Marantz 1530 running hot.

[jrdrbtbrr]

Hi all,

I'm hoping I can get some help with this, as I've looked online and at my .pdf of the service manual and I'm still pretty unsure of myself. So, I picked up a Marantz 1530 from a local seller and it sounds great. Cleaned the whole thing with deoxit and everything. Now, when I leave the unit on for awhile, it gets pretty damn hot. The heatsink on the power amp board (P700) is hot and I can only touch it for about 1-2 seconds. There is a black burn spot on the underside of the metal case (directly above where the heatsink is). It also smells pretty bad and seemed to get kind of smokey when I had my speakers connected to it (didn't realize there was an issue until after I had my speakers hooked up). I got lucky that I didn't burn them out.

I know I need to check the DC-offset and then the bias of the unit. But, I'm very new to this and honestly I don't want to fry anything. Is this something a novice can do? And, if I need to tweak these, should I do it or have it sent to a tech? I'm picking up a voltmeter tomorrow to have a look, but any help is appreciated. Including explaining where to affix the voltmeter on the board and such. Hopefully someone else here has done this on their 1530!

 

[Baron K Rool]

See the small heatsink with a single transistor mounted to it, in the corner of the main PCB? That thing gets hot a as a mainiac. I recommend replacing it, or at least giving it a clean up and new thermal compound (the original should have turned to something like ash by now). New mica insulator might be nice, too.

The real culprit however is the big 150 ohm resistor right next to that. It gets SUPER hot, and REALLY stinky to boot (it probably produces some degree of smoke).

It MUST be replaced - with a 3W metal oxide resistor, or a 5W ceramic resistor. It dissipates almost 3W during normal use, so it needs to be one of these (or higher). The service manual specifies a 5W resistor, but 3W is what's actually used.

That'll take away terrible smoke smell this amp produces.

As for the hot main amp heatsink, that sounds like you have too much idle current going. Adjust that as per the service manual (available online for free), it's not difficult.

Good luck!

EDIT: Sorry, I didn't actually read your post in full. Adjusting the idle current is easy, but you do run the risk of killing the amp. Make sure you clean the adjustment trim pots on the board, before you even touch them. Do not actually MOVE them when cleaning, just give them a quick spray and leave them alone until you're ready for some adjustment action.


To do that, hook up a volt meter (set to 200mV DC range, or similar) across the emitter resistors (specified in the manual) with MINI GRABBER LEADS (Google it). You can buy these very cheap on Ebay. You must use these, otherwise you will kill the amp, guaranteed. It's very difficult to measure this without slipping (and thus, likely killing it) on this model without those leads, please don't even attempt it.

A last resort would be to solder wires directly to those resistors, and then measure on those (outside of the amp). That way, you won't need the hooks, I suppose.

When that's set, just turn the trim pots, ridiculously slowly, until you get whatever reading the manual says you should get. You can destroy the amp during this procedure too, so be very careful here.

But don't let that scare you. It's actually very easy! Just take it slow.

 

[elnaldo]

+1 on the mini grabber. I always measure with that in circuit. Put the probes with the unit OFF, be sure they are steady and fixed, and then power-on, and read the measurement in your device. Don't play with the probes when the unit is ON. Power-off, place the probes in the next point, power-on again and read.

Follow service manual instructions to adjust DC offset and Idle (bias) current. It's very easy, just do it slowly. Let the unit ON for 30 minutes, measure again to check. Some manuals advice about a warming time, or to measure when the unit is cold.

You can destroy the amp during this procedure

Even if this is true, I don't see this adjust so dangerous if you take this cautions: mini grabber probes and adjust the pots very slowly, not 1/4 turn at a time, just minimal movements while looking at the voltage in your multimeter.

 

[jrdrbtbrr]

Thanks! I really appreciate the input. And, it's good to know that the stock heatsink on the PCB runs hot normally. I will definitely replace that 3W resistor and the heatsink, after making the adjustments to the trim pots.

 

[dr*audio]

These:
http://www.mcmelectronics.com/product/201-016
http://www.mcmelectronics.com/product/201-015

 

[jrdrbtbrr]

Follow service manual instructions to adjust DC offset and Idle (bias) current.

The service manual is really vague on this. The only thing I can find is "audio alignment procedure" and has me adjusting the current to 10mA between two points. I'm assuming this is the bias adjustment procedure.

from the manual:

"DC voltmeter in 100mV or 50mV range to R741 (R743) and R742 (R744). Adjust R717 and R718 until current is 10mA"

I'm assuming the different sets of numbers are for right and left channels (ones in parentheses are 1 channel?). So I'd connect the DCVM to R741 and R742. Then R743 and R744.

Haven't found any info on where to find a trim pot for adjusting DC offset, but I'll keep hunting.

Thanks for the replies everybody!

 

[elnaldo]

You need to know the value of R741 and R742 to calculate the current (mA, milli Ampere). You are measuring mV (milli Volts),

you need to calculate : Ampere = Volt/ohm

I don't have the schematic, but I GUESS the numbers in parentheses means you can measure in one resistor or in the other. I guess it means R741 OR R743, and R742 OR R744 in the other channel. Look at the schematic to be sure.

Many amplifiers don't have a DC offset adjustment (like mine).

 

[rotco]

Hi all

i wanted to perform the bias adjustment for my 1530L.
i read this post. and i see that they say to calculate the current from the measuring voltage.

on the s/m (and on the resistors themself) it's written 0.47ohms.
and i connect the probes on both ends of the resistor, so i actually measuring the voltage across the resistor itself.
(while measuring the resistance of the resistor on board, powered OFF, the measuring resistance is not the actual resisor itself.. goes up to tens Kohms).
before i did any change to the varistors, i just wanted to measure to see how "close" it to the desired value.

edited: so i measured about 50mV on one channel, and 87mV on the other.

while i did the calculation, i see that the desired voltage is:
V=I*R, i.e V=0.01*0.47=4.7mV.

so, it about 10-20 times higher.
currently i dont feel like the receiver plays bad or getting over heat. but. of course, while i touch the heat sink, is hot... not in a way that i cant put my hands on for no more then 10 seconds..

should i adjust it?
am i correct with this? than factor of 10-20 is normal and still not in dangerous status?
the s/m doesn't says nothing for the specs, or the delay time to measure the voltages.

thanks

 

[elnaldo]

Hello. You need first to have an accurate measurement of the resistor. You mentioned you are not measuring 0.47 ohm. Please let the circuit to discharge, and measure the resistance again. Anything below 1ohm should be OK (a regular meter is usually not accurate at those low values)

What's the target value for idle current in that amplifier?

 

[rotco]

Hello. You need first to have an accurate measurement of the resistor. You mentioned you are not measuring 0.47 ohm. Please let the circuit to discharge, and measure the resistance again. Anything below 1ohm should be OK (a regular meter is usually not accurate at those low values)

What's the target value for idle current in that amplifier?

10mA

But, ia that type of resistors i could measure their resistance on board? Powered OFF?

 

[elnaldo]

You should measure the resistance in circuit. POWER OFF AND UNPLUGGED FROM THE WALL. Measure each resistor, or measure across 2 resistors to have more resistance, the error will be lower. Also when adjusting, measuring across 2 resistors lower the error.

 

[rotco]

Thanks!

Appearntly, the "legs" of the resistors are bit rusty and coroded, plus, the contact point of the tiny probe grabbers touch only tiny spot on the resistor legs.
Thats why i got the reading of about 70kohms of one resistor.

I was lucky, that when switching to mV the DMM respond better and i adjusted it to 10mV as @petehall347 recommended me.

Now the unit running w/o getting warm almost at all.
Thanks!

I was lucky, only later i understood that altough i connected the mini grabbers stable on the legs, doesn't says that the contact point is healthy!

 

[SteveAndBell]

The real culprit however is the big 150 ohm resistor right next to that. It gets SUPER hot, and REALLY stinky to boot (it probably produces some degree of smoke). It MUST be replaced - with a 3W metal oxide resistor, or a 5W ceramic resistor. It dissipates almost 3W during normal use, so it needs to be one of these (or higher). The service manual specifies a 5W resistor, but 3W is what's actually used.

Legendary! Thanks Baron K!!! I just picked up a 1530 and this is exactly what's going on with it! It was a barn find so I slowly powered it up with a Variac & DBT and eventually managed to get sound from it... but within about 2 minutes I began to see whisps of smoke coming from that 150R resistor. There was also a fair amount of glue in the vicinity so I removed all caps from that section, tested them, cleaned the glue off the PCB and surrounding components, replaced the 2SD313 & thermal grease on the heatsink and a couple of resistors that had been affected by the glue then put it all back together and it worked fine but that resistor was still smoking.

Thanks so much for mentioning this because it didn't take me more than a quick Google search to discover your comment above. I'll be ducking out to grab a 5W ceramic tomorrow but to be absolutely sure the problem is solved do you think I should bolt that to the chassis or a small heatsink to keep it super cool or would that be overkill?