Golborne Vintage Radio

Split from the Rigol thread:

As said, delivery time was good, less than 24 hrs from confirmation of order (Farnell) This 'scope started life out as the GW Instek GOS 630FC.

All workie workie so far, the only waveform I had to hand for checking was the 'scopes Cal and the AC mains, the 'scopes Cal waveform is a bit shy of its target frequency, but by not a lot, inside it looks like two or three transistors for the Cal osc, there's a couple of presets close by, it's hard to say what one was labeled up as but the other is Cal, possibly one for set frequency and one for amplitude.

Everything looks reasonably accessible inside, just two main boards, the only surface mount stuff is on the LCD driver board, I suspect the timebase is derived from that also, the only sure fire signal I could use to check the frequency counter was AC mains, it checked out accurate against the digital readout on an earth loop impedance tester I have.

It's 2Kv or so for the CRT.

All the other internal pots etc are labeled with there functions, highest winding on the mains tranny secondary is about 200 volts.

No one off special IC's that I could spot on the main boards, all looks like standard stuff.

The panel/knobs case etc are a nice pro finish, the detents on the timebase and Y amp selectors are are just about right, not too heavy or stupidly light, nice action on the other controls, nothing feels scratchy.

The probes are typical generic types, nothing to write home about, 1x/10x, 6 Mhz/60 Mhz, the coax is not stiff, quite floppy, compensation adjustment is smooth.

[attachment=13848][attachment=13849][attachment=13850]

So far so good.

Next mission is to get some radio bits to fiddle with, I used to have a fair bit but had to flog stuff off including the receiver collection to raise funds towards a major purchase...Long story.

First project will be a 12 volt valve regen detector tunable from 3 to 3.5 Mhz with as little amplitude variation as possible across the range at the sweet spot for SSB/CW etc.

A likely valve candidate will be an ECC82, I can run that with 12 volt heaters, it's low mu with a low input capacitance and according to the Ia/Va curves against bias it should be ok with low bias at 12 volts HT:

http://www.r-type.org/pdfs/ecc82.pdf

I'll report on that project as and when, It'll be a few weeks yet.

Lawrence.

(02-02-2016, 04:20 PM)pwdrive Wrote: [ -> ]All workie workie so far, the only waveform I had to hand for checking was the 'scopes Cal and the AC mains, the 'scopes Cal waveform is a bit shy of its target frequency, but by not a lot, inside it looks like two or three transistors for the Cal osc, there's a couple of presets close by, it's hard to say what one was labeled up as but the other is Cal, possibly one for set frequency and one for amplitude.

I really wouldn't worry about that.

The main purpose of the Cal output is to allow you to adjust the HF compensation on a x10 probe.

The voltage and frequency accuracy is secondary to this - good enough to check you're in the ballpark, but that's all.

Some 'scopes have a preset to allow you to find-tweak the voltage, but I've never seen one for frequency; you're at the mercy of the caps used in the astable multivibrator (usually 10% or worse IME).

Some really posh models will generate the cal signal more carefully - dividing down from a stable clock that is used elsewhere, but presumably not here...

Generally, it looks quite reasonable, but I'm surprised the EHT is only 2kV. You'll be OK with basic stuff, but low duty-rate signals will be hard to see. The Philips PM3215 uses 10kV IIRC.

I note from the manual that it has a Y-out on the back. This is incredibly useful (though many folk ignore it). If you didn't have the built-in frequency counter, that would be an obvious use. I connect mine to a powered 'speaker by default, and use it a lot for all sorts of work beyond the obvious...

They've replaced the Volts/Div rotaries with relays and programmable gain amplifiers under control from the rotary encoders and a microcontroller. Likewise for the Time/Div control. Assuming they used half-decent encoders, this ought to be more reliable in the long term - switches are the weak point most modern gear.

Quote:I note from the manual that it has a Y-out on the back. This is incredibly useful (though many folk ignore it). If you didn't have the built-in frequency counter, that would be an obvious use. I connect mine to a powered 'speaker by default, and use it a lot for all sorts of work beyond the obvious...

The Y2 output of my main scope (2465B) is connected to a little monochrome monitor. This monitor is locked to mixed sync from my man lab SPG. Incredibly useful when working on video equipment. For example I can see the bits of a digital video bus. Address buses that are locked to video are easy to see too.

The monitor is interesting too. It's a 9" soild state Ikegami dating from the late 1960s. Pretty horrible as a monitor, AC coupled video with no attempt at black level but that makes it ideal for this application. It went wrong just once, many years ago. The insulation around the LOPT started to fail. Took it apart, patched it up and it's been fine ever since. Shouldn't say things like that

Look a good value scope to me Lawrence.

Gary

I have just had a look at the service manual for my Tek 7000 series and the calibrator is marked ~1Khz and the only adjustment is amplitude. There is a direct output of 4 volts and two divided outputs giving 0.4 volts and 40mv.

As Mark said, the main purpose of the calibrator output is to compensate scope probes. The voltage is often held within close limits. Some older Tek scopes had a switch for multiple voltages. On a few scopes it's a loop with a defined voltage on it and a defined current flowing through. So can be used for calibrating current probes

On some scopes (such as my 2465B) the cal frequency is related to the timebase speed. It's not usually intended as a timing calibration.

If you wanted to calibrate the timebase of an oscilloscope, use a timemark generator like the Tektronix TG501 - I still have one of these (plus a lot of Tek 500/5000-series kit), but for accurate calibration I have a 10MHz/1Hz Rubidium atomic clock module from eBay (FE-5680 ex cellular base station kit).

Thanks for the input folks, after the 'scope's been on for a while the cal frequency settles at around 995Hz + - a Hz or so, good enough for me.

Lawrence.