Hi folks, recently bought a Marantz 3800 preamp and after some tests I noticed a 10dB difference between left and right channels.
I started changing all eletrolytics at the main preamp board and doing a deoxi it at the pots but the problem persisted.
So I took a look at the service manual and get some voltage measurements. F749PC and LM301AN op amps. and RC4194TK power supply regulator are within the voltages expected.
So I turned to the npn and pnp transistors. 2N5210 npn and 2N4250 pnp. A few are showing some discrepancies at the base voltage, so I am wondering if I need to also change these or am I missing something?
I do not have them, instead I have the BC550 and BC560 in stock for change. I would like to hear some clues for fixing this problem.
Cheers
I started changing all eletrolytics at the main preamp board and doing a deoxi it at the pots but the problem persisted.
So I took a look at the service manual and get some voltage measurements. F749PC and LM301AN op amps. and RC4194TK power supply regulator are within the voltages expected.
So I turned to the npn and pnp transistors. 2N5210 npn and 2N4250 pnp. A few are showing some discrepancies at the base voltage, so I am wondering if I need to also change these or am I missing something?
I do not have them, instead I have the BC550 and BC560 in stock for change. I would like to hear some clues for fixing this problem.
Cheers
Not sure why people replace everything in sight, that just masks the issue.
What you should do, in my opinion, is insert an equal value mono signal to both channel inputs at the same point and follow the signal path until you find the drop out. There will be the issue point. Working from output towards the input works best for me.
Makes life so much easier.
What you should do, in my opinion, is insert an equal value mono signal to both channel inputs at the same point and follow the signal path until you find the drop out. There will be the issue point. Working from output towards the input works best for me.
Makes life so much easier.
^ 100% agree on that. Changing random parts in hope is a recipe for disaster.
If you have a 'few' transistors that seem to have unexpected voltages around them then you need to re evaluate the situation... either they are genuinely incorrect (and its most unlikely you'll have a handful of faulty devices in there) or the readings are incorrect because of a single (usually) simple cause that is altering the biasing point on the whole circuit.
If you have a 'few' transistors that seem to have unexpected voltages around them then you need to re evaluate the situation... either they are genuinely incorrect (and its most unlikely you'll have a handful of faulty devices in there) or the readings are incorrect because of a single (usually) simple cause that is altering the biasing point on the whole circuit.
You are right. Never replace anything before exhaustive analysis of the issue. Signal tracing is the obvious solution, very easy because one channel is in (apparently) good condition.
Ok, just a newbie question, if the transistor voltages are within the service manual specs, does it necessarily be ok? What about the hFE?
I am checking as.you advise me, just noticed that the two 2N5210 that goes after the X10 amp are not original, someone changed them for some BC, so I started to look at this point, also some resistors are also changed near them... So there is a lot to investigate.
As for changing parts, IMHO, changjng eletrolytics from a 40+ years old piece is not a dumb thing. I do not touch other capacitors, there are numerous topology motives to respect their original values and types, but for eletrolytics they usually get out of specification under age.
Another good upgrade is for some faster diodes.
Thank you all for the comments. They are really welcomed.
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Voltages agreeing with the service manual are a pretty good indicator that all is OK around that area. Low hFE can happen but is very uncommon, and there are usually other problems showing as well.
Replacing very old electrolytic caps is good practice but it is not necessarily the first thing to do when faced with an unknown fault.
Always try and fix the fault first by normal fault-finding methods... the reason is simple... if changing the caps seems to fix the fault you still don't know whether you have genuinely fixed the unit, or whether the heat and mechanical disturbance of working on the unit has temporarily masked a problem only for it to return later.
A gain difference between channels should be easily identifiable using a scope and signal generator (or use a test CD/MP3 file).
Replacing very old electrolytic caps is good practice but it is not necessarily the first thing to do when faced with an unknown fault.
Always try and fix the fault first by normal fault-finding methods... the reason is simple... if changing the caps seems to fix the fault you still don't know whether you have genuinely fixed the unit, or whether the heat and mechanical disturbance of working on the unit has temporarily masked a problem only for it to return later.
A gain difference between channels should be easily identifiable using a scope and signal generator (or use a test CD/MP3 file).
+1
From what the OP says it seems there's just a level difference.
Realistically faulty / old caps and worn pots are not going to give this. You would expect noise / distortion / crackling etc.
And it would be unlikely to be one channel only.
Nothing wrong with recapping / cleaning etc. but it muddies the water when dealing with a specific fault.
The level difference seems quite large so should be relatively easy to trace where it is being caused.
Don't discount that it may be that the one channel is 10dB too high rather than the other too low
First place I'd check - Balance Control Pots
First place I'd check - Balance Control Pots
Nothing wrong with it.
Will trace the circuit to find out what's going on.
BC General Purpose Transistors
First Character - Material
A:Germanium
B:Silicon
C:Gallium Arsenide
R:Compound materials
2nd Character - Type of Device
A: Diode - low power or signal
B: Diode - variable capacitance
C: Transistor - audio frequency, low power
D: Transistor - audio frequency, power
E: Tunnel diode
F: Transistor - high frequency, low power
G: Miscellaneous devices
H: Diode - sensitive to magnetism
L: Transistor - high frequency, power
N: Photocoupler
P: Light detector
Q: Light emitter
R: Switching device, low power, e.g. thyristor, diac, unijunction
S: Transistor - switching low power
T: Switching device, low power, e.g. thyristor, triac
U: Transistor - switching, power
W: Surface acoustic wave device
X: Diode multiplier
Y: Diode rectifying
Z: Diode - voltage reference
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Oh, I didn't know it stood for general purpose. I was actually thinking about it's full denomination, to look at gain groups. To perhaps pose the question, would a lower gain substitute cause this. I don't know, but would look it up, if nobody seemed interested (meaning it's probably a bad idea)
Spent the afternoon tracing the signal but appears to be no voltage drop.
Maybe this approach of changing all transistors saves me a lot of time.Now the relay is not arming. So I checked the voltage regulator it is with unexpected voltages, I will have to change it. The voltage in the case is correct 29V, but the pins measure no correct values, maybe this is the cause of all this issues. It is a RC4194TK. Can I use some equivalent? This part is very hard to find.
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Due to file size I can not post it here. But it is easy to find it hifiengine, look for Marantz 3800 service.manual. thx for your help.
and thanks for updating the thread.- Status
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