Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => The Newcomer's Forum => Topic started by: fuzzbuzzfuzz on October 19, 2015, 07:01:21 AM

Title: Univox 65 - Hissy Reverb?
Post by: fuzzbuzzfuzz on October 19, 2015, 07:01:21 AM
Well firstly - Hello All  :) My frist post, and I hope not my last.

I have a couple of Japanese SS amps - ELK 103 Viking (Head), ELK 75 Bass Head and Cab and as from yesterday a Univox 65. And I love em!

The last one is my query of today which perhaps experienced minds can help me with  ???

It has a slightly higher than normal (what I consider to be normal) background hum when in use, but in particular the Reverb is rendered almost unusable when turned on, even a 1/4 of the the dial leads to a pretty loud hiss/white noise.

I just took the whole shebang apart. Pretty clean and tidy inside, no loose cables or dodgy connections when poking as far as I can see. The reverb unit (which I believe to be spring reverb) is housed in a rubber/foam sealed housing for isolation.

So where should I perhaps start. Cap replacement? Or some amateur shielding?

Here's a YouTube video highlighting the problem:

Many thanks!
Title: Re: Univox 65 - Hissy Reverb?
Post by: Enzo on October 19, 2015, 02:33:28 PM
Bad caps generally result in hum.  Lack of shielding usually results in hum.  You have hiss (white noise).  So your recovery amp is noisy, and most likely is a noisy semiconductor.  You did not post or link a schematic, so I can't recommend specific parts to consider.  I am guessing it is an op amp, if not then a transistor and a simple substitution will tell us if I am right or not.  Does the amp use ICs or is it all transistors?
Title: Re: Univox 65 - Hissy Reverb?
Post by: galaxiex on October 19, 2015, 09:09:36 PM
Is that a U65RN?

Sure looks like it, and I think it is all transistors, no op-amps.

I might be able to dig up a schem...

ahhh, here it is...
Title: Re: Univox 65 - Hissy Reverb?
Post by: fuzzbuzzfuzz on October 21, 2015, 10:11:08 AM
Yes! Thank you, that is indeed the model  :cheesy:

All transistors.

Looking at the circuit board/scheme what would perhaps be the best place to start?

If necessary I shall disassemble it once again, unscrew the board, flip it and take a picture.

Many thanks gentlemen, please keep the advice coming, much appreciated. :tu:

Title: Re: Univox 65 - Hissy Reverb?
Post by: phatt on October 22, 2015, 06:16:48 AM
Thanks to *galaxiex* for the schem :dbtu:

My first impressions; I doubt this reverb design will ever be great,, I'd say it will always tend to be noisy.  Not enough drive and too much gain on pickup end and you just get NOISE. :grr
The pickup end of reverb is rather odd in that the transducer is floating above ground and that in itself could cause hum buzz issues. (better minds may know more)

Meantime stuff to check,,
Ground plane and the way they are wired is very important here.
Note how the Drive circuit goes to common? Well the Rev tank Case must Also be at common. (check for continuity between circuit board common and reverb case.
If that case is not grounded well it will certainly make lots of noise. Also Check the cables are not broken.
Title: Re: Univox 65 - Hissy Reverb?
Post by: Enzo on October 22, 2015, 05:14:38 PM
The pickup coil has a relatively low resistance, and directly coupled to the transistor base, so they lifted it off the ground with the voltage divider so as to bias the transistor base.

The design may not be optimal, but there is normal noise, and then there is excess noise compared to normal.  A noisy transistor will add a lot of hiss to whatever naturally exists.  For the few cents a transistor costs, it is worth substituting a new one in there.