Author Topic: Trying to fix a Marantz PM 35 amplfier  (Read 1730 times)

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Offline Yamin

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Trying to fix a Marantz PM 35 amplfier
« on: March 14, 2019, 07:33:51 am »
Hi, I’ve got a Marantz PM 35 amplifier which I’m trying to restore. It was in real bad condition when I got it full of dust, and there was no output. Cleaned the whole thing touched up the dry joints. It looks like some else has gone in before me as I can see lifted pcb traces which has been fixed and some parts are filled with dried up solder flux. I wasn’t able to get a service manual for it however I was able to find a detailed schematic which was very accurate except for some componet values. It is at http://akdatabase.org/AKview/albums/userpics/10007/Marantz%20PM-35%20Schematic.pdf
Anyway here is what I have done so far.

1. Touched up all the dry joints that I could find (feels like I have re soldered the whole board back again).

2. Changed the 24V regulator.

3. Changed some leaky electrolytic capacitors.

4. All the output transistors and everything seemed to be in working order meaning couldn’t find any shorts etc.



The problem

The audio output is distorted on the left and right channel. Same for the speaker and headphones.


Voltage measured across components/test points seems ok relative to the schematic nothing too out of the ordinary. But I am not able to get 35V at the main bridge rectifier output. It only went upto 25V! Normally it’s even below that around 20V. This is what I have found so far – It goes upto 25V when one side of the amplifier was not turning on (left output side). I measured 300mV at (Q711) which is half of what it should be. When it rises up to 600mV the output of the rectifier goes down to 20V. I touched up the solder joints of the amplifier IC (uPC1270H) and now I get steady voltage of 600mV at the base of Q711. But now I get 20V at the bridge rectifier.

Following this I get 18V at the input of the 24V regulator! And only have 16V at the output of the regulator :S. I didn’t measure 24V at the output of the regulator at any instance the highest it went was around 20V.

Another thing I remarked was R805 which is a safety resistor gets quite hot and it has got burn mark. It checks out ok I have checked after it and before it but couldn’t find any shorts. I measured 10V across it which seems reasonable, its well within the power rating of the resistor.

When I first powered it on the speaker protection relay didn’t engage, but now after all the work done the relay turns on but it takes quite a significant time to turn on.

I checked the output of the transformer - it measures at 19Vac in circuit and out of circuit too (measured across winding and the center tap)

I am using the series light bulb trick - the light bulb turns on briefly when the amp is turned on but remains off afterwards.

I would really like to get this to work please advice on what else to do and other test I could do.


Thanks so much for the help.
 
« Last Edit: March 14, 2019, 10:38:36 am by Yamin »
 

Offline soldar

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Re: Trying to fix a Marantz PM 35 amplfier
« Reply #1 on: March 14, 2019, 10:20:06 am »
I was able to find a detailed schematic which was very accurate except for some componet values. It is at http://akdatabase.org/AKview/albums/userpics/10007/Marantz PM-35 Schematic.pdf
Linky no worky for me. Try http://akdatabase.org/AKview/albums/userpics/10007/Marantz%20PM-35%20Schematic.pdf
All my posts are made with 100% recycled electrons and bare traces of grey matter.
 

Offline Yamin

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Re: Trying to fix a Marantz PM 35 amplfier
« Reply #2 on: March 14, 2019, 10:37:17 am »
Yep sorry attached the wrong URL, original post is edited now
« Last Edit: March 14, 2019, 10:39:08 am by Yamin »
 

Offline Circlotron

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Re: Trying to fix a Marantz PM 35 amplfier
« Reply #3 on: March 14, 2019, 10:49:18 am »
CN01 just above QN01 is drawn with reversed polarity on the schematic.  :--
 

Offline shakalnokturn

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Re: Trying to fix a Marantz PM 35 amplfier
« Reply #4 on: March 14, 2019, 12:53:04 pm »
A very average amp with what looks like some serious butchering work done to it!

A quick image search with your model name makes me think that the butcher may have replaced the transformer by a not quite equivalent model. If this is the case look for other windings that may be appropriate either finding more turns on the secondary or less turns on the primary (I.E. moving from 120 to 110 or 240 to 220 you'll gain some juice on the secondary).
If too many secondary voltages mismatch, may as well look for another transformer.

Of course supply voltage being much too low can lead to incorrect bias and crossover distortion. Do you have a decent (dual) lab supply you could feed it with to check that all works as intended when fed with 2x35V DC?
« Last Edit: March 14, 2019, 01:00:28 pm by shakalnokturn »
 

Offline Yamin

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Re: Trying to fix a Marantz PM 35 amplfier
« Reply #5 on: March 16, 2019, 04:32:49 pm »
Hi guys, some new updates. I touched up some more joints and removed both the VRs which is used to set the bias for Q703 and Q704 cleaned them and put them back in. Turned the power on with the light bulb in series. The light bulb had a faint glow, changed the bulb to a higher power bulb and it also had a faint glow (I didn't get that before). After that I removed the bulb and powered up the amp directly. Nothing blew! :D
Anyway still I get a voltage reading of 22V at the main rectifier output.

Somethings I found out:
1.The two main filter caps had a hard time holding a consistent voltage across them.
2.Still the 24V regulator input reads 18V and the output reads 16V.
3.The heat sink on one side got really HOT (it has got Q712, Q710, Q704). The other heat sink remained 'coldish'
4. Some of the voltages at the pins of the amplifier IC was a bit off probably due to bias being a bit off.
5. The protective relay turned on a little bit faster than before, but as I turned off the amp and turned it back on again the realy took some time to engage.

I feel that the biggest clue is the delay in the protective relay turning on.

What could be the possible issues I could look for? Unfortunately I don't have a bench power supply :( ....

Further questions
1. How can I check whether amplifier IC is not faulty. I might be able to get one spare IC but I'm afraid that by changing it I could damage the one spare I have.
2. The TA7317P IC is also a mystery to me. How can I check whether its faulty.
3. Would an analog multimeter be good to measure voltages across caps - suppose to be more responsive right? Generally I thought that jumping voltage across caps indicated either a leaky cap or some other issue with the components close to it.

The main reason to trying restore this amp is mainly for emotional reason rather than for its fidelity :)
Hope you guys could help me out
Thanks
 

Offline Yamin

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Re: Trying to fix a Marantz PM 35 amplfier
« Reply #6 on: March 16, 2019, 04:49:51 pm »
I forgot to mention about the audio quality - The quality seems a bit better now on one output - the side which gets hot, but still its distorted.
 

Offline shakalnokturn

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Re: Trying to fix a Marantz PM 35 amplfier
« Reply #7 on: March 17, 2019, 01:46:06 am »
You'll not get 24V output on the regulator if you have less than that on the input, you'll need at least 27V input to regulate anything.

As I suspected you had the wrong transformer (but could be wrong myself), I'd recommend that you remove the secondary wires and measure ACV unloaded, that will at least give you some clue if your transformer is fit for the job.

If you do have a capable transformer and one side of amp is getting hot, it's also possible that the abnormal current draw is causing your supply to sag.
Warm is OK, hot is bad...

Measuring voltage at the output transistors emitter resistors (R731-R734) gives you the bias current (expect 10-20mV in normal conditions).
For messed up bias check the components linked to pins 7&8 of µPC1270H, if nothing is wrong there, it's likely the I.C. itself that is dead.
 

Offline Yamin

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Re: Trying to fix a Marantz PM 35 amplfier
« Reply #8 on: March 17, 2019, 06:20:45 pm »
I have checked the transformer out of circuit and unloaded - I get about ~21Vac between the center tap. It looks like its the original transformer.

I have adjusted the variable resistors so that across R734 and R733 its now 10mV. I had some trouble getting 10mV across R733 (right channel). I flipped the µPC1270H of that channel and was able to  adjust the voltage. I would like to ask why 10mV - 20mV though?

Still the main output of the rectifier remained at ~24V but it did go upto 25 volts at one point but is steady at 24.xxV.

I monitored the 2nd pin of TA7317 during power on. The relay engages only when the voltage of that pin goes to -8mV. I found that once the power is on the voltage at that pin is about 47mV and takes quite a significant delay to go down to -8mV. I did change the CN02 capacitor hoping that it was the issue, but there was no difference.

Regarding the 24V regulator - C805 capacitor was burned when I first opened the unit. I removed it to find that it was 22uF but the schematic had a 47uF capacitor so I changed it to a 47uF cap. Could this be an issue?

Still the audio output is distorted. I played through some sine waves and clearly I can hear the distortion sounds like a square waves :(

I'm  |O right now...


 

Offline ExtraThiccBoi

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Re: Trying to fix a Marantz PM 35 amplfier
« Reply #9 on: March 18, 2019, 06:57:15 pm »
I am currently tuning and fixed before a vintage amp as well.
If you are having problems with power supply voltage, and it is possible, try disconnecting power to all active circuits except the power indicator and main power amp. If not all of them, kill just the phono.
There could be a short or something and it's drawing too much current.
I'll have a better look at the schematic and post if anything comes to mind.
Extra:
Those polystyrene capacitors near the heatsink IC are most likely going to be fried, they dont like hot environments and that spot seems like exactly that.
Extra extra:
If you have a transistor tester, i'd desolder the mains and drivers and check them, if the Beta or Idss are the same (small testers with big transistors are not very accurate but should be able to show differences)
« Last Edit: March 18, 2019, 07:17:54 pm by ExtraThiccBoi »
 

Offline Yamin

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Re: Trying to fix a Marantz PM 35 amplfier
« Reply #10 on: March 18, 2019, 07:58:37 pm »
Some updates:
I measured the voltage at R733 and R734 to ground and it measures -0.011V and -0.013V (why do I get a minus voltage here)

Some more findings
I had a look at the other boards like PV01 to find out if anything was shorting out - as this could bring down the 24V output. I couldn't find anything but what I noticed that voltages are slightly off but very close to the marked values of the schematic. But what stood out was at Q405 pin 1 schematic says -25mV but I measure -47mV close to double.
Also with the other boards the ground relative to chassis ground is also a bit off, I connected alligator clips to make the chassis connection the same but still the ground voltages were off by couple of volts. (its a little of a concern right?)
I removed the bridge rectifier to clean it up etc. decided to power on the amp without it - I noticed that at diode D802 it measures 18V (looks correct as the transformer out is around 19V) which goes to pin 1 of the voltage regulator, I found this a bit interesting too - if the main rectifier is in I would get 20V. Whats happening here :S.
Another alarming point - I measured the voltage of negative pin of C805 and it kept jumping around.
and finally could you help me figure out what this part of the circuit does -

Thanks so much
 

Offline ExtraThiccBoi

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Re: Trying to fix a Marantz PM 35 amplfier
« Reply #11 on: March 18, 2019, 08:20:57 pm »
Some updates:
I measured the voltage at R733 and R734 to ground and it measures -0.011V and -0.013V (why do I get a minus voltage here)

Some more findings
I had a look at the other boards like PV01 to find out if anything was shorting out - as this could bring down the 24V output. I couldn't find anything but what I noticed that voltages are slightly off but very close to the marked values of the schematic. But what stood out was at Q405 pin 1 schematic says -25mV but I measure -47mV close to double.
Also with the other boards the ground relative to chassis ground is also a bit off, I connected alligator clips to make the chassis connection the same but still the ground voltages were off by couple of volts. (its a little of a concern right?)
I removed the bridge rectifier to clean it up etc. decided to power on the amp without it - I noticed that at diode D802 it measures 18V (looks correct as the transformer out is around 19V) which goes to pin 1 of the voltage regulator, I found this a bit interesting too - if the main rectifier is in I would get 20V. Whats happening here :S.
Another alarming point - I measured the voltage of negative pin of C805 and it kept jumping around.
and finally could you help me figure out what this part of the circuit does -

Thanks so much
That part of the circuit is most likely a step down regulator, you can see some zeners (might need replacing, i did that to be safe, BZX85 series zeners should be enough) and series pass transistors in there.
The diode voltage you mentioned there is most likely act of what looks like a power monitoring & protection circuit, does the amp have soft start? My amplifier uses similiar circuit to slowly bring up the power amp circuit and not cause pops in the speakers (no output relays). It's done discretely there though, in your case it's probably controlled by that TA7317P speaker protection IC.
I include datasheet for the protection IC so you can check.
Q405 is an opamp for the phono amplifier board, seems kind of irrelevant when you can't get the amp to play clean sound. I would cut power to the phono circuit. You can see it's powered by the voltage regulator you asked about, i think just removing those resistors could disconnect it from power but i would like that confirmed by someone more pro than i am.
 

Offline shakalnokturn

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Re: Trying to fix a Marantz PM 35 amplfier
« Reply #12 on: March 18, 2019, 11:11:46 pm »
With 2x 21V AC at the transformer only expect around 2x 30V DC after the diode bridge. Again you may be able to gain a little playing with the primary if you have an extra winding there but even with 2x 30V DC it should be good enough.

10-20mV was just a usual range for similar powered amplifiers, in this case 10mV is 100mA bias through the output transistors.

Heatsinks are now only warm and you still can't get more than 2x 24V DC supply?

C805 has nothing to do with the 24V regulator, C805 is on the front panel LED supply. Anyway 47µF shouldn't be an issue.

If you have nothing over-heating any more and sound is still distorted maybe it's time to buy a couple of µPC1270H, if I understand well at least one is dead as it causes excessive bias current.
 

Offline shakalnokturn

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Re: Trying to fix a Marantz PM 35 amplfier
« Reply #13 on: March 18, 2019, 11:42:10 pm »
The circuit around QV01/QV02 is +/-15V regulation for the phono preamp.
I wouldn't be to fussy on all the voltages noted on the schematic, I seriously doubt those values where noted in the Marantz service manual from the start. Whould they go to the trouble of noting mV differences between L/R channels? Just giving voltages at a few reference points is more common.

Good luck.
 

Offline Yamin

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Re: Trying to fix a Marantz PM 35 amplfier
« Reply #14 on: March 24, 2019, 07:29:12 pm »
Just an update
I had a potential breakthrough - I was playing music through my phone and was using the phono input of the amp. I had to bring down the output of the phone way down and I was able to get  audio output with out any distortion. It was working ok-ish. So I was hoping that I was heading in the right direction.
So I thought I will put back the amp as the boards were all over the place. So I put the amp back together not all the way but tidied up a bit. I turned the amp back on and the relay didn't click :(
I checked voltages at TA7317P pins, the voltages seemed to be in order apart from the pin going to the relay. Then I thought it must be the TA731P IC so I replaced it with a new one. No luck checked the voltages again and now I measure -0.900V at pin 2 and its suppose to measure -9.9mV! The biasing voltages of the transistors also are in order.
I have gone further back then when I started :'(
 

Offline shakalnokturn

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Re: Trying to fix a Marantz PM 35 amplfier
« Reply #15 on: March 25, 2019, 01:19:01 am »
.... I was playing music through my phone and was using the phono input of the amp....

 :o
 

Offline Yamin

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Re: Trying to fix a Marantz PM 35 amplfier
« Reply #16 on: March 26, 2019, 08:18:26 pm »
.... I was playing music through my phone and was using the phono input of the amp....

 :o
Hahaha I know the mistake I made, but I used the Phono input because the person who brought it in used it. The 'CD' input works perfectly.

I was able to get the amp fixed :D . Turns out that some wire connections broke off due to corrosion. The relay didn't turn on mainly because the wire connections going to the headphone output snapped off.

However I'm still wondering why the relay takes 7 seconds to turn on (I think its a bit long). Is it because of a chosen capacitor value?

Thanks so much for the help everyone really appreciate it :D
 


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