Welcome, I agree. I said above that I had read about that issue in a forum as I knew I had seen a reference to it somewhere. In actual fact he has just reminded me that I had actually read it in his listing! In fact, that appears to be the only place where I have read this. He claims the information is in the manual (which he did not supply because he does not have it) however you are not the only one to point out that this seems a rather odd arrangement for the manufacturer to implement. There is no warning on the amp itself.
There was a small dent on the side of the case and one of the feet came off very easily, so it may have taken a knock during transit. It was easy to put right cosmetically, but you may have a point. If it was knocked in transit, this may have caused some internal damage as well. He seems to have overlooked these cosmetic damage observations I made in my first message.
I had a look at the circuit diagram which shows that the button switches a relay that alters the supply voltage to the final stage transistors. Also, via an opto coupler arrangement, it changes the biasing current level. I can't see how switching it could cause a failure, but I thought I'd ask as someone more expert than myself may be able to confirm.
I have so far found only a German user manual which I can't read. I could not see any obvious mention of this issue in the service manual.
Would I be cynical in suggesting that the comment was put there as a bargaining chip or get-out?
There was a small dent on the side of the case and one of the feet came off very easily, so it may have taken a knock during transit. It was easy to put right cosmetically, but you may have a point. If it was knocked in transit, this may have caused some internal damage as well. He seems to have overlooked these cosmetic damage observations I made in my first message.
I had a look at the circuit diagram which shows that the button switches a relay that alters the supply voltage to the final stage transistors. Also, via an opto coupler arrangement, it changes the biasing current level. I can't see how switching it could cause a failure, but I thought I'd ask as someone more expert than myself may be able to confirm.
I have so far found only a German user manual which I can't read. I could not see any obvious mention of this issue in the service manual.
Would I be cynical in suggesting that the comment was put there as a bargaining chip or get-out?
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Was it well packed? This is a very heavy amp and damage in transit is can very easily happen unless it is super well packed (double boxed, no ability to flop around, etc). I think it may have simply taken a bad hit. I think you should have a look on the inside, there may be a small crack in a PCB or something such (or some solder joint popped loose).
This is a really cool amp and I wouldn't let it go that easily. I think it's a keeper, even if it would take a lot of effort to get it back in action.
This is a really cool amp and I wouldn't let it go that easily. I think it's a keeper, even if it would take a lot of effort to get it back in action.
Yes, it is a big heavy lump! It was packed in a large sturdy box with plenty of filling (various bits of bubble wrap, boxes, foam etc) placed in a somewhat haphazard fashion around it. Adequate enough but perhaps a little loose maybe. The top, bottom, back and front were well protected, but the sides not quite so much. The dent was a bit odd, as a small corner section (left hand side, front edge, near the top) was buckled (very slightly) outward rather then inward, like maybe something landed on top of it, which might also explain why the plastic around the retaining screw securing the foot in the same corner was damaged slightly as well. Since the box does not appear to show any sign of being damaged or dropped, this may have been there already or maybe happened during packing.
I know that among the older Marantz amps, the pm45 tends to get sought after. The pm80 I believe is higher up the range and much more expensive, but I figured it was worth a punt. Whether its worth keeping or whether to send it back is the dilemma I have been agonizing over for the last few days, especially as I liked the sound it produced for the short time it was working!. I got the impression from comments so far that it may be a bigger gamble that I may have imagined, so I have been pursuing a refund with the seller, but it has not been an easy decision. I still have a few days to wait before I can escalate my case, so I can always change my mind....
PS, I contacted Marantz technical support. They had to contact their HQ first, but the technician I spoke with came back to me and said it was unlikely that pressing the mode switch would have had any adverse effect on the amp. He offered to supply me with a copy of the user manual so that I can check for myself whether there are any warnings is this respect, but he concurred that it would have made sense to put a warning on the equipment if that was the case.
It could be a cracked PCB or maybe a heat expansion related issue or some other problem, but its looking increasingly unlikely that it was a user error....
I know that among the older Marantz amps, the pm45 tends to get sought after. The pm80 I believe is higher up the range and much more expensive, but I figured it was worth a punt. Whether its worth keeping or whether to send it back is the dilemma I have been agonizing over for the last few days, especially as I liked the sound it produced for the short time it was working!. I got the impression from comments so far that it may be a bigger gamble that I may have imagined, so I have been pursuing a refund with the seller, but it has not been an easy decision. I still have a few days to wait before I can escalate my case, so I can always change my mind....
PS, I contacted Marantz technical support. They had to contact their HQ first, but the technician I spoke with came back to me and said it was unlikely that pressing the mode switch would have had any adverse effect on the amp. He offered to supply me with a copy of the user manual so that I can check for myself whether there are any warnings is this respect, but he concurred that it would have made sense to put a warning on the equipment if that was the case.
It could be a cracked PCB or maybe a heat expansion related issue or some other problem, but its looking increasingly unlikely that it was a user error....
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PS, I now have the user manual supplied by Marantz. There is nothing mentioned under Precautions (which covers other matters such as not obstructing vent, not operating near excessive heat or moisture, electrostatic discharge etc), however under the Class A switch description there is this one liner in standard (i.e. not bold or emphasized in any way) print:
It does not however detail why or what the consequences might be. There is only a warning about the operating temperature while in class A and not obstructing the vents which, of course, is eminently sensible. Very odd for the statement to be made in such a very low key manner.
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Ok, I am now dekinitely stuck with repairing this so I have taken the cover off today to have a look.
I have checked that all fuses are OK.
As per JohnSnell Electronic, I also checked the relays. L851 is kicking in as expected on class A mode. However the output relays are not switching at all. I applied a signal to the input of Aux1 and traced it all the way past the input and output of the STK3102 right up to the output relays. Both channels are functioning and I get an AC ouptut which varies in amplitude with a turn of the volume control. I did however also notice that there is a negative DC voltage of -11.8v on L702, whereas on L701 it is 0v as expected. This DC voltage is also present across R756 depite there being two capacitors in series? I'm curious as to where this voltage is leaking in from? Will keep digging further but I suspect that the presence of this DC voltage is causing protection to kick in so that the current is no passed through the speakers.
I'm also curious as to the difference in resistor designation. Some have an 'R' prefix, whereas others have an 'RN' prefix, e.g. R754 vs RN20?
I have checked that all fuses are OK.
As per JohnSnell Electronic, I also checked the relays. L851 is kicking in as expected on class A mode. However the output relays are not switching at all. I applied a signal to the input of Aux1 and traced it all the way past the input and output of the STK3102 right up to the output relays. Both channels are functioning and I get an AC ouptut which varies in amplitude with a turn of the volume control. I did however also notice that there is a negative DC voltage of -11.8v on L702, whereas on L701 it is 0v as expected. This DC voltage is also present across R756 depite there being two capacitors in series? I'm curious as to where this voltage is leaking in from? Will keep digging further but I suspect that the presence of this DC voltage is causing protection to kick in so that the current is no passed through the speakers.
I'm also curious as to the difference in resistor designation. Some have an 'R' prefix, whereas others have an 'RN' prefix, e.g. R754 vs RN20?
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Ok, moved on with this today. The negative DC voltage tracks right back to pin 14 of the STK3102 and also appears on pins 10 and 11 (which should be at 1.7 and -1.7v respectively). The voltages around Q702 are all abot -10v lower than they should be, I decided to desolder pins 10, 11, 14 and 15 of the STK3120 in order to decouple that channel of the STK3102 from the circuit. When I powered the amp up again, the speaker relays relaxed after a few seconds and I could now hear my injected signal loud and clear on the opposite channel!
I checked the voltage on the output side of R756 and found it to read 0v which was as expected. I also checked the voltages around Q702 and they were also near 0v. It now seems clear that the excessive negative voltage was down to a problem with the STK3102. It couldn't leak from anywhere else (e.g D704 was good). I then checked the voltages on all pins around the STK3102 again. All except pin 11 were correct. Pin 11 was reading -56v instead of the expected -1.7v. I reckon that the transistor junction it connects to is leaking current from the negative supply line.
I will now order a replacement STK3102 and see how we go. I'm also thinking about how to improve the heat dissipation around that chip.
I checked the voltage on the output side of R756 and found it to read 0v which was as expected. I also checked the voltages around Q702 and they were also near 0v. It now seems clear that the excessive negative voltage was down to a problem with the STK3102. It couldn't leak from anywhere else (e.g D704 was good). I then checked the voltages on all pins around the STK3102 again. All except pin 11 were correct. Pin 11 was reading -56v instead of the expected -1.7v. I reckon that the transistor junction it connects to is leaking current from the negative supply line.
I will now order a replacement STK3102 and see how we go. I'm also thinking about how to improve the heat dissipation around that chip.
The STK3102 was the problem and once this was replaced, the operating voltages were as expected and amp was working again. Just wanted to say thanks to all who contributed and provided pointers. Since power to the STK3102 is fed from a seperate power rail, and this is not changed when the mode is switched from A to A/B and vice versa, am I correct in assuming that switching modes had nothing to do with this failure and that it was perhaps due to a combination of age and heat? Whatever the case, the amp is now up and running again and although it cost me a few pounds more to get sorted, I'm glad its now over and done with and I can enjoy using it!
That's great work - an excellent result, considering just how many possible faults there could be and the final cost. I linked another part manual here because it's a good copy, in case anyone hasn't found this one: Marantz PM80
The STK3102 is the front end (the voltage amplifying part) of both power amplifiers in one chip. The output stages, bias controller and protection circuit are external to it. You are right in assuming it was not directly involved with controlling the mode of operation. The relays connect the speakers and muting for amplifier/speaker protection and should be dropped out by any significant DC fault appearing at the amplifier output. This is the job of protection IC QN04 (TA7317P) and will prevent output in either mode. The amplifier mode is controlled by (1) a fourth relay (LO51) switching high/low voltage transformer windings for the main power supply and (2) low/high bias setting - optically switched by Q719,720, as far as I can tell.
Anyway, 'hope you do get to enjoy it for some more years at least
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The STK3102 is the front end (the voltage amplifying part) of both power amplifiers in one chip. The output stages, bias controller and protection circuit are external to it. You are right in assuming it was not directly involved with controlling the mode of operation. The relays connect the speakers and muting for amplifier/speaker protection and should be dropped out by any significant DC fault appearing at the amplifier output. This is the job of protection IC QN04 (TA7317P) and will prevent output in either mode. The amplifier mode is controlled by (1) a fourth relay (LO51) switching high/low voltage transformer windings for the main power supply and (2) low/high bias setting - optically switched by Q719,720, as far as I can tell.
Anyway, 'hope you do get to enjoy it for some more years at least
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