An Auction Buy
July 2025
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I was searching for something
interesting the other day when I spotted a dismantled metal detector
in a forthcoming auction. I've been interested in this subject
for over 55 years when I built my first detector copied from
a tiny circuit in a Heathkit catalogue. I'd actually visited
Heathkit in Gloucester in early 1970 and found that they couldn't
supply me with the one in their catalogue so took the advertising
page for their metal detector because it had the circuit diagram,
albeit extremely tiny. Within a few weeks I'd built a copy and
looked around a nearby lost Roman Villa near Cheltenham where
I was living at the time. With a pal from Plessey we hunted around
and found lots of interesting stuff. When I was packing up in
December 1970 and off home I actually stopped outside the museum
in Cheltenham (from which the assistant curator had given me
a map of the villa drawn up over a hundred years previously)
to hand over my finds but found I couldn't park and it was late,
raining and I'd got 100 miles to go.
I still have that box of finds.
However I'm getting distracted... I looked for information on
the dismantled detector due to be auctioned and discovered that
even a second hand example on Ebay was £1,399. I entered
a bid and of £60 (hoping it was a "sleeper")
and waited for auction day. Near the end of the auction the lot
showed up on our TV (it was a "live" auction), but
to my surprise bids arrived within mSecs upping the price from
£25 to £200. The thing ended at £380 plus add-ons
(ie. nearer £500 than £400).
To cut a long story short I
bid for a few later lots viz. a Cambridge Audio amplifier and
then a matching CD Player, plus a lot comprising several boxes
of integrated circuits and a pair of probes. After a quick drive
of 197 miles there and back (at 62.7mpg) I'm now able to check
over the purchases. Fuel economy nowadays is amazing but driving
on congested roads full of pot holes and many 20mph speed limits
is tiring. Most of the time the speed limit is a mystery as my
satnav tells me 30mph or whatever and a sudden glimse of a worn-out
road marking declares 20mph. My car has difficulty in maintaining
these low speeds (insisting on climbing up through its 7 gears)
so I'm forever having to spend half my time watching the speedometer
instead of watching out for traffic etc. I've also noticed that
many road markings are in a dreadful state with box junction
lines worn out and direction information in lanes hardly visible.
I had to go around the ginormous roundabout at the A34/M3 junction
twice to figure out the correct turn without getting boxed in.
I noticed too that lots of roadside signs are overgrown with
foliage. Alas.. the country has gone to the dogs!
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You'd think the seller
would have taken a few minutes to dab a damp cloth over the things
to make them more presentable but they chose not to. I was very
happy to discover the remote for the "boxed" CD player
(left) which had been overlooked by the auctioneer's photographer..
and is good for the amplifier also, but that for the amplifier
(right) was recorded. The AAA batteries were all less than a
volt so the things musn't have been used for decades. |
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I half expected that at
least the CD Player would need a new laser assembly, but maybe
not.. I powered it up and inserted a CD. It does skip a little
so the laser may need cleaning? The open/close button needs relacing
or cleaning as it's intermittent (the button on the remote proves
the mechanism is perfect), but the CD was read and it seemed
to play OK.
The amplifier powered up although
its minimalist front panel design doesn't let on much that it's
powered up. There are miniscule blue lamps above the various
buttons but little else to indicate that it's working.
The amp and CD are almost in
perfect working condition so, after a little attention, maybe
I'll offer them for sale but only after a good clean.
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The audio output devices
are a pair of complementary NPN/PNP Darlington transistors (SAP15N
& SAP15P) per channel. Each is rated at 150W, Vce 160V and
an Ic of 5A. However, with all semiconductor devices the over-riding
parameter is temperature.
This means there's a balancing
act between the desired audio output, the magnitude of the power
supply voltage and power, plus the size (and performance) of
the heatsinks and of course here, I reckon the Cambridge design
engineers were constrained by a key parameter. That being the
size of the outer case. That would dictate the design of the
heatsinks. A "nice to have" output power of 2 x 50W
sounds really good and this reflects on the voltages and currents
required from the mains transformer.
The circuit diagrams show plus/minus
40 volts as the main supply voltages. Given a total of 100W output
(two channels at 50W each) and a supply voltage of 80 volts together
with an efficiency of the circuit of 50% the current required
is about 2.5A. However, given a DC power supply efficiency of
say 75% and subsidiary power requirements, this current may well
be over 2.5A (eg, plus 40V@ 4A and minus 40V @ 4A). This equates
to an input from the mains supply given 75% efficiency of 75%
making say 430W.
The rear of the case has a label
with "Max power 515W" so maybe 50% efficiency is an
overstatement?
I'll carry out some measurements...
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The next day I catalogued the
chips in my third lot. Many are very odd types but there are
dozens of 2716 EEPROMs.
Two very large "chips"
shown opposite are a pair of new "ILP HY50" amplifier
modules. These must have been purchased by the original owner
of the boxes as a future "project" but alas he'd never
got around to this.
The modules measure around 2
x 4 inches and an inch in height.
Click
the picture to read about the remarkable HY50 dating from 1973.
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The 1983 catalogue prices
of £24.93 and £25.80 without VAT, which was then
15% makes them about £28 and £30 respectively. 2025
prices are circa 3.6 times those in 1983 so an engineer in 2025
would be looking at prices, compared with the money in his wallet,
of over £100 each. |
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My third Lot, apart from
the HY50 amplifier modules and integrated circuits contained
two logic probes. Although they looked nearly identical they
carried different RS numbers viz. 423-942 and 424-103.
Searching through my RS catalogues
I discovered them both in the March-June 1983 edition. I've copied
the details below the two pictures.
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I powered up the amplifier
using a workshop loudspeaker and turned it on. There was nothing
to indcate that it was on except a faint click from a relay inside
the case. I also powered up the CD player and connected this
via twin phono lead to the CD input on the amplifier. One of
the two remotes caters for both amplifier and CD player and I
used this for testing.
Initially I pressed the Open/Close
button on the player but found that this only worked about one
time in ten. One of the stock faults, if not the only stock fault,
is failure to open the CD tray, but if I used the remote control
the tray worked perfectly, suggesting the button on the front
panel was bad rather than a loose drive belt.
I opened the drawer and inserted
a CD. Closing the drawer resulted in the CD spinning up with
the display correctly indicating the CD information. Pressing
play started Track 1 but I noticed that the slightest bump would
make the CD skip. The CD also skipped intermittently during play.
I removed the lid (this, like
that on the amplifier, is secured by eight T7 Torx screws.
Below is a view of the CD player
electronics. Click to see full size.
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Both problems were easily
fixed. At first I intended to change the Open/Close button but
it seemed tricky to get to (probably detaching the mains transformer
to reach the circuit board securing screws) so I decided to use
switch cleaner, As shown below I poked the plastic tube down
behind the front panel, pointing at the switch button, and gave
it a few squirts whilst operating the button. This magically
cleared the intermittency.
Next, I removed that black panel
from over the laser (just two screws) and applied a little isopropyl
alcohol to the laser lens. I used a cotton bud with its end fluffed
up a bit and carefully wiped the surface of the lens. Although
the lens surface didn't really look any different the CD worked
perfectly once reassembled and gently tapping the case no longer
made the CD skip.
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