HIWATT 1977 SA112 Custom 50 HUM

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HIWATT 1977 SA112 Custom 50 HUM

Postby HIHO on Thu Jan 26, 2017 1:00 pm

Good day

First timer here, and I am enjoying reading all the posts.

I recently got a 1977 HIWATT sa112 Custom 50 combo and I love it, when I tried it before bringing it home I did not notice anything out of the order.
At home my basement is relatively quiet and I am noticing when I flip on the standby to start playing there is this constant hum , not the 60hz cycle Hum it is louder and more like a fridge hum, so when the amp is on standby there is no hum.

Volume up or down do not affect it, guitar plugged or not plugged do not affect it. Always the same constant hum.

The amp does not get hot and do not buzz pop sparkle or anything, the amp sounds killer and it still has the Partridge transformers with the Fane Crescendo speaker.

I should mention that:
-Leaking filter caps replaced in 2016
-Canadian mods reversed
-Bias adjustment added by a great tech

I had an agreement that I can bring it back if anything not proper but I like it and would like to keep it. Any Ideas please?
HIHO
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Re: HIWATT 1977 SA112 Custom 50 HUM

Postby OldSchoolDave on Thu Jan 26, 2017 3:45 pm

HIHO wrote:I had an agreement that I can bring it back if anything not proper but I like it and would like to keep it. Any Ideas please?


Welcome!

A properly working vintage HIWATT is quiet at idle. Since the amp has been recently serviced, I'll give the internals the (temporary) benefit of the doubt.

Swapping out the tubes, one by one, with known-good models would be the first step in troubleshooting the hum. Another easy thing to check would be moving the amp to a different location to determine if something external is causing the noise. Try these and report back.

a fellow SA112 owner,
Dave
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Re: HIWATT 1977 SA112 Custom 50 HUM

Postby Porge on Tue Jan 31, 2017 3:36 am

Another fellow owner :)
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1979 Hiwatt SA112 50w Combo
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Re: HIWATT 1977 SA112 Custom 50 HUM

Postby Sloclone100 on Fri Sep 22, 2017 6:35 pm

Hiwatts are known for their impeccable wiring but the design has some flaws in making 'the prettiest amp on the planet'. To retain their 'military style' appearance, they made a few mistakes along the way. One ground point was connected to another by ground wires hidden underneath the preamp circuit board (turret). What can be done is to locate all grounds in the amp and disconnect them from the main ground wire under the main turret board. Separate the main ground wire under the board and run each ground wire separately to the front of the chassis. You will find this will reduce your hum dramatically, maybe even for good.

IMPORTANT: If the screen resistors are 1 watt carbon type, pull them ASAP. At full volume the power tubes exceed the resistors rating and they will blow. Replace all 1 watt carbon screen resistors to 5 watt wirewound. I also took an extra precaution. The stock DR103 has a 470R 10 watt resistor feeding the screens. Since I had one lying around, I replaced it with a 25 watt metal type. This maybe overkill but I won't be blowing up my Hiwatt anytime soon.
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Re: HIWATT 1977 SA112 Custom 50 HUM

Postby pckpat on Sat Sep 23, 2017 10:29 pm

I also have a SA-112 from '77- and I was wondering, did any others come with the Crescendo Fane? Mine just has the regular cast frame one. (with the label missing)
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Re: HIWATT 1977 SA112 Custom 50 HUM

Postby OldSchoolDave on Mon Sep 25, 2017 5:50 pm

Can't say for sure, but it wouldn't surprise me if a small number of SA112's shipped with Crescendos.

My '80 SA112 came with a heavier-than-normal cast frame HIWATT Fane model 122918.

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Re: HIWATT 1977 SA112 Custom 50 HUM

Postby Vortexion on Mon Sep 25, 2017 9:51 pm

HIHO wrote:Volume up or down do not affect it...


Just so we can narrow down the source of the hum, does that include the Master Volume control?
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