Marantz 8b power transformer B+ down 30% - diyAudio
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Old 1st July 2013, 11:07 PM   #1
TBL is offline TBL  Hong Kong
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Default Marantz 8b power transformer B+ down 30%

Hi All

My 8b was newly acquired and its serial no. Is around 9xxx, prob an early one.

While it had been replaced with two new voltage doubler caps and bias circuit rebuilt with new resistors, it variacced up fine, and played for an hour. Then it smoked an doutput was seriously distorted. My electrician stopped it and invetigated. He found:

1 The B+ preiously had measured 221vac/445 vdc, right on spec. Now it measured 152vac/325vdc. It mesured low as well at all the other power supply points. He didnt open up the transformer cover but it was felt the power tranny was hotter than normal, given a hort operation time only.

2. The induction coil (choke) was the smoking part. It was replaced wuth another 250ma/15Henries one. The B+ measured the same at a low 352 vdc which was lmost 100vdc lowrr than before. Now it measured 68 ohms in resistance, and carried 0.5 Henry. I will check with it later to confirm this. This replacement one got to abnormally high temp after three mins. Possibly it could get toasted as well just like the original choke did.

He hasn't tried replacing those diodes yet coz the power ripple seemed fine at less than 0.5vdc. The bias outout was the same at 5.7v like before.

So my question is, assuming the power transformer is shorted, is it recommendable to have it rewound, or is it easier to buy a replacement? I know my electrician friend is competent at this but what other things peculiar to a Marantz 8b should I suggest that he check?

Thanks to all in advance for helping. This one is really a lemon.

I am negotiating with the seller. Hope I dont have to return it.

Last edited by TBL; 1st July 2013 at 11:21 PM.
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Old 2nd July 2013, 12:32 AM   #2
DUG is offline DUG  Canada
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Check all the diodes?
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Old 2nd July 2013, 12:52 AM   #3
TBL is offline TBL  Hong Kong
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Not yet with the diodes. I ll ask him to try replacing them. 600v at 2 Amperes will do?
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Old 2nd July 2013, 02:01 AM   #4
DUG is offline DUG  Canada
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Quote:
Originally Posted by TBL View Post
... 600v at 2 Amperes will do?
Can't say...don't know the circuit.

What is in there now?

Use equivalent or better.
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Old 2nd July 2013, 08:27 PM   #5
TBL is offline TBL  Hong Kong
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I installed new diodes. No improvement. Now it would pop the fuse every time. But when all three power supply points at the big can cap is disconnected, it's fine again. I'm still diagnosing, comparing with my other 8b.

Interesting, I found the 4 nos of 1.2 meg ohm resistors on the board supplying nos 2 and 7 of the 6CG7, in open circuit. What is that board for?
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Old 3rd July 2013, 09:24 PM   #6
TBL is offline TBL  Hong Kong
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OK Folks.

I have tried several days to diagnose why the B+ falls like that.

I have another 8b so I tried this tfmr outputs one by one to the other 8b. No problem whatsoever. I also did the other way round. No. It kept blowing the fuse now.

So.... guess what? Long story short, the primary coils of the output transformers are shorted to ground at a resistance 45 ohms!!!! BOTH channels!! YUCK!

What do I do now? Rewind? Second hand trfmr? I suppose either way, I cannot have the original sound back. Ah Geee......
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Old 7th April 2014, 02:00 AM   #7
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Tried to get 2 8B outputs rewound by a guy in Maine, put power on them and they both showed shorted. I going to rebuild with another type of OPT.
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Old 7th April 2014, 04:43 AM   #8
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TBL,
Slow Down - I strongly doubt that both output transformers are blown. That 45 Ohms you are measureing is much more likely to be something in parallel with the OT primary connection. A blown filter capacitor is my first guess but it could be a shorted tube too.

Pins 2 and 7 of a 6CG7 are the grid connections of the 2 triodes - this doesn't make a lot of sense.

First:
Confirm that the 45 Ohms is there without power on.
Then pull all tubes and see if it is still there - if not => dud tube
Then disconnect B+ side of each filter cap in turn - if teh 45 Ohm disappears then that last cap is the culprit.
etc.

Cheers,
Ian

Last edited by gingertube; 7th April 2014 at 04:45 AM.
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Old 7th April 2014, 05:02 AM   #9
AJT is offline AJT  Philippines
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Quote:
Originally Posted by TBL View Post

1 The B+ preiously had measured 221vac/445 vdc, right on spec. Now it measured 152vac/325vdc. It mesured low as well at all the other power supply points. He didnt open up the transformer cover but it was felt the power tranny was hotter than normal, given a hort operation time only.
a sure sign of overloading...

Quote:
Originally Posted by TBL View Post
2. The induction coil (choke) was the smoking part. It was replaced wuth another 250ma/15Henries one. The B+ measured the same at a low 352 vdc which was lmost 100vdc lowrr than before. Now it measured 68 ohms in resistance, and carried 0.5 Henry. I will check with it later to confirm this. This replacement one got to abnormally high temp after three mins. Possibly it could get toasted as well just like the original choke did.
the choke was smoking because of the overloading, it was the victim not the culprit...

what i do in these situation is to temporarily pull out the output tubes of both channels, and then measure the voltages on power on and see if the overload is gone....if overload still persists then probably one channel opt is bad or both....this time you can cut the B+ leads and try again...

if voltages return normal, then put back the output tubes on one channel and try again, if overloading comes back, then that set of tubes are bad....

did you see any sparking inside the tube when that happened?
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Old 7th April 2014, 05:26 PM   #10
kevinkr is offline kevinkr  United States
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Bias voltage quoted sounds wrong. Have you (or previous owner) replaced bias rectifier, electrolytics and coupling caps to the output stage? Diodes in main supply need to be 1kVpiv or better.

Most likely one of the supply capacitors is partially or fully shorted, excessive current demand destroyed the original choke. Other things to look at is the bias on each of the output tubes - it should be somewhere between -34V and -40V typically depending on the tubes installed. (When amplifier functioned correctly and bias was set prior to the current events.)

Unlikely but not impossible for the output transformers to be shorted winding to winding, but highly unlikely to core (gnd) - look for an external short like a bad electrolytic or tube (less likely)

The quest to keep 50+ yr old amps all original which seems common with "collectors" these days is at the root of these sorts of problems. The passive parts were not intended or designed to last beyond what was then considered a normal service life. (10 - 15yrs at most) That many did is a tribute to their original quality or relatively modest usage.
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