Marconi 995b/2 R.F. Gen High H.T. current.

[vintage_8bit]

Hi all.
Firstly, thanks to a fellow forum member, Mike for forwarding the correct manual to me. Applicable extracts attached below.


See attached image of V11, it's a solid state device, don't know if its home made or not, it's been done very well. I intend to put the right valve in there anyway as I'm not sure if the resultant higher H.T. is a good thing. There no series resister added.

My fault is...... With the PSU disconnected HT1 measures only about 400 Ohms to chassis. The generator was working until I noticed the input current had shot up, followed by a sound from within the chassis. No smoke or smell. I think I've traced the short to filter L2 or (C10 which I couldn't find.) I cant see any trapped wires. L2 is buried deep in the chassis and before I try and get to it I wondered if anyone here has tried to get to this filter. Not sure if it's a front panel off or remove the R.F. box. I might bypass it for now to see if everything else is ok.

I also found an intermittent connection to the feed of neon stabiliser V9, a connection (R49 4KOhm) that had never been soldered!. This would of caused HT3 to be rather erratic. I cant see any connection to the main fault though, other than it must of been upsetting all the H.T. lines and coupled with the higher voltage from V11.

[Silicon]

I have a different version of the 995.
I removed the rectifier valve and made up a solid state rectifier to cut down the amount of heat generated. I added a simple voltage regulator.

Have you replaced the paper insulated capacitors and checked the electrolytics on the main chassis?

[vintage_8bit]

Thanks for your reply Silicon.

No not yet. There doesn't appear to be to much ripple on HT1 & HT3 from the PSU assembly. But I will be going through those caps and the main chassis electrolytics, when I've fixed the over current, I don't believe it's the electrolytics, wish it was!

I have just finished temporarily bypassing the filter L2/C10 and the generator appears to be working. So I fear I've got to get at that filter, as now it's isolated and still showing just a few hundred Ohms to chassis. I was hoping that someone on the forum may have gained access to these filters before. So just looking now, to see the best way.

Colin

[vintage_8bit]

I've got access to the filter, hooray. See attached. It was a bit of work but more because I wasn't sure about the order of stripdown. I can see C10 now as well. I've not investigated further yet, as its Tea time.

[vintage_8bit]

The fault was a breakdown of the mica insulator. A black tell tale mark can be seen on it. It took 2 soldering irons on each filter pin, in turn before I could melt and remove the solder and lift off the silvered plate.

[Silicon]

Could that be a combined mica capacitor and spark gap?

Some transistorised car radios had similar devices.

[vintage_8bit]

I dont think so. I think it's just the way they chose to shield the connection. Although you can see there are unused holes in the mica. This would give around an 8 tho" spark gap? As you can see in the photo there is a capacitor on that side of the filter. I have found some insulating material and made up a replacement without the extra holes. I gave it a 500V insulation check after assembly and all good. So just got to fit everything back together. Then see if it still works.

[vintage_8bit]

So the generator is now functioning. But I have low output on a couple of ranges. Whilst investigating this and looking at the HT voltages, I noticed 3 off resistors on the HT2 rail, these are switched in and out depending on frequency range selected. These resistors top right on the circuit are R35,36, & 37. On my generator these resistors seem to be fitted in the reverse order to the circuit. I assume these are fitted to adjust the HT2 rail voltage by performing a potential divider with R16?. This voltage is applied to the screens on the multiplier stages that are not on the selected frequency range. HT2 is derived from HT1a via R16 and HT1 which is also unregulated. So has anyone else noticed this reversal of the above resistors. My generator output is very dependent on the incoming mains. In my case varying from about 220 to 240V.

[vintage_8bit]

Just looked on an earlier model circuit. Here the circuit layout is drawn differently. It is however connected the same way in the area concerned, with the exception that the resistors mentioned in post above are drawn in the same order as fitted in my generator. So I think this is a drawing error. I would still be interested to hear from other owners on the stability of the output voltage when the mains fluctuates and how good there generator voltage stability is when moving between ranges and through the frequency spread within a range.

[vintage_8bit]

Basically its all working now but....

I am now investigating the wave shape of the output, with no modulation. In the attached image you can see the master oscillator output and the output from the X3 stage. I expected a constant amplitude here. As you can see the X3 stage oscillation reduces for 2 cycles before it gets the next kick from the master oscillator. This happens through the entire frequency range of the X3 stage. Having no experience on this type of R.F. gen, I'm not sure what to expect. It doesn't look right to me. I've only worked on turret tuned gen's before so any comments would be gladly welcome. Colin

[frsimen]

Colin,

What you are seeing there is breakthrough of the master oscillator signal. It's normal to see that on the TF995 family of signal generators.

Paula

[vintage_8bit]

Thanks for a quick reply. I will probably not improve on that waveform then. I've just found a post, from a few years ago where the chap had the same problem and posted exactly the same waveform as mine, on the same type of generator. He seemed to get some improvement by adjusting the relationship between the vanes on the main tuning condenser. I did try adjusting C47 trimer instead, but no improvement in wave shape could be made. See attached extract of circuit for all interested. Colin

[vintage_8bit]

So 2 more questions for the Marconi users.

1. See attached for image of output, this time range 3 at 50MHz. Do you think that waveform is also normal for these generators?.

2. On the 1st range when you get down to around 400KHz the output falls away so you can't maintain the output at the "Set RF" point. It does however continue to go down to around 30KHz. So is this fall in amplitude normal?. I imagine this generator would not normaly be used at these extremes though. Colin

[frsimen]

Yes to both questions. You may get a small improvement by cleaning the contacts on the variable capacitor, but that result is much the same as on my TF995.

[vintage_8bit]

Ive spent quite a bit of time on this. I have a pile of high value resistors and faulty capacitors that have been changed. I found a few of the valves had inter-electrode leakage as well. It's a shame that the output waveform is not better. I will look at those contacts and probably go through the allignment procedure again. So time then, to call it a day on this one. Colin