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Marantz 8B bias(?) issue question.

fogf1

fogf1

New Member
Hi all,
First let me say that I think this site is a fantastic resource for gear suggestions and troubleshooting help!
I've picked up a lot of good advice just by reading through the threads.
So here's the gig....
I've been given a Marantz 8B to work on for someone and I've hit a brick wall with it.
Audio output is almost inaudible and quite distorted.
There appears to be no bias reading on the meter even though there is -47V coming from the bias power supply and going to each of the bias pots. The bias pots test ok and voltage is going to the tubes.
I've tried two different sets of output tubes with the same result. Audio quality is best, but not acceptable, when the bias pots are turned fully clockwise - minimum bias voltage going to the tubes but the output waveform is terribly distorted.
B+ voltages are where they should be. The one thing I have noticed is that when power is turned off to the amp, for a split second the audio sounds perfect.
All electrolytics have been changed.
I'd appreciate any suggestions and I'll gladly provide more info if I can
Thank you.
 
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Might want to ohm out that meter selector switch and see if its actually working like it should. If I muddle through the switching arrangement correctly here the entire B+ runs through that thing. It looks like it switches the meter in the circuit between B+ and the output transformer per-tube depending where you have it set. If the meter shunt resistor toasted it probably killed the meter itself and it will not read anything. You also won't get proper voltage through it so the output tubes will not perform like expected. If you've got plate voltage and want to confirm if the tubes are conducting somewhat, first verify the 6.8 ohm resistors on the EL34 cathodes are good, and then measure voltage across them. A little math magic turns that into current. Open cathode resistors will also cause no current to flow and some very heavy distortion.

Also worth making sure you're getting about the right voltage to the grids of the output tubes. If its too negative the tubes won't conduct and you'll get performance like you are getting. Schematic says -36v at the grids, which sounds about right.
 
Note to users of these series amps:
Do not ever check bias with the meter with a signal, it could/will toast the meter.
I'm trying to remember the actual bias current for this amp, it may be searchable.
 
Ok, so I've had a look around with a meter last night.
Seems the B+ is wildly off spec. I'm getting +244V. The schematics say I should be getting +435.
I've also looked at the voltages on the output tubes.
Negative lead on chassis ground.
Cathode(pin 8) 0V
Plate(pin 3) 242V
Grid(pin 5) -34V
Grid(pin 4) 242V

I'm also getting -247VDC on pins 2 & 7 - (filament), as well as 6.3AC.
 
Low B+, how about the filter capacitors? In what condition are they, especially the first one? I just searched the web (since I don't know this amp) and it appears that it uses silicon diodes. They are usually still good, even when they are older, early manufacture. But who knows.

Another thing, leaky filter capacitors can bleed off the B+ so I would start at the diodes and work my way to the tubes. Possibly, remove the tubes first?
 
Just looked up a schematic, is it a voltage doubler? It appears that the diodes are in series, might be an open one?
 
Not sure why you'd have -247v on the heater supply, it shows no connection to anything that would do that.

More importantly your HV supply is about 150v low. Schematic says you should have about 390v on the plate and screen. That'll be why it doesn't flow any current, the bias supply is too negative for how how the HV supply is. Time to start poking around in there. If the first two capacitors in the supply are not good, you'll have this issue.

Its a voltage doubler with series diodes to deal with high PIV. You could sub that for singles of a higher voltage now, but it won't really matter. Pretty sure if one was open you'd get nothing though.

**edit** Had a thought, does the heater supply center tap happen to share a connection with one of the voltage doubler caps? An open ground connection maybe could cause the odd voltage readings on the heater supply as well as the low B+. I've never been into one of these and have no clue about the physical construction.
 
gadget73 said:
...

Its a voltage doubler with series diodes to deal with high PIV. You could sub that for singles of a higher voltage now, but it won't really matter. Pretty sure if one was open you'd get nothing though.
Click to expand...

I am sure that you are correct. That power supply is rather straight forward. Kinda reminds me of Fisher and Sherwood. But certainly not most of HH Scott. (FWIW).

Keep us posted please, I suspect you can get it going with little trouble.
 
It's a bridge doubler supply, a problem with one or both of the bridge elyticaps will cause low B+. The B rectifiers are doubled up, a problem there is unlikely.
 
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Thank you all for the leads and suggestions.
All the electrolytic caps have been changed in this unit. Done prior to me getting it.
Not sure if the unit worked prior to me getting it also.
It looks like the rectifier diodes have been changed as well. Looks like there are 1N4007's installed.
I'll have to run down all the suggestions listed and see where it brings me.
I'll start with the high voltage and see where that brings me.
Then I'll have to hunt down why there is such a high DC component on the filament voltage.
I'll be back!
Thanks again.
 
Finally got the chance to revisit this unit
All was not as it seemed on my first run through with this unit.
B+ voltage was way off which was the reason I was getting no audio output.
After some poking around and tracing of the circuit I found the culprit.
There was a missing ground connection from C20.
The multi-cap cans were bypassed as they had started to show signs of leakage and the ground was overlooked.
I now have the correct B+ voltage, 435V, and no DC voltage riding on the heater lines.
Amp is fixed. Sounds great.
Thank you all for your suggestions.
Peace and Love.
 
The 8B is a great amp. While you are in there, change the diodes for low noise diodes and consider changing the coupling caps. I'm using a mix of PIO's and film and really like the sound.
 
fredtroy said:
The 8B is a great amp. While you are in there, change the diodes for low noise diodes and consider changing the coupling caps. I'm using a mix of PIO's and film and really like the sound.
Click to expand...
What low noise diodes do you recommend?
As for the coupling caps, why wouldn’t you use all the same type? What are you using and where?
Thx.
 
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