I have been lucky enough to score said amp for a really good price, but sold as defective because the transformer humms - you can hear it from at least 2m (7 feet). The seller, knowleadgeabli in tube amps said a new transformer is needed (also mentionet that all s150 he encountered hummed).
My first question would be regarding the humm - is there something i can do to improve the situation? Also might it have something to do with the voltage in the EU (currently 230 as opposed to 220 in the past).
Another question would be regardin the mod - moving the transformer further away from the left sided pcbs.
As i have more projects started, this will have to wait a bit, until i fill a mouser cart at least, for the ageing parts (caps, diodes and trimmers).
The sad (or is it funny?) part is i have no means of hooking it up at home, as i have no preamp or amp with jumpers to use as a pre.
Pictures will have to wait.
My first question would be regarding the humm - is there something i can do to improve the situation? Also might it have something to do with the voltage in the EU (currently 230 as opposed to 220 in the past).
Another question would be regardin the mod - moving the transformer further away from the left sided pcbs.
As i have more projects started, this will have to wait a bit, until i fill a mouser cart at least, for the ageing parts (caps, diodes and trimmers).
The sad (or is it funny?) part is i have no means of hooking it up at home, as i have no preamp or amp with jumpers to use as a pre.
Pictures will have to wait.
it's easy - sort of - to make simple and crude "passive pre", which is nothing else than small box with in RCA pair an out RCA pair and 10Klog stereo pot in between
then you'll know is there hum in speakers too
anyhow, best to revitalize it - new electrolytics and thorough inspection of all solder joints, including possible change of thermal goop on outputs, change of diode bridges
then, when you know rest of electronics is OK, you'll have proper conditions to evaluate mains xformer
then you'll know is there hum in speakers too
anyhow, best to revitalize it - new electrolytics and thorough inspection of all solder joints, including possible change of thermal goop on outputs, change of diode bridges
then, when you know rest of electronics is OK, you'll have proper conditions to evaluate mains xformer
Managed to take a look at it a bit today.
I was able to change the bridge rectifier, and check Bias with a dmm. Both channels were off, around 80mv on cold turn on. Did not have time to check temps.
The transformer buzzez exactly as before. The metal cover also vibrates, can be felt by hand. Next step will be to change the filter caps and hook it up. A new transformer might be next on the list (if a suitable transformer can be sourced) if the issue is not resolved. Anybody has any info regarding transformer specs?
I was able to change the bridge rectifier, and check Bias with a dmm. Both channels were off, around 80mv on cold turn on. Did not have time to check temps.
The transformer buzzez exactly as before. The metal cover also vibrates, can be felt by hand. Next step will be to change the filter caps and hook it up. A new transformer might be next on the list (if a suitable transformer can be sourced) if the issue is not resolved. Anybody has any info regarding transformer specs?
Managed to do another round of troubleshooting. In the meantime also got a preamp and both are currently playing music.
To cross off possible fault of the list, i did a bucking transformer and reduced mains voltage to aprox 220v. Sadly it did not eliminate the buzzing. If no music is playing it can be heard from at least 3m (12 ft).
Adter about one hour the heatsinks are just a bit warm to touch. I assume this is not quite right. The bias is set to 170mv ( was set to 180v without the bucking transformer)
As I have a few other projects I have to tend first, i did not acquire filter caps until now but they will be included on my next order.
Any pointers on what to check next are appreciated.
To cross off possible fault of the list, i did a bucking transformer and reduced mains voltage to aprox 220v. Sadly it did not eliminate the buzzing. If no music is playing it can be heard from at least 3m (12 ft).
Adter about one hour the heatsinks are just a bit warm to touch. I assume this is not quite right. The bias is set to 170mv ( was set to 180v without the bucking transformer)
As I have a few other projects I have to tend first, i did not acquire filter caps until now but they will be included on my next order.
Any pointers on what to check next are appreciated.
Another day another another step forward:
Powered it up with a dbt and the transformer was silent. Without sbt the buzz came back with no audible change.
Noticed some burn marks on the back plate on the side of the 15nf cap - I assume something made it blow - and possibly taking more parts with it, but this is only an assumption.
Here are a few random fotos I took
- new filter caps of higher capacitance (to maintain size mostly)
- two electrolitics swapped (47uf)
- tantalums swapped one was 15nf and the other 150nf - somebody was already inside (only had 220nf on hand but will replace with 150nf after my next parts order)
Powered it up with a dbt and the transformer was silent. Without sbt the buzz came back with no audible change.
Noticed some burn marks on the back plate on the side of the 15nf cap - I assume something made it blow - and possibly taking more parts with it, but this is only an assumption.
Here are a few random fotos I took
Exactly, speakers are dead silent with no music playing. Buzzing can be heard feom far avay and can also be felt to touch
Although the hum persist in two different locations, you might check dc offset on the AC power into the amp. Just a thought.
yeah, making DC blocker is easy, even if using it just for test
besides, re-check xformer bolts ...... sometimes tightening the packet is all what's needed
besides, re-check xformer bolts ...... sometimes tightening the packet is all what's needed
Mine does as well. I assume it is because the transformer is mounted to the front panel, amplifying the noise. It dimishes after a few hours, but never totally subsides. Amp is entirely original afaik, and still sounds good otherwise.
Dunk it / pour in varnish, many posts here.
Best done on load, hum acts as vibratory feeder.
And tighten the mounting bolts later.
A gasket / grommet between transformer and body may help.
And do check for supply and earthing issues.
Best done on load, hum acts as vibratory feeder.
And tighten the mounting bolts later.
A gasket / grommet between transformer and body may help.
And do check for supply and earthing issues.
There could be bolts holding lamination stack together. Tighten them after varnish.
And see if the coil bobbins are fixed, or are slightly wobbly, hot glue in place for the coils.
And see if the coil bobbins are fixed, or are slightly wobbly, hot glue in place for the coils.
Exactly, the transformer is sealed.
Dc offset is 8 and 27 mv, so no problem there.
proceeded to take the front plate off and removed the transformer. When it is placed on on a soft surface the transformer barely makes a buzzing sound - can not be heard in silence from half a meter, so I assume the noise is amplified by the case, and possibly the mounting can be somehow improved.
I also wanted to distance the transformer from the pcbs - this is a good opportunity to modify how the transformer is mounted.
Does anybody know what screws are holding the front plate to the rest of the chassis? Are they unc 10-24?
Dc offset is 8 and 27 mv, so no problem there.
proceeded to take the front plate off and removed the transformer. When it is placed on on a soft surface the transformer barely makes a buzzing sound - can not be heard in silence from half a meter, so I assume the noise is amplified by the case, and possibly the mounting can be somehow improved.
I also wanted to distance the transformer from the pcbs - this is a good opportunity to modify how the transformer is mounted.
Does anybody know what screws are holding the front plate to the rest of the chassis? Are they unc 10-24?
The DC problem is in relations to your mains supply, not output offset. Toroids don't like DC on their primary.
OK, it's EI xformer, but anyway
worth checking with DC blocker
easy to find how to; if in doubt, link here what you found
now, regarding mounting of xformer in case, I don't know details, but this is worth checking
just random link and pic from there
https://www.amazon.co.uk/Anti-Vibration-Rubber-Isolator-Compressor-Absorber/dp/B07Q29YCXS
worth checking with DC blocker
easy to find how to; if in doubt, link here what you found
now, regarding mounting of xformer in case, I don't know details, but this is worth checking
just random link and pic from there
https://www.amazon.co.uk/Anti-Vibration-Rubber-Isolator-Compressor-Absorber/dp/B07Q29YCXS
Attachments
Its actually Toroidal, so it might be dc, but I am not entirely convinced (had other gear with toroid and had no buzzing problems). I will put in on the list of stuff to try.
Does anybody know how can I tell if it is an optical bias unit or not? Have read that some have been upgraded.
Does anybody know how can I tell if it is an optical bias unit or not? Have read that some have been upgraded.
I stand corrected
give us proper pics of boards
if you don't see 4 or 6 legged IC, there is no optocoupler present, so no optobias
heck, you can always make new FE
