I recently picked up a non-working Marantz PM-34 amplifier for cheap.
I received it yesterday and done a quick visual check before powering it on.
Powered it on and nothing! Tested the internal and it was knackered. I ordered replacement fuses (glass fuse, 20x5mm, type T, 1A). I fitted the fuse this morning and when I turned it on the fuse blew straight away.
1st problem:
========
I have tested the main mosfets/transistors for shorts and found that Q764 (PNP) on the the right channel has a C-E short.
Q764 = 2SB1254
Q762 = 2SD1894
First questions:
1) Am I correct in thinking that I should I be replacing Q762 (NPN) at the same time as replacing Q764?
2) Would a FJA4210OTU be a suitable replacement for a 2SB1254. I chose this transistor because it has the same "Vceo" voltage, is available in the package and has a higher "Collector current".
2nd problem:
========
The two white cement resistors (2x0.18 ohm) seem to be reading out of spec.
R767 (L-ch) readings are as follows:
L - R = 1.88 ohm
C - L = 1.50 ohm
C - R = 1.63 ohm
R768 (R-ch) readings are as follows:
L - R = 1.56 ohm
C - L = 1.66 ohm
C - R = 1.97 ohm
Questions:
1) Should I replace the two resistors (at the same time as the 2x mosfets on the Right channel)?
or
2) Should I replace the two resistors, after:
a) replacing the Q764 and Q762, then
b) retesting both cement resistors?
Thanks in advance for any help.
I received it yesterday and done a quick visual check before powering it on.
Powered it on and nothing! Tested the internal and it was knackered. I ordered replacement fuses (glass fuse, 20x5mm, type T, 1A). I fitted the fuse this morning and when I turned it on the fuse blew straight away.
1st problem:
========
I have tested the main mosfets/transistors for shorts and found that Q764 (PNP) on the the right channel has a C-E short.
Q764 = 2SB1254
Q762 = 2SD1894
First questions:
1) Am I correct in thinking that I should I be replacing Q762 (NPN) at the same time as replacing Q764?
2) Would a FJA4210OTU be a suitable replacement for a 2SB1254. I chose this transistor because it has the same "Vceo" voltage, is available in the package and has a higher "Collector current".
2nd problem:
========
The two white cement resistors (2x0.18 ohm) seem to be reading out of spec.
R767 (L-ch) readings are as follows:
L - R = 1.88 ohm
C - L = 1.50 ohm
C - R = 1.63 ohm
R768 (R-ch) readings are as follows:
L - R = 1.56 ohm
C - L = 1.66 ohm
C - R = 1.97 ohm
Questions:
1) Should I replace the two resistors (at the same time as the 2x mosfets on the Right channel)?
or
2) Should I replace the two resistors, after:
a) replacing the Q764 and Q762, then
b) retesting both cement resistors?
Thanks in advance for any help.
From the datasheets:
2SB1254 = "2SB1254 Silicon PNP epitaxial planar type Darlington"
FJA4210 = "FJA4210 PNP Epitaxial Silicon Transistor"
So how is a "FJA4210" not a suitable replacement for a "2SB1254"?
... and ...
What would be a "suitable" replacement (and why)?
Thanks
I've checked most (if not all) transistors and have found any more with shorts.
Can't find a service manual or schematics for this model.
The only anomaly I found was the 2x 0.18ohmx2 cement resistors, (see first post).
Don't be offended if you know this, but measuring small resistors with 2 wire DVMs is difficult.
Touch the probes together and record the residual value which should be subtracted from the apparent reading.
Those white cased resistors are heavy gauge wirewound and very robust. If they are not scorched in any way they are not suspect.
Build and use a light-bulb mains connection. Search here for it. It will save further blow ups.
Resist the temptation to wade in and replace lots of stuff - one at a time, does it!
Touch the probes together and record the residual value which should be subtracted from the apparent reading.
Those white cased resistors are heavy gauge wirewound and very robust. If they are not scorched in any way they are not suspect.
Build and use a light-bulb mains connection. Search here for it. It will save further blow ups.
Resist the temptation to wade in and replace lots of stuff - one at a time, does it!
Dfear-- I checked and the PM-32 uses the same BJT,s ( transistors ) you are talking about at the output
and that circuit diagram is available on hifi engine ,at least it will give you an idea .
CliffForest is right you have to take the multi-meter lead resistances into consideration when checking very low values very expensive bench multi-meters have 4-wire capabilities or some can be zeroed either automatically or manually.
My old HP bench model has 4-wire.
and that circuit diagram is available on hifi engine ,at least it will give you an idea .
CliffForest is right you have to take the multi-meter lead resistances into consideration when checking very low values very expensive bench multi-meters have 4-wire capabilities or some can be zeroed either automatically or manually.
My old HP bench model has 4-wire.
Will do more testing.
Thanks for the info.
I have one of these I made myself; it comes in handy with these types of repair.
I've already learned my lesson on this (...with my first amplifier repair.. a Marantz pm66 se ki)!
Difficult to check the resistors without specialized equipment, but if they all read about the same
(after subtracting the resistance of the leads), it's likely they are ok.
id say your best option for replacing these Darlingtons is going to be something from Sanken. I'd probably go with 2SD2389/2SB1559.
The first thing I would try is to remove the output transistors from the bad channel, then power up and see if the other channel is working correctly. Judging by the PM-32 schematic, which is similar, the amp's frontend is an AN7062. if this is damaged, the amp is pretty much a write off.
The first thing I would try is to remove the output transistors from the bad channel, then power up and see if the other channel is working correctly. Judging by the PM-32 schematic, which is similar, the amp's frontend is an AN7062. if this is damaged, the amp is pretty much a write off.
They have some AN7062 in stock.
AN7062 Panasonic - Distributors and Price Comparison | Octopart component search
AN7062 Panasonic - Distributors and Price Comparison | Octopart component search
Would the output transistors (for the right channel - which is the bad channel) be Q764, Q751 and Q762?
Would I need to remove all 3 (only Q764 has a short)?
Or would it be better to just disable the right channel completely? Would removing Q702 do this?
The 2SD2389/2SB1559 transistors don't seem to be easily obtainable, so do you have suggestions for other transistors I might buy; preferably from UK suppliers?
My main suppliers are: RS or Farnell.
An =FJA4210OTU is not a Darlington transistor so its gain (hFE) is very low in comparison --look for the word=DARLINGTON.
https://uk.farnell.com/c/semiconduc...ipolar-junction-transistors-bjt?st=darlington
https://uk.farnell.com/c/semiconduc...ipolar-junction-transistors-bjt?st=darlington
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For the necessary Allegro Sanken audio darlingtons, which you'll no doubt have to use unless you think fakes from on-line sellers are worth trying, you probably need to deal with Digi-key. Yes, I believe they have a freight charge of some 20UKP but if you look at other replacement parts likely needed, you will likely have exceeded their threshold amount and qualified for free delivery anyway.
Farnell have 2SD2390/2SB1560 but they are listing the 2SD as "Available until stocks exhausted". Digikey have them. You could also try Profusion.
edit: High speed power Darlington's suitable for audio use is kind of a niche product, one which Sanken has specialised in. I also notice Farnell is getting even worse when it comes to their stock! This is why I switched to Digikey - they have similar or better prices to Farnell, similar minimum order, and free shipping from the US. There was really no reason to keep using Farnell!
edit: High speed power Darlington's suitable for audio use is kind of a niche product, one which Sanken has specialised in. I also notice Farnell is getting even worse when it comes to their stock! This is why I switched to Digikey - they have similar or better prices to Farnell, similar minimum order, and free shipping from the US. There was really no reason to keep using Farnell!
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