HK 330B after recap: No power to FM, crummy sound, help needed!

[exwhyzed]

Hi All,

Thanks to this site (and also avionic), I was able to follow along and repair a non-working left channel of a Harman Kardon 330B. When i picked up the unit, I never paid attention to the sound as it only worked through one side of the headphones at the time. So i can't tell if it had a crummy sound beforehand.

I replaced the large main capacitor, the capacitors and transistors on the amp board, and the board right beside it, and all parts on the two power supply boards underneath.



But now I've created a couple of new problems.

1. There is no longer power to the AM or FM board. This was working perfectly before. I get sound on both channels when i use the AUX input. but its totally dead, not even the signal meter moving, when i choose AM or FM.
I took out each part of the primary power supply board and tested them and they are fine. I even put back in the old transistor and diode but still nothing.




2. The sound of the unit sounds coloured in a weirdish way. like 5% reverb has been added...highs a bit rolled-off. Is this because i need to follow through and swap out the green polyester capacitors on the bass/treble tone board as well?
These capacitors were never mentioned in the (dlucy) guide that i was following, but I read about them from another post, where someone commented that swapping them out to newer films caps brightened the sound.

Could this be happening because i have yet to do the Idling Adjustment and DC Balance adjustment to the unit? I can only do the first one myself. and will have to take it to a technician with an oscilliocope to do the second.

Any insights and help would be much appreciated!

 

[JohnT123]

Re examine your work paying close attention to the soldering. Bridging traces and connections can kill things. Follow back through the supply for the boards with no power and see if you don't have a break in continuity at some point.

There really isn't anything that requires a scope to set up. The idle current and DC offset can be done with a DMM.

 

[CosmoK]

Whenever I perform surgery I always power the unit up on a dim bulb tester. If I messed with the power supply or main amp, I check the idle current to make sure it is extra low. Then I plug the unit directly into the wall and set the idle current. NOTE: I've had some trouble setting the 330B idle current (basically both trim pots on the channel have to be adjusted, do a quick search and you should find the threads). Only something to worry about if the idle current doesn't change when you turn the trim pot. Also, be careful with those trim pots, they can fall apart. I replaced mine with the nice new ones.

To find the problem, I'd get the schematic and start checking DC voltages from the bridge rectifier toward the problem. Voltages are usually listed at transistor legs. When you find a voltage that is way off look around that area. You could have a backward capacitor or an incorrect value. If you have pictures of the working unit verify the capacitor polarity using those. Schematic and silkscreen markings often have errors. Also check continuity on any wires that you tugged on when moving boards.

I wouldn't be too worried yet, almost everything in the 330B can be replaced with a modern part.

I'd be pretty careful about messing with the AM & FM sections, since it's not possible to re-align them without lots of equipment. I.e., think twice before turning trimmers in those circuits or replacing parts.

 

[CosmoK]

Also, I've done 6 or 7 restorations now on different receivers. I am finally starting to recognize what the hell is going on in a schematic. The output of one section is the input of the next and each section is probably a standard circuit that you can find on the internet with an explanation/analysis of how it works.

The reason I say this is because I find it useful to follow the signal or power supply voltage on the schematic, identifying the path and parts in the receiver as I go. This will give you a better idea of what went wrong and why. I.e., if you understand the circuit, it may be obvious that a capacitor is backward regardless of what the silk screen or schematic says.

Anyway, detective work is fun, until it's not. Good luck and keep us posted.