Marconi TF2015 Fault

[Timmus66]

Hello, One of my Marconi TF2015’s has developed a fault that I am struggling to sort out, the problem is very low RF output from the front BNC but not on all ranges, it works perfectly on the 10 and 50 MHz ranges but very low on all the rest.

I do not own an RF millivoltmeter but do own a VTVM and RF probe so can take relative readings, TR8 on the A16 board is giving a good RF output throughout the ranges, increasing and decreasing as it should with the use of the +6db switch, the problem seems to be between the output of TR8 and the front panel BNC output which led me to suspect the A22 attenuator unit however the unit works fine on the two ranges previously stated and also attenuates the low output on all other ranges relatively.

I have carried out various DC voltage checks etc. but feel I am going over and over the same ground, I have been working on it all day trying to solve the problem so any fresh ideas for a tired old brain would be most welcome, I feel like I have missed something simple.

Thanks.

Tim

[MrBungle]

I just got one of these the other day. Rather like it. The attenuator was toasted on mine though and required a replacement resistor. Do the attenuators work properly on the lower ranges? Mine was identifiably knackered by switching from 0 to -10dB attenuation. Worth starting from there and working back.

[Terry_VK5TM]

As each range is a different oscillator, are their outputs all approximately equal into the rest of the stages.

Also, as these units are getting older, don't discount caps in the power supplies, the set-up margin for the voltages is only +/-100mV on the 22v and 19v rails.

[MrBungle]

Mine was surprisingly accurate and the caps were still good which was a surprise!

Aforementioned toasting:

[Timmus66]

Thank you both for the replies, I think my job for the day will be to remove the attenuator unit and check that out.

I do believe there is also a connector and resistor under the A16 board which I think I will also have a look at just in case.

Tim

[Terry_VK5TM]

The coarse attenuator can be checked without removing it.

Details in the manual, page 22 I think.

There are two attenuators, the coarse one is resistors, the fine one is electronic.

[MrBungle]

Yeah you can check without moving. I only got mine out because the checks failed. Was quite easy getting it out.

[Timmus66]

All repaired

The culprit was A0 R39 a 50 ohm resistor that is a bit of a pig to get at, it sits in a brass fitting under the A16 board, after removing the attenuator unit I tested R39 in situ and it was giving a reading of 285 ohms, the manual states it is a quarter watt resistor but to be on the safe side I replaced it with two 100 ohm 1/4w resistors in parallel to aid with any dissipation.

Not a job I would like to repeat too often but at least she is back in working order.

Thank you both for your help.

Tim

[MrBungle]

Ugh hope mine doesn’t do that. Well done though

[G0HZU_JMR]

That 50R resistor is used to define the output impedance of the RF source so it will probably be a fairly decent resistor. It ideally wants to look like 50 ohms resistive over the full frequency range of the generator. However, the sig gen only covers up to 520MHz so I would expect that a couple of small (low capacitance) 100R resistors in parallel will be OK to use here. It could even be better than the original 50R resistor but a lot depends on the parasitic inductance and capacitance and the technology used inside the 100R resistors. If you have used a typical pair of small metal film resistors here I'd expect it to be fine up to 520MHz.

I've had my TF2015 since the 1980s. I rarely use it these days but the tuning cord snapped on mine about 20 years ago. I managed to find some black lacing cord and used this to repair it. The lacing cord is still working fine 20 years later!

[MrBungle]

Tuning cord on mine seems ok so far. Fingers crossed. I need to do some work to calibrate each range as they are all completely out. I use it with a counter mostly so not end of world but would be nice to be in spec.

[Terry_VK5TM]

One of the jobs on the 'to-do' list for mine is to build a phase lock device to keep it on frequency as mine tends to wander a little bit.

I know there was an add-on for the TF2015 (the TF2171) to do that but in all the years I have had mine, have never seen one come up for sale.

[MrBungle]

Yeah it drifts terribly on the higher ranges. Mine settles down after about an hour. I was planning on doing the same thing if I couldn’t get a synchroniser. There is an interesting technique described here: http://www.hanssummers.com/huffpuff.html

[Timmus66]

The 50R that was originally fitted did look a bit more ‘beefy’ than a typical 1/4w modern day metal film resistor, I had nothing like it in my parts box, I have attached a photo of it as my knowledge of vintage components is very limited I’m afraid.

After using the Marconi TF2015 for over a year for CB radio repair and servicing I managed to get a TF2171 along with a spare TF2015 on a well known auction site recently, It was sold as untested but only required a small amount of work to get working correctly, It does make a big difference to the usability of the TF2015, Unfortunately they seldom seem to come along as an individual unit and usually sold as a pair, well worth having though.

Tim

[MrBungle]

Looks like it was once a 1/4W carbon composition resistor to me.

I think I shall look out for a pair. I got my TF2015 from the same place. Looking at some of the parts inside the unit I could do with a spare "just in case" (any excuse to collect more test gear! )

[G0HZU_JMR]

I also mainly used my TF2015 for CB and ham repair work. If left on for about an hour mine was stable enough for this kind of work. In order to connect a frequency counter I made up a screened attenuator to connect to the 'counter out' connector at the back. I found that I needed to attenuate this signal because it is so large and it could leak out into a radio and cause problems.

The main niggle with the sig gen (for me) is that it only covers down to 10MHz so I made up an external downconverter for mine. I still have it somewhere. It has a 30MHz LO and it downconverts 30.001 - 40.000MHz to 0.001 to 10.000MHz with a conversion gain of 0dB. This allowed me to generate signals down at 455kHz or across the ham bands in the lower part of the HF band and the sig gen levels were the same as the raw sig gen. It worked down to about 1kHz at the RF output.

[Timmus66]

You can never have too much test gear, I find most of the time equipment listed as untested or for parts requires very little work to get functioning again, and can be got for a reasonable price.

When I bought the pair it was my honest intention to sell the spare TF2015 however like you say, a spare is always handy

Good luck in your search for a TF2171

Tim

[MrBungle]

G0HZU_JMR: I'm doing exactly the same thing with mine already. I built the 10MHz source and multiplier last week. I may have got the idea from one of your posts



(also found the AGC output on my counter is good for peaking tuned circuits!)

Timmus66: Yes I've got a test gear addiction going here. I have stacks of it all over the place