Wednesday, 12 July 2017

'Scope repaired, and some fun with the Rectum Paralyzer

After some time spent blasting every moving contact in sight with Servisol, and resoldering any connection that didnt look pin sharp, my Hitachi V-212 20MHz Dual Trace Oscilloscope is now back working how it should be.


Inside the scope is very tidy, but clearly many years of grime had built up on the controls, causing all manner of irregular operation. Since I had to remove many of the plugs, these were all numbered with a Sharpie type pen so I knew where to put them back!


These two photos might seem pointless - actually, they are visual references as to where the correct control knobs and nuts were to be refitted! I also found that the protective film was never removed from the front panel, and was now in a poor state, Removing the film has brought the front panel back to near new looking condition!

After putting the scope back together, and replacing all the test gear on the bench where it should be, I started to have a play with the Spectrum Analyzer. (the weight of my analyzer is why it gets the name in the title! If your not careful with your manual handling these things can give you some interesting ruptures). I had heard that it was possible to get demodulated audio out, but wasnt sure how. Several youtube videos 'almost' helped, except that the described outputs were not as described on my instrument!

I sent a post in to G-QRP forum for assistance, but in the meantime, Sam came home from school, so I commandeered him for a few minutes to swap BNC connections around on the back of the analyzer, whilst I looked at what was on them on the 'scope. My connections are marked X, Y, and Z. One of these, Z I think it was, was tried first, resulting in a pulsing DC level. Clearly not that then. X was next up and showed a large triangle wave with a long 0v period, clearly the timebase! So the final Y connection was tried - yes! The 'scope trace showed just what the zero-span display of the analyzer did. Swapping the connection from the 'scope to the audio input of the 2955B, we had demodulated audio out of the test sets loudspeaker.

Using the 2955B isnt really convenient for this though, so I found a general purpose AF amp, based on an LM386, that was kicking about the bench, and connected that up. Perfect. I now need to find a box to put the audio amp in, so I can connect it neatly to the spectrum analyzer Y output for monitoring.

I hope eventually to us it for narrowing down airband transmissions, but im not accurate nor fast enough on the analyzers controls yet to pinpoint a transmission!


Car Stereo fault - Beat 485 repair

Well, marvelous isnt it? The Marconi 2955B goes on the blink, but before I can fix it the oscilloscope requires an overhaul. Get the 'scope dismantled for cleaning, lubricating and repair, and the flipping 2955B decides to start working again, leaving me with no idea whether something ive done, like reseating the cards, has fixed it, or whether the fault is indeed temperature dependent and likely to be a real twat to narrow down,

And while all this is going on, to really piss me off, the volume control on my car stereo fails! It randomly decides whether to go up or down whichever way I turn the knob!

I have a cheap after market DAB stereo, a Beat 485. This does DAB and DAB+ (why I bought it!) plus CD, USB etc. The volume control is not a traditional potentiometer, but a rotary encoder.

No photos for this fix im afraid! But it really doesnt need any, if your capable of doing this repair then you dont need a pictorial guide!

Opening the head fascia unit up is fairly simple - four screws and case clips, easily separated using a finger nail or similar thin object. The encoder turns out to be a mechanical type. This was to be expected as optical units are far more expensive. It also is a type with a central push button action (which I didnt know about its function on the stereo - turns out it selects the tone and balance options).

Once inside the head unit, the next job was to remove the encoder. This isnt particularly easy - there is a sizeable metal casing to it soldered down at either side, plus the five switch contacts (three one side, two the other). Ideally use a good hot iron here with plenty of heat capacity! I just plodded on with the 18W Antex, but the 45W iron would have been more sensible! Once desoldered, there are four folded tags on the underside that have to be lifted to dismantle the device.

Inside, the main encoder track could be seen, with two sliding contacts either side of the shaft. The pattern of the track could barely be seen it was so dirty! A blast with Servisol and a good scrub with a brush had it clean and shiny again. A gentle push with the end of a terminal driver reset the sliding contacts.

Reassembly was just the reverse of the above, ensuring of course that everything stayed in alignment, and then giving the securing tabs a good push down. After cleaning up the terminals, resoldering the encoder to the board was simple, as was rebuilding the head unit. I removed any other muck and dust at the same time.

A quick test back in the car shows that I now have a properly functioning volume control again!

Thursday, 6 July 2017

And now the Marconi 2955B is on the blink again!

Well, this just about takes the biscuit!

Not only do I discover that my understanding of the use of my spectrum analyser is probably woefully inadequate, and that my oscilloscope is playing up and needs a service, but now my 2955B test set is on the fritz! Again!

This time, I know im at serious risk of screwing the firmware if I dont fix it. So, its time to open her up.

The 10MHz Oven Compensated Crystal Controlled Oscillator (OXCO) unit, is mounted, horrifyingly, on the same board that generates the EHT for the CRT!

Rear of the Reference/EHT board
I very much dislike playing near EHT supplies! I know from experience what a belt from the anode cap of a brand new never been connected TV tube feels like - and dont fancy experiencing the same from an actual supply! So I very carefully ensured (with a pair of long nose pliers) that the reservoir capacitors were properly discharged!

Inside the Marconi 2955B. Note the warnings!
The actual OXCO's six pins desoldered very easily, and the whole module came out smoothly. I opened it up to see if there was anything obviously wrong inside

The 10MHz OXCO...
...is bloody complicated inside!
But other than the trimmer capacitor (which I seem to remember being told could be the problem) it all looks ok, albeit extremely complex! So, I connected it up on the bench. 12v at 120mA steady (450mA initial) for the heater, 5v via a 22ohm current limit for the oscillator. 

On the bench...
 And the damn thing seems to be working perfectly!

...and seemingly working perfectly!
 I have had it powered now for about an hour. Ive even abused it - heating it with a hot air gun until it was too hot to touch, whereby it crept up to 10.00015MHz, and freezing it until the heater current maxed out again, and it barely dropped below 9.99998MHz!

Im now at a loss, but my suspicions center around either a power supply fault, or something as insidious  as a dry joint or leakage due to old flux! Where I go from here, im not sure, and ive a ton of work piling up that requires a working test set!

Spectrum Analysis - Am I doing it Wrong?

After a lot of puzzling and pondering over the transverter, ive started to come to a slow realization that much of the problem might actually be with me!

What I mean is, im starting to think that my knowledge of how to correctly read the spectrum analyser, and hence my interpretation of the results of tests on the transverter, may be lacking in understanding.

Sadly, when I try and find tutorials online, they are either far too in depth, such as guides to measuring phase noise, or based on much more modern and advanced LCD FFT instruments, rather than the 'classical' CRT superhet instrument I have.

So I guess what I need to do is get to very thorough grip with my SA instrument and its operation, before I progress with the transverter.

Tuesday, 4 July 2017

FARS Rally - Sunday 23rd July

A quick and blatant plug for this exceptional radio rally!

The Finningley Amateur Radio Society rally is fast approaching! http://www.g0ghk.com/rally/

Located at the club HQ near Sandtoft, this rally is one of the best component and 'junk' sales around! I myself, with Bob M1BBV, will have a stall again this year, where I hope to be able to sell on some interesting items, big on our stall this year will be batteries for the Clansman PRC-349 radios (12v in a handy sized screw in pack), 'covert' diplexers, 'Panorama' professional antenna mounts, and Kenwood TK-349 PMR handhelds. Plus all manner of other junk!

I only hope to make enough money to cover my stall fees and the cost of whatever junk I buy!!!

More Transverter Trouble

Coming back to the 4m transverter after a short while doing other things, I found the bias current still unstable! So, I bit the bullet and added a 10ohm emitter resistor with a 1nF bypass. I can now get either an immediately stable standing current, or, it quickly (a few seconds) reduces down from around 40mA to a suitable 20mA as the diode stabilizes the bias.

Me, My Boys, Beer, Camping, and Radio

After a bit of playing around with Clansman radio in the garden, I moved on to build the PA section of the transverter. This involves the use of my one and only MRF237 RF power transistor... and it didnt go well!

First go at the 1W PA

 In the above photo,  the PA is built, and the transistor heatsink attached. On switching on - the PSU pegged the current limit!!!

Oh heck, not good! Lots of things were tried, resulting ultimately in nothing being connected to the transistor other than the collector to the tank coil and the emitter to ground - and it still pegged the PSU! Yet the transistor tested out ok every time! What on earth was going on?

Checks of the datasheet showed I had it connected properly - or so I thought! I asked on the G-QRP club forum, and they agreed the datasheet. Yet a check of the old Marconi RC-690 RF board the transistor came from, showed otherwise! It turns out that every manufacturer builds these with the expected 'tab to emitter' device layout - all apart from the original designers Motorola! Genuine Motorola parts, which this is, have the tab on the can next to the collector!

So the whole layout of the PA has to be redesigned! Luckily it seems the transistor has survived, possibly because I was very quick in turning the thing off before it got too hot.

BBQ fried eggs - new uses for old equipment panels
 There are a few other issues with the transverter though which im not so happy about. Firstly, I seem to be getting far too much leakage of the local oscillators 42MHz signal through the mixer and into the Tx chain - enough that it is present as an amplified carrier! Likewise, when the 28MHz carrier is fed in for testing, that shows too much leakage. Either there is a mixer problem (doubtful with an SBL-1 DBM) or I need better filtering before the PA stages. Im also worried about correctly terminating the PA during testing, before I have the LPF built. Im not sure of the correct impedance to add a temporary load.

Another issue, noticed yesterday - the Tx buffer transistor gets too warm! This should be a BF199, but as I didnt have any ive been using a 2N3904. Ive now ordered some BF199's to put the correct specified part in place.

The past few nights have seen amazing displays of noctilucent cloud over Europe. The image below was taken at 00:30UTC

NLCs

Ive also done a little bit towards finally finishing the TDOA direction finder, by making the main boom section of the antenna array. 

VHF TDOA RDF ANT

Whilst working on this boom, I discovered two of the elements of the UHF section of my tape-measure crossed Yagi for satellite work were off-center. Ive now fixed that, and tested the UHF beam with my MVT-7100 receiver on a number of passed of SO-50. Next step is to add a pair of headphones, couple the UHF beam to the VHF beam, attach the VHF transmitter - and see if I can actually get into the satellite!