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Old 6th October 2014, 08:34 AM   #1
contax is offline contax  Spain
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Question Musical Fidelity power amplifier trim pots

Hello,

I have fixed a MF Elektra E30 Power amplifier.

It has 2 trim pots (I have enclosed one picture):

- one (labeled PR102), I imagine is for adjusting the DC offset and I thnik it must be set trying to reach the lowest value of DC in the output. I measure it in the speaker binding posts. Is it correct?.

- I think the other one (labeled PR101) is for biasing the output mosfets current, but I don't how to measure it and the desirable value. In the picture, I can see a point labeled TG104. I don't know the purpose of it. Does anyone know?.

Thanks for your help!

Best regards
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Old 6th October 2014, 09:27 AM   #2
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A circuit would help. Can you post it?
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Old 6th October 2014, 09:40 AM   #3
contax is offline contax  Spain
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I have searched the E30 schematic or Elektra series schematics, but I have not found them.
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Old 6th October 2014, 11:21 AM   #4
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I don't know of any schematics or service manuals for early MF available on the internet. Any that I have seen are hand drawn diagrams only. However, PR102 does adjust offset voltage like it says but this is not usually important. It is measured as you say, across the output terminals of the appropriate channel when there is no signal input and no speakers connected. Make sure of this condition for both tests.

The bias current is measured as the voltage across either source resistor(s). They will be large resistors of 2-5W rating and very small value. They appear to be the 0.22R resistors R115/6 in your pic. so there should be another 2 for the left channel. If there are 4 output Mosfets for each channel, there should also be 4 resistors to choose an average reading from in either channel (check this please). You can then calculate bias current by using Ohm's Law. So, if for example your source resistor is 0.22R and you measure an average 22mV, you have an average 100 mA bias current flowing through each pair of output Mosfet(s).

The correct bias for a Mosfet amplifier is not fixed because there no optimal current level - just enough of it. Measure and check what it is now. If the left and right channels are within 20% of each other, I suggest you set both to the highest value you measure and no more. (as long as it does not exceed an estimated 100 mA per complementary pair of output Mosfets). Without a manufacturers spec. or an experienced repairer's advice, there is not much more you can do for your own adjustments.



A word of warning for people looking to fix or just make trouble: Don't test with both meter probes whilst you are trying to do or read something. Use insulated clip leads, good quality IC clip adaptors or even solder insulated wires to where you want to test because with one slip (very common!) it can be goodbye amplifier.
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Last edited by Ian Finch; 6th October 2014 at 11:30 AM.
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Old 8th October 2014, 10:33 AM   #5
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They are bias pots. As i recall there is no dc offset adj.The amp is bipolar and uses mje 15003/4 output devices with the part numbers replaced by mf ones. I had a e300 which is almost the same . A bloke called roar malmin on here gave me a few pointers years ago when repairing mine.
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Old 8th October 2014, 02:49 PM   #6
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It seems I was quite wrong, having trusted a couple of reviews that described the amp as Mosfet. Apologies, folks. Is this the thread you referred to, raymondo? Musical Fidelity e300 power amp trannys help :(

That means bias should still be measured and compared carefully in both channels because I think the emitter resistors are different values to your E300 but the correct setting could require some better advice for contax. There didn't seem to be any report of success at the end of the thread either.

However, from contax's E30 PCB pic, PR102 is clearly labelled "offset" on the PCB and is a 10k value. PR101 (1k) can't be doing the same job and it's label appears at a guess, to say "idle adj." That pic shows only the right channel since there are only a couple of duplicate parts visible from the left channel.
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Old 8th October 2014, 06:21 PM   #7
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Sorry doing this off my phone from hospital . Can't see pictures . Im sure the e30 is almost the same as e300 which is definitely bipolar . It would be odd if it was mosfet when from the same family. I did repair my e300 successfully. I sold it a friend years ago but its just come back yo me broken!
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Old 9th October 2014, 02:55 AM   #8
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Yes raymondo, you were right about bipolar, that's how I recall E series in the 1980s. Roar Malmin made that clear in your old thread. What I saw in the pic was the offset adjust and bias pots. Offset trimming isn't essential and later versions designs such as yours could well omit it.

Thanks for the update - if you are a patient and not hospital staff, best wishes for recovery.
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Last edited by Ian Finch; 9th October 2014 at 02:57 AM.
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