Hacker (?) Powerpack

[Johnbarn]

I have just picked up a working Hacker VHF Herald along with which came a black plastic box the size of a PP9 battery with a mains lead going in and two two-pair conductors coming out. Each pair is fitted with 9v battery-type connectors. It obviously purports to be a power pack for a Hacker radio but before attempting to use it I opened it up. It is commercially made - but not very professionally made. There is a mains transformer with two secondary windings, each with an o/c voltage of 17.2v ac. Each is rectified by a single 1N4005 diode feeding across a 1000uF 25v electrolytic. One of the outputs is also fed through a coil (choke) which looks like the armature of an old headphone; the other output has no such coil and feeds out simply across the electrolytic. The whole business is wrapped up in pvc tape and the soldering is poor. On delicately connecting to the mains, the o/p voltage at each pair of outputs measures as 24.5v dc. Photo below.

I plan to rewire the whole thing on Veroboard to fit in the plastic box but I have a few questions first. Why would only one output have the choke wired in? There is no sign of there ever being one on the other one. And what would your opinion be of the half-wave rectification? Surely it's going to be hummy - even across that big electrolytic. And is that o/p voltage not far too high, even on open circuit? I am reluctant to connect it to the Hacker which otherwise looks in good condition - although it might well have been used with this powerpack arrangement all along so it might be ok. But I am suspicious of the circuit and the quality of the manufacture. There is no manufacturer's name anywhere on it. In my rewire I will probably dispense with the coil/choke.

[TonyDuell]

Many, (if not all) Hacker radios that took 2 PP9 batteries simply connected them in series to give 18V with no connection to the midpoint. So in the case of this PSU the 2 supplies would be linked in series. A single choke would thus end up in series with the supply to the radio and would presumably be all that is required.

Personally I'd rebuild it using bridge rectifiers (that is if I was going to use it at all). That would give you full wave rectification which should reduce the hum level.

[Johnbarn]

Thanks Tony,
putting in a couple of full wave bridges seems the best thing. It was probably built like it is for cheapness - but nothing excuses the soldering! It might have been made for a Hacker - but it certainly wasn't made by Hacker. Your observation on the single choke also makes sense, I'll get to work, John

[Nuvistor]

Hacker had their own psu's, the VP408 had an adjustable output to 18volts, cannot remember the minimum but a wide range, fully regulated.
They also produced a 9 volt fixed supply, exactly the same size as a PP9, both made to the quality expected from Hacker.
The one in this post would be an OEM and I don't think it would have been acceptable to Hacker.
Frank

[Herald1360]

Paradoxically half wave might be useful in this cheap'n'nasty effort- 50Hz hum will be quite a bit less audible than 100Hz since it'll be fairly well down below the speaker resonance.

I'd be inclined to convert to full wave using the two windings in series with centre tap, then feed the rough dc to a regulator. For simplicity a 7815 will give adequate voltage for an 18V nominal Hacker (they normally work quite happily down to less than 12V).

[mhennessy]

Most of the earlier Hacker sets are really quite sensitive to ripple on the supply rail - if the VHF Herald is OK with this PSU, then that's quite surprising.

From what I can see in the picture, I'd have serious concerns about the safety of this device as it stands, and I'd probably dispose of it. But if you wish to make use of the transformer, I'd consider putting the two windings in series to make 17-0-17, then use a pair of diodes to give full-wave rectification. Following a smoothing capacitor, I'd use a 7818 or 7815 (with a heat sink) to regulate the supply to clean up the ripple and prevent any damage to the radio. But prior to doing all that, the primary side needs to be 100% safe (sorry to state the obvious, but anyone could be reading this in the future).

[Herald1360]

Great minds?


(Well greater ones mention the safety side too.....)

[mhennessy]

Oh yes - our posts must have crossed

Two batteries (or a doubly connected AC adaptor) in series has one great advantage, it is very unlikely both will be connected the wrong way round at the same time, saving the transistors.