Marconi RF sig gen - no signal.

[Diabolical Artificer]

I was given this big old boat anchor of a RF sig gen by Frank ont forum. It's a Marconi TF144H. After a suitable soft start on a variac and LL I found there was no OP.

I checked the PSU and established that it had HT and LT. I also tested most of the valves and thought V101 a QV03-12, the oscillator was kaput. I tried a known good one, still no RF out.

I found there was no HT present at the anode of V101 or any voltages on g1 or 2. I traced the source of HT and after taking the RF box lid off and removing the coil turret I found a resistor R195 (47r) was completely burnt out. It seems to supply the coils and caps and hence V101's anode etc. I've replaced it with a 3w metal film resistor (original 1/4w MF) as that's all I have.

One other thing I've noticed. There are two neons used on this sig gen, one (Im sure it's a neon, described as "Type A15 in parts list but symbol is different to a neon - see pic 4) is TH201 . this goes to the cathode of the 6U8 ( mod osc + crystal cal) On checking I'm getting no crystal signal when I select "crystal check". It wasn't lighting up, so I soldered on a temporary neon - a bog standard indicator, N-2 I think. This now lights on various settings when the function knob is turned. V104 doesn't light either, though it may do after I power it up with R195 replaced.

I also found that R233 which goes to the carrier interrupt SW was scorched too. Before I power up the sig gen after replacing R195, I'd like to have an idea why it burnt out in the first place, lest I do any damage.

I'm not that familiar with valve oscillator circuits, what should I be looking for as a possible cause for the R195 burning out. Also how does V101 work? I have an idea that it's a colpitts oscillator "driven" by the LC tank circuit made up of coils and variable caps on the coil turret and that its screen grid is modulated to get a modulated signal.

This is a beautifully made bit of kit, the coil turret is a work of art.

TFL, Andy.

[Diabolical Artificer]

More schematics.

[ms660]

I think TH201 is a thermistor.

Lawrence.

[WME_bill]

TF144H series II.
The TH201 is a thermistor, type A15, made by STC. Its resistance is 100K at 20degC, reducing to about 330ohm when dissipating 60mW, ie when hot.
I suggest leave it alone for the moment. When you have HT back and the thing is more or less working, feel it with Carrier Off, when it should be hot. It serves to control the cathode voltage of the Amplifier when the carrier is switched off (S200B. position 1) to help keep a more steady load on the power supply. Not vitally important. They didn't have it on the seriesI version.

If that resistor feeding from HT to the oscillator is burnt out, it suggests that something is shorting from the coil turret assembly to chassis.
Have you checked for shorts from the 250V rail, top end of the resistor R195 to chassis at all positions of the wavechange switch. It could be just one position; when check the coil, capacitors and turret contacts for that range.
If all ranges, it could be shorting blades on the variable capacitor Main Tuning. There is a point on the wavechange between ranges where no contact is made to any of the studs, where exixtence of a short could indicate the capacitor.

I don't see any other neons. The panel light is a standard 6v 0.15A bulb.

Be especially careful with the Level Monitor assembly, and particularly the thermocouple TC102, which looks like a glass blob with wires. It is very fragile (electrically) and unobtainable anywhere in the world. I suggest unsolder the feed R186 at C195 while you are working on the machine.
The diodes on the Automtic Level Control are no trouble HD1870, Ge small signal, as GEX66 / AAZ13 / OA47, or small modern schottky.

I assume you have the manual. I have the manual and details of the thermistor, diodes etc if needed. I sold my version of this some years ago when I found I could no longer lift it on the bench. Lovely piece of kit.
wme_bill m0wpn

[Diabolical Artificer]

I mistook TH201 for a neon a it's a glass bulb with two rods inside like a neon. I'll leave it alone for now.
V104 also looks like a neon physically. It comes off g2 of V101 part of the ALC circuit I think. Although it could be a VR. I'll also leave this be.

Thanks Bill for that advice, I'll check for shorts before powering up and disconnect the thermocouple. It says in the manual it is fragile.

A.

[Diabolical Artificer]

I now have a good signal on the direct output but nothing on the RF OP. It could be ignorence on my part, will read manual. I also have a crystal OP. I tested all ranges and have 6v P-P on all apart from J - 8 - 16 Mc's when it jumps to 12v P-P. I could find no reason for the blown resistor unless it was caused by the interupt carrier switch being left on for too long?

I disconnected the thermocouple as directed. What is best avoided in order not to damage this?

Andy.

[Diabolical Artificer]

Still finding my feet as regards using a RF sig gen, so I may get things wrong here.

I've now got an OP on the RF BNC connector. Switching the "direct out" to normal from high and voila, OP out, however the "set carrier" knob although it increases the amplitude of the OP somewhat,isn't normal, I think. Am sure it should do more.

I set up as per manual, EG set range to F (535 - 1605kc's), set dial to 1000, set func knob to CW (what does CW mean? Carrier width??) now bring meter pointer to set carrier mark by adjusting set carrier knob. When I do this the needle barely moves and as mentioned above barely increases amplitude.

There is a lot of gumph on loads in the manual, some of which is heavy going. While I understand that at RF any reactive load will have an effect on amplitude, should I be sticking a load on the OP? I tried a 50 ohm terminator with no change.

I suspect another stage in the path of the signal is kaput. The modulation knob doesn't work either. That said I'm getting a max of 4.8v P-P OP. So maybe the metering circuit is at fault. thinking out loud here. The problem is I havn't used a RF sig gen before so have nothing to compare to.

Any thoughts comments welcome, Andy.

[ms660]

CW means Continuous Wave, ie: no modulation.

Lawrence.

[Dickie]

If you search the forum for "TF144 thermocouple" you will find more than enough information on this generator!

[Diabolical Artificer]

Thanks Lawrence.

Will do Richard.

Swapped out a few valves, IE V102 a 6U8 for an ECF82, no change. Then tried swapping out the others, again no change. Not very systematic I know, but worth a shot.

As I'm not sure which section or circuit is the cause of problem, I'll try a quick setup/calibration to see if that gives me some clues. As I understand it, V101 oscillates, then it's screen grid is modulated by V204. The other sections/circuits keep everything like gain, time etc bang on. Again thinking out loud, so V204 is worth a look see.

I've checked the crystall osc and am getting a 1kz tone at the phones out, so that bits working.

Onwards, A.

A.

[Diabolical Artificer]

Read about the TC, scary stuff, so I'll disconnect it again. Will an AC meter do in order to check if carrier OP is going up while tweaking things? I could really do with some guidance here lads as I do not want to be responsible for killing the thermocouple.

I'm stopping work on this until I have a better idea of the craic.

Andy.

[Biggles]

There is always one thing that RF generators are prone to, namely someone transmitting into the output by mistake and damaging output circuits or attenuator networks. I've seen it done a few times in radio maintenance outfits. It may be worth checking that area for obvious signs of distress. Just a thought.
Alan.