Hi
The Marantz PM-6A has an automatic shifting system from Class A to Class B. Class A and Class B run with different voltages (+-26,5 V and +- 63V) and different bias current (370mV vs 14 mV) in the poweramp.
Bias procedure is described in the Service Manual, page 3. While the bias for class B is easily adjusted, in Class A the bias stays far to high and with the trimmers (RL39 and RL40) I cannot reduce it further. The trimmers run out of way. 30 Watt Class A is also over the top so I would like to reduce it anyway - to three to five watts or so.
I have following ideas:
1. Change the values of the resistors/trimmers (RL39, RL 40) in the Class A bias part until I can reduce the bias to the values wanted?
But bias components for Class A and Class B seem to be interlinked (Service manual page 11) so I do not know whether the switching system will blow then or not work properly?
2. For Class B bias adjustment I have to connect two contacts together (JL15 - only on PCB not in schematic - and JL12). Can I leave this connection permanent and bias the Class B system a bit more or will the amplifier blow in the end?
More ideas? What should I do?
The Marantz PM-6A has an automatic shifting system from Class A to Class B. Class A and Class B run with different voltages (+-26,5 V and +- 63V) and different bias current (370mV vs 14 mV) in the poweramp.
Bias procedure is described in the Service Manual, page 3. While the bias for class B is easily adjusted, in Class A the bias stays far to high and with the trimmers (RL39 and RL40) I cannot reduce it further. The trimmers run out of way. 30 Watt Class A is also over the top so I would like to reduce it anyway - to three to five watts or so.
I have following ideas:
1. Change the values of the resistors/trimmers (RL39, RL 40) in the Class A bias part until I can reduce the bias to the values wanted?
But bias components for Class A and Class B seem to be interlinked (Service manual page 11) so I do not know whether the switching system will blow then or not work properly?
2. For Class B bias adjustment I have to connect two contacts together (JL15 - only on PCB not in schematic - and JL12). Can I leave this connection permanent and bias the Class B system a bit more or will the amplifier blow in the end?
More ideas? What should I do?
I'd fix the root cause rather than try and get round the seemingly inherent current fault by altering circuit components. ... but that's just me.
How far too high? is it over 370mV?
Last edited:
Hi Mike
When I got the amplifier I gave it a listen. Sounded good but after some time the liquid cooler made mechanical noises and it smelled like overheated electronics. So the idea was to reduce the bias until it runs hot but not so hot to selfdistruct.
For the bias adjustment class A I used a variac. When I powered the amplifier up slowly I could see that I could not adjust down to the level I wanted. I do not remember what AC Voltage it was. May be about 170 VAC - far away from the 230V we have. I do not remember the bias current at that voltage.
My electronic knowledge is limited and the circuit of the Marantz seems to be overly complicated.
When I got the amplifier I gave it a listen. Sounded good but after some time the liquid cooler made mechanical noises and it smelled like overheated electronics. So the idea was to reduce the bias until it runs hot but not so hot to selfdistruct.
For the bias adjustment class A I used a variac. When I powered the amplifier up slowly I could see that I could not adjust down to the level I wanted. I do not remember what AC Voltage it was. May be about 170 VAC - far away from the 230V we have. I do not remember the bias current at that voltage.
My electronic knowledge is limited and the circuit of the Marantz seems to be overly complicated.
If you bought it secondhand it may well have loads of dust inside which will "smell" especially when you're running it at "class A"
You need to run it back up at the full rated mains voltage and come back here with what it does adjust to....
Class B adj sets up ok, but what is the minimum the Class A setting can be adjusted down to ?? (at the correct mains voltage)
Then we'll know positively if there's an underlying fault or not
I bought it used and cleaned the inside. Looks clean now. Amplifier was not used for about four years.
With 230 Volt mains left channel bias stabilized after two to three minutes to 366 mV, right channel was 395 mV. These are the lowest settings.
At two minutes the liquid filled heat sink startet to make ticking noises as if it was boiling. Heat finns (?) were slightly warm.
With 230 Volt mains left channel bias stabilized after two to three minutes to 366 mV, right channel was 395 mV. These are the lowest settings.
At two minutes the liquid filled heat sink startet to make ticking noises as if it was boiling. Heat finns (?) were slightly warm.
I can't think marantz ever used thermal pipe heatsinks like some technics, it could be oil filled which would explain the ticking.
You need to monitor the mV closely as things come up to temp and stabilise, it'll prob need quite a while to get to adjustment time (30 mins..... 3mins isn't enough), then see what the class A adjustment range is.....
I'd also double check any heatsink grease hasn't hardened/dried up , it's worth cleaning and reapplying on such an old amp to get it to it's most efficient.
If I look at the values there seems to be no underlying fault with Class A bias even if the right channel is a bit high? I have no idea whether the heat sink leaked or the noise will go away with more use. As I wrote the amplifier was not used for several years.
But I am still interested to reduce the bias for Class A or drive the amplifier in Class A/B with enhanced bias. As I have neighbors I am listening normally at levels below 1 Watt so 30 Watt Class A is excessive and will shorten the life of the components with no benefit.
But I am still interested to reduce the bias for Class A or drive the amplifier in Class A/B with enhanced bias. As I have neighbors I am listening normally at levels below 1 Watt so 30 Watt Class A is excessive and will shorten the life of the components with no benefit.
Hi Mike
It seems our mails have crossed.
According to the Service Manual the time for stabilization of bias is 30 to 60 seconds. That seems very short but I found after the 2 to 3 minutes it is stable. In Class B I tested it for an hour - once set it does not move at all (I have two Yamaha poweramps with the same phenomenon - once set the bias is and remains stable. Different to my diy Pass F5 where it wanders for hours).
As the amplifier gets so incredible hot and the liquid filled heat sink makes so strange noises I do not want to risk to run it in Class A for any length of time If the heat transfer in the liquid filled cooler - which should be noiseless - is compromised I fear the amplifier will self-destruct.
By the way Mike you seem to have experience with liquid coolers. Does the noise go away with use or will it be permanent?
You are absolutely right I should check the heatsink grease, that is another argument no to drive the amplifier now with 30 Watt Class A.
My main question remains:
How can I get rid of 30 Watts Class A as in my situation it has several disadvantages and no benefit at all?
Should I wire the amplifier permanently as for adjustment for Class B or will this harm the amp or the automatic Class A to Class B sliding system?
Or should I try to reduce the Class A bias in changing the resistive elements in the bias adjustment system or will this do harm to the amp or the quarterly bias system?
Or what else can I do?
It seems our mails have crossed.
According to the Service Manual the time for stabilization of bias is 30 to 60 seconds. That seems very short but I found after the 2 to 3 minutes it is stable. In Class B I tested it for an hour - once set it does not move at all (I have two Yamaha poweramps with the same phenomenon - once set the bias is and remains stable. Different to my diy Pass F5 where it wanders for hours).
As the amplifier gets so incredible hot and the liquid filled heat sink makes so strange noises I do not want to risk to run it in Class A for any length of time If the heat transfer in the liquid filled cooler - which should be noiseless - is compromised I fear the amplifier will self-destruct.
By the way Mike you seem to have experience with liquid coolers. Does the noise go away with use or will it be permanent?
You are absolutely right I should check the heatsink grease, that is another argument no to drive the amplifier now with 30 Watt Class A.
My main question remains:
How can I get rid of 30 Watts Class A as in my situation it has several disadvantages and no benefit at all?
Should I wire the amplifier permanently as for adjustment for Class B or will this harm the amp or the automatic Class A to Class B sliding system?
Or should I try to reduce the Class A bias in changing the resistive elements in the bias adjustment system or will this do harm to the amp or the quarterly bias system?
Or what else can I do?
Closely monitoring that bias as it runs up to temp will ensure you stop it self destructing.
The clicking should just temporary, ever heard an oil filled radiator as it warms up from cold ?
Looks to me like QL04 is a monostable multivibrator (74LS122) set to 0.5 second pulse (with RL34 and CL14) when triggered and QL05 is a dual optoisolator on the output shorting pins 8-7 and 6-5 when Class A is triggered
Just spotted the Bias setup page has crucial typo's (not untypical) - Great job Marantz 😀
AND I can't see JL15 in the circuit (JL12 is ground, but what is it grounding when you're in setup mode.) Maybe it should be JL14? to prevent the monostable triggering the Opto into Class A mode?
The setup page also says adjust the class B pots when adjusting the Class A - this isn't right
RL43 and RL44 are for Class B adjustment, and RL39 and RL40 are for Class A....
The Class B pots should be left alone once set for ClassB
So with these typo's in mind how have you been adjusting it ?
If you really don't like Class A mode, as far as I can see you only need to remove QL05 completely and then you'll just have Class B adjustment active, and you should be able to replace RL43 and RL44 to suit your bias choice.
Once QL05 is removed the Class A pots will have no effect
I may have got all that the wrong way round, becase the amp isn't in front of me, I'm relying on a rubbishly set out service manual - with technical typo's !
I'm continually scrolling from page to page and can't paper print out the various areas to make more sense out of it
Attachments
Last edited:
Oh I found that typo too and checked the schematic on page 11 of the service manual. So I adjusted the pots RL39, RL 40 for Class A.
JL15 is not in the schematic but on the pcb and connecting it to JL12 (ground) I could adjust the bias for Class B to the required 14 mV. So my initial idea was leave this connection and increase the bias to 50mV. Then the amplifier runs with +- 63V, about 3 watt Class A. Fine.
But then I read another service manual for another Amp (forgotten the model) in which this was only recommended for a short time.
JL15 is not in the schematic but on the pcb and connecting it to JL12 (ground) I could adjust the bias for Class B to the required 14 mV. So my initial idea was leave this connection and increase the bias to 50mV. Then the amplifier runs with +- 63V, about 3 watt Class A. Fine.
But then I read another service manual for another Amp (forgotten the model) in which this was only recommended for a short time.
I meant the operation of the opto..
If all four pots were set to max for instance (and the opto isn't operating) then the resistance seen at JL05/06 (left) and JL07/08 (right) would be 44K on each channel
If the opto comes on, then the resistance drops to 14.66K
I can't see for the life of me where these four terminals connect to the amp section, the JL markings aren't consistantly present.... (again great job marantz 😀)
More usually it would say (goto page xyz) and page xyz would have corresponding JL (or something similar) numbered connectors.
Like I said I'm at home and don't have the luxury of being able to print it out and make easier sense of it.
May be I can be of help here. Please go to HiFi Engine/Library/Marantz/Integrated amplifiers.
The service manual of the PM-6A you will find below PM-68 and load it down to your computer. Now you an enlarge the pictures a will on the screen.
Chapter 11, page 32 of the SM contains the whole schematic with all the connections shown. In the scan page 32 is divided into four overlapping pages. Fortunately the parts of interest are all on the fourth page including the connection to the power amplifier.
I can only look at it and think Oh that's awful complicated.
I hope this is of help.
The service manual of the PM-6A you will find below PM-68 and load it down to your computer. Now you an enlarge the pictures a will on the screen.
Chapter 11, page 32 of the SM contains the whole schematic with all the connections shown. In the scan page 32 is divided into four overlapping pages. Fortunately the parts of interest are all on the fourth page including the connection to the power amplifier.
I can only look at it and think Oh that's awful complicated.
I hope this is of help.
I've looked at that just now Marantz PM-68 Stereo Integrated Amplifier Manual | HiFi Engine, and sadly it's not the same scheme as the PM6A.
The PM68 simply had a manual Class A switch on the front panel, not this auto affair on the 6A
Sorry this is a misunderstanding. In the listing of the Marantz integrated amplifiers . The Information for PM-6A you find just below the PM-68. (so not near PM-6 as I had expected)
I finally found the crucial board interconnect diagram on the last few pages of the 6a manual (missed that the first time I visited this problem)
I'll try and look at it again armed with that.
I'll try and look at it again armed with that.
Many thanks. The circuit diagram is the most complicated I have ever seen.
The PM6 had a switch to decide for Class A or Class AB. That was a fine solution.
The PM6 had a switch to decide for Class A or Class AB. That was a fine solution.