DC-2 Rhythym Channel Sounds Weak / Sick

[lookstoomuch]

Hi All! New here, and in need of a bit of help. I turned on my studio caliber which sat for a while and all was normal then eventually the Rhythym channel started sound a bit weak / sick almost like a bit of tremelo was applied. I replaced both power tubes with some JJ's and also did the bias mod by piggy backing the 470K resistor, with no change. So I did a quick recording to demonstrate where I toggle between lead / rhythym.

https://www.youtube.com/watch?v=MhCvysuB1L8

Any ideas?

 

[lookstoomuch]

Well, even though I had replaced all preamp tubes a few years back, decided last night to swap one by one with a new batch I just purchased.

After doing the first one (closest to the reverb connections), believe this is V6 the P/I tube, everything cleaned up and the amp sounds great.

Guess this makes sense as the initial testing I did involved removing one Power Tube at a time and at the time concluded it was a power tube as it sounded OK with one tube in but not the other.

Think this P/I tube issue would also lead to the "tremelo" like sound it was making. I don't think it explains why the lead channel sounded oK but perhaps the distortion just better masked it better.

 

[lookstoomuch]

And just like that, the issue returned. Looks like I jumped to a conclusion to quickly. Seems to be intermittent and happens when toggling between rhythm / lead.

I'll keep hacking away, in the mean time if anybody has any tips....

 

[lookstoomuch]

As a bit of an update, I have since replaced:

- VTL5C9 LDR
- 4N33 Optocoupler
- Relay 1 & Relay 2

I thought this was certain to resolve it based on typical channel switching issues reported elsewhere and the role these play, however, no luck.

To better describe the sound, it is almost a "tremelo" style effect. Pretty sure it's happening in the pre-amp section somewhere as when I plug directly into the return of the effects loop it is not happening.

Back to the schematic....

 

[Monsta-Tone]

You might try a patch cable from the Send to the Return jack on the FX Loop.
Chances are, the loop has been converted to Series (it is Parallel in it's stock form).

 

[lookstoomuch]

You might try a patch cable from the Send to the Return jack on the FX Loop.
Chances are, the loop has been converted to Series (it is Parallel in it's stock form).

Thanks MonstaTone-

I did this and there was no difference, when I patched the the Send / Return the rhythym channel sounded the same....."tremelo" and slightly distorted.

 

[lookstoomuch]

One thing I just noticed when messing with the amp, when I turn the gain up on the lead channel, it introduces some distortion artifacts into the rhythym channel.

Anyway, frustrating. It's so close and can tell it has a better sound hiding in the amp but can't manage to pin it down.....

 

[Monsta-Tone]

Ah well. I was hoping it would be an easy fix!
I'm trying to think back. I know that I had this problem in a DC about 10 years ago.

I think that it was a switching problem.
I'm pretty sure that I replaced the 4N33 opto isolator chips.
There should be 2, most of them are white.

These control the switching in the amp.
It sounds like they are only partially switching the LDR's.

 

[lookstoomuch]

On my DC-2 there is one 4N33 which I have replaced. I also bench tested the LDR's and the one's which came out were OK.

As a bit more information, discovered that when I use a guitar with some hotter pickups and turn the volume all the way up, things improve.

If I turn the guitar volume back down below 7 or so, the tremelo sound comes back.

I've ordered new filter caps and will replace them (and all electrolytics). Yes I'm throwing parts at it but figured it won't hurt given the age of this thing.

If that does not work I talked to a pretty competent sounding tech in the Chicago area and will bring it to him.

Will post back when / if I get this thing repaired and what the resolution was.

 

[lookstoomuch]

In hopes this will help someone in the future, I got the amp back from a tech yesterday. Below is the repair he performed:

"Find and repair open diode in +/- 3 volt power supply". I brought the schematic which is available for ths amp and he said this particular diode was not on the schematic.

Here's a picture of the diode replaced:

 

[Monsta-Tone]

Right on, glad you got it figured out!