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Old 4th Apr 2017, 3:03 pm   #1
milannoir
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Default Hacker GP15 distortion

Hello everyone

I have this Hacker GP15 with Garrard TT, GP91 3SC cartridge. I have serviced the mechanism to get it working but the records play distorted on the mid-range and treble. I put a new stylus on but no improvement. Would this be cartridge or capacitors? Many thanks in advance for advice on this one.

Pete
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Old 4th Apr 2017, 4:18 pm   #2
Edward Huggins
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Default Re: Hacker GP15 distortion

The problem is that the Acos cartridge is the wong type for your Hacker. It's overloading the amplifier and causing the distortion (who fitted that?). The Acos GP91-3SC is a high-output cartridge and very valuable in its own right for low gain amplifiers as found in Dansettes/Fidelitys.
It could be worth up to £40.00 in it's own right so take care of it!
You need a medium-output Mono stereo-compatible or Stereo cartridge like an Acos GP91-1C or BSR X5M or BSR SC12M. If you fit a stereo cartridge you must bridge the channels.
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Old 4th Apr 2017, 4:59 pm   #3
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Default Re: Hacker GP15 distortion

The grid coupling (THAT) capacitor - x2 in this case, are usually plastic cased paper supamolds or dubiliers in the gp15 in which case they should be changed for reliable modern types. The similar gp42 usually has reliable Mullard mustards
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Old 4th Apr 2017, 8:37 pm   #4
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Default Re: Hacker GP15 distortion

If you are looking for a replacement for the missing treble knob (they tend to crumble if you try to remove them), I had a design made up in CAD for the collar, it worked a treat (see photo). It cost me about £6 I think to have the replacement collar 3D printed. It then slips into the transparent knob. Let me know if you want the file.

Mike
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Last edited by Armbored; 4th Apr 2017 at 8:42 pm. Reason: Clarification
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Old 7th Apr 2017, 8:16 pm   #5
milannoir
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Default Re: Hacker GP15 distortion

Hi Mike, the 2 on are warped and have already broken and restuck collars so I need to replace all 3. Where would I find some, or do I need to go the CAD route?

Pete
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Old 7th Apr 2017, 9:18 pm   #6
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Default Re: Hacker GP15 distortion

Quote:
Originally Posted by milannoir View Post
Hi Mike, the 2 on are warped and have already broken and restuck collars so I need to replace all 3. Where would I find some, or do I need to go the CAD route?
Hello Pete,

That's not so good. What I have is a design for the grey part you see in the third picture. This is a collar with four fins which fits over the spindle coming out of the deck and will accommodate the actual volume knob, basically a connecting piece.

You do need to have the plastic knob. I Dremelled out the old adhesive from the inside of the knob to allow the new 3D printed piece to fit.

It's all a bit difficult to explain but looking at the photo, you should see how it fits. Let me know if you want the file, I can't view it myself because you need the CAD software to see it. This is the correct file format for any printers to work from though (Solidworks).

Regards
Mike
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Old 7th Apr 2017, 10:22 pm   #7
Edward Huggins
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Default Re: Hacker GP15 distortion

What about all that distortion problem and in getting the correct cartridge fitted?
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Old 23rd Apr 2017, 12:35 am   #8
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Default Re: Hacker GP15 distortion

Edward,
That, I'm afraid is beyond my scope of knowledge, so cannot help.

Regards
Mike
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Old 23rd Apr 2017, 8:48 am   #9
Edward Huggins
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Default Re: Hacker GP15 distortion

Thanks for the further response. So did you change the cartridge? That should solve the problem as advised under Post #2. That rare Acos GP91-3SC you have in it now can be easily sold to pay for it.
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Old 23rd Apr 2017, 12:48 pm   #10
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Default Re: Hacker GP15 distortion

Mike isn't the OP - these question should be directed towards Pete.

I've been scratching my head about this one, Edward. Basically, I fail to see how a high output cartridge can cause distortion at normal playback levels given that the first thing the signal hits is the volume control. The OP would have to be turning up the volume far enough to clip the power amplifier, which would result in an extremely loud playback level - which surely would have been mentioned? The only difference caused by the output level of the cartridge is how far the volume control would need to be turned before clipping. As was said in post #3, the amplifier needs to be checked out and serviced - as a matter of some urgency, frankly - irrespective of the cartridge.

Of course, the incorrect cartridge could be distorting because of mechanical factors like tracking weight or mistracking, but that's not a function of the output voltage.

Curious about the cartridges, I found a spec sheet of the GP91 3SC and GP91-1C in this post: https://www.vintage-radio.net/forum/...67&postcount=6 - according to that, the output level is 500mV and 178mV respectively. That's really not a big difference - just 9dB. To put that into context, with a typical logarithmic volume control, there is about 20dB between mid-position and maximum.

To put it another way, consider this: the sensitivity of the amplifier is given as 250mV. If fed with 2.5V, clipping would occur at 12 o'clock approximately.

So while it'll be good to get the correct cartridge fitted eventually, let's not distract Pete from the amplifier - as we know, leaking grid couplers could be disastrous. ECL86s aren't getting any cheaper!

Mark
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Old 23rd Apr 2017, 1:59 pm   #11
Edward Huggins
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Default Re: Hacker GP15 distortion

I fully take your point and your update is relevant. But a 2 x ECL86s amp has a typical sensitivity of 250mV, but with that Acos now fitted, it is offering double the input mV required.
With the amp to be attended to, and an appropriate cartridge, all should be well.
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Old 23rd Apr 2017, 2:37 pm   #12
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Default Re: Hacker GP15 distortion

My point was that the cartridge output voltage is utterly irrelevant.

Things might have been different if there was amplification before the volume control - amplification that could perhaps be driven into clipping with a high output cartridge, irrespective of the setting of the volume control. Even so, that would be a spectacularly bad bit of design, given that designers are aware of words like "headroom" and "overload margin". But as I say, in this case, the cartridge goes straight into the volume pot which has, for all practical purposes, infinite headroom.

Pete, are you still here?
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Old 24th Apr 2017, 4:30 pm   #13
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Default Re: Hacker GP15 distortion

Sorry, but I agree with Edward on this one. I'm not technically-minded, so can't give a reason for it, but fitting a higher output cartridge to an amp that should have a medium output version definitely results in distortion.
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Old 24th Apr 2017, 5:16 pm   #14
mhennessy
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Default Re: Hacker GP15 distortion

The short answer is this:

All that changes is the position of the volume control where the distortion starts. Even the correct cartridge will distort when playing a "hot" disc at a setting of the volume control that is too high (remember - you need spare "gain in hand" for quiet recordings). The high output cartridge will simply distort sooner - perhaps at 5 out of 10 rather than 7 out 10?



Here's the long answer:

Every amplifier has a point at which the output clips. That is basically fixed by the power supply voltage. The input voltage required to provoke clipping depends on the gain of the amplifier. Agreed?

The specification of the Hacker amplifier states "250mV for 7W". 7W is the maximum output power; above that the amplifier clips.

So, any more than 250mV at the grid of V1a will cause the amplifier to clip. So let's look back in the chain...

Immediately prior to that is the 1M volume control and R1 (a 470k resistor - let's call it 500k) in series.

Let's adjust the volume control to about half way. Because it's a log pot, that actually means we'll have 100k to ground, and 900k between the wiper and the top, and we have to add R1 (500k) to that. This gives a potential divider where the top resistor is 1.4M and the bottom resistor is 100k. This divides down the signal (from the cartridge) by a factor of 15. In other words, the signal needed from the cartridge to cause the amplifier to clip is now 3.75V. You can apply any voltage you want, and provided it doesn't exceed 3.75V, there will be no distortion.

Let's adjust the volume control such that there is 500k between the wiper and each end (about 8/10). So now, we have 500k to ground, and 1M to the cartridge. The new potential divider is reducing the signal by a factor of 3, and we would need 750mV to clip the amplifier. Again, any voltage up to this amount will be clean and undistorted.

Key to understanding all this is seeing that the amplifier comes after the volume control. You could feed 100V to the volume control, and the amplifier would only clip if you turned up the volume control enough. Before that point, there would be no distortion whatsoever. Sure, it wouldn't be very far around the dial - barely off the end stop in practice - but the point remains. There is no pre-amplifier stage before the volume control that could be driven into clipping, so it comes back to the simple message: it's all about the position of the volume control.

I hope that the above makes sense, and I'm happy to explain further if you'd like.

Mark
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Old 13th Jun 2017, 2:17 am   #15
milannoir
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Default Re: Hacker GP15 distortion

This is an interesting discussion. I did swap the cartridges round but now it is humming badly, and is just the same with the head shell removed. What could be the cause? Pete
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Old 13th Jun 2017, 2:28 am   #16
milannoir
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Default Hacker GP15 humming

I am trying to get this record player back working after years of disuse. All tidy inside and out but there was distortion. I swapped the cartridge, now its humming loud and also hums with the cartridge removed. Thanks in advance for any guidance. Pete
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Old 13th Jun 2017, 7:30 am   #17
leslie5555
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Default Re: Hacker GP15 distortion

Quote:
Originally Posted by milannoir View Post
This is an interesting discussion. I did swap the cartridges round but now it is humming badly, and is just the same with the head shell removed. What could be the cause? Pete
Most likely the capacitors need changing, also check resistors are within tolerance.
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Old 13th Jun 2017, 10:31 am   #18
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Default Re: Hacker GP15 distortion

The usual question. Does the hum vary with the setting of the volume control?

Removing the head shell will increase hum due to stray pickup, as the amp's input is unterminated.
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Old 13th Jun 2017, 1:59 pm   #19
milannoir
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Default Re: Hacker GP15 distortion

Yes the hum increases with the volume.
Pete
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Old 13th Jun 2017, 2:31 pm   #20
milannoir
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Default Re: Hacker GP15 distortion

I have just tested the tonearm pins. The hum remains when I touch the two right pins with copper wire but ceases completely when I touch either of the two left ones.
Pete
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