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HD 130 diagnosis?


Gary Chapman
Posted on Monday, September 20, 2004 - 12:01 am:   

Got my amp back from the repair shop. Got it home, plugged it in, and it distorts. I can hear the fundamental but their is an unpleasant sound. Like a bad fuzz box. With the volume on three, at any kind of playing volume.

I did all the other things to check it out, different speaker cords, instrument cords, tried both 4 and 8 ohm speakers, and different basses. I almost get a clean tone at 2 in the normal guitar channel. Not near loud enough to play on stage with. The bass channel distorts no matter what.
Its going back to the repairman. Would anyone have a suggestion on what the technician should be looking for? I figured that if they couldn't get it right the first time I might get someone here, who has worked on a few of these to offer a suggestion? Any help would be appreciated.

I know it will be worth all the effort, once its fixed right!

Steve Kennedy
Posted on Tuesday, September 21, 2004 - 10:48 am:   

A problem like this should be easy to replicate with an audio generator and visible an oscilloscope. The tech should be able to walk through the circuit (using the schematic) to isolate the stage causing the problem.

Make sure he checks the power supplies first! All it takes is one low voltage supply being too low and everything in the preamp will distort early!


Gary Chapman
Posted on Tuesday, September 21, 2004 - 11:41 pm:   

Steve, thanks for the tip. It will be passed along.

Gary C
Posted on Tuesday, October 05, 2004 - 01:20 am:   

Its been worth the wait . The amp thundered at rehearsal. Any suggestions on favorite volume/tone settings for bass? Can I daisy chain my bass input signal (input#1) from the #2 input to another amp? Is their an owner's manuel available? Again, thank you for your help!
michael kaus
Posted on Wednesday, October 06, 2004 - 03:47 am:   

Well, what was wrong with it? We like hearing about success! Mike.
Steve Kennedy
Posted on Wednesday, October 06, 2004 - 12:06 pm:   

Cool! I'm glad it miraculously starting working as it should! What happened? Did the same tech find the last problem or ????.

It would be most helpful to know what problems were responsible for your symptoms... that way we can all learn from each other's experiences!

Yes, you can daisy chain to another amp from your #2 input. However, you can gain GREAT tone variations by strapping the driven channel to the OTHER channel of the same amp!

This puts two separate preamp sections of your HD130 in parallel. They will interact with each other to a great degree and you can get tonality variations that are impossible to get any other way!

Try it, it is an awesome way to use all those knobs & switches to your advantage! You could even do this AND strap the #2 input of the second channel to an external amp (if there is enough signal level left from your instrument... not a problem if preamped or if you have active electronics).


Gary Chapman
Posted on Friday, October 08, 2004 - 12:46 pm:   

Steve thanks for the tip I will try daisy chaining the channels right away.

Mike and Steve, here are the tech notes from the first repair ?? and then the follow up (warranty) repair.

1st Service Performed:

Tested Unit and confirmed symptoms. Disassembled and troubleshot unit. Replaced transistors, IC chip, in biased circuit. Replaced and rebiased output tubes. Verified all inputs, and outputs. Performed general cleaning, burned unit. Parts replaced: 4 EL34BSTR tubes, 2 NTE331 Power transistors, 1 LM1558 IC Chip.

After returning amp for the distortion problem,

Warranty Service Performed:

Tested Unit and confirmed symptoms. Disassembled and troubleshot unit. Replaced VEAK Zener diode that changed voltage setup during time. Verified inputs and outputs. Reassembled and retested. Unit funtions as designed. Parts replaced: 1N4745 Diode

Any comments?

Gary Chapman
Posted on Saturday, October 09, 2004 - 07:41 pm:   

Round 3: After getting a chance to really play with the amp last night, band not present, I found out that when a note fades out, there is still a real faint distortion that cuts off cleanly before the note stops ringing. Is this OK? I'm thinking it should go back one more time? Am I expecting too much of this amp to produce a clean tone?
michael kaus
Posted on Sunday, October 10, 2004 - 05:39 am:   

There should be no distortion after the fact. I assume you have tried other speakers? It's a long shot but you have to start somewhere. It is possible that you have some transient dist coming form one of your new tubes. Does this amp have a 12ax7 PI tube or is it a SS driver? The driver section can induce some distortion but it's usually of the constant type. I would suggest that it go back one more time. Did the "after" distortion start after the trip to the tech? Check to see if he cleaned and re-tensioned the sockets. These types of things are hard to locate because of the short duration that they occur. You will have to show him and maybe get him to look at the output AS the note dies. Mike.
Gary Chapman
Posted on Monday, October 11, 2004 - 12:42 am:   

"I assume you have tried other speakers?"

Yes, both an 8 ohm 15", and a 4 ohm 2-10" cabinet, switching the ohms to the appropriate setting for each cab.

"Does this amp have a 12ax7 PI tube or is it a SS driver?"

Its is a SS driver, no 12ax7 tube.

"Did the "after" distortion start after the trip to the tech?"

Hard to say because the original distortion I returned it for was so bad I never noticed this before. I did kind of notice it when I tried it at the store when I went back to pick it up but it was explained away by the store repair department manager as a sympathetic vibration in the repair department. It is a very faint distortion. Of course the day I went to pick it up was the stores annual tent sale and the place was a zoo.

Thank you for your assistance on this, I will pass on your suggestions.

Gary Chapman
Posted on Thursday, December 16, 2004 - 05:29 pm:   

I've had the amp back a couple weeks now. They got it fixed right. Quiet, clean tone, the harder you press it the better it sounds. The new tech replaced the original tubes they installed, said they were mismatched. The new set is matched. I believe he also replaced 2 diodes. Jammed with 2 guitars, keyboards, loud drummer, and I had plenty of power.

Guys, I cant thank you enough for your insight and suggestions. You have been very helpful.