@anatech One other random post I found mentions the heatsink mounted transistors being mounted backwards. I checked mine and they are indeed mounted with the heat plate of the transistor AWAY from the heatsink. It apparently gives a symptom like you have mentioned. Maybe worth a check in case it has been put in wrong. Cheers, Glenn.
Vintage audio don't seem to want to reply to queries. Two emails and an online chat and no response. Doesn't bode well, but I bit the bullet and maybe AU$76 the manual should be on its way. Amps are cleaned up and dried out ready to be tested.
Hi Trackhappy,
Why wouldn't that surprise me? An idiot had been working on the amps I am working on, so nothing would surprise me.
No, the designer was zero help. Not impressed. Adcom in the USA is also of no help recently.
Why wouldn't that surprise me? An idiot had been working on the amps I am working on, so nothing would surprise me.
No, the designer was zero help. Not impressed. Adcom in the USA is also of no help recently.
I have started working on mine seriously. I ordered the service manual anyway. They responded to say he was on holiday for 7 days so there would be a delay in postage. I have asked if he can scan the schematic when he gets back. I have tested almost every component on the boards and nothing amiss, yet I have DC on the outputs. I suspect wild oscillations happening. One of mine has a pair of capacitors missing and a track cut with a resistor added underneath. Need the schematic now or I make one. I have made one for the driver board, and already I think there should be a capacitor across the rails at each op-amp to avoid oscillation (according to the datasheet). I also don't like where they have mounted the board and I might move it central, might even make a new board yet.
Well, possibly correct. If you add supply capacitors, be mindful where you connect the ground as the signal ground isn't the place for supply current to go through.
The layout is convoluted enough so that I gave up trying to trace it. These are a repair for a customer and I have to be very mindful of charges. I have already lost money on these but would like to get them fixed for him. Time will tell I guess.
The layout is convoluted enough so that I gave up trying to trace it. These are a repair for a customer and I have to be very mindful of charges. I have already lost money on these but would like to get them fixed for him. Time will tell I guess.
You had me scared, the diagram says 741 op amps! lol!
The 5534 may need stability adjustments. You can bypass the supplies, but watch out where you return those caps to common. You may have to experiment. Watch for low level (background) noise.
I would have to study to see if your offset adjustment is correct. You might want to consult the data sheet in the app notes section to make sure on this.
The 5534 may need stability adjustments. You can bypass the supplies, but watch out where you return those caps to common. You may have to experiment. Watch for low level (background) noise.
I would have to study to see if your offset adjustment is correct. You might want to consult the data sheet in the app notes section to make sure on this.
Hi,
We usually add each opamp two 100nF capacitors which directly solder to the opamp supply pins and the near gound.
If your amp have DC on the outputs, please check each 5534 outputs, if 5534 has DC output then you can add a 4.7uF bipolar cap between 5534 output to the final amp
Well. 21 days later and we have a service manual. It covers both series 1 and series 2, mine seems to be series 2. Good news, it has the full schematic and parts list and some description of the operation. Bad news the layout only cover the series 1, so I will have to work out where the components are.
Previous repairer had replaced one bank of 2SD1046's with 1047's which on paper looks fine, but in reality they test hfe 1/4 of the 1046's. anywhere from 10-15 where the originals are 48-65. I ordered a batch of 1046's only to be sent 6 of them from 3 wildly different batches varying from 30-103. So mix and match I have a roughly matched set. I also purchased an Atlas ESR/capacitance tester to test the (very few) caps around this thing. The main filter caps are a bit perplexing, measuring <0.1 ESR, which is fine, but they measure more like 8000uF than the 6800 they should be. That implies leakage, but all 8 of them? And putting 30VDC across them I can't measure any leakage current, but I haven't hooked up a uA meter to measure that low. Maybe time to sign up for Mr Carlson's lab Patreon and build his leakage tester... 🙂. Anyhow, we have progress and if work and life get out of the way for 5 minutes maybe I can get onto the next steps.
Previous repairer had replaced one bank of 2SD1046's with 1047's which on paper looks fine, but in reality they test hfe 1/4 of the 1046's. anywhere from 10-15 where the originals are 48-65. I ordered a batch of 1046's only to be sent 6 of them from 3 wildly different batches varying from 30-103. So mix and match I have a roughly matched set. I also purchased an Atlas ESR/capacitance tester to test the (very few) caps around this thing. The main filter caps are a bit perplexing, measuring <0.1 ESR, which is fine, but they measure more like 8000uF than the 6800 they should be. That implies leakage, but all 8 of them? And putting 30VDC across them I can't measure any leakage current, but I haven't hooked up a uA meter to measure that low. Maybe time to sign up for Mr Carlson's lab Patreon and build his leakage tester... 🙂. Anyhow, we have progress and if work and life get out of the way for 5 minutes maybe I can get onto the next steps.
That all sounds like good news.
Don't get hung up on 6800uF capacitor measuring 8000uF. Electrolytic capacitors have exceptionally wide tolerance - often +100%-50%. That they are only 18% high in value is pretty good!
Where did you get a service manual?
Don't get hung up on 6800uF capacitor measuring 8000uF. Electrolytic capacitors have exceptionally wide tolerance - often +100%-50%. That they are only 18% high in value is pretty good!
Where did you get a service manual?
Yes, seconded. Capacitor tolerance is wide, plus most cap checkers may not be accurate. Today's new capacitors normally measure low (using accurate equipment). However the value is not critical at all within reason.
Hi Sawyers. Yep, good news so far. Vintage audio had it. Original paper version with pencilled in notes and all. Very cool. Now I need to work out how to hook my scope up as this is a full bridge configuration. Isolation transformer I guess. Will keep you updated.
Do you have two probes for your scope? Most scopes have an invert button. So connect one probe to one amp output, the other probe to the other amp output, and invert one channel. Then you are measuring the difference between the two bridged outputs.
Hi Trackhappy,
Where did you say you got your manual from? There is something in there I am missing and I need to have the details of what they did. I have a really good idea, but the design is not what I would call the greatest. They have done something odd here.
-Chris
Where did you say you got your manual from? There is something in there I am missing and I need to have the details of what they did. I have a really good idea, but the design is not what I would call the greatest. They have done something odd here.
-Chris
Thanks for the tip @sawyers.
Update:
Well. I got some time in the weekend and now have two working amps. Both have new power cords and strain reliefs. I stupidly only ordered 2 x 220uF BP caps and I need 2 for each amp so have to revisit that on the second one. Fuses were all over the place so replaced those. Should be 5A quick blow on the outputs, although there is a note to use 8 amp if 4 ohm speakers are used. Main problem on one unit was dry/loose speaker terminal. The other had been got to and they had used the wrong transistors in the outputs of one side. I have replaced that bank and the smaller transistor that senses the output stage temperature with the correct ones, fixed a few suspect joints and it now functions correctly. I don't unfortunately have any distortion measurement equipment, but a sine wave on the scope looks great. Of note is that one unit has been modded, with track cut and resistor added in each channel. I will have to work out where and document it, then make them both the same (once I understand what it is for). Anyhow, success for the moment. Gotta be happy with that. Shame I don't have a suitable preamp to drive them. I have a valve based phono preamp, but it doesn't have a volume control.
Update:
Well. I got some time in the weekend and now have two working amps. Both have new power cords and strain reliefs. I stupidly only ordered 2 x 220uF BP caps and I need 2 for each amp so have to revisit that on the second one. Fuses were all over the place so replaced those. Should be 5A quick blow on the outputs, although there is a note to use 8 amp if 4 ohm speakers are used. Main problem on one unit was dry/loose speaker terminal. The other had been got to and they had used the wrong transistors in the outputs of one side. I have replaced that bank and the smaller transistor that senses the output stage temperature with the correct ones, fixed a few suspect joints and it now functions correctly. I don't unfortunately have any distortion measurement equipment, but a sine wave on the scope looks great. Of note is that one unit has been modded, with track cut and resistor added in each channel. I will have to work out where and document it, then make them both the same (once I understand what it is for). Anyhow, success for the moment. Gotta be happy with that. Shame I don't have a suitable preamp to drive them. I have a valve based phono preamp, but it doesn't have a volume control.
One other possible "improvement", the inputs are either direct coupled or via a 4.7uF "MKT style" cap. Tempted to replace that with a higher grade audio rated cap maybe. I still might make a new driver board to shorten and balance the driver to output wiring and fit the input cap on it. Oh, I also carefully routed the long driver to output cables as far from the transformers as I could. They are just a twisted pair strangely enough. There is also a 22 ohm resistor from audio ground to chassis earth. Best practice I think is more like 150 ohms shunted with a 100nF cap, which my main valve amp has, but open to comments on that one.
Don't mess with the twisted pair wiring and be tempted to replace it. There is an induced distortion mechanism that has nothing to do with real distortion.
Thing is with a class AB amp (which your amp is) the power supply takes half wave current. Think about a sine wave input. The + voltage supplies a current half cycle, then the - supply provides a half cycle current. So each supply provides current to the amplifier like an unsmoothed half wave rectifier - and that has a whole host of harmonics. That can easily couple into the input wiring, circuit board traces, and speaker wiring. Tightly twisting the wiring helps cut down the coupling of the current flow harmonics.
Even if you don't have induced distortion, an amplifier's distortion increases by a factor of ten or more at high frequencies, because the voltage amplifier stage runs out of steam driving the output transistors - it cannot properly drive the capacitance of the output devices at high audio frequencies, and distortion goes up. One reason that amp manufacturers tend to quote distortion only at 1kHz. You can get around that somewhat, but at the expense of significantly increased circuit complexity.
Induced distortion just adds on top of that.
It is a real sod to sort out and get distortion low across the audio band. Been there, got the badge. Even the wiring to a dummy load has to be very carefully routed. And even so it is nearly impossible to achieve - the wiring is a 3D space in which you are trying to route wires, and some form of compromise is needed.
And that is the reason that Class A amps don't have induced distortion. The power supply currents just don't have discontinuities that give rise to induced distortion.
Thing is with a class AB amp (which your amp is) the power supply takes half wave current. Think about a sine wave input. The + voltage supplies a current half cycle, then the - supply provides a half cycle current. So each supply provides current to the amplifier like an unsmoothed half wave rectifier - and that has a whole host of harmonics. That can easily couple into the input wiring, circuit board traces, and speaker wiring. Tightly twisting the wiring helps cut down the coupling of the current flow harmonics.
Even if you don't have induced distortion, an amplifier's distortion increases by a factor of ten or more at high frequencies, because the voltage amplifier stage runs out of steam driving the output transistors - it cannot properly drive the capacitance of the output devices at high audio frequencies, and distortion goes up. One reason that amp manufacturers tend to quote distortion only at 1kHz. You can get around that somewhat, but at the expense of significantly increased circuit complexity.
Induced distortion just adds on top of that.
It is a real sod to sort out and get distortion low across the audio band. Been there, got the badge. Even the wiring to a dummy load has to be very carefully routed. And even so it is nearly impossible to achieve - the wiring is a 3D space in which you are trying to route wires, and some form of compromise is needed.
And that is the reason that Class A amps don't have induced distortion. The power supply currents just don't have discontinuities that give rise to induced distortion.
Hi Trackhappy,
The coupling capacitors will not have any effect on sound quality unless they are truly bad or ill-suited top the circuit. Smaller ones (within reason) will not pick up noise, or interfere with surrounding stuff. But change them with similar types (size and value) if it makes you feel better.
Induced distortion is only an issue when you have layout issues. If you do, I doubt the rest of the amplifier is designed well (talking in a general sense).
As far as class "A" designs are concerned, they absolutely can have current discontinuities. They also suffer from other class A-B issues, the bias is simply cranked way up there. In addition they have serious issues with heat and are generally very hard on filter capacitors. I'll take a well designed class A-B amplifier over any class A design any day! Extremely high bias current doesn't actually solve anything, but brings other factors into play. Never mind the increased electrical power cost and much higher air conditioning costs.
The coupling capacitors will not have any effect on sound quality unless they are truly bad or ill-suited top the circuit. Smaller ones (within reason) will not pick up noise, or interfere with surrounding stuff. But change them with similar types (size and value) if it makes you feel better.
Induced distortion is only an issue when you have layout issues. If you do, I doubt the rest of the amplifier is designed well (talking in a general sense).
As far as class "A" designs are concerned, they absolutely can have current discontinuities. They also suffer from other class A-B issues, the bias is simply cranked way up there. In addition they have serious issues with heat and are generally very hard on filter capacitors. I'll take a well designed class A-B amplifier over any class A design any day! Extremely high bias current doesn't actually solve anything, but brings other factors into play. Never mind the increased electrical power cost and much higher air conditioning costs.