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Tek 2712

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vk3gjm
Posts: 2
Joined: July 2nd, 2009, 10:37 pm

Tek 2712

Post by vk3gjm » February 2nd, 2016, 4:29 pm

Hi All,

I am in the process of trying to fault find a potential issue with my 2712 Spectrum Analyser.

Basically what happened, a colleague :cry: input about 2 watts from a 50 Ohm source into the main input of the analyser while power was turned off. Reviewing the schematics for that assembly Attenuator "A18" shows while the SA is powered down, no RF could possibly get to the 1st mixer and effectively 2 watts has dissipated into a 5.6K resistor, however the Analyser now reads about 5dB down at 1.8GHz.

I removed the attenuators assembly and removed the RF shield and did a quick hardware loss measurement with the relays all energised accordingly and I end up with an approx. 1.36 dB loss at 100MHz and a 5dB loss at 1.8GHz.

I assume the schematic stated loss of 0.5dB is at 100MHz, however I am getting approx. 1.33dB loss at 100MHz whereas the Teledyne 712-6 datasheet implies a loss at 100MHz should be about 0.05 dB, therefore with 7 relays in series ( 0.05*7 = 0.35dB) therefore 0.5dB is not unreasonable, thus the 1.33dB is uncharacteristic. Naturally I woudl expect a much higher at 1.8G, I would assume at 1.8G the loss should be about 3 dB+/- 0.5 this is about 0.5dB per relay but the value is far greater.

Removing the first TO5 Teledyne relay is extremely hard to do due to final position and excessive heat would be required, I guess this mail is the ask if others have experience with reliability of the Teledyne 712-6 relay and would/could 2 watts cause the relay to exhibits losses?

I have a feeling I may be looking for something that this not there and the normalisation process takes care of losses. However the normalisation process is only available at 100MHz, therefore any gain adjustment/normalisation process at 1.8GHz in not possible?

Has anyone ever measured the loss of a working attenuator?

Could you use a 2465B 400Mhz Oscilloscope to look at respective losses of each relay input/output in a relative sense and try to isolate one or more that may have a greater variance?

I love this analyser and tracking Gen with the options, I am 100% keen to restore it back to factory default with some TLC.

Regards


Gerald

VK3GJM

Gerald
Posts: 1
Joined: September 9th, 2021, 2:05 pm
Country: Australia

Re: Tek 2712

Post by Gerald » September 9th, 2021, 6:30 pm

Hi All,

How quick time travels. Moving ahead. I decided to tackle the 2712 again.

After trying to adjust and normalise many times, I continued to get multiple failures. I put a post out and found another user who had similar issues, but he discovered a part that failed in the options VR that caused loss of level in 2 x 2711 units. Q350 MMBT3904 SOT-23 get hot by design, the solder pads are a bit to small, the SOT-23 case with lid open after 1/2 hour warn up sits at 46.1 degrees C.

This device gets quite hot even touching it with your finger. Removed the device and did a basic test, I did find the hfe to be [email protected] I batch tested a bunch of new devices and found then all to be 150 and above.

I replaced Q350, I created a heatsink by way of a standoff screwed to the top EMC aluminium cover positioned above the SOT-23 device, added a heat transfer rubber. Anything is better than leaving it unheatsinked, and the lack of solder pad size inhibits better heat transfer.

The attenuator is fine, I mapped out all losses at 20+ spot frequencies and the teledyne relays all meet there spec, also all parts around the attenuator are in fine working order.

As I left this unit sitting around for so long, the RTC battery finally gave up, 430mV, the NVR battery was fine. Both were replaced anyway. When replacing the RTC batt, you must re-initialise the RTC, if not it will not come up properly on its own.

After replacing Q350, I spent hours setting up all (9) tuneable VRBW filter, using a stable external source, in my case HP-8663A. Most 6-50pf trimmer became scratchy and they could be tuned up for amplitude and shape, but the did jump around given the 1995 vintage.

These are no longer available, and I did not want to use cheap imitation products, so decided to use Deoxit D5 to help lubricate and clean. Both VRBW PCB were removed, each trimmer sprayed, left over night upside down to leak Deoxit out onto a cloth, leave 24 hours, use dry air from a can to remove ant Deoxit content. When you tune, that scratchy jumping has gone. I am keeping an eye out for VRBW filter movement due to Deoxit residue, so far it is good.

I was able to normalise after tuning. It is critical to ensure all digital setup and respective screen centre, time marker setup and amplitude step levels are accurate, the 2715 service manual is superior in the way it explains the adjustment process, however I have found some methods to be confusing and have found the right steps.

There is one outstanding issue, the 200Hz VRBW filter is out when tuning to 2715 manual, this can be very subjective to the internal 100MHz ref, my 2712 measured internal ref is 99.999223Hz, although setting up the 200Hz with an external ref like I did, the normalisation process dac cannot pull the 200Hz dac value in, therefore after checking with debug tool, the 200Hz fails as the 200Hz is about 1 division out, this needs manual intervention.

I am still not 100% clear on the internal ref normalisation and setup, I find the setup and setting the bits via dip switches somewhat strange, but it passes.

As my internal ref passes, and if I view my ref values, both the cal and cf ref freq are 99.999223. I would assume the centre frequency ref to be 100MHz , equal to the external ref it asks you to enter and assume it measures when you add a external stable 100MHz ref at -30dBm.

I would be keen to hear from other on their util 5,5,4,6 value?

Regards

Gerald
VK3GJM

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