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Ferrograph RTS2 restoration

Purchased this at the last Harpy' complete with matching ATU (Auxiliary Test Unit).
Spent a bit of time bringing the unit up to peak performance.
Here's a brief run down of the work carried out so far.
1). Clipping of scope output - re-biased amp by adding 82k in parallel with R174.
2). Squegging of oscillator output - replaced R7 which had gone high value.
3). Several noisy pots - carefully drilled hole in casing to allow contact cleaner to be sprayed on the track.
4). Inability to null phase control - replaced and adjusted RV121 which had gone intermittant.
5). Pickup of oscillator output at input socket - fabricated shield to reduce pickup.
6). Pickup of mains hum - re-dressed mains wiring away from pcb's.
7). Pickup of W&F 3,150 Hz tone - disconnected +30 V supply to W&F board*
8). Pickup of scope output by mV meter causing self oscillation on lowest range - connected scope output direct to pcb, by-passing W&F switch.
9). Noisy scope output - bypassed R150 with 82 pF to reduce the HF gain. Now 1dB down at 100kHz which is acceptable and has dramatically reduced the noise level. Also replaced MR31 which had gone noisy.

I've manipulated the diagram to make it easier to follow and print out, see attachments.
* I only intend to use this unit for distortion measurements, so can live without the W&F functions. The changes I've made can be easily reversed.
BTW there's an error on the meter_amp schematic, the moving contact of SW6b should go to the contact j/o R154 + R155 in the position of SW6a shown.
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Last edited by glowinganode; 25th Mar 2013 at 1:05 pm.
 
25th Mar 2013, 1:52 pm   #2
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Re: Ferrograph RTS2 restoration

I remember that Wilmot Breeden owned both Ferrograph and Wayne Kerr around this period, so I've got a feeling who did the RTS2

David
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25th Mar 2013, 6:21 pm   #3
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Re: Ferrograph RTS2 restoration

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It seems it didn't want to give in without a struggle !
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What sort of residual distortion does this read with a direct OUT / IN link ?

PS. whats "Squegging" ?

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Re: Ferrograph RTS2 restoration

Hi Dave,
I managed to get the residual distortion down to approx 0.015 % THD @ 1 kHz.
Squegging occurred due to too much LF feedback, causing pulse modulation at around 10 Hz PRF of the output signal. It's a bit of an old fashioned term really, more applicable to RF instability in a regenerative receiver.
David, going by the age of the components, this was manufactured late '73 which I think pre-dates the Wayne-Kerr / Rendar merger. The badge on the rear states London and South-Shields, I believe Wayne Kerr were based in Bognor Regis. I do have an Audio Response Analyser which was built a few years later ('77?) which was marketed under the Ferrograph name as well as Wayne-Kerr.
See attached photo's of the holes I drilled in the Colvern wire-wound pot to allow lubrication, and the shield around the input socket (made from a bit of Quality Street tin). The trace nearest the edge of the pcb is the oscillator output which was being picked up by the input socket.
Rob.
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25th Mar 2013, 10:49 pm   #5
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Re: Ferrograph RTS2 restoration
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Neat work with the drill, Rob.

Yes, I think that's earlier than the merger. My W-K bridge has Ferrograph 7 knobs on it.

Squegging is where an oscillator is starved of DC power and it pulls down its supply when it runs until it stops, the supply goes higher and it restarts. Hysteresis makes it work as a relaxation oscillator.

A more complex version is 'Motorboating' in a superhet where the audio in the output amp pulls down a weak battery, the LO stops, so the sound stops, and the battery recuperates a little.... and so on.

Cheers
David
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