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Messages - DrGonz78

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1
Amplifier Discussion / Re: Marshall 3005/Lead 12 Power Trans Question
« on: August 31, 2021, 03:46:55 PM »
I am not saying the 1458 is healthy by any means. I would remove that 1458 opamp and then measure for voltages at the pads for pins 4 & 8. If the opamp was to blame for missing voltages then with it removed you would still see voltage present at those points in the circuit. If the voltages are missing then we must figure out why.

2
Amplifier Discussion / Re: Marshall 3005/Lead 12 Power Trans Question
« on: August 31, 2021, 03:21:08 PM »
since i can't seem to get a reading for any voltages at pins 4 and 8, and no output after the 1458, should i assume the opamp is toast? a shorted opamp would make it so i get no voltage readings anywhere, wouldn't it?

Actually I would think just the opposite and if the opamp had shorted internally then it would lead to unexpected voltages present on many legs of the IC. I would think there is actually an open condition that exists in your circumstance. Meaning the missing bi-polar power supply that feeds pins 4 and 8 points to a power supply issue.

P.S. Why not start a fresh thread for this repair? It will make the repair easier to follow along as well.

3
Amplifier Discussion / Re: Fender Champion 40 as a modding platform
« on: April 28, 2021, 03:09:00 PM »
Where would the low ass filter go?

I can only guess...??? But please don't post any pictures of it.

4
Amplifier Discussion / Re: Ampeg SS-150H cutting out
« on: April 11, 2021, 03:22:38 PM »
I tried the cable jump method and it seems to work okay, channel a is good and channel b seems a little off but is doable. What causes this issue?

I assume this means you are placing a cable connecting the effects loop in and outs. Dirt, corrosion and worn out metal switching contacts are all possible reasons for the cutting out. But I don't see effects loop on the schematic so I am not sure if this is what you meant.

If the audio cuts out like clock work then it is always possible there is a cold solder joint. I would then input a test signal into the amp and when it cuts out I would scope or trace the signal in the amp. I would confirm preamp and power amp voltage while it is working and after it cuts out.

5
Amplifier Discussion / Re: 1983 Fender Showman Channel Switching Issue
« on: February 26, 2021, 01:59:40 PM »
Post pictures of the board and all the caps that you replaced. Which exact caps were the one's you were unsure during installation?

6
Amplifier Discussion / Re: 1983 Fender Showman Channel Switching Issue
« on: February 13, 2021, 02:23:08 PM »
I think that whoever had opened up the amp before me was trying to figure out if the TL604 or TL607 chips were a problem. I got the amp that way and made for a confusing moment indeed.

Side Note: If you are looking for replacement TL604 or TL607 chips I found a good reputable source. I bought a few to have on hand after this repair. I made a test setup to make sure the analog switching was working and that they were actual genuine chips. The chips passed the test with flying colors.

https://www.utsource.net/itm/p/147198.html

https://www.utsource.net/itm/p/713686.html

7
Amplifier Discussion / Re: 1983 Fender Showman Channel Switching Issue
« on: February 12, 2021, 02:35:21 PM »
The first thing to do is to make sure all those grey colored ribbon cables are making connections from each end to the other end. I would literally check for continuity between each wire on all of those ribbon connectors. They tend to bend and wear out badly. What happens is that the connections become unreliable and introduce many problems between each board's connections. Only after making sure that these ribbon cables are making good connections on each wire can you expect to know they are not the main problem at the start. I attached a few pics of the ribbon cables that I am referring to on that amp.

Update: Make a note that Pic 2 and Pic 3 show a different ribbon cable. I took another cable I had and soldered the each leg to the corresponding pin. I left the ribbon connector on the logic board side and soldered the wires directly to the preamp board. This was the most important factor when fixing this amp in my opinion. There was no way to depend on that original ribbon cable after I ohm out each connection on each wire only to find the connections were failing. Also, note that the original ribbon cable in that spot was actually white.

8
I could not see the pictures you are referring to... However I do see the links...
https://stefancarey.zenfolio.com/p553178115/hF606202E#hf606202e
https://stefancarey.zenfolio.com/p553178115/hF6062034#hf6062034

2. Reverb cables to the reverb box, does it matter which they they go on?
Are the red and black cables on the reverb box (RCA connectors) reversible? That is, does the circuitry mind which way they are plugged in? Tried both ways and it does not seem to make any difference. (I think the term for this is ‘polarity’.) Just want to be sure.

Each RCA cable has a signal and ground feeding signal to and from the reverb tank. Lets guess that the RED is sending signal IN (input on the reverb tank) and BLACK is sending signal (output on the reverb tank) back to the circuit in the amp. Drive circuit (I think is what it's called) sends to the input of the reverb tank and Recovery circuit (again this is what I call it) is the return signal from the output of the reverb tank.

Now as far as troubleshooting if you have tried cables both possible configurations then it appears, at a minimum, that the recovery circuit is not working.


9
Those 1458's are very much the only opamp that will work with that amp. All who try to replace it with a different opamp run into trouble.

10
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. 

11
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.

12
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.

13
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

14
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...

15
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?

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