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Messages - DrGonz78

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1
Tubes and Hybrids / Re: Vox vt120+ combo schematic needed
« on: September 29, 2020, 01:57:13 PM »
Looking through the VT80+ schematic C184 & C185 are near the speaker output. Obviously they are different circuits but I am curious where in the circuit those two capacitors are located on your amp board. Look at page 4 quadrant H2 of the VT80+ schematic and locate C184 & C185 the 0.68uf 100v film capacitors. Look on your board near the burnt out caps to locate two 1000pf 100v caps (C146 & C147) that run in parallel with those caps. May be a long shot here but perhaps these two schematics share something in common or possibly not. Can't hurt to check. 

2
The Newcomer's Forum / Re: Roland JC-120 with weak reverb
« on: September 18, 2020, 02:33:32 PM »
What is the serial number on the amp? I ask only that we can first help identify the correct schematic to use.

3
The Newcomer's Forum / Re: Fixing a Yamaha G100 112 ii
« on: July 16, 2020, 02:35:22 PM »
UPDATE: I had posted not accurate information about this amp, refer to post 12. This amp has a cap coupled output and without a load on the output it will float at 50v as the cap will not be able to charge up. I must have missed this bit on the schematics and did not realize. Hopefully this post will prevent any further confusion if referencing this thread while working on their amp.

4
Guitar News / Re: Coronavirus hitting the music industry
« on: March 13, 2020, 02:53:25 PM »
There have been reports of patients that were deemed recovered from the coronavirus only to test positive for it again after being discharged. This could of been missed the first time since the testing equipment might be to blame and over-crowding hospitals led to a desire to discharge patients too quickly.

However, there is one article I remember reading in which they stated "the body does not become immune after infection." Something about antibodies produced during infection not being strong enough to provide future infection from the same strain. Honestly, I think it's all too early to say since most of that data came from China, who were very overwhelmed by the virus.

https://www.businessinsider.com/wuhan-coronavirus-risk-of-reinfection-2020-2

5
Amplifier Discussion / Re: Marshall MG 50DFX transformer
« on: February 28, 2020, 03:06:27 PM »
One more thought about this... I am not certain but does this transformer have a thermal fuse on the primary section built into the tranny? Since the primary is reading open it makes me wonder if there is a thermal fuse that blew open. I have read many times where people went into the transformer and either replaced/removed the thermal fuse to get it working again. I have never had experience dealing with such problems but I know others here have. Sometimes it's even a bad connection point right at one of the wires leading into the tranny. Just a thought...

6
Amplifier Discussion / Re: Marshall MG 50DFX transformer
« on: February 20, 2020, 02:48:25 PM »
This question will be asked 100% of the time and should be... Is the transform bad or missing? If bad>> How have you tested to know its a bad transformer?

7
Amplifier Discussion / Re: Peavey Stereo Chorus NEW noise problem
« on: February 15, 2020, 02:34:57 PM »
Here is the Stereo Chorus 212 1989 version...

8
Amplifier Discussion / Re: Ibanez TSA30 - buzzing
« on: November 10, 2019, 02:58:37 PM »
Welcome to the site  :dbtu:

Well here is the schematic... Please look at the components where there is a capacitor between signal ground and chassis. Please let us know which caps on the schematic so we might have a better way of visualizing what you are thinking about that one. Also, I am confused by " amp randomly buzzes when a cable is connected to TS/boost jack" since all I can see is there is a boost switch but not a boost jack.

Anyway we know there is a random buzz occurring and that you twist some cable to make the buzz disappear. Which cable is this one? The Input cable from guitar or cable for footswitch?

Schematic>>>
https://www.ssguitar.com/index.php?action=dlattach;topic=4011.0;attach=6242

9
Amplifier Discussion / Re: Acoustic 220 quieter volume than expected
« on: November 07, 2019, 03:23:10 PM »
I am actually not familiar with this model and have not worked on one before. Basically we just need to break the amp up into sections, Preamp & Power amp. If the preamp is putting out a good healthy signal then the problem is pointing to the power amp and vice versa. Conquer and divide is the term for such an exercise and we want to know where in the circuit to focus our attention.

Edit: Typical issues are sometimes good to look at but can easily get us looking the wrong direction for the fault in the amp. Best to troubleshoot in a logical way first. Once we determine if the amp has a preamp problem or power amp problem then the next steps to to troubleshooting will follow. For example, we might have a power supply issue where signal is not passed through either the preamp or power amp. So honestly testing for power supply voltages might actually be the very first step in the process. Since testing preamp and power amp is simple we sometimes just jump right in to check that first.

10
Amplifier Discussion / Re: Info on Sunn Stinger 100 amp
« on: November 06, 2019, 02:53:46 PM »
I think that EL34World website download is the Japan version. Either way just email Stratopastor and he'll help you out, he's a nice guy.

11
Amplifier Discussion / Re: Acoustic 220 quieter volume than expected
« on: November 01, 2019, 03:49:22 PM »
First thing to check is the preamp output and poweramp in mono jacks. The main instruments signal path passes right through those jacks. Often those jacks get dirty and the signal starts to fade as heads through to the output section of the amp. Easiest way to test the jacks is to just simply connect the two jacks together using an instrument cable. Does plugging this patch cord from preamp out to poweramp in make any difference the volume?

Later you can connect the preamp out (Acoustic 220) to the poweramp of another working amp to test the preamp section. Then take the preamp out of another amp (Guitar signal, iphone preamp works too) and input it into the power amp input of the Acoustic 220 amp. That way you can find out if there is a problem with the preamp or poweramp.

https://elektrotanya.com/acoustic_220_guitar_amplifier_sm.pdf/download.html

12
The Newcomer's Forum / Re: Peavey Audition Plus
« on: October 16, 2019, 03:43:19 PM »
Look inside the amp chassis and confirm if there is a 2030 type opamp power IC. Not sure how closely related your amp is to the schematic posted but we'll find out as we go. Start by confirming the 2030 and then we're off to the races.

13
Amplifier Discussion / Re: Match Output Transistors?
« on: August 15, 2019, 03:51:49 PM »
Recently, I did a purchase order from Peavey and was a bit surprised at the cost of the MJ15024 & MJ15025 transistors, $10 each. I ended up buying them from Mouser for about $5-6 each.

14
Amplifier Discussion / Re: Match Output Transistors?
« on: August 14, 2019, 04:09:24 PM »
When I have one failed output transistor failed in a set I typically will replace the entire set. Not only that I will replace the drivers and pre-drivers that associate with those output transistors. You want to get it just working again but you want it to keep working for many years to come. There was one case where I had replace 3 output transistors on one side of a PA system and left the working originals in place. That amp still works to this day and that was over 5 years ago. In that case I was feeling mighty frugal and got really lucky. I still recommend against trying to get lucky. 

15
Amplifier Discussion / Re: AMPEG B2R Problems-Help Please!
« on: August 13, 2019, 03:39:16 PM »
Amps with actual electronic problems generally don't "magically come back to normal".

I sure do miss Roly. Very good quote to get stuck in your head. I have many statements like this one in my memory bank from so many great techs I have met here and other places online.

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