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Marantz 140 amp DBT dead short question

Richs_Trains

Richs_Trains

Well-Known Member
Hey all,

I have a Marantz 140 amp that I replaced some diodes on several years ago because the peak indicators were stuck on. After this fix, the unit worked fine and I had it in the living room on a 3200 for several months with no issues. A couple of years ago I swapped the unit out for a Marantz 4140 and boxed it up. It has been sitting in a closet boxed since then. I had decided to recap some of the units I never worked on and ordered the parts to recap, change out the drivers, relay, and new thermal paste. When I unboxed it I plugged it into the DBT just to be safe with a 100-watt bulb, well the bulb never dimmed and I powered it down immediately. The unit never lit up and the relay never pulled. My question is the 100-watt bulb too much for this unit and should I try with a 40-watt or do I have a direct short somewhere? If it's a short, I am sure it is beyond my meager abilities and I will have to send it off to be repaired. Thanks, Rich
 
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Not the bulb. That just limits the amount of current it can draw and if lighting up a 100 watt it definitely has an issue. Get it to a tech or if you have capabilities I would check the Output Transistors first. Start by pulling covers, shake upside down to make sure nothing fell inside and shorted across something over the years. Look for any heat damaged resistors or burn marks on boards or caps that look bulging. Could be something simple if lucky.

Here is LINK to basic testing of transistors in DIY section. You should be able to get plenty of help here if you have basic testing equipment and service manual downloaded.
 
jd3rdcoast said:
Not the bulb.
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Thank you, JD. I decided to give it a try. I have disassembled the unit to pull the amp boards and with both amps and meter boards out I still get a DBT with no dimming. I did purchase 2 new filter caps to replace the existing dual cap if you feel this may be the problem.
 
I made the mistake of running a 40W bulb in my DBT, the result was not enough current being passed to the power supply board, and all kinds of weird symptoms. I put the 100W bulb in and it works exactly like it should.
 
At this point, the only things left in the chassis is the filter cap, transformer, rectifier, and the power supply board.
 
Ummm, so why don't you measure across the caps and check for a short or excessive leak, and put your meter on Diode Test and check the rectifiers? Stop guessing and do some troubleshooting.
 
dr*audio said:
Stop guessing and do some troubleshooting.
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Thanks for your reply dr*audio, unfortunately, I am not good at troubleshooting without the help of the good people of AK. Can I test the filter cap with my DMM and the rectifier is somewhat buried in this unit so some coaching on where to connect the DMM in diode test would be quite helpful. I pulled the power amp boards to allow for more room and eliminate them from the equation. I do have a cheap cap tester, but I don't know if the 9v battery inside is enough for the filter cap. Thanks again for any help.
 
Get copy of service manual at hifiengine.com
I am not an expert tech so don't want to steer you wrong. But below is the rectifier section including the main caps. You can google how to test a Bridge Rectifier. Will need to disconnect it from the circuit so make sure to note your wiring and take pictures.

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See, you were doing so well, disconnecting the amps and meter board that I thought you knew what you were doing! Ok, some help, then. Your meter is good enough to tell if the caps are a dead short or very leaky. The reading should climb slowly until it reads over range if the cap is good. If it does that it probably isn't lighting up the bulb.
Check both caps, observing polarity. If that's ok, then check the diode as explained above. If everything looks ok, it could be inrush current causing the bulb to light. To check that, leave the amp and meter board disconnected and power it up with the dim bulb. Leave it on for 10 seconds. If the bulb is bright but goes out then the power supply is ok. While it's on, with the bulb out, measure the voltages on the power supply with respect to ground, where the 2 big caps are connected together. The big caps should have equal voltages, + and -.
 
dr*audio said:
If the bulb is bright but goes out then the power supply is ok.
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The bulb stays bright, the rectifier appears to be okay if I'm doing it right. One thing I noticed and did not mention is the meter lights never come on. It looks like they are directly connected to the transformer. Could the transformer have failed just sitting boxed in a closet? I seem to remember when I have had DBT failures in the past, I still got dial lights. Thanks, Rich
 
A little out of left field, but look for pinched wires. As you mentioned you changed lamps previously and they aren't on, so I would be checking wiring to the lamps first. It could have been pinched, but not shorted when you put it away. Good luck with it.
 
sregor said:
A little out of left field, but look for pinched wires.
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Thank you for your reply. Wiring for the lights appears all okay, in its current state the wiring to the lamp housings is all free and clear and the supply voltage comes direct from the transformer to the lamps. Rich
 
dr*audio said:
Your meter is good enough to tell if the caps are a dead short or very leaky. The reading should climb slowly until it reads over range if the cap is good.
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I’m curious how a large value Leaky vs. Shorted capacitor behave when testing with DMM capacitance check when value is higher than available scale? I have struggled in past trying to determine if Filter caps need replacing. I typically just check voltage and look for high ripple with a scope. I’m guessing that does not tell everything.
 
You could start to disconnect the secondaries one set at a time to see if the short goes away.

Tony
 
tonebells said:
You could start to disconnect the secondaries one set at a time to see if the short goes away.
Click to expand...

Thanks, Tony, last night I pulled the filter cap and was preparing it for the 2 new caps when I thought about the fact that I had no meter lights and became distressed that the transformer might be bad (as I said earlier, other times I had a DBT failure, I still had dial lights). So, I posted this morning hoping for some encouragement and scoured the internet for a possible transformer replacement. I saw your post and figured, what the heck, I'll isolate the leads to the filter cap a power it up on the DBT just to see. Well, I have meter lights and relay click! So I must have had a bad filter cap and will proceed to complete the installation of the clamps for the new caps. Thanks so much for prodding me on! Rich
 
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