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May 08, 2021, 07:13:23 AM

Author Topic: S.E.X monoblocks - filter caps  (Read 332 times)

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Offline Natural Sound

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Re: S.E.X monoblocks - filter caps
« Reply #15 on: April 25, 2021, 08:43:27 AM »
That certainly embodies the "E" in SEX amp. 

Don't you mean "X"? 8)

Online Paul Birkeland

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Re: S.E.X monoblocks - filter caps
« Reply #16 on: April 25, 2021, 09:26:10 AM »
True!
Paul "PB" Birkeland

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Offline Psimon

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Re: S.E.X monoblocks - filter caps
« Reply #17 on: April 28, 2021, 04:52:58 PM »
Okay, I finally was able to replace the batteries, same Ni-MH type, but the hum is still present in one channel. Also put some music through it and the sound seems richer. Any suggestions on what I can try next?

I also measured the voltages between pins and chassis again and there are some changes:

Good amp:
Pin  1st tube (by battery)   2nd tube     
1:   32.7mV                      37mV
2:   303V                          304V
3:   12.79V                       12.8V
4:   Almost no voltage       145.7V
5:   135.9V                       273V   
6:   4.86V                        149.7V     

Amp with hum:
Pin  1st tube (by battery)   2nd tube
1:   131mV                       131mV
2:   296V                          297V
3:   13.3V                         13.3V
4:   Almost no voltage        146.1V
5:   136.2V                       267V   
6:   4.7V                          150V     


Foreplay II, S.E.X Monoblocks, Whamodyne speaker towers with subwoofer.

Offline Paul Joppa

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Re: S.E.X monoblocks - filter caps
« Reply #18 on: April 28, 2021, 07:21:14 PM »
It appears to me that the 9v battery replaces a resistor and capacitor on the upper triode of the mu-follower driver. If that is the case, these voltages look good and the circuit is operating properly. Some of the solder joints to the ground buss look marginal; I remember having a heck if a time getting those hot enough back in the day! Your hum might be just a failing solder joint to the buss. Poking things with a chopstick while listening might help.

Still, the power supply is always a candidate for hum. It's harder to see it; there are two chokes and three capacitors but the initial 100uF seems to be bypassed by one of the 22uF films - and then I get lost. There's a muddle of components near the film caps which I can't figure out. The perf board over the power transformer appears to be a reverse-recovery spike filter, which solves a potential buzz problem but has little to do with hum.

From the power transformer label, this amp seems to be wired for 100v power, as used in Japan., with a replacement power transformer.

By the way, NIMH batteries lose their charge fairly fast - 10% loss per month, I think.  It's not carrying any current in this design, but you'll still want to charge them up every few months.

Paul Joppa

Offline Psimon

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Re: S.E.X monoblocks - filter caps
« Reply #19 on: April 28, 2021, 11:25:13 PM »
Paul,

Yes I'm in Japan so 100V from the power transformer. I have confirmed as you say "the 9v battery replaces a resistor and capacitor on the upper triode of the mu-follower driver" - correct. And the perf boards are indeed reverse spike filters.

I checked the joints and re-soldered anything that looked marginal, poked around with a chopstick but no difference was made.

I also used a frequency meter app and the hum is 100Hz. I'm sure it's getting louder as well.

Any other suggestions?

Thanks,
Simon





Foreplay II, S.E.X Monoblocks, Whamodyne speaker towers with subwoofer.

Offline Paul Joppa

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Re: S.E.X monoblocks - filter caps
« Reply #20 on: April 29, 2021, 05:56:38 AM »
Considering the age of the am, it is possible the 100uF electrolytic filter capacitor is getting old and not functioning as it should.
Paul Joppa

Offline Psimon

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Re: S.E.X monoblocks - filter caps
« Reply #21 on: April 29, 2021, 06:01:55 AM »
That would be this one? Any recommendations for sourcing them?
Foreplay II, S.E.X Monoblocks, Whamodyne speaker towers with subwoofer.

Online Paul Birkeland

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Re: S.E.X monoblocks - filter caps
« Reply #22 on: April 29, 2021, 06:53:56 AM »
You live in the country of origin.  The part number for that cap is likely EKMM451VSN101MQ30S.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline Psimon

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Re: S.E.X monoblocks - filter caps
« Reply #23 on: April 29, 2021, 02:35:14 PM »
Thanks again guys! Will update once I get the part in there.
Foreplay II, S.E.X Monoblocks, Whamodyne speaker towers with subwoofer.

Offline Psimon

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Re: S.E.X monoblocks - filter caps
« Reply #24 on: May 06, 2021, 11:35:46 PM »
Update:

I replaced the 100uF electrolytic filter capacitors but it made no difference. I've again checked all the joints, measured the voltages in L & R monoblocks but there's no significant differences that I can find. Resistors seem to be functioning at the right ohms.

One thing I have noticed is that when I flick the switch of the power amp, the hum starts off louder for a second or two and then settles to about half the volume.

Does that suggest anything?

Cheers,
Simon
Foreplay II, S.E.X Monoblocks, Whamodyne speaker towers with subwoofer.

Offline Paul Joppa

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Re: S.E.X monoblocks - filter caps
« Reply #25 on: Yesterday at 05:34:39 AM »
OK, so we've tried the easy guesses and not found the problem. Time to get more methodical.

Has the amp always hummed, or did it start doing so recently?

Does it hum when nothing is connected to the input? You will probably have to short the inputs temporarily to do this test - a shorted RCA plug is a convenient way to do this, but a clip-lead can do the job if that's all you have.

We'll do what we can to help, but there is a limit to how much help we can provide, since the circuit and most of the components are changed from the original and unknown to us. The filter choke on the top side is not original, and the center tube socket is also not in the original, though it appears to be used as a terminal strip. We may need more pictures, too. Just so you know what to expect.
Paul Joppa