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Hi Guys,I've been restoring a Marantz 8B power amp. This is a unit that has seen better days cosmetically, so collectors value is not an issue. I just wanted a reliable daily driver hence the full restoration I performed on it (see pic).
I replaced all molded caps in the circuit and added new axial electrolytics under the chassis. The circuit itself has not been modified. Rectifiers were replaced with 1N4007s.
After parts replacement, I've been getting very weak signals on both channels and motorboating on channel "A". I also get no bias readings on the meter.
After double checking all the wiring for errors I did a voltage check. Heater and bias voltages were right on the money.
However the measured high voltage points are nearly half the nominal voltages indicated in the schematics.
Measuring AC on the secondary high voltage winding gave me a ridiculously low 180 volts!!! After rectification, I was getting 220 volts instead of 447, etc.No wonder the amp is not working right, and this explains the motorboating.
What could possibly be wrong with the power trannie? Putting a light bulb in series with the primary shows no signs of a short. It does not blow any fuses during testing. It has normal voltages on the bias and filament windings. It appears the HV winding is not putting out enough voltage.
I've seen blown power trannies but never anything like this, very strange indeed.
I've noticed in the schematic that this power transformer actually has two primary connections. Perhaps these connections are at fault.
Your help is greatly appreciated!!!
Best regards,
crooner
Follow Ups:
It almost sounds like you don't have the voltage doubler hooked up correctly... I had this same problem working on a filament supply lately...check your wiring... and take some measurements of the AC on the secondary.
Kevin
Double check the polarity of your diode in the rectifier.
I doubt it's the tranny, as they usually open when they go bad. It could also be one of the power supply caps going bad.
Hi,My schematics show only one primary winding for the 8b. Unless you have some other later model with international voltage capabilities(doubtful) or some one has replaced the power trans. with a substitute.International transformers parallel primaries for North America and series windings for Europe....this is doubtful unless the transformer has been changed and came back from Europe etc... check without the circuit connected to be sure
Check your voltage doubler caps 60mf/300v ea one's on top one's on the bottom clamp...ISOLATED from the chassis if you see the schematic you'll know why. If these are not hooked up correctly...or you have tacked them across faulty existing cap you will get the results you mention...Suspect wiring error... lift the caps off the supply and check voltages..Check your diode orientation and condition. good luck.
The revision corresponding to my serial number clearly shows two separate windings in the power supply primary.I am looking at my unit, and it appears the fuse holder was replaced at some stage. The wiring might have been disturbed here.
I see that C19 is isolated from chassis ground, but C20 is connected to ground. May sound strange but it is. I have two different versions of the schematic and both show the grounding symbol at the negative leg of C20.Regards,
crooner
I replaced the original metal top hat diodes with 1N4007s from Radio Shack, I was careful about following proper orientation. Perhaps one of the diodes went bad because of the heat applied during soldering.I will double check the rectifiers and recheck.
.
Existing AC voltage in the transformer primary, should be doubled by the doubler rectification and filtering.I'll check my wiring. Thanks!
There is just something about connecting the positive side of a cap to ground/common that makes one do it backwards. Besides doublers can be confusing if you haven't seen one before. From reading your posts I would bet the problem is related to the new diodes and how they are installed but do take a close look at cap polarity as well. I am sure you will have it figured out in short order.
Russ
Hi, I guess I would start from the 1N0007s, right polarity, bad units etc etc, since it is there where the HV been produced.
cheers
george
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