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Help with Amp please

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bristol188

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Help please. My own amp. a Marantz mod. 1070 (bought new, sentimental value) has developed a quite loud mains hum. It is independent of volume control and
channel slider, just constant. Amplification is perfect, everything works perfectly. I have a schematics and have some knowledge of
electronics. Played with 240 - 415v and dangerous machines all my life, no fingers missing and no electric shocks! Oh just a few- shocks that is. (70 years young) I have checked out the two mains smoothing caps. (10000uf) and they seem
ok (nice fat spark when disconnected and shorted) Is that a good test? I would like some advice as to what else to check . Thanks Rog.
 
Mains hum, eh? Caps in the power supply could be bad (drying out, leaking, etc), decreasing capacitance, increasing ripple, etc. Just a guess. If you had an oscilloscope, you could check the voltage ripple from the output of the power supply, but if you're not huge into electronics, you probably don't have an oscope.
 
Caps

Mains hum, eh? Caps in the power supply could be bad (drying out, leaking, etc), decreasing capacitance, increasing ripple, etc. Just a guess. If you had an oscilloscope, you could check the voltage ripple from the output of the power supply, but if you're not huge into electronics, you probably don't have an oscope.

Thank's Are there any other smoothing caps in the power supply that I should check?
I do have Winscope which I use for low voltage (5v). I would have to buy probs for 240v but that would be ok.
Or I should just buy two new caps and see how that goes? Rog.
 
Thank's Are there any other smoothing caps in the power supply that I should check?
I do have Winscope which I use for low voltage (5v). I would have to buy probs for 240v but that would be ok.
Or I should just buy two new caps and see how that goes? Rog.

If it *is* the caps in the power supply, then it'll probably just be "the biggest ones", and if it's cheap enough, you should just shotgun replace them and hope for the best. Might as well, right? :)
 
Hi there u need to use an ESR Meter if you are checking capacitor, Normal capacitor meters sometimes give bad readings.
 
Hi there u need to use an ESR Meter if you are checking capacitor, Normal capacitor meters sometimes give bad readings.

It's not really a problem in this case, as they are LARGE electrolytics working off a normal 50/60Hz supply - it's mainly switch-mode supplys where ESR meters are absolutely essential.
 
I am replying to all helpers thanks, I have replaced the caps with new ones but no joy, the hum is still present. I agree a oscope. would be good to have
but the hum is coming out of the speakers! what would I do trace the ripple all the way back to the source! It's definitely mains hum. As I said already
I have a schematics. Amp. was bought in the days when they where supplied! Ha. I can understand the power supply to a point, but after that I'm lost. The replaced caps are 50v rated but I have not checked the secondary volts,
no point.
 
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If the output transistors go short you get a LOUD hum, so presumably if the amplifer is taking far too much quiescent current this could cause the amp to hum as well?. Are the output transistors getting hot at all?.
 
Thank you Nigel. I will investigate that to day (morning here) I have a temp gun, will that be a way to go? The output trans are the ones attached
to heat sinks true. I burnt one out a few years ago and was able to replace it successfully myself. Thanks again to all ongoing helpers, we will win.

Added
Ambient temp. 24.6c max rise output transistors 34c no load no volume, input added some volume 36c 5mins
max temp one resistor 42c went down a bit with input. (and that was playing B Dylan) Ha. Mains transformer
couple degs.
 
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Thank you Nigel. I will investigate that to day (morning here) I have a temp gun, will that be a way to go? The output trans are the ones attached
to heat sinks true.

No need for that, just lick your finger and touch the transistors - if they are drawing enough current to cause hum they wil be obviously VERY hot (which is why you lick your finger first).
 
No need for that, just lick your finger and touch the transistors - if they are drawing enough current to cause hum they wil be obviously VERY hot (which is why you lick your finger first).

Yea. That's OK for you! I'm not touching anything with my finger, not even just after turn off!
But no all jokes aside there are no hot spots at all. And if it were an output board prob. would it not be affected by the volume control? which it is not, the Amp. works perfectly.
And must point out the speaker wires not to blame.
Still to early here but I will try connecting the main speakers to the ambiance output see
if that has an effect.

The DMM on speaker connections shows 100 Hz is that significant?
 
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I would be looking for pre-amp regulation and making sure the pre-amp supplies are clean and ripple free. Also check the capcitors around any interstage coupling.
Without seeing a schematic for it, I can't really suggest anything more.

rgds
 
I would be looking for pre-amp regulation and making sure the pre-amp supplies are clean and ripple free. Also check the capcitors around any interstage coupling.
Without seeing a schematic for it, I can't really suggest anything more.

rgds

Yes thanks I will attempt to carry out those tasks. I could post a schem. but it would have to
be photograph as I don't have a copier, don't know how it will come out. I have "Winscpoe" on
computer, it is suitable for signal testing and up to 10v (tried it). It uses the ADC inc. in
sound card.
 
Yes thanks I will attempt to carry out those tasks. I could post a schem. but it would have to
be photograph as I don't have a copier, don't know how it will come out. I have "Winscpoe" on
computer, it is suitable for signal testing and up to 10v (tried it). It uses the ADC inc. in
sound card.

I wouldn't be so much worried about the win scope thing, as I would be for the amp.
I would be more worried about applying an offset voltage to the amp with the scope probe and driving something into cut-off accidentally. If you accidentally turn a transistor hard on you may well pop something in the amp, and then you will have two problems to deal with!
Not to mention that there may be a ground potential problem. Far better to use a real scope for this. I would recommend that you befriend a local electronics enthusiast/ geek as they will likely have a scope and a nose for troubleshooting. Failing that, join a local radio club, there's always a geek in there :) or speak to the tech at you're local secondary school and see if they would be interested in helping you with it. A last resort would be to take it into you're local repair shop. If it really means a lot to you, it's got to be worth investigating other sources of help.

rgds
 
It might also be the 40V supply, so try C803 and C804.

100 Hz hum is most likely to come from unfiltered full wave rectified mains.
 
Thank you Mr Wolf. Your points are all taken and I will not play around with W/S. I have not had a very good experience with electronic
repair shops over the years but will try again.
 
OK HD. Your on the same time frame as me! (near enough) I will check out those caps in the morn. they could be the ones I have
already replaced, back to bed. 2am.


Yes. I will replace the caps you suggest on the power board. It is better I just keep replacing parts
because to test I need to unsolder anyway as far as I'm concerned. Cost is small compared to L/E
tech. shop. (I'm learning "never to old")

Found the prob. or I'm very shore I have, put the iron on one side of C806 and it fell off
no connection + side. () I could not get 220uf 50v, only 2 off 100uf or 2 off 470uf which way
would you recommend? they are all 65v. Thanks again for all your help.
 
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OK now, quiet as ssssss no humming like a B/Bee. And thats thanks to all my Cyber helpers AAA+.
Turned out pretty basic in the end, but we can nearly always say that at the end.
 
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