I've just signed up in the hope that someone here might be familiar with this amp ?
It came to me powering on but no output. My tests have shown the power amplifier board [P701] is 100% fine & B+/0v/B- is 65vdc. There is a small "regulator" board [P851] which has regulated +24v, +15v, +/-18v & unregulated 57v/0v/57v outputs. They all output the correct voltage apart from the +/-18v so I checked every component in that circuit & found five 1R resistors [R851, R852, R859, R860 & R861] were open circuit... Replaced them & tested P851 board in isolation, +/-18v now working ! Great I thought 🙄 Connected it to the preamp board [P601] and after 1 second those five 1R resistors are open circuit again.
Any clues where to look next friends ?
Service manual can be found here: Marantz PM-80 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine
It came to me powering on but no output. My tests have shown the power amplifier board [P701] is 100% fine & B+/0v/B- is 65vdc. There is a small "regulator" board [P851] which has regulated +24v, +15v, +/-18v & unregulated 57v/0v/57v outputs. They all output the correct voltage apart from the +/-18v so I checked every component in that circuit & found five 1R resistors [R851, R852, R859, R860 & R861] were open circuit... Replaced them & tested P851 board in isolation, +/-18v now working ! Great I thought 🙄 Connected it to the preamp board [P601] and after 1 second those five 1R resistors are open circuit again.
Any clues where to look next friends ?
Service manual can be found here: Marantz PM-80 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
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Have you done resistance checks across the rails of the preamp board. You might get lucky and find a near short. More specifically the supply looks to loop through that board and go on somewhere else which I suspect is where the problem will be. There is nothing that could draw the current needed to zap those resistors on that PCB as shown.
Hi Mooly, thanks for the advice. I will check for shorts now... The supply loops through to power the phono stage which is currently disconnected so surely the fault must be within the reg or preamp boards ?
No [obvious] shorts 🙁 The reg board only dies when under load - Could there be component/s on that board that are measuring fine but breaking down once stressed ? Maybe I should strip it & rebuild... Its pretty crappy quality & looks to have spent most of its life very hot.
No [obvious] shorts 🙁 The reg board only dies when under load - Could there be component/s on that board that are measuring fine but breaking down once stressed ? Maybe I should strip it & rebuild... Its pretty crappy quality & looks to have spent most of its life very hot.
What is puzzling is the fact that the resistors aren't all directly related. A short on the -/+18 volts rail for example looks as though it shouldn't take out R861 and R862.
Have all those resistors failed again ?
I wouldn't go for rebuilding anything at this stage. You need to find the fault. If you really think it is the 18 volt rail drawing excess current then you could try disconnecting just those rails. You could also try fitting a bulb tester (100 watt bulb in series with the mains) and power the whole amp up that way. That might help locate a fault as any problem rail will be pulled down.
I would still do cold resistance checks first though. You might find a short on a rail.
There are still other ways and means of finding the fault but lets try and find it scientifically first.
Have all those resistors failed again ?
I wouldn't go for rebuilding anything at this stage. You need to find the fault. If you really think it is the 18 volt rail drawing excess current then you could try disconnecting just those rails. You could also try fitting a bulb tester (100 watt bulb in series with the mains) and power the whole amp up that way. That might help locate a fault as any problem rail will be pulled down.
I would still do cold resistance checks first though. You might find a short on a rail.
There are still other ways and means of finding the fault but lets try and find it scientifically first.
All the aforementioned resistors failed O/C apart from R851.
I took your advice & checked for shorts... I found one on Q601 [opamp] continuity between +/- pins ! So I desoldered it & replaced the resistors [again!] now when the preamp board is connected the resistors stay alive.
Thats all until boxing day, I'll update & let you know if that was the only issue. Thanks for your help [thus far 😎]
Have a good Christmas 🙂
I took your advice & checked for shorts... I found one on Q601 [opamp] continuity between +/- pins ! So I desoldered it & replaced the resistors [again!] now when the preamp board is connected the resistors stay alive.
Thats all until boxing day, I'll update & let you know if that was the only issue. Thanks for your help [thus far 😎]
Have a good Christmas 🙂
Hmm, I couldn't leave it alone 😀
The 18v rails are currently reading 20.3v... Too high for the opamps I guess that's what killed one of them. Any idea what might be causing such a thing or the easiest way to drop it ?
The 18v rails are currently reading 20.3v... Too high for the opamps I guess that's what killed one of them. Any idea what might be causing such a thing or the easiest way to drop it ?
You need to measure the voltage across each of the two zeners, D863 and 864. They are 20 volt devices so you should see around 20 volts -/+5%. The two series pass regulator transistors that each zener connects to are darlingtons and so you should see the zener voltage less around 1.4 volts on the emitter of each.
These are not regulators with feedback. The voltage you get out of them depends purely on the voltage on the base of the series pass transistor less the 1.4ish volts (the two B-E volt drops in series)
These are not regulators with feedback. The voltage you get out of them depends purely on the voltage on the base of the series pass transistor less the 1.4ish volts (the two B-E volt drops in series)
Okay, I'll check that tomorrow & report back. If need be I can use a couple of 1N400* in series to drop it down to a safe level but that's a bit Heath Robinson for my liking 😀
overall feedback said:
Will do !
If your rails are reading 20 volts as you mentioned earlier then you need to find out why. It can only be the zener or series pass transistor at fault.
I must admit that the voltages shown on the diagram are right at the opamps max limits. 18 volt zeners would be a safer option but you must find the original fault first. 20 volt rails with 20 volt zeners isn't possible due to the "double" vbe drop of the darlington transistors. The most you should see is the zener volts less around 1.4.
I must admit that the voltages shown on the diagram are right at the opamps max limits. 18 volt zeners would be a safer option but you must find the original fault first. 20 volt rails with 20 volt zeners isn't possible due to the "double" vbe drop of the darlington transistors. The most you should see is the zener volts less around 1.4.
Yes, it is not possible to read higher voltage than zenner 18V, it is only possible if reversal EB voltage is added. If output darlington transistor is blown it could be the case on EB junction of input transistor. If supply resistance is high enough to cause voltage drop that is possible but something will probably heat, transformer or failed device.
if the fault re-occurred after hooking up the preamp board i'd be looking there for an active device(op-amp)that's gone south
Well this is becoming irritating, just went to test it & 4 1R resistors are open circuit again ! The board isn't even connected to anything other than the transformer... I'll have to change them again before checking the voltage across the zeners as its currently a big fat 0v.
Maybe you have a problem elsewhere. Remember I mentioned earlier that there was no connection in all of the resistors failing and the 18 volt rails. Perhaps there is some other issue.
with no power applied i'd start looking for shorted capacitors!(which zeners in specific)
i think Mooly mentioned a "dim bulb tester"(homebrew current limiter) this would be helpful in the test phase to allow you to get some "on" time to figure things out.
i think Mooly mentioned a "dim bulb tester"(homebrew current limiter) this would be helpful in the test phase to allow you to get some "on" time to figure things out.
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I agree with what your saying Mooly but cant see where the fault could be as all other voltages are spot on 😕
I've replaced the resistors & taken the suggested measurements:
D863= 20.5v
D864=20.2v
D859=37.2v
D860=37.3v
So from what you've said that suggests the zeners need replacing as there's no 1.6v drop...FWIW Q853 & Q854 are both cold which doesn't seem right as they have big copper heatsinks attached with signs of past heat. I see the input is 7v too hot but that's probably due to my 259v mains supply 🙄 Something I cant do anything about... Well I could buy a variac !
I've replaced the resistors & taken the suggested measurements:
D863= 20.5v
D864=20.2v
D859=37.2v
D860=37.3v
So from what you've said that suggests the zeners need replacing as there's no 1.6v drop...FWIW Q853 & Q854 are both cold which doesn't seem right as they have big copper heatsinks attached with signs of past heat. I see the input is 7v too hot but that's probably due to my 259v mains supply 🙄 Something I cant do anything about... Well I could buy a variac !
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20 volts across the zeners is fine. Off load I suppose its just possible you might measure something higher than normal (in theory you shouldn't but in practice you might) across the outputs so I would suggest loading each rail in turn and checking the output voltage. Even a 10k should pull the voltage to a correct figure. Just dab a resistor across each rail and check the output.
Okay done, 10k across the + rail and the voltage instantly dropped to 9v before slowly draining down to 0.2v... 4x1R resistors are open circuit again 😀
It shows that stabilization circuit is ok. Problem could be in front of it, like turk said capacitors should be checked (replaced? c851, c852), they are rated 35V vs. 37V measured.