Tuesday, 4 July 2017

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


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


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!

No comments: