nt seem very much but it's enough to deflect the meter from the zero position. Just to prove the point, I shorted the filter down to chassis and the front meter then showed zero deflection.

All the capacitors in the modulator circuit were the originals so I decided to give it a birthday and change them. They were mainly 'Dubilier metalmite' types but there was also a 300pF Hunts in the filter. Checking the 0.5 coupling cap on the bridge, showed it to be very lossy although the value wasn't too bad. The 300pF Hunts was the worst and I couldn't balance the bridge at all. All the other 'Metalmite Dubiliers' tested poor so I changed them all for new polyesters. I even managed to find a 300pF (as opposed to a 330pF) capacitor to replace the Hunts with.

When I tried the generator again, I measured the same 110V on the cathode but now there was nothing at the other end of the capacitor and nothing through the filter. What's more, the front meter was now reading zero in the 'MOD' position with the modulation control turned right down and it responded normally when the control was turned up.

Job done and all I have to do is put it back in the case. Too late now though so that'll be for another day.


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The original problem, indicating 30% mod with control at zero!


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Part of the filter circuit


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The original 0.5uF capacitor that was leaky. There are two of these in the filter circuit.



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The modulator section resplendent with all new capacitors.


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The Hunts cap was replaced with this little silver job.


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The six capacitors changed.


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The meter now showing zero at zero modulation.


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General view of the innards. Power supply at the top and modulator at the bottom. large screened RF box to the left.

All in all, a successful and satisfying repair to a useful piece of equipment. I hope it was of interest.


SB

 

Post by tunneldiode » Sat Sep 17, 2011 10:15 am

Nice work.
I too, enjoy repairing test equipment.
IME, pre-Gould Advance is of pretty much bomb-proof construction.

British engineering at it's best.

 
Posts: 2853
Joined: Wed Jul 27, 2011 7:13 pm
Location: Croydon

Post by sideband » Sat Sep 17, 2011 10:29 am

The date on the front meter is 1957 so I assume it was built around that time so only a youngster at a mere 54 years old! You can 'feel' the quality when working on equipment like this.



SB

 
Posts: 2853
Joined: Wed Jul 27, 2011 7:13 pm
Location: Croydon

Re: Advance B4A signal generator.

Post by sideband » Sun Sep 18, 2011 9:46 pm

Just to wrap this up, today after the afternoon deluge, I went into the workshop and put the Advance back in it's case but not before changing another Hunts hiding at the side of the modulator box. It's only a heater decoupler with 6.3V AC across it so doubtful it would have actually failed s/c but as it was there and easy to get to, a new .047uF cap was fitted in it's place. Checking it on the bridge showed that it was hoplessly lossy and the bridge wouldn't balance at all so I was justified in changing it. According to the circuit there are two more of these lurking in the screened RF box but I decided not to venture further into it's vitals. I know one is easy to get at after removing the first cover but the second is buried under the ECC91 valveholder and another screening cover. Chances of them failing s/c is remote so I'll take the risk.

When I came out of the workshop, it was too late to go back in the garden.......! ;)



SB


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