Hafler XL280 repair

[thomasdf]

Hello everyone

I am the happy owner of a Hafler XL280 I bought (2nd hand) some time ago to power my B&W Matrix 805. I was working perfectly fine, but a few days ago when I turned up the amp one morning a hiss had appeared overnight on the right channel. I opened up the amp and discovered quite a lot of dust and prob humidity marks from the previous owner especially on the right channel... grr.. Upon a quick inspection I realized that the "phase setting" 120pF variable capacitor C11 is quite corroded so I unsoldered an measured it and it's dead and tests as a resistor. Someone before me bypassed it with a 50pF cap.

Now to my question : what's that thing about the "phase" setting in the Hafler amp ? It sounds quite esoteric to me. What's the role of that variable cap, and can I replace it with a same value fixed cap on both channels ? If not how can I set the phase up without the devide described in the manual ?

I am not sure my hiss problem comes from this, but this is a problem for sure, so I'll start there

Any help / opinion / experience is welcome !

Cheers & thanks

Thomas

Joined is a schematic minus the PSU, and PDF of the full Service Manual / Assembly Guide where the "phase" procedure is described.

 

[Tubetec]

I have the Hafler XL600 ,
in the manual it describes a test set up for adjusting the capacitor , its a form of nul test using both amp channels and a speaker or headphone to monitor the output .

 

[thomasdf]

Yes, it's in the XL280 manual too, but I don't quite understand it and they say that it can be done with a device borrowed from a Hafler dealer ?!

 

[Gus]

R26, C8, C9 have one RC time constant
R27, C10 and C11 have another RC constant

I would guess the there are two different frequency turnover points (I am having a problem opening the manual)

Or another way to think about it is they increase the gain starting at 1/(2 pi RC)
They are part of the feedback network for a noninverting amp
However
R25?, C7 form a shelving LPF in parallel with R28 in the network because it is a noninverting setup

I wonder why the feedback network is made this way

 

[Tubetec]

I dont remember anything special being needed to perform the tests ,
Maybe a plastic trimmer adjuster , dont go in there with a metal screwdriver .

The spec on these amps is amazing , there capable of driving a 2 ohm load and power output is roughly double what it develops into 4 ohms . The guy I bought it off told me he was forever burning up tweeters , he used run a pair of XL600s in bridged mono mode , the tweeters used glow red hot he said . He asked me why it happened as no one on Tottenham court road could explain what was going on back in the day . I told him he probably would have been better off if he had passively biamped and added a simple high pass filter on the input to the tweeter channel .

 

[JohnRoberts]

IIRC He went through a phase where he voiced (tweaked) his solid state amps, to mimic high end audiophile amps... this is probably related to that tuning, and null test with another properly tuned amp is to dial it in.

JR

 

[thomasdf]

I have replaced the variable cap with a 100pF cap for now on. My thought is maybe this is not set up properly but it will help me figuring out problems. I also renewed the thermal compound paste for the 12 output transistors. And now the amp has no output and fried a suicide speaker I had hooked up to it for testing purposes... probably DC on the output.
The TO3 output transistors had both mica insulator and thermal paste, and some had no mica insulator.
Anybody know what’s the stock config for them ? Isn’t their body supposed to be grounded ?
The thermal paste I used is also insulating, maybe that’s a mistake

 

[JohnRoberts]

I have replaced the variable cap with a 100pF cap for now on. My thought is maybe this is not set up properly but it will help me figuring out problems. I also renewed the thermal compound paste for the 12 output transistors. And now the amp has no output and fried a suicide speaker I had hooked up to it for testing purposes... probably DC on the output.
The TO3 output transistors had both mica insulator and thermal paste, and some had no mica insulator.
Anybody know what’s the stock config for them ? Isn’t their body supposed to be grounded ?
The thermal paste I used is also insulating, maybe that’s a mistake

The schematic posted above shows the drains of the power fets hot (connected to power) so not grounded. I would expect them to all have mica or some kind of electrical insulation from heatsink. BUT I have never peeked under the hood of that amp...

JR

 

[Disco Volante]

Sounds like failing power transistors. Test 'em all, make sure they're all wearing mica...

 

[Ivan K.]

Hi, I've restored a number of Hafler amps including two XL280s. It is a great amp. On my amps, the tuning adjustment actually boosts or cuts the HF around 15- 20 kHz about +/- 1 dB, presumably to add a bit of "air".

Check all the power FETS for shorts. They probably have to be removed, especially if there's no insulator. If you need replacements, buy matched FETs from Exicom.com in the UK. Every TO3 FET should have a mica insulator. I'd also use a good thermal paste on both sides of the mica. Once installed, test each FET from the case to chassis ground for shorts.

There are many easy upgrades such as a better grounding scheme, better PS caps: 105C, 20-50% higher cap values, low ESR types. Add an appropriate MOV on the AC input to protect the power transformer from turn-on surges. Replace the small PCB caps with PP types, replace the gate resistors with 1% metal film types, etc.

If you really want to upgrade the amp, Bob Cordell has redesigned the amp, only keeping the PS and FET output transistors. A Canadian fellow designed new PCBs. They were available on eBay. See: CordellAudio.com - The DH-220C MOSFET Power Amplifier
Best, Ivan

 

[capacitorless]

Hey Ivan! Just a quick thanks for the DH-220C info. I have a stock 220 which will get that treatment, now that I know about it

@thomasdf if it's useful, I can look at the physical configuration of the mosfets in this amp for you and report back.

 

[thomasdf]

Hello JR, Ivan K. an Capacitor ! Thanks for your kind help.
I have ordered new mica insulators and will check all the transistors. I can't see why they would have suddenly failed on both side so I hope the problem is due to my renewal of the thermal paste, maybe too much... I'll clean this up, measure and will re do this properly. I should have stick to the variable cap issue ! In French we have this saying "Le mieux est l'ennemi du bien" which would translate by "Better is the ennemy of already good".
In that same spirit I think I won't mod this amp because to me it already sounds amazing : virtually no noise (at least in my actual control room in my appartment), power for days, gorgeous tone and drives anything with ease... Wonderful match with my B&W Matrix 805 speakers.

I don't know what's a MOV but I was planning on adding a soft start circuit on the mains as there is a huge current draw when I turn it on ! My lights blink and my computer screen turns off ... oh well ...

Edit : GREAT read from Ivan K. 's link : http://www.cordellaudio.com/poweramp/Testing_Lateral_Power_MOSFETs.pdf

 

[Tubetec]

'If it aint broke dont fix it ' would be the closest to the French saying you mentioned .

Regarding the null test I dont think having a pre tuned amp channel is a prerequsite in this situation , although some fine tuning back and forth across the channels might get better results . From my memory of the procedure adjusting the capacitor was more about balancing out harmonic distortion to a finer degree than adjusting frequency response .

 

[thomasdf]

Absolutely ! And if I had focused solely on the variable cap I probably wouldn't have a silent amplifier now haha.
I'll check everything and will report back !

 

[thomasdf]

Update :
One of the output transistor's body was grounded by the radiator, for some reason.
I unmounted and checked all transistors (all good), cleaned all the thermal paste, and replaced all mica insulators and thermal paste on all of them (thermal paste on both side of the mica insulator).

The variable cap is still being replaced by a 100pF cap, and everything sounds perfect, clean and powerful. The amp is back in business, and so am I

For now on, I'll keep that 100pF cap in place on both channels. The oxyded variable cap (C11) was causing intermittent noise and hiss on the right channel.
Next and last mod is a soft start

Thanks all for chiming in !

 

[capacitorless]

Great news! From http://www.exicon.info/news/end-of-an-era-for-to3-lateral-mosfets/ it sounds like TO3-cased lateral mosfets may be harder to find in the future.

 

[thomasdf]

Seems like it indeed. Fingers crossed !

 

[Tubetec]

great news Thomas ,

I guess you could do the null test, measure the varicap ,then replace it with the nearest value fixed cap if you wanted .
I never bothered with the null test on my XL600 but it would be fun to try with real world speaker cables and crossovers added into the equation , then use REW for the visuals and start tuning things .
I also didnt get carried away with doing any kinds of mods to mine , just left it stock ,
I probably would like to replace the now 30 or more year old PSU caps ,but there not causing any problems, backround noise levels are very very good.

The special part mentioned is only a resistive divider , easily enough put together ,

 

[thomasdf]

Thanks Tubetec ! Maybe one day haha.. Right now it's working fine... and my slow start module has arrived. Hopefully the last time I open up the amp before a loooong time