Hacker GP15ST Amplifier/Speaker

[Kat Manton]

Hi,

Prompted by this thread I decided it was time I checked this amplifier/speaker over. I'd been given it some time ago (thanks Jim) but hadn't done anything with it.

Note: To make the following more concise and save typing, assume that while I'm working on it or connecting/disconnecting my DMM, I've switched it off and disconnected the mains every time. That's just basic safety; I just think it's best to be clear about these things.

Fuses...

Checking it over, the first thing which came to light was that one fuseholder had a broken clip and had wire wrapped around it (shorting to the chassis screw for good measure.) The other fuseholder had one loose clip and a strip of metallised plastic (crisp packet?) alongside the fuse. The fuse itself tested ok for continuity and was the correct 500mA antisurge type.


The broken fuseholder was the one in the neutral side of the mains. As I'm not keen on this arrangement I linked it out with a short piece of insulated wire.


Is the mains transformer ok?

To make sure nothing was amiss, I removed all three valves and connected my DMM on the AC 200mA range across the other (live side) fuseholder. My DMM has a 630mA fuse in series with the 'amps' terminal, so there was a fuse in circuit. After checking the voltage selector was on the '240-250' setting I plugged it in and switched on. I was rewarded by the control panel lighting up and a reading of 44mA, which seemed reasonable. That, at least, proved that the mains transformer was ok.

Next, I carefully bent the fuseholder clip back into shape and replaced the 500mA antisurge fuse.

HT smoothing capacitor health

I didn't feel like mucking around with variacs and lamp limiters, sorry! I connected my meter on the DC 1000V range, negative to a convenient chassis connection and positive to the red tag on the HT smoothing capacitor. With only the EZ80 fitted, I switched on and watched the HT rise steadily to 330V and settle there. Switching off, I watched it fall fairly slowly. Even after a minute or so there was still around 100V present. This quick test confirmed that the HT smoothing capacitor was fairly healthy, or at least not seriously electrically leaky.

So far, so good...

Two of 'That Capacitor'

The next thing I checked was the output stage coupling capacitors. These are C6 and C7 on the R&TVS diagrams available from this site.

They're the two yellow capacitors on the tag strip to the upper right in this photograph.


I mentioned elsewhere that I've never had these give trouble in these amplifiers but this time I wasn't so sure. This is the first time I've seen something other than those nice epoxy-encapsulated types which usually last well. In this amplifier, they were TCC Supamolds for a change. Now these do misbehave...

...and misbehave they did! Still with only the EZ80 fitted, the voltage on the output valve control-grid ends measured over 150V on both.


If the ECL86s were fitted with the amplifier in this condition, this would cause both output pentode sections to conduct heavily. This would cause excessive current to flow through both valves, the output transformer primary and the HT rectifier. This excessive current could damage any of the aforementioned components.

Suitable replacements turned up after a bit of rummaging, these being RS 0.047uF 1000V types. Lower voltage ones would suffice but I didn't happen to have any.

Once fitted, I made the same check as before and found 0V on the control grid ends, as should be the case. (Yes, it really was switched on when I took the photo!)


Put it back together and...

Now it was safe to refit the ECL86s without risk of severe damage and see what happened. Once it'd warmed up there was a very faint hum audible from the speaker (I'd class that as 'acceptable') and a few crackles when rotating the controls. Promising...

Finally, I hooked it up to the line output of my PowerBook, fired up iTunes and it's been playing away happily for a couple of hours now. It sounds very good, as expected based on my experience with the record players

Incidentally, I checked the TCC Supamold capacitors I'd removed on my DMM. There's no reading whatsoever, even on the 20M range; they appear to be fine. Clearly that is not the case, though. This goes to show that capacitors really need to be tested with something closer to their rated voltage across them than the few volts a DMM provides - and if you're not sure, replace them anyway, especially when they're in as critical a position as these were.

All that remains to be done now is give the pots a quick squirt of Servisol and that completes the electronic side of things.

Cosmetically it's a bit rough and is missing one foot and the back panel but at least it works now

Cheers, Kat

[Oisin69]

Kat, Unbelievable..that's incredible. I'm just off to sleep now but I'll give this a long hard read at work tonight. Thanks you so much for your help. I will try and post some photos and how it sounds when I (hopefully) fix it.

Thanks so much for this.

Oisin

[Kat Manton]

Hi,

I've just been investigating a little, it turns out that this is actually a Hacker GP15ST, the amplified extension speaker intended to provide stereo reproduction from the GP15 Cavalier 'record reproducer'. (I'd have known that if the rear cover wasn't missing.)

The cabinet matches the styling of the GP15 Cavalier (and doesn't match my mostly-GP42 Gondolier-with-an-AT6...)

The circuit and layout appear to be near-identical to those in the GP42/AL42 R&TVS documentation, so everything I've written still applies to the GP42/AL42 as well as the GP15/GP15ST.

I'm not sure as I haven't got schematics, but I'd expect there to be similarities with the audio output stages in the RV14 Mayflower and RV20 Mayflower II VHF radios and the RG16 Serenade radiogram. I have a feeling that Hacker got some use out of this amplifier circuit

Cheers, Kat