FT 102 Pre Amp Fault

[ChristianFletcher]

I have just bought an FT102 and by my standards this must now qualify as vintage its over 26 years old and has a valve PA stage. (was 1980 really that long ago)

I have a problem with the Preamp push button. When activated the incoming signal level actual falls. I assume that this is a problem. The preselect TX tune does not apear to do much and the TX HUM increases with the frequency selected.

I think this set has a few faults does anyone own or has experience with them ?

Regards Chris.

M3SNC as of december 28th 06.

[Geoff G4ICD]

Usually a relay problem in the front end, very common with these sets, see the workshop manual for details.......Geoff G4ICD

[Sean Williams]

AARRRGGHHH......

Well, once you have sorted out the countless Relay faults you should be OK...

Have spent countless hours trying to sort these out (Where are you Onewatt - time for you to comment)

They are a half decent unit once you have sorted out the iffy rx......

Good luck - you will need it!

Sean
M1ECY

[ChristianFletcher]

Yes these are prone to relay problem from the amount of web space devoted to changing them. Its obvious what relay is brought in with the preamp so this is perhaps a good place to start. I believe there are also a couple of FET transistors in the front end that are prone to failure.

Can anyone recomend a good rig doctor in the north of england. I unfortinately paid far to much for this set and would rather not make the problem any worse.

Regards chris.

[Sean Williams]

Hi Christian,

I doubt you will find many service centres that will bother to quote you for repairs on this radio - The estimate charge will probably be around £75.00, and that is before any work is carried out!

Ok, so, there is nothing in the radio that you can ruin, if you are careful.

On the receiver board there are several relays, one of them is wedged below some control shafts - this is the most likely culprit - it is also different to all the other relays - this is the one that switches the signal path via the Preamp.

You will need to remove this relay, and have a go at cleaning it first - this might work, as they were not hermetically sealed devices - so oxidisation of contacts can be a problem.

There are several tips on the web, along with an owners club.......

Do be very careful when using this radio - It is not designed for low power use, and will not appreciate being throttled back to 10w output - you will need to learn how to "tune up" the PA stages in quite short time, otherwise you will cook the PA bottles, and risk flashing over the tuning capacitors.

There is also a "fuse" (Bulb) on the antenna changeover relay board (located in the PA compartment - Be careful, as the power supply generates about 600v at half an amp - I still have the burn scar on my hand from my last encounter.....) This "fuse" can be a source of trouble, as can the ACO relay...

Feel free to ask more questions - it is all coming back to me.....

Oh, Get a service manual - you will need it.

73
Sean
M1ECY

[ChristianFletcher]

Yes thanks for the info, I guess I will have to open it.

Thanks Chris

[johnib1]

Hi
Have you seen this site?
I have a FT102 my self and after fitted new relays it works just fine.

john
http://www.jvgavila.com/ft102_1.htm

[jim_beacon]

Chris,

if the relays are only passing received RF, it is probably worth removing them, and passing a DC current of a few milliamps through the contacts (try swithcing a LED on and off a few times with the relay). This will reduce the contact resistance back to a suitable level, and improve the performance.

Measuring the contact on resistance can give a reading of around 20 Ohms, which is enough to kill most front end circuits, after the DC treatment, the resistance should drop back to less than 1 Ohm.

I discovered this when I had a Marconi TF2016 signal generator to repair - in these, the relays switch around 30mV of RF, and once the contacts start to oxidise, the oscillators stop running correctly. Most small relays rely on DC wetting to keep the contacts clean, and this is often absent in RF designs.

Jim.

[ChristianFletcher]

I have now replaced all the relays and the set still goes deaf when the RF Amp is brought in. The amp circuit is quite simple and I can only suspect a fault with the RF JFETS and this is a known problem with the FT102.

However I am trying to source two replacements and have not found a supplier, can anyone help. I have tried RS, RAPID and Farnell.

FET 2SK125Y

Best Regards Christian.

[Sean Williams]

Hi Christian,

Ok, Well, replacing the relays is still a sound idea.

I seem to remember that there are some of these fets elsewhere in the front end board - try swapping them over to confirm the fault.

I have a scrap FT101 here - I will have a nose and see if there are any of these devices lurking about....

I see that JAB electronics still have a listing for them - Give Peter a shout and see what he can do.....

Cheers
Sean

[ChristianFletcher]

Thank Sean

I have emaile JAB.

Regards Chris.