PIN diode band switching yaesu ft-767

[Steve G4WCS]

Evening all, hope everyones not suffering too badly with the weather and cabin fever setting in.

Ive been doing some further investigation with the 767 receiver. Using the crystal marker as a signal generator, I have confirmed it is profoundly deaf between 0.5 and 1.5 Mhz.

Geoff at Castle Electronics had said that they can suffer from PIN diode problems on the RF unit, and if there is a leaky one on one of the filters it can leak signal on the other bands. Ive had a look at the rf unit diagram and one of the filter sections does cover 0.5-1.5, so Im suspecting this (however havent discounted a switching relay yet).
Unfortunately he has stopped servicing these, and the days of nipping over to see Harry Leeming are long since gone

Can anyone explain to me in simple terms how the PIN diode switching is implemented, how they can be checked (ie can they be checked in situ or do they need removing) or shall I just bulk replace them ?

They are an ISS83, is there a readily available equivalent ?

Thanks in advance

[kellys_eye]

Q31-Q39 are used to apply a DC bias to the filter diodes forward biasing them and allowing the signal to pass through the relevant L-C filter.

Check for the relevant DC levels on the transistor collectors and that it reaches the relevant filter.

The diodes can be checked in-situ with a standard test meter (assuming it has a diode tst function) and should read around 0.2V or 'infinite' depending on polarity.

[Radio Wrangler]

A PIN diode is a deliberately spoiled diode with a layer of undoped silicon in between the P type and N type doped parts. They are slow. Very slow.

At low frequencies, they act like a diode, though not a terribly good one. At higher frequencies they look like a resistor. What makes them valuable is that the resistance value can be controlled by varying the DC forwards current through the diode.

Some are optimised for use as RF variable resistors. PIN diodes have a parameter 'Carrier lifetime' which essentially says how long it keeps on conducting after you take the forwards voltage away - Yes! PIN diodes do carry current in the reverse direction, for a little while. The carrier lifetime is a marker sor there the bounadry between diode and variable resistor action can be found. It isn't a hard boundary. For use on the HF bands, you need a diode with a carrier lifetime of at least a few microseconds for it to be reasonably linear. Once you are into the variable resistance frequency band, DC forwards current will program the effective RF resistance for you. You can get values from 0.1 Ohm to kilo Ohms. The more DC bias current, the lower the resistance.

Other PINs are optimised as switches A typical small signal one will look like an Ohm with 20mA forward current and you'll get a couple of nH stray from the package. Reverse bias it with 20v and it'll look like a couple of pF.

They have no moving parts, don't wear out and are quick in operation.

In that receiver, you could see if they are the cause of your deafness by lifting one end of each diode for each filter you don't want in, and fitting a short in place of each diode for the signal path of the filter you do want in.

You will see DC bias current paths switched by transistors or switch contacts supplying DC current to the diodes through resistors and sometimes chokes. At the same time you will see the RF paths connected in and out via DC blocking capacitors.

Tedious, but it gives a definite result.

But you don't just want to make sure the wanted filter is in, you need to be sure the unwanted ones aren't shunting it.

David

[Terry_VK5TM]

1SS83's are not PIN diodes, they are HV switching diodes and are a known problem in both the '767 and '757.

If you are lucky, you may get some genuine ones on the usual auction sites otherwise commonly used replacements are 1N4148's (they are lower voltage spec but seem to hold up) or 1N4007's. Any HV switching diode of similar spec should be OK.

The common servicing regime used to be to replace all the switching diodes rather than just the faulty ones (they go leaky) and it may not be the ones for the band that is deaf that are faulty but ones in another band.

To test, snip one end and test for leakage, easier than removing the board, but you will have to remove the board any way to replace them, hence the blanket replacement approach.

[Radio Wrangler]

Some high voltage switching/rectifier diodes like the 1N4007 (..8..9) are PIN diodes internally. The manufacturer's data sheet says nothing about this. You can use them as RF switching devices, but their behaviour is not optimised to that purpose. Mostly the capacitance will be somewhat higher than you'd want in a small signal application, giving poor isolation in the 'off' condition.

Quite a few of the Japanese HF transceivers used diodes that were not RF type PINs as a cost saving measure. Ulrich Rohde (who owns 50% of Rohde & Schwarz as a day-job) wrote an article about substituting proper intended for RF PINs in the Icom IC-765 including measurements of intermodulation distortion, blocking etc. I think it was published in QEX.

Unfortunately, the diodes he used, after trying many were the HP ones . HP's semiconductor arm became part of Agilent, got sold off to appear as Avago, owned by a financial consortium and then they took over Broadcom. It then obsoleted its entire range of superb RF diodes and MMIC amplifiers etc. No longer available, and the industry is still looking for alternatives. No other supplier has anything like such a comprehensive range.

Other diodes are available from the likes of Skyworks and M/A Com but they take more finding and are generally more expensive.

NXP (nee Philips Semiconductors) do some candidate parts but you have to search through a lot of data sheets to find them. Their website is terrible and seems determined to hide all sorts of things. Prices are good, but you may have to buy unwanted quantities unless Digikey or Mouser stock them.

What the PIN can do that the ordinary diode cannot is to carry a far higher RF current than the bias current turning it on. The transponders I've spent the last teens of years working with turn a PIN on with 200mA, and then put about +/- 10A of RF through it... at a GHz.

I definitely agree with blanket replacemant in this case.

You may also find others switching your IF filters.

David

[Terry_VK5TM]

And all the Philips ones are SMD, so somewhat fiddly to use as replacements.

Sent Steve a link privately as to where he can get the originals. but as i said in the message, some others that are advertising the 1SS83's are showing pics of ones that are definitely not them.

[Dual Standard]

Have a shufties here https://qso365.co.uk/2011/03/replacing-the-schottky-diodes-in-my-ft-847
I haven't compaired the FT-847 & FT-767 circuits but Yaesu were never ones to re-invent the wheel so I would be surprised if the section in question was radically diffferent. The BAT85 pin diodes used are still readily available from Mouser and probably elsewhere too.

Mike

[Radio Wrangler]

If getting PIN diodes, make sure the carrier lifetime is long enough to cover your lowest frequency. Some are good for VHF/UHF/microwaves only.

David

[Steve G4WCS]

Thanks all. Will have to have a check over when I can be bothered

[G0HZU_JMR]

Hi Steve, I'm not familiar with the FT767 radio but I had a quick look at your schematic and I can guess a few things.

Probably the most significant thing is that it looks like this radio is meant to be deaf across 0.5-1.5MHz as it has a resistive (21dB?) T attenuator ahead of the relevant filter section using series resistors R176 (470R), R177 (100R) and R178 (470R).

There has been a lot of misinformation in ham circles about diode switching in radios like this and there seems to be a bit of a fad for replacing the factory diodes with PIN diodes. There have been lots of arguments over the years if PIN diodes offer any benefit (in terms of IMD) over regular silicon diodes.

However, one way to boost the distortion performance of a diode switch like this is to raise the impedance of the switched filters well away from 50R. This reduces the RF signal currents in the diodes. The diodes can be thought of as non-linear resistors and it makes sense to minimise the RF signal current through them.

Therefore, I think the designers of your FT767 have chosen to do the Rx preselector band switching at an impedance of 450R. The writing is fuzzy on your schematic but It looks like there is a 9:1 transformer at the input to these filters.

This means the T attenuator has been designed to add 21dB of fixed attenuation inline for the 0.5-1.5MHz frequency range. These filters do appear to have been designed as 450R bandpass filters. The 21dB attenuator is probably there on purpose because this is the MW band and there could be huge signals on this band at night.

Therefore, I'm not sure if your radio is faulty. How deaf is it on this range?

MW attenuation is common practice for (ham) transceivers, they are designed down to a price with good low signal performance (just what you want on the amateur bands). MW has some whacking great signals and can overload such a set, funny it is only MW, must be for the American market. Here in Europe LW can give whacking great signals too, mind you most of these radios don't go that low.

[Steve G4WCS]

That would make sense Jeremy. It is less sensitive on paper over that band. Did a quick re run of the crystal marker test and it seems to be receiving better than last night.

https://youtu.be/6BVLrUU4Szo

Perhaps I should get it checked out on a calibrated signal generator before pulling it to bits. Maybe I expected more activity down that end than there actually is, or my wire isnt long enough

[G0HZU_JMR]

Looking at your youtube video it does look like there could be >20dB attenuation below 1.5MHz and this could be normal because of the T attenuator on this range. Probably best to check it out with a sig gen as you say.

Looking again at your circuit there is at least one series diode that still runs in a 50R section of the signal path. This diode is just ahead of the 9:1 transformer and will probably be the one most vulnerable to generating IMD distortion and it would ideally need to be a very good diode. This is one place where a suitable PIN diode could offer lower distortion although it would need to be a fairly exotic PIN diode with a long carrier lifetime as it would be exposed to LW and MW signals.

I did look at this stuff a few years ago and I did manage to find some chunky PIN diodes from Macom that worked quite well with very low distortion below 1MHz in a 50 ohm system. However, these diodes cost about £20 each.

I'm not sure PIN diodes would be needed for the preselector switching diodes because these diodes are switching 450 ohm impedance filters. The RF signal currents passing through these diodes will be a lot lower compared to a 50 ohm system. I'm not sure at what point the larger RF voltages (associated with a 450 ohm system) impact the IMD of the diode switches but in the past I noted a huge improvement when comparing the IMD in diodes in a 200 ohm system compared to diodes in a 50 ohm system.

[Steve G4WCS]

Been perusing the circuit again and noticed it does use the same lamp “fuse” as the FT-757. (Pl01) . Again though its a sig gen job before taking it apart.

[space charge]

harry leeming covered this problem in his pw column . i would expect its on the cd covering his columns available from pw which is indexed. rather sadly last month ended his regular pw column
.regards pierce ei7ka

[Steve G4WCS]

Originally Posted by space charge

harry leeming covered this problem in his pw column . i would expect its on the cd covering his columns available from pw which is indexed. rather sadly last month ended his regular pw column
.regards pierce ei7ka

Thanks for the heads up on the CD. Will definitely be treating myself

[G0HZU_JMR]

If I get a chance this week I'll compare a few diodes in a 50 ohm RF switch for loss and distortion. It's also worth checking the datasheet for the capacitance of each diode type. The 1SS83 has about 1pF capacitance for example.

A few years ago I did similar tests and I made up a test jig using a 1MHz LPF + 1.9MHz notch filter and also a 1.9MHz narrowband diplexer to allow fairly serious testing for IP2 and 2HI distortion. The idea was to combine signals in the MW band to generate a distortion term at 1.9MHz in the RF switch. The various filters ensure that the weak link for distortion is the diode and not the test gear. I think I posted up some of my results on QRZ.

I'll warn in advance that I recall that the 1N4148 diode performs really badly in this 50 ohm test. The winner will be the £20 Macom PIN diode but I do also have some nice PIN diodes from Microsemi that are cheaper and also other classic diodes like the BA282, 1SS53, BA479 and 1N4007 to try.

Testing with big signals down in the MW band really is quite harsh but it will show up the differences in these diodes. I don't have the 5082-3081 PIN diode that Ulrich Rohde used but looking at the datasheet I think this diode would have the highest insertion loss. The RF resistance of this diode at 10mA bias is 10 ohms. This would be OK in some radios but if the whole signal path and band switching is done at 50 ohms this would become quite lossy with several of these diodes in the signal path.


I recall some of the older Yaesu HF radios used low voltage Schottky diodes for the band switching and these were prone to failure. A Schottky diode seems an odd choice of diode for this application and I think Yaesu stopped using them in newer radios.

Looking at the datasheet for the 1SS83 silicon switching diode it looks to be a fairly rugged diode. I'm not sure why these would prove unreliable in this application. I'd expect this diode to be more robust than the earlier Schottky diodes.

[Radio Wrangler]

Philips also had a number of diodes that they described as 'Band Switching'. No mention of PIN. They were intended for multiband tuners in TVs.

Navigating their website to find just what they have is a nightmare, everything is presented through mass market product marketing goggles. You have to think not of what something is, but of where it might be used.

David

[Steve G4WCS]

Ive some hp 5082-2800 and 2810 diodes I can send over if they are suitable for your testing

[mickm3for]

Hi i can vouch in4148 work well in this position change them all Mick