Hi Everyone,
This is my first post so please be considerate !
I am an amateur & just picked up an old Rotel RA712 very cheap which was very dirty but has cleanned up nicely. It had obviously been sitting in a damp enviroment since a long time.
No problem with the dim bulb tester, so I started her up and found the dc offset to be OK, so hooked her up to some speakers. The sound is really nice ...... but then there is an annoying intermittent issue with the sound on the left channel. On start up or suddenly while listening to music there is an audible hum suddenly coming from the left speaker. It is definately on the power board as input source or volume makes no difference
The dc offset at each speaker terminal (not pot to adjust it) is 30mv & 34mv, so not great but OK. The bias can be adjusted easily to within spec and is stable on each channel, except when the hum starts in which case the bias on the left channel increases significantly.
The strange thing is if i give a rapid burst of volume, the hum goes away or if if I touch the bias adjusting pot VR601 very lightly with a metal screw driver or the speaker fuse the hum will suddenly dissapears, only to come back a few minutes later, which would suggest an earthing problem.
I am at a bit of a loss what to do with my limited means. I checked all solder joints, cleanned all pots, fuse holders and compared all resistors/diodes on the left & right channels with no discernible difference found so far.
I only have a DMM & a capacitance meter, with no other tools.
Can anyone suggest what I should be looking for ?
Many thanks
This is my first post so please be considerate !
I am an amateur & just picked up an old Rotel RA712 very cheap which was very dirty but has cleanned up nicely. It had obviously been sitting in a damp enviroment since a long time.
No problem with the dim bulb tester, so I started her up and found the dc offset to be OK, so hooked her up to some speakers. The sound is really nice ...... but then there is an annoying intermittent issue with the sound on the left channel. On start up or suddenly while listening to music there is an audible hum suddenly coming from the left speaker. It is definately on the power board as input source or volume makes no difference
The dc offset at each speaker terminal (not pot to adjust it) is 30mv & 34mv, so not great but OK. The bias can be adjusted easily to within spec and is stable on each channel, except when the hum starts in which case the bias on the left channel increases significantly.
The strange thing is if i give a rapid burst of volume, the hum goes away or if if I touch the bias adjusting pot VR601 very lightly with a metal screw driver or the speaker fuse the hum will suddenly dissapears, only to come back a few minutes later, which would suggest an earthing problem.
I am at a bit of a loss what to do with my limited means. I checked all solder joints, cleanned all pots, fuse holders and compared all resistors/diodes on the left & right channels with no discernible difference found so far.
I only have a DMM & a capacitance meter, with no other tools.
Can anyone suggest what I should be looking for ?
Many thanks
The noise is after the volume control, but could still be in the preamp etc.
If you want to localize it to the power board, disconnect the input and jumper it to ground.
Sounds like it could be a bad bias pot from your description.
Clean all the controls, bias pots, and switches, as well as the pots and fuse holders. Use only this:
https://www.amazon.com/Hosa-D5S-6-D...ocphy=9018873&hvtargid=pla-382003658380&psc=1
If you want to localize it to the power board, disconnect the input and jumper it to ground.
Sounds like it could be a bad bias pot from your description.
Clean all the controls, bias pots, and switches, as well as the pots and fuse holders. Use only this:
https://www.amazon.com/Hosa-D5S-6-D...ocphy=9018873&hvtargid=pla-382003658380&psc=1
Last edited:
Thanks rayma, I did as you recommended and still reckon its coming from the power board, so in the region of the bias control. I did clean up the contacts but will try with some deoxite (I am using something else).
If it was a faulty transistor, resistor or cap, the fault wouldn't be intermittant, but a permanent hum right ? Just want to know before I start unecessarily desoldering anything.
I will start by changing both bias trimmer pots. The problem is that they are flat metal ones that stand up from the board, and there isn't a much space to put a conventional round one, anyhow I will find a solution. The original spec is 5KB, sorry for the dumb question, but does that mean a trimmer with resistance 0-5KOhm or 0-500KOhm ?
Many thanks for your advice
If it was a faulty transistor, resistor or cap, the fault wouldn't be intermittant, but a permanent hum right ? Just want to know before I start unecessarily desoldering anything.
I will start by changing both bias trimmer pots. The problem is that they are flat metal ones that stand up from the board, and there isn't a much space to put a conventional round one, anyhow I will find a solution. The original spec is 5KB, sorry for the dumb question, but does that mean a trimmer with resistance 0-5KOhm or 0-500KOhm ?
Many thanks for your advice
Use only the smallest possible amount of DeOxit, and exercise the control. If still noisy,
repeat one more time. More is not better. I,n fact their slogan was: the less you use,
the better it works.
repeat one more time. More is not better. I,n fact their slogan was: the less you use,
the better it works.
Yes, this is called vertical mount. The hole spacings do vary a lot among various brands and series,
so check carefully to see if a new one will fit. Keep the old one until you are certain that
you don't have to reinstall it.
Also look for bad connections or bad solder joints.
so check carefully to see if a new one will fit. Keep the old one until you are certain that
you don't have to reinstall it.
Also look for bad connections or bad solder joints.
The fact that turning up the volume clears it makes it sound like a dry joint near the bias pot. Best to remove the old solder from the pads around there with a sucker, then resolder and make sure each joint 'wets' properly with the melted solder; 'wetting' is the moment when the solder wicks into the gaps and forms concave surfaces.
Convex shaped solder joints means either the joint hasn't alloyed properly or possibly there is too much solder which will mask whether the joint is faulty. Remove it and start again.
Convex shaped solder joints means either the joint hasn't alloyed properly or possibly there is too much solder which will mask whether the joint is faulty. Remove it and start again.
Hello, just some further feedback after a few months.
I had to change the speaker select switch which was broken (not easy to find an equivalent) which was the reason the amp was so cheap in the first place (30 Euros). I then checked again all solder joints on the main board, but couldn't really find anything wrong.
Anyhow on the left channel there was no way of adjusting the bias current as it was completely up & down all over the place even though the amp sounds great ! RH channel is perfect with no issues. At the same time it had a loud background hum through the (suicide) speakers. Anyhow throwing caution to the wind I made large variations on the bias pot and found extremes from an ocillating hum, to loud hum, quiet hum, then silence when the pot was almost fully round clockwise. Whatever position doesn't seem to affect the sound quality. This is not the way to do things I know, but I get the feeling there is nothing wrong with the bias pot.
So I decided to change all the small caps in the near vicinty of the bias adjustment pot, and hey presto much better ! Now the bias is steady and it is possible to adjust to the correct bias (6.6mv).
I figured I fixed it and as the dc offset at 30-35mv on both channels seems acceptable, I decided to hook it up to my main speakers. The sound is really great, and I actually prefer it to my sansui 317. Annoyingly though after a few minutes of silence or non-use, the left chanel starts to hum again, although not loudly. A quick burst of volume and it dissappears only to come back a few minutes later.
Can anyone advise the next step ?. All transistors on the main board seem OK, and the amp runs resolutely cold even after a couple of hours. Is it possible that the left channel is under-biased, or should I just go & change all electrolytic caps on the main board ? Would a defective cap cause these symptons ?
I am a bit of a noob & are not sure what to do next. Thanks
I had to change the speaker select switch which was broken (not easy to find an equivalent) which was the reason the amp was so cheap in the first place (30 Euros). I then checked again all solder joints on the main board, but couldn't really find anything wrong.
Anyhow on the left channel there was no way of adjusting the bias current as it was completely up & down all over the place even though the amp sounds great ! RH channel is perfect with no issues. At the same time it had a loud background hum through the (suicide) speakers. Anyhow throwing caution to the wind I made large variations on the bias pot and found extremes from an ocillating hum, to loud hum, quiet hum, then silence when the pot was almost fully round clockwise. Whatever position doesn't seem to affect the sound quality. This is not the way to do things I know, but I get the feeling there is nothing wrong with the bias pot.
So I decided to change all the small caps in the near vicinty of the bias adjustment pot, and hey presto much better ! Now the bias is steady and it is possible to adjust to the correct bias (6.6mv).
I figured I fixed it and as the dc offset at 30-35mv on both channels seems acceptable, I decided to hook it up to my main speakers. The sound is really great, and I actually prefer it to my sansui 317. Annoyingly though after a few minutes of silence or non-use, the left chanel starts to hum again, although not loudly. A quick burst of volume and it dissappears only to come back a few minutes later.
Can anyone advise the next step ?. All transistors on the main board seem OK, and the amp runs resolutely cold even after a couple of hours. Is it possible that the left channel is under-biased, or should I just go & change all electrolytic caps on the main board ? Would a defective cap cause these symptons ?
I am a bit of a noob & are not sure what to do next. Thanks
