Newbie repairing Hafler DH220 advice needed

[Simbl]

Hi this is my first post but i read on this forum alot.

Basically my left channel has gone dead and my local electronics store quoted me £100 just for parts alone. I don't trust them and they said they'd never heard of hafler and gave me a funny look when I asked them to bias the amp. Anyway I have it back now and I want to tackle it my self. Anyway tomorrow I'm going to see the guy who fixes things there and find out whats round with it. But i'm guess all he's going to do is replace the whole power output module as he said you can only get parts in america?

I have a very basic knowledge I know what component types there are and I can measure most with my dmm and i'm ok at soldiering. I also have the hafler schematics but the output caps are modified i couldn't tell you how.

Assuming it is the power output module should I go through and measure everything to see if its shorted and replace with new parts. Any general electronic repair advice would be great or possibly any books that would be helpful to me for repairing things.

[jaycee]

Hafler is more well known in the US than the UK. The schematic is readily available. There are no "output caps" in this amp, it is possible the main power supply capacitors have been replaced. Pictures will help!

If you find that the output MOSFETs have blown... you will have to get a modern replacement as the original Hitachi devices are discontinued. The best replacement is the Exicon devices sold by Profusion PLC. Unfortunately these will not be cheap - probably why you were quoted £100 for parts.

[Simbl]

ye it has four instead of two capacitors and some little extra ones stuck on top joining them together. I heard the mosfets are the least likely thing to blow. Well I'm hoping they will tell me exactly whats wrong tomorrow.

[Simbl]

So I spoke to the guy at the electronics shop and he said that the left channel was outputing like 5 watts which is wrong and that is it. Apparently they don't need parts from the us that was a lie. He wanted £100 to find out what was wrong with it not including parts so I said don't worry as I paid £120 for it.

So does that 5wats mean anything to anyone? I guess i should test the mosfets. I found these instructions on another thread but I'm not sure if they apply to this type of mosfet.

Originally Posted by ilimzn

To measure an Nch MOSFET:
1) Use DVM in diode test position
2) Holding red lead on S
3) Put put black lead on D. Reading should be between 500 and 800, you are measuring the forward voltage drop in the body diode.
4) Put black lead on G. Reading should be 1... (open circuit)
5) Hold black lead on S
6) Put red lead on D. You shuld read 1... (open circuit) as the gate should be retaining negative charge from step 4.
7) Put red lead on G, you should read 1... (open circuit)
8) Put red lead backon D, you should read a low value (close to zero initially, possibly rising very slowly). You are now measuring the D-S voltage drop with the gate positively charged in step 7.

In any case, source to gate and drain to gate should alsways read open circuit. If you get low readings in steps 4 or 7, it's dead for sure (in either case, not a MOSFET... any more ). If you get low readings in step 3 or 6, there is a good chance it's dead but you can test this later on, by doing it again with G and S shorted. If you get anything but the body diode reading in this case, the FET is dead. Also, you can try to measure the drop between shorted D+G and S, you will get the gate treshold voltage for very small Id provided by the DVM - however, most DVMs don't provide a high enough voltage to get over the gate treshold of VMOS, so you are likely to still measure 1... (open circuit), the exception being lateral MOS (will emasure low, about 1V, useful for testing for fakes), and PI-MOS such as the Toshiba Audio MOSFETs which will measure about 1-2V dependant on DVM.

BTW that schematic for a phones amp cannot work right as the required heater current for the 6922 is 300mA, and the current source connected LM317 only gives 150. It appears to me this was originally designed for a ECC81/82/83 12AU/AX/AT7 type tube with center tapped 12V filament, which is divisible into two 6.3V 150mA filaments by grounding the center tap - but then you have a problem with the too low anode voltage.

[jaycee]

Hmm looking at it, someone has definitely modified this amp. The 4 power supply capacitors + bypass capacitors is not stock, but this is an improvement and as long as the supplies are OK, it should be fine.

Note that the input capacitor on one board is a new yellow capacitor compared to an older looking blue one. Which one is the faulty board?

[c2cthomas]

You will need to join HiFi Engine to pull down the documents - but it's free so go do it.
They have a schematic (hopefully it agrees with yours) as well as the assembly manual. On page 12 of the assembly manual is a voltage chart for checking various values which should help you in finding the problem.......... Hafler DH-220 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine

Also - here is a link to trouble shooting amps http://www.repairfaq.org/sam/audiofaq.htm#audaaboco

[jaycee]

Don't even need to subscribe to that - It's on the Hafler site http://www.hafler.com/techsupport/pd...20_amp_man.pdf

edit: just to clarify, the above is not just a "user manual". The early Hafler stuff like this was also sold in kit form, so the manual includes "how to build" and the complete schematic. It also includes voltages at test points, very handy for fault finding.

If memory serves a common fault on these amps is for one of the current sources to fail. This is usually the diodes D1-D6. They are just cheap 1N4148's so easily sourced. Pretty much all of the parts apart from the original output MOSFETs are easily sourced.

[c2cthomas]

Some output devices - but located in US 2SK134 2SJ49 BUZ900 BUZ905 ECF10N16 ECF10P16 EXICON SET | eBay

[Simbl]

I do have the assembly manual and schematic but i get those as well in case I'm missing something.

The board with the old blue cap is the left side and that is the one thats not working properly. The right side has some polyester film caps on which the left doesn't. I did start checking all the resistor values and then realised they would be the last to blow.

I will have to reassemble to read voltages as i have the mosfet and heatsink off ready to test them I just need to know the correct way of doing it. Plus I've never measured voltage before so I'll have to figure that out to. Do you know what the capital letters above voltage mean in the manual on page 12?

Should i just check everything on the board on that channel as it's so old? Then all I have left is the mosfets and the power supply caps to check.

[Simbl]

Originally Posted by c2cthomas

Some output devices - but located in US 2SK134 2SJ49 BUZ900 BUZ905 ECF10N16 ECF10P16 EXICON SET | eBay

That doesn't seem to bad around £40 incl delivery not incl import tax