Tech: Hill DX700 amplifier part needed

Subject: Tech: Hill DX700 amplifier part needed
From: Richard Bridge
Date: 02 Mar 10 - 11:40 AM

Well, there are probably half a dozen posters where who know what I am talking about.

The Hill DX700 was born in the late 70s and in its day was a mighty beast of a power amp producing 625 + 625 watts RMS into 4 ohms. Mine is sick, producing a mighty DC thump from one channel at five second intervals. Other channel OK.

The problem appears to be the interstage transformer that goes between the 3020 chip that is used as a driver stage and the power stage. The 3020 is an audio Op-Amp that can produce up to 20 watts of audio. In the Hill it drives a 1:1:1 transformer that produces symmetrical drive for the output stage which is therefore close to class A.

Taking out fuse to input stage produced a thump-free situation - therefore fault was driver or earlier.

With 3020 in circuit, (and fuse to output stage removed) supply voltages to 3020 on the dud channel read -9 0 +9.

WIth 3020 out of circuit (and fuse to output stage removed) supply voltages to 3020 on the dud channel read -15 0 15 (which is I think as it should be and is as the other channel is).

Replacing the 3020s results in a pop and a destroyed 3020 (twice so far).

So it's probably a largely short circuited input winding on that transformer (if it was the output winding there would be much less drive to the output transistors).

Regrettably the PCB on which that 3020 is mounted has been done so many times in the past that that 3020 has to be connected with fly-leads, but I can live with that.

My usual repairman (he is good) is talking glumly about custom wound transformers at a price of rather too many beer tokens.

Anyone know of anyone with any of those transformers or any Hill DX700s going cheap for spares or repair?

Subject: RE: Tech: Hill DX700 amplifier part needed
From: Richard Bridge
Date: 03 Mar 10 - 07:53 AM

Refresh

Subject: RE: Tech: Hill DX700 amplifier part needed
From: GUEST,Phil B
Date: 03 Mar 10 - 10:29 AM

Remember the problem well.
Try this. He has one which can be gutted for spares.


John Godsland
Orchid Electronics
3 Chester Close
EXETER EX4 2EJ

01392 438844



www.orchid-electronics.co.uk

Subject: RE: Tech: Hill DX700 amplifier part needed
From: Richard Bridge
Date: 03 Mar 10 - 06:18 PM

Thank you lots!

I thought that transformer failure was a relative rarity on them, the usual problem being excessive therms since the quiescent current was higher than the full load current.

I will ring Mr Gosland in the morning.

Subject: RE: Tech: Hill DX700 amplifier part needed
From: GUEST,Phil B
Date: 03 Mar 10 - 06:28 PM

I seem to recall that it was a regular problem with these amplifiers as was the infamous output transformer problem with the otherwise excellent old HH d500s which tended to fry your loudspeakers. Good luck. Hopefully John will sort you out. He's a good man.

Subject: RE: Tech: Hill DX700 amplifier part needed
From: Richard Bridge
Date: 03 Mar 10 - 06:35 PM

I am a bad man with speakers - frequently bouncing Eminence kilomaxes' endstops off!

Subject: RE: Tech: Hill DX700 amplifier part needed
From: Richard Bridge
Date: 04 Mar 10 - 09:13 AM

I have traced Malcolm Hill, and spoken to John. Malcolm says the transformers were nearly bullet-proof. John says that his transformer will fit (it's from a different amp) but thinks a new amp might be best - of course I agree but not sure that my bank manager does. Anyway, John also has a full schematic which he will kindly copy for me.

Progress?

Thanks all.

Subject: RE: Tech: Hill DX700 amplifier part needed
From: GUEST,Nick
Date: 13 Jul 10 - 08:04 AM

Dear Richard Bridge,

I would appreciate it if you could let me have a copy of the DX-700 curcuit diagrams please.

I have one that is sick. The output stage is OK but I have a problem with the drive circuit to the transformers, two 15 ohm resistors have burnt!

Thank you Nick.

[e-mail]

Subject: RE: Tech: Hill DX700 amplifier part needed
From: Richard Bridge
Date: 13 Jul 10 - 06:51 PM

Hi Nick. I will have to rummage in the innards of the computer!

It sounds as if your root cause maybe similar to mine - excessive current draw by the drivers.

Maybe between our two machines there is the makings of one working one. But maybe not: I still think my interstage transformer (on one side) is shot. Is yours one channel good?

Subject: RE: Tech: Hill DX700 amplifier part needed
From: GUEST,NIck
Date: 14 Jul 10 - 10:35 AM

Hi Richard,

One channel is good, the other has a problem.

I have not had much time to look at it, however I feel it is unlikely that a short circuit on the transformer is unlikely. It seems to be a problem after the amp has been turned on for a couple of minutes.

I owned a lot of these amplifiers a few years ago; I picked up a couple faulty off Ebay. They had both been modified to work in Bridge mode. both not working.

The first one was an easy fix, one of the 3020's was faulty. the second amp has a different fault.. I am confident I will fix it but a daigram will be helpful.

Best regards,

Nick

Subject: RE: Tech: Hill DX700 amplifier part needed
From: GUEST,Nick
Date: 14 Jul 10 - 10:39 AM

Dear Richard,

If you feel the transformer could be a problem it may be an idea to swap it with the adjacent channel.

Does your transformer smell as if it may be burnt?

Best regards,

Nick

Subject: RE: Tech: Hill DX700 amplifier part needed
From: Richard Bridge
Date: 14 Jul 10 - 11:31 AM

Hi Nick - I have the schematics - they are for a bigger Hill but the principle is the same. If I am right there is bugger all between the 3020s and the balancing transformer. On mine the supply voltage to the 3020s that were blowing was getting dragged way down (even when the output side of the transformer was disconnected)indicating excessive current draw - and the only thing that could be causing that since nothing else is attached is a transformer fault. It's with an alleged expert now.

Did you know Malcolm Hill is still about in Yorkshire? If all else fails you could ask him.

So how do I get the schematics to you? You aren't a member, or you could PM me your email address. You may not want to disclose your email address on a public board for spambots to gather etc.

Subject: RE: Tech: Hill DX700 amplifier part needed
From: GUEST,Nick
Date: 14 Jul 10 - 11:49 AM

Dear Richard,

Please Email me on [e-mail]

I will let you know how I get on.

Thank you,

Nick

Subject: RE: Tech: Hill DX700 amplifier part needed
From: Richard Bridge
Date: 14 Jul 10 - 12:31 PM

Done. Elf, can you delete Nick's email address above for his own protection please?

Subject: RE: Tech: Hill DX700 amplifier part needed
From: GUEST,Nick
Date: 15 Jul 10 - 05:09 AM

Dear Richard,

Thank you for the drawings. This is just what I needed.

Nick

Subject: RE: Tech: Hill DX700 amplifier part needed
From: Richard Bridge
Date: 15 Jul 10 - 10:11 AM

My pleasure. Let us know if it works!