The wattage of the pots is of zero concern... just get ones that fit. Even if the 100k one was directly across 100volts it would calculate out at only 100mw... and looking at the circuit and chip I bet there is less than 10 volts across it, which is 0.001 watts.
The 4K7 could approach the dizzy heights of around 0.002 watts or so depending how it was set
Does the amp as a whole get hot or is it mainly that PSU generating the heat.
What I would definitely do is adjust now the pot R301/2, firstly to make sure it's set correctly now, but more importantly so you know which way to be able to set the new pots to give minimum current when you replace them. That's very important... you don't want to power up and risk blowing something because they were set to maximum.
When you adjust the pots keep rechecking... it's normal for it to drift upwards as the transistors get hotter... and usually I would do a final adjust when the amp is quite hot.
Make sure you have no speakers attached when setting as that affects the reading.
Hard to say what the fading sound could be... I would have suspecte switches/vol control etc tbh as the main suspects.
Headphone socket maybe ? try a little WD40 on the phones plug and rotate it in the socket.
The 4K7 could approach the dizzy heights of around 0.002 watts or so depending how it was set
Does the amp as a whole get hot or is it mainly that PSU generating the heat.
What I would definitely do is adjust now the pot R301/2, firstly to make sure it's set correctly now, but more importantly so you know which way to be able to set the new pots to give minimum current when you replace them. That's very important... you don't want to power up and risk blowing something because they were set to maximum.
When you adjust the pots keep rechecking... it's normal for it to drift upwards as the transistors get hotter... and usually I would do a final adjust when the amp is quite hot.
Make sure you have no speakers attached when setting as that affects the reading.
Hard to say what the fading sound could be... I would have suspecte switches/vol control etc tbh as the main suspects.
Headphone socket maybe ? try a little WD40 on the phones plug and rotate it in the socket.
My plan is to measure the resistance of the pots and to duplicate that on the replacements.
It's the SMPS that gets hot, and as a day passes the rest of the amp heats a fair bit because of that. It never gets too hot to put one's hand on, but it is quite toasty I must say! I was abit worried it might have something to do with the recent repairs I'd done to the SMPS, but TheFragger said his gets hot to the touch too so I guess it's running as designed. No wonder the SMPS is the first thing that seems to go on these amps, especially with the stock caps which are rated 85C!
No I've tried all the switches and knobs and nothing effects the signal at all bar the odd bit of crackle now and again. I am sure it's a cap somewhere that's dried out and is on its last legs - perhaps a signal cap or the 1000uF reservoir cap on the power-amp board if it's directly effecting the sound like that?
I would have thought if it was something more serious like a transistor or something it wouldn't fade in and out like that and would be more of a constant problem...
It's the SMPS that gets hot, and as a day passes the rest of the amp heats a fair bit because of that. It never gets too hot to put one's hand on, but it is quite toasty I must say! I was abit worried it might have something to do with the recent repairs I'd done to the SMPS, but TheFragger said his gets hot to the touch too so I guess it's running as designed. No wonder the SMPS is the first thing that seems to go on these amps, especially with the stock caps which are rated 85C!
No I've tried all the switches and knobs and nothing effects the signal at all bar the odd bit of crackle now and again. I am sure it's a cap somewhere that's dried out and is on its last legs - perhaps a signal cap or the 1000uF reservoir cap on the power-amp board if it's directly effecting the sound like that?
I would have thought if it was something more serious like a transistor or something it wouldn't fade in and out like that and would be more of a constant problem...
The setting of the pots in an absolute sense is quite critical and pots may have a tolerance of as much as -/+20% so "duplicating resistance" isn't quite so easy. If you set it the same from wiper to one end, the wiper to other end will be very different.
I wouldn't even think twice... go through the adjustments now to get a feel for what happens and then with the DC Bias pot adjust preset noting which way to set it for minimum voltage across the test points (that 22mv will or should adjust down to zero or very nearly).
It makes a surprising difference to the heat generated in the output stage...
The 22mv is just the volt drop across one of the emitter resistors in the output stage. That give a current of 100ma, a typical "safe" figure for any amp. 50 volt rail, 0.1 amp flowing gives 5 watts dissipation per transistor. And you have four outputs so that is 20 watts. Now imagine it was nearer 27 mv across the resistors. That gives 123ma and 6.13 watts, total of nearer 25 watts... a big jump.
In practice even 2 or 3 milliamps flowing is usually enough to eliminate crossover distortion on an audible test using a pure sine at low level so my advice is set it for no more then the recommended value and even then only set it to that when the amp is fully up to temperature.
It's worth noting the behavior of the current. Is there much difference in the voltage reading (the 22mv) from a stone cold condition to hot.
I wouldn't even think twice... go through the adjustments now to get a feel for what happens and then with the DC Bias pot adjust preset noting which way to set it for minimum voltage across the test points (that 22mv will or should adjust down to zero or very nearly).
It makes a surprising difference to the heat generated in the output stage...
The 22mv is just the volt drop across one of the emitter resistors in the output stage. That give a current of 100ma, a typical "safe" figure for any amp. 50 volt rail, 0.1 amp flowing gives 5 watts dissipation per transistor. And you have four outputs so that is 20 watts. Now imagine it was nearer 27 mv across the resistors. That gives 123ma and 6.13 watts, total of nearer 25 watts... a big jump.
In practice even 2 or 3 milliamps flowing is usually enough to eliminate crossover distortion on an audible test using a pure sine at low level so my advice is set it for no more then the recommended value and even then only set it to that when the amp is fully up to temperature.
It's worth noting the behavior of the current. Is there much difference in the voltage reading (the 22mv) from a stone cold condition to hot.
I'm going to give the bias a check tomorrow to see that it is set at .22mv. It's got new(er) output transistors so I hope the guy who did the replacements reset the bias accordingly.
I think that fading channel was now the volume 'pot' It happened briefly today and whilst turning the pot vigorously left & right for a few seconds it seemed to go and hasn't happened again today. Not sure how to clean this pot out as the 4-gang monster is housed in a totally sealed die-cast housing, rather than the usual plastic. They obviously spared no expense with this amp back in '78! I'm hazarding a guess if I desolder it from the EQ amp-board there might be slots underneath it which I can spray some contact cleaner into. If not I'll just have to live with it.
Decided to try out the phono input today. I guess I shouldn't be so surprised, but it sounded VERY good indeed. In fact it sounded as good as my Trichord Dino mk.2 if I'm being totally honest. I'll give it a few more LPs yet, but I can see the Dino ending up on Ebay at this rate.....
Incase nobody had guessed yet, I'm loving this amp
I think that fading channel was now the volume 'pot' It happened briefly today and whilst turning the pot vigorously left & right for a few seconds it seemed to go and hasn't happened again today. Not sure how to clean this pot out as the 4-gang monster is housed in a totally sealed die-cast housing, rather than the usual plastic. They obviously spared no expense with this amp back in '78! I'm hazarding a guess if I desolder it from the EQ amp-board there might be slots underneath it which I can spray some contact cleaner into. If not I'll just have to live with it.
Decided to try out the phono input today. I guess I shouldn't be so surprised, but it sounded VERY good indeed. In fact it sounded as good as my Trichord Dino mk.2 if I'm being totally honest. I'll give it a few more LPs yet, but I can see the Dino ending up on Ebay at this rate.....
Incase nobody had guessed yet, I'm loving this amp
Well that had me looking at the manual, the 4 gang pot! The block diagram is easier to follow.
It looks as though the volume control attenuates both the input to the tone control stage and also the output. They must have had their reasons. To attenuate and then amplify is frowned on a little today as it increases noise. And tbh noise is the least of the worries I find in audio (that's white noise, not hum etc) and I suspect it can even help in the hearing process... like dithering of a digital signal at low levels.
WD40 is pretty good as a cleaner and it doesn't attack plastics etc which can't always be said for others (in the past anyway). Turn the amp upside down maybe and see if you can get any in there.
Follow the manual for the bias... and just make sure you are on the correct test points for the channel you are altering. Don't measure on one channel while adjusting the pot for the other lol. If it's easier to measure you are aiming for 22mv across either of the 0.22 ohms in each channel. It doesn't matter which you measure over as they are effectively "in series" with the outputs.
As you are finding out the old saying "Good sound today will be good sound tomorrow" holds true.
It looks as though the volume control attenuates both the input to the tone control stage and also the output. They must have had their reasons. To attenuate and then amplify is frowned on a little today as it increases noise. And tbh noise is the least of the worries I find in audio (that's white noise, not hum etc) and I suspect it can even help in the hearing process... like dithering of a digital signal at low levels.
WD40 is pretty good as a cleaner and it doesn't attack plastics etc which can't always be said for others (in the past anyway). Turn the amp upside down maybe and see if you can get any in there.
Follow the manual for the bias... and just make sure you are on the correct test points for the channel you are altering. Don't measure on one channel while adjusting the pot for the other lol. If it's easier to measure you are aiming for 22mv across either of the 0.22 ohms in each channel. It doesn't matter which you measure over as they are effectively "in series" with the outputs.
As you are finding out the old saying "Good sound today will be good sound tomorrow" holds true.
Thankfully in practice there's no hiss/hum. If you want to hear a combo that takes the hiss, my recently sold Naim 72/140 combo was unbelievable in that regard. One of the reasons I had to sell it was it was setup in my bedroom, and the hiss was audible on anything that wasn't rock/indie/dance. Very bad electrical engineering!
Regarding that pot I'll have to actually desolder it to get into it (if it's got some openings underneath and isn't sealed there too). It's directly soldered into the EQ board PCB so one can't get to it any other way. I'll do that when I replace the caps. I've got some de-oxit, plus some Servisol Super 10 which usually does the trick.
That's so true - if it sounds enjoyable, and realistic today (as realistic as reproduced music CAN sound anyway) then it will in 10 years time (servicing aside) unless we suddenly developed bionic hearing or instruments suddenly sound better and the goal posts were moved forward haha! I am so glad I got off the upgrading/reading all the mags treadmill and discovered DIY plus the second-hand/vintage gear scene. So much more fun and sonically rewarding. I used to write for one of the UK magazines, and you know it's time to hang up the quill when 99% of the megabucks gear doesn't sound any better than an average Gainclone setup!!!
Regarding that pot I'll have to actually desolder it to get into it (if it's got some openings underneath and isn't sealed there too). It's directly soldered into the EQ board PCB so one can't get to it any other way. I'll do that when I replace the caps. I've got some de-oxit, plus some Servisol Super 10 which usually does the trick.
That's so true - if it sounds enjoyable, and realistic today (as realistic as reproduced music CAN sound anyway) then it will in 10 years time (servicing aside) unless we suddenly developed bionic hearing or instruments suddenly sound better and the goal posts were moved forward haha! I am so glad I got off the upgrading/reading all the mags treadmill and discovered DIY plus the second-hand/vintage gear scene. So much more fun and sonically rewarding. I used to write for one of the UK magazines, and you know it's time to hang up the quill when 99% of the megabucks gear doesn't sound any better than an average Gainclone setup!!!
Going back to all those back-to-back electrolytic caps which are in the signal path (and which I'll be replacing with Panasonic polyprolylene film caps) I've noticed that in each instance there is a 0.056uF film cap used as a bypass for each pair. That's a technique I thought was fairly 'modern' to get the best out of electrolytics when used in the signal path. Yet again my respect for the designer(s) of this circa 1978 amp has increased!
As I'll be using high(er) quality film caps to replace the electrolytics anyway I'll remove those 0.056uF bypasses in each instance (unless anyone can think of a good reason to leave them in?). Bypassing a film cap with a film cap seems a little pointless... So instead of 3 components (2 back-to-back electrolytics and one film bypass) in the way of the signal in each case that's been reduced to one film cap only. I prefer to keep things simple
After asking a few pages back which caps where in the signal path, I noticed today that these are actually marked with a red dotted line in the service manual - doh! Interestingly they don't highlight C205/255 as being in the signal path
- John
As I'll be using high(er) quality film caps to replace the electrolytics anyway I'll remove those 0.056uF bypasses in each instance (unless anyone can think of a good reason to leave them in?). Bypassing a film cap with a film cap seems a little pointless... So instead of 3 components (2 back-to-back electrolytics and one film bypass) in the way of the signal in each case that's been reduced to one film cap only. I prefer to keep things simple
After asking a few pages back which caps where in the signal path, I noticed today that these are actually marked with a red dotted line in the service manual - doh! Interestingly they don't highlight C205/255 as being in the signal path
- John
Last edited:
C205/255 are in the feedback network around the opamp. R210 allows for DC feedback to set the DC conditions, while 99% of the audio passes via those caps from the tone control network. You'll have to look up opamp circuit design In the example below R2 is would be AC coupled and connected to the tone network. So they are very definitely in the signal path, but not in the obvious sense of just being "in series" with the normal audio path.
Op-amps - The non-inverting Amplifier
I suppose it is strange to see the electro caps bypassed although I guess that at 100khz there could have been noticeable effects. Strange really when the source equipment of the day was vinyl or FM radio that wouldn't come close to those limits.
Just to wander off for a moment, old gear like this was superbly designed and built. I used to have a Sony SLC9UB betamax recorder, I built a Nicam decoder for it (it was one of the first stereo VCR's albeit using the linear track. Anyway the IC count from memory was something like 400+ IC's and a 1000 or so transistors and I think as many diodes. It was an amazing machine (about 1982 ish). I kept that running for 18 years of constant daily use... and it only suffered two electronic faults. The small DC/DC convertor for the display and a TL494 IC in the servo. I recapped the SMPSU (as you do) and apart from that it was all original electronically.
http://www.palsite.com/slc9tech.html
In the example below R2 is would be AC coupled and connected to the tone network. So they are very definitely in the signal path, but not in the obvious sense of just being "in series" with the normal audio path.Op-amps - The non-inverting Amplifier
I suppose it is strange to see the electro caps bypassed although I guess that at 100khz there could have been noticeable effects. Strange really when the source equipment of the day was vinyl or FM radio that wouldn't come close to those limits.
Just to wander off for a moment, old gear like this was superbly designed and built. I used to have a Sony SLC9UB betamax recorder, I built a Nicam decoder for it (it was one of the first stereo VCR's albeit using the linear track. Anyway the IC count from memory was something like 400+ IC's and a 1000 or so transistors and I think as many diodes. It was an amazing machine (about 1982 ish). I kept that running for 18 years of constant daily use... and it only suffered two electronic faults. The small DC/DC convertor for the display and a TL494 IC in the servo. I recapped the SMPSU (as you do) and apart from that it was all original electronically.
http://www.palsite.com/slc9tech.html
Yeah they're extremely well built. I'm wondering how much an amp like this would cost in today's money. Not sure how much it cost originally for that matter.
Even though I now have all the caps to make a start on the power-amp board, Ihaven't been able to tear myself away from listening to it all week. I almost don't want to touch it it sounds so good. Still I'm sure with new caps it'll sound better still, and - hopefully - be good for another 30 years of use (scarey thought I'll be in my late 60s by the time I need to do that!).
Ahhh Betamax - such a shame VHS took off instead the picture on Beta was far better!
Even though I now have all the caps to make a start on the power-amp board, Ihaven't been able to tear myself away from listening to it all week. I almost don't want to touch it it sounds so good. Still I'm sure with new caps it'll sound better still, and - hopefully - be good for another 30 years of use (scarey thought I'll be in my late 60s by the time I need to do that!).
Ahhh Betamax - such a shame VHS took off instead the picture on Beta was far better!
They are certainly products that have stood the test of time. Just take it slowly with the recap and if any caps don't quite fit lead spacing wise it might be worth drilling new holes in the print rather than bending leads etc .
I have no idea what the original price was... must be googleable somewhere lol
I have no idea what the original price was... must be googleable somewhere lol
John, you and I should start an 'I brought a TA-F6B back from the dead' club!
Sony TA-F6B Integrated Stereo Amplifier | Mike's Vintage Stereo Equipment
"Original price in 1978: $650"
Sony TA-F6B Integrated Stereo Amplifier | Mike's Vintage Stereo Equipment
"Original price in 1978: $650"
Wow! That's equivalent to $2340.94 according to an online inflation calculator - no wonder this thing is built/sounds great! That's equivalant to £1,452.32... though knowing the way we're screwed with pricing here in the UK it would probably be far closer to the US figure, but in pounds
Not bad for something that cost me £40
Not bad for something that cost me £40
Noticed a barely audible high-pitched whistle about an hour back. Turns out it's coming from the Sony amp unfortunately - hope this doesn't mean there's a cap overheating somewhere after it's all been running nicely for the past few weeks
Anything I should do - I presume have a look to see if there's any leaking caps is a good first start?
- John
Anything I should do - I presume have a look to see if there's any leaking caps is a good first start?
- John
I guessed you had been enjoying listening to the amp.
I take it you mean the sound is physically coming from the amp rather than via the speakers.
Well it may be nothing at all, I say may... SMPS are noted for being noisy. There's such a lot of energy flying around in all those wound components and ferrites. Very difficult to say really. You would have to pinpoint the source of the noise, then see if the drive waveform was correct which is all getting pretty involved.
Maybe some other users of the amp could comment if theirs is noisy too.
Things to watch out for are whether it's running hotter than it should etc which would indicate a problem. I know you replaced all the caps so would be surprised if there were a problem in that area.
I take it you mean the sound is physically coming from the amp rather than via the speakers.
Well it may be nothing at all, I say may... SMPS are noted for being noisy. There's such a lot of energy flying around in all those wound components and ferrites. Very difficult to say really. You would have to pinpoint the source of the noise, then see if the drive waveform was correct which is all getting pretty involved.
Maybe some other users of the amp could comment if theirs is noisy too.
Things to watch out for are whether it's running hotter than it should etc which would indicate a problem. I know you replaced all the caps so would be surprised if there were a problem in that area.
Hey Mooly
Yes the whistle is coming from within the amp itself. Typically it isn't doing it now, but it's only been on again for 5 minutes. At any rate have had the lid off and cannot see any bulging and/or leaky caps. It still sounds fab and seems the same temperature as its always been.
Of course I still have to replace the caps in all locations other than the PSU/SMPS, so there's a chance it could have been one of those after 30+ years of use. Impossible to pin point the whine really as it was so quiet. It it comes back and/or gets louder I'll take it apart and have a thorough check - don't want to risk any damage to the SMPS & those transistors after all that work (and your help!)
- John
Yes the whistle is coming from within the amp itself. Typically it isn't doing it now, but it's only been on again for 5 minutes. At any rate have had the lid off and cannot see any bulging and/or leaky caps. It still sounds fab and seems the same temperature as its always been.
Of course I still have to replace the caps in all locations other than the PSU/SMPS, so there's a chance it could have been one of those after 30+ years of use. Impossible to pin point the whine really as it was so quiet. It it comes back and/or gets louder I'll take it apart and have a thorough check - don't want to risk any damage to the SMPS & those transistors after all that work (and your help!)
- John
Yep not a bad idea (the lid is fortunately held on with only two bolts from checking it earlier for leaking/bulging caps). I shall do that now.
- J