Metalwork - Lathe and Mill topics

Tuesday, 1 July 2025

Re: [Amateur-repairs] Old test eqpt? Chuck it? Refurbish it?

Well, you're in luck!

Marconi 2955 Communication Service Monitor

$1,990 - https://www.ebay.com/itm/284689545136

$1,950 - https://www.ebay.com/itm/356941529543

$1,750 - https://www.ebay.com/itm/356583617089

$1,523 - https://www.ebay.com/itm/286439999912

$1,340 - https://www.ebay.com/itm/386832700583

73 de Andrew/N5ASE

-0-

On Tue, Jul 1, 2025 at 2:56 AM maxwelloau via groups.io <maxwelloau=gmail.com@groups.io> wrote:

At an Amateur convention recently I picked up two Marconi 2955 units that had been stored in farmer brown's shed for years.

1) All self test failed. Required 4x 4k7 resistors + 2x 220uf 25V capacitors. Unit is in calibration when compared to a 2024 NATA calibrated 2955R and Agilent E4421B signal generator.

2) Full of millipede carcases. No corrosion except small section on rear panel, was fully operational (some scope waveform trigger problems). Old FW was updated but had to include the CAL chip all from another unit. Scope problem was edge connector cleaning. So some calibration was required and manually its a painful process but as my time is retired and worth Zero, its a labour of love.

A testament to the good design Marconi, their designers and the reliability of normal components like R, C, IC and relays. I marvel at the green CRT quality and both 100% usable after 30+ years.

End result is two units destined for the metal scrap bin are now usable and in fact more than very good. Sought after for anyone testing RF and radio equipment.

With the good info from groups.io and using the technician brain you cant beat repairable in my books.

maxwelloau

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Monday, 30 June 2025

Re: [Amateur-repairs] Old test eqpt? Chuck it? Refurbish it?

At an Amateur convention recently I picked up two Marconi 2955 units that had been stored in farmer brown's shed for years.

1) All self test failed. Required 4x 4k7 resistors + 2x 220uf 25V capacitors. Unit is in calibration when compared to a 2024 NATA calibrated 2955R and Agilent E4421B signal generator.

2) Full of millipede carcases. No corrosion except small section on rear panel, was fully operational (some scope waveform trigger problems). Old FW was updated but had to include the CAL chip all from another unit. Scope problem was edge connector cleaning. So some calibration was required and manually its a painful process but as my time is retired and worth Zero, its a labour of love.

A testament to the good design Marconi, their designers and the reliability of normal components like R, C, IC and relays. I marvel at the green CRT quality and both 100% usable after 30+ years.

End result is two units destined for the metal scrap bin are now usable and in fact more than very good. Sought after for anyone testing RF and radio equipment.

With the good info from groups.io and using the technician brain you cant beat repairable in my books.

maxwelloau

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Re: [Amateur-repairs] Old test eqpt? Chuck it? Refurbish it?

A vintage German broadcast receiver that I have uses a grounded metal vane on a rotating shaft in the vicinity of the main tuning capacitor to create a fine tune control. Moving the vane closer to or farther away from the tuning capacitor changes the capacitance enough to make a decent fine tuner. One would need to take care that it can't actually touch the main capacitor of course. I thought it was pretty darn clever when I saw what was going on.

73

-Jim
NU0C

On Mon, 30 Jun 2025 19:19:13 -0600
"Bob Isselhard K5INW via groups.io" <robertisselhard=gmail.com@groups.io> wrote:

> It should. It will probably vary more the higher frequency you are working with. But try to find a small value cap. Might be fun to try different ones to see which one is best for you in your application
>
>
>
> On Jun 30, 2025, at 4:16 PM, Alan Majeski via groups.io <arcadiaresearchgroup=gmail.com@groups.io> wrote:
> >
> >
> > Hello Bob
> > I love your idea, it appears to e the simplest solution, I was just wondering if it would work on all bands.
> > 73
> > --
> > Alan
> > AK6MF

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Re: [Amateur-repairs] Old test eqpt? Chuck it? Refurbish it?

It should. It will probably vary more the higher frequency you are working with. But try to find a small value cap. Might be fun to try different ones to see which one is best for you in your application

On Jun 30, 2025, at 4:16 PM, Alan Majeski via groups.io <arcadiaresearchgroup=gmail.com@groups.io> wrote:

Hello Bob

I love your idea, it appears to e the simplest solution, I was just wondering if it would work on all bands.

73

--

Alan

AK6MF

Re: [Amateur-repairs] Old test eqpt? Chuck it? Refurbish it?

Hello Bob

I love your idea, it appears to e the simplest solution, I was just wondering if it would work on all bands.

Alan

AK6MF

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Re: [Amateur-repairs] Old test eqpt? Chuck it? Refurbish it?

Craig

As I recall, HP used the same setup on their 200 series oscillators. You can fabricate one using a long shaft potentiometer and a rubber grommet.

make sure the potentiometer is the type with the metal housing held on by folded metal tabs.

make sure the grommet fits snuggly on the shaft of the potentiometer, I am sure you can see where I am going with this.

disassemble the potentiometer and discard the housing which holds the resistive wire element

cut the wiper end off of the shaft with a hacksaw, keep the shaft as long as possible, you can trim it later after the final fit.

file off the burrs and reinstall the shaft into the potentiometer collar and push the grommet onto the shaft at the end where the element used to be. The knob goes on the other end.

once you see how this works, it is a simple task to determine proper placement and drill the hole for mounting the home made vernier.

I hope this helps

Alan

AK6MF

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Re: [Amateur-repairs] Old test eqpt? Chuck it? Refurbish it?

Why not do it electronically rather than mechanically? Mount a small value variable capacitor on the main capacitor to keep stray inductance from the leads down, and tune it thru an extension, either plastic or an insulated cable type. Simply put a collar on the front panel to run a shaft through and put a knob on it. Start your tuning with it in the middle so you can vary it equally either way to walk your frequency right to where you want it. This of course assumes you have waited the appropriate amount of time so the internal heat has stabilized and is not a factor. And it assumes your generator is not drifty due to other factors. My .02.

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