Hacker Centurion MK11 (GA500A)

[Techman]

How do you get inside it?

This is the model without the screws around the edge of the motor board.

There's three screws holding the lower part of the front trim on the underside and removing these allows the trim to be pulled forward when the control knobs are removed, but what's holding it at the top edge?

Am I missing a trick here or does it just need a bit of brute force?

I've removed the ventilation grille from the bottom of the cabinet and interestingly there's three screws that have a left hand thread that do absolutely nothing with nothing on the inside - observed through the grille hole with a mirror and light and by feel.

I can get to one (and possibly the other with a long implement, or perhaps through the rear fuse holder cover which I haven't removed yet) record deck hold down flick round spring clips, so at a push could perhaps remove just the record deck and get at things this way, but there must be a better and more correct way.

I need to get to the innards, as although both power amplifiers seem to be working, there's absolutely no signal getting through from the inputs, so some sort of pre-amp fault or disconnection. No noise or crackle when any of the buttons are pressed, so really need to get to this part of the circuitry.

I thought I'd ask before applying extra force and risk busting (or bending) something - I bet someone will know.

[Paul_RK]

Hacker were really tricky with this one The board that surrounds the deck is held on by bits of Velcro - prise it gently upwards and lift it away, then you have access to the fixing screws for the motor board proper.

Paul

[Techman]

Originally Posted by Paul_RK

Hacker were really tricky with this one

Cheers for that, Paul, they certainly were, who would have thought! Glad I didn't try to prise that front facia any more than I did. I did notice that it almost seemed to go under the front edge of the motor board. Now to do some careful prising of the motor board without doing any damage.

[Techman]

All sorted.

Just the deck needs a bit of a service.

Looks like it hasn't seen any use in decades and probably hasn't had a lot even then, as the stylus in the Goldring G850 cartridge looks like it's probably the original and seems to be good (still got its protector on it), which isn't always the case.

Speaker interconnecting leads are missing, so replacements need to be made up - the speaker cabinets have inset din plugs, so leads need to have a female flying socket on the speaker end and obviously a standard male plug on the unit end. I've got to see if I've got any kicking around, otherwise I'll have to send off for a couple. Speakers measure expected continuity, but obviously haven't yet been tested for sound. Unit itself was tested with speakers from another unit (Ferguson) that accompanied this one, just for the sake of convenience.

Yes, the top black trim which looked like part of the motor board is just a cosmetic trim with, as you say, the motor board proper held down with the six screws below. Velcro pads on the four corners.

[Techman]

The deck has been serviced and works very well. It was solid with hardened grease and needed heat from the heat gun to remove the cam gear and the trip pawl - heat from the soldering iron wasn't enough. It's strange working on this SP25 MK111, as I bought one of these brand new when I was a teenager. It was used in a couple of home built systems and finally in a radiogram to replace the original single speed 78rpm deck that was fitted originally. It was finally removed from the radiogram when I refitted a more period, but 3 speed deck in the radiogram around 12 years ago and at that time it was still working faultlessly. It's been in storage in the loft since then, so whether it's now got the hardened grease syndrome I don't know. So it's really down to decades of non-use that causes these decks to seize up.

The treble control had absolutely no effect, with permanent minimum treble. Contact cleaner had little effect and the double pot had to be flooded with pure IPA from a pipette to restore operation - to be honest the control really needed complete dismantling to do a thorough job, but it seems to be working for the time being, so we'll see.

While the amplifier output modules were just idling with no music being played through them, I noticed that the heatsinks run quite warm and that one of them ran considerably warmer than the other, but both work fine and give good clear output. Although it may be imagination, I seem to detect that the 'warmer' one gives slightly louder output than the (slightly) cooler one. I seem to think that this particular unit is known for biasing issues, and that they're designed to run like this.

The speaker socket connections are going to be a problem, due to the fact that they're in a deep well and have a metal surround right down inside the plug hole in the wood of the speaker cabinet, a standard type of plug that I had, although just about made the connection, it wouldn't push far enough on to make a reliable connection - and looking at those on-line seem to be the same type as the one that I've already got. I may have to either do a modification or perhaps cut the surround off a couple of old chassis sockets and grind them down to fit in the holes and solder and shrink wrap connections to them, using them as flying sockets to fit in the recesses.

Picture below of standard standard flying socket plug and recessed plug with metal surround in speaker cabinet back cover:-

[mhennessy]

While you're in there, do give a bit of time to the power amp modules. Yes, they do run rather warm, but if they are notably different to each other, it's possible they need attention. The pre-set resistors can play up, as can other components.

I normally set them for a lower standing current than is recommended. I check their distortion performance as part of that, just in case. I'm struggling to remember what I settled on for my GAR500A, and can't check it because it's on loan to a family member, but the recommended setting of 150mA is rather high IMHO.