Marconi TF2603 RF Millivoltmeter

[Phil G4SPZ]

This ex-Royal Navy instrument works on all ranges, but the 'balance' control seems to have no effect. The net result is that it's impossible to zero the meter on the higher sensitivity ranges, as there's a negative bias off towards the meter's left-hand end-stop.

I have an instruction manual from 'electrojumble.org.uk' which contains a rather poor quality copy of the circuit diagram. I found a slightly better A4 copy at RM.org and this shows the balance pot being fed with DC via what looks like a 2.2 Meg resistor, which is my first suspect. But this instrument looks very complicated... I've read about it containing a chopper circuit, and a driver board for which I don't have a circuit.

I'd like to get this instrument working properly again. Please can anyone help with a better circuit, a manual, or even an explanation of how it all works? Many thanks in advance,

Phil

[G4XWDJim]

Phil,

I have the manual for the TF2600 Sensitive millivoltmeter - I think. It's in my storage facility so I won't be able to look for a few days. I don't know whether it will be similar enough to help but you're welcome to anything I have.

Jim

EDIT,

I've just looked at the 2603 manual on Electrotanya and it's completely different from the 2600. It looks to be a good clear copy so you should be able to download it yourself. http://elektrotanya.com/marconi_tf26.../download.html

Jim, any assistance short of actual help!!!

[ronbryan]

Jim

The TF2603 is an RF milli-voltmeter and is all transistor. Even the diode RF probe uses crystal diodes rather than an EA52. The TF2600 is a different animal, intended for AC measurements to about 5MHz, but more commonly used for audio work. The TF2600 is all valve, although a later re-vamp, the TF2600B was transistorised.

Ron

[Phil G4SPZ]

Jim,

Thank you very much for that link, I'll download the manual this evening.

Thanks to Ron also. Cheers both,

Phil

[WME_bill]

TF2603 manuals.
If you are still stuck, I have the original manual for the original 1965 version, with germanium transistors (mainly), and copy manuals for the later 1977- 1984 version with silicon transistors. The circuit diagram from the earlier version is much clearer, and I mainly used that when working on my later version unit.
Troubles with the balance control makes me wonder if one of the diodes in the input rectifier stage has been damaged. They are small signal high speed germanium: AEI type CG91H, which I believe is a renumbering of the earlier GEC GEX66, and later became Mullard AAZ13. All 8V, very fast, very low capacitance. Hughes HD184x family similar. If you want a pair, I may be able to help, as I seem to have a junk box full of small diodes. I used these when repairing my TF2603, and the end result was fine.
Modern substitute are schottky, such as BAT17 or 5082-2800, or what you can get. wme_bill

[karesz*]

Hi Jim
I loaded up today a more sharper and full SM over TF2603 as you knowed!
Take it yourself for usage please...
K.

[Phil G4SPZ]

Karesz*,

Whilst acknowledging with appreciation the work done by those who make this service manual available, is it possible to simply download the PDF file without having to install the "DailyLocalGuide extension"? I'm reluctant to install any such unwanted third-party software without knowing the reason for having to do so.

Bill,

If this download fails to materialise safely, could I take you up on your offer of a few pages from your printed manual, please? Basically I need the circuit diagram and a description of operation or block diagram, or whatever. My meter is the later style, dated 1982.

Thanks for the tips about the diodes. Are these the ones in the probe itself? Apparently the probe can withstand 350 volts "up it" so I didn't anticipate anyone would have blown them, but possibly applying the output from an unmatched and unterminated high power transmitter could have upset a few things.

Who knows the history of some of this vintage test gear? I always find it interesting to speculate why it was taken out of service. Mine was last calibrated in 1995 but the seals have been opened since then.

[karesz*]

Phil, you can download it on the link_without some other things, or even send me pls pm with your email...
Please find (under the "first doc-side") and click on "get manual"_these is all to do for you and be hapy :
>...This file is downloadable free of charge: Get Manual
>(You are not logged in, only 2 downloads per day possible for you.)
Sincerelly K.

[G4XWDJim]

Phil,

You have a PM.

Jim

[Phil G4SPZ]

Karesz* - Many thanks! I have now downloaded the manual. Sorry I didn't spot the link to "get manual" on the page when I first viewed it.

Jim - PM received, thank you, and I'll reply in due course.

Bill - I have checked the 2.2 Meg resistor (it was fine) and the associated 100uF electrolytic (also fine). I've cleaned up the range switch wafers and the zeroing is now marginally better, but not perfect. However the meter does work on all ranges. I'll spend a bit more time reading the service manual and have another go at the meter in due course.

Thanks for everyone's help and interest so far.

[Radio Wrangler]

If you need detector diodes, Phil, I think Farnell stock the HP fast, low-threshold Schottky diodes which are probably the best modern alternative. I can't check right now because their website is off-air for maintenance.

David

[Phil G4SPZ]

Thanks for that, David. I've checked on CPC's website but they don't offer any HP diodes, though there are dozens of others. I'll try to dismantle the probe and look at the diodes that are fitted.

[Radio Wrangler]

What were HP diodes re made by a firm now called Avago. Same people, just now a spin-out from a spin-out. The 5082-2800 part mentioned earlier by Bill is a 70v schottky diode with wire leads. There are lower voltage, more sensitive parts than that

The 2800 number is a code, the last digit, zero, means one diode per pack. The next to last means 70v. -2820 is a single 15v low threshold diode the next grade is 4v and so might not be a good substitute for an 8v Germanium part.

The first 4 digits point to the package amongst other things

5082-2820 would have been the old HP branded wire leaaded 15v diode.

Today, you're more likely to find HSMP-2820 which is a single diode in a SOT23 package, available from suppliers, you can solder wire leads on, but it's fiddly and needs a magnifier.

David

[G0HZU_JMR]

Here's a bit of nostalgia for you David

See below for an old bag of HP2800 diodes I have here. 1974 vintage!

I've been slowly eating my way through these in various projects since my youth

I can send a couple to Phil if they are any use but they probably aren't the right voltage rating etc.

[Radio Wrangler]

Eeee that fair takes me back!

I also remember the little white envelopes containing four matched ones for making mixers.

Those are the 70v ones. If Phil wants to replace low-barrier Geraniums, then he may get better results with the 15v or 4v Schottkys.

David

[Phil G4SPZ]

Update. I've done a bit more testing, and have come to the reluctant conclusion that there is probably something wrong with the diodes in the probe. Chapter 4, Section 24 of the excellent manual describes how to check the functioning of the diode probe. The forward resistance check should show around 50 kohms. Mine measures over 200 kohms, and varying.

My next problem is, how does the probe come apart? I've pulled off the rubber shroud and removed the small bolt, but the innards stubbornly refuse to come out of the probe tube. Any advice from Bill or someone who has done this before would be most welcome!

[WME_bill]

TF2603 diodes.
I will have a look at my probe.
But to cheer you up, I can let you have a couple of HD1871 ge diodes, which are similar to the AEI CG91AH and Mullard AAZ13. I can select from the couple of dozen I have, and I'll match as suggested in the manual if you want them.
They were rescued from a fast pulse generator 30 years ago, before schottky diodes became common. The Bradley CT471 multimeter uses the CG91 in the probes, and the HD1871 appear as the modulator in the Advance SG69 uhf sig gen and the Avo CT378 / TFM / AFM2 suitcase signal generators, so use was well established for uhf use.

If you want data sheets or summaries, I can post those for the AEI 1965 CGxx range, the Hughes 1965 HD18xx range, Mullard 1975 AAZxx range.

My thanks to David Radio Wrangler for information about the HP diode naming convention. I mentioned the 5082-2800 as I have half a dozen, but as Jeremy G0HZU has 250, I am quite put in the shade. The best of the HP to use would be the 5082-2835, which has a specially low turn-on voltage intended for uhf mixers, as has the comparable Mullard BA481. I have information upon both these families: HP 1990 and Mullard 1985.
I have always found it useful when replacing impossible to get parts to have the specification of the original, as then I have a better idea of what to look for that will do. wme_bill

[Phil G4SPZ]

Bill, thank you for the kind offers to (a) look at your probe, and (b) supply me with some diodes. I can save you the trouble on both counts, because the meter is now working as it should.

I did what I should have done first - RTFM - and found Chapter 4 paragraphs 44 to 47, which deal with leakage effects. This meter is ex-HMS Orwell, a "River" class minesweeper operational during the 1980s, and I did find evidence of some internal corrosion. So I cleaned range switch wafers SA1, SA2 and SA3 and the other sensitive areas with isopropyl alcohol, and left the meter in front of my gentle 1kW fan heater for an hour. After this treatment, I was able to adjust RV401 and obtain correct operation of the 'balance' control and a stable zero reading on the 1mV range.

I left the meter to settle down for 15 minutes, then ran through the set-up procedure using signals from my restored Marconi TF1370A wide-range oscillator. Most ranges were within tolerance, but a couple of ranges couldn't be brought within calibration although they were quite close.

One strange phenomenon puzzled me for a few minutes, in that the meter would sit steadily at zero for a while, then dance wildly about. It eventually dawned on me what was causing this, and I cured it by switching off the mobile phone in my pocket! This meter is sensitive enough to pick up the RF radiation from a cordless DECT phone. An extremely well-built and very sensitive bit of kit.