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Messages - Dimi Pana

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The Newcomer's Forum / Amp repair?!
« on: January 23, 2019, 12:35:01 AM »
I have an Epiphone Valve Junior Combo amplifier that needs repair. It used to play fine until I decided to swap in/out a few different tubes both at pre and power section. I suspect the repeated removal/insertion of tubes and retention spring must have done something to the socket. I simply cannot turn the amp on and start playing and in less that a few minutes there is a weird crackling noise, then some hiss/static noise and then silence. I then check the tube with a tester it's dead, both the heaters (pins 4 & 5) show no continuity. I have ..."lost" two brand new tubes so far, so I am not risking putting another in there unless I identify the problem. This is a very simple design the only thing that could be the culprit is a resistor and cap and of course the voltage that is provided to the power tube. BUT, there is absolutely no problem at all with the preamp tube (which I also swapped in/out many times) so I am kinda stuck. I can follow instructions, I am good at soldering, I have a decent DMM and tube tester (albeit borrowed from a friend), I understand electronic schematics although definitely not an expert. I believe I can do the troubleshooting with someone's guidance. It is simply not worth paying a tech to look at it, it will probably cost me more than the amp's value. I am also posting the schematic, however at this early point I am primarily concerned about asking questions in a Solid State forum about a tube amp. Let me know, I hope I did not break any rules. Thank you!

EDIT (1/23): I uploaded another schematic I found online, the tube section is identical but there are some "interesting"(?!) differences in the power supply. Indeed the transformer outputs 6.3 VDC for the heaters obviously after the bridge it should be a bit higher, so indeed the first schematic is wrong (we already know that).  ;)

This is a truly priceless relic you got there!!! I suspect you have some flavor of the early IBM "memory units", namely starting in the early '60s with the IBM 1311 all the way to late '80s the last iteration of 14'' disk modules, the IBM 3390, I go by memory so take this info with a ...grain of salt. Indeed, these (aluminum) disks used -pretty much- the same technology (Ferrite) found in tape recording. I'd love to see a good resolution picture especially with those scraped areas clearly visible.


Anyway, back to our "parts salvaging" discussion, I am posting pics from a basic power supply I removed from a portable Karaoke box. Under no load the output is 10.09 Volts DC. As you can see it is a pretty basic design for a switching p/s based on a SVD7N60F and a MBR20100FCT.

So, am I right to assume that this is a (nominally) 9V p/s? What else can you tell me about it? How can I test/estimate its max power output? A sticker on the back of the Karaoke box stated as its power requirement 45W. Logically the p/s should be able to provide up to that much, so at 9V current should be no more than 5A, correct? Come to think about it, do you think this can be "used" as a pedal board power supply? I mean if it powered a 45W karaoke it should be more than enough for up to 12 stompbox pedals, right/wrong?

Thank you!

For a smaller project, I sometimes tear down a hard drive.  The disc is so shiny.

I am sure you had a lot of success finding new uses for these "shiny" disks. Personally, I like making "wind chimes" with them. Care to share your experiences? Cheers!

Thanks for the thoughtful reply, honestly, it never crossed my mind you were being harsh, actually adherence to verbal accuracy and proper choice of words is something I always admire.  ;)

To the point, I work as an IT guy for a company that buys, refurbishes and resells, wholesale, equipment from restaurants that closed. So you are 100% correct, these PAs came from old restaurants.

The PA amps I've "salvaged" are indeed NOT obsolete so I stand corrected.



To my understanding, obsolete means "no longer produced or used; out of date." Which in this case is absolutely true. If an amp "does not amplify" then I guess is not necessarily obsolete but ...well defective or broken.

Anyway, I read somewhere in this forum by one of the more experienced members that salvaging parts from obsolete, unused or in some sort of disrepair, old equipment might be worthwhile, either for using as hard to find old spare parts in repairs, or simply for building projects without spending much time or money searching.

So, about the D718: Is that a chip worthwhile salvaging? Can it be used to build a simple power amp with minimal components?

Hello -

I have recently come across a few old RadioShack mono PA amplifiers at my work which were supposed to be thrown away so I decided to take them home myself and see what's inside.

Each of these PA amps had (among many others) the following parts inside:

1. Two power output transistors of the D718 type mounted on a huge (e.g. worth saving) heatsink.

2. One NJR4558D dual op amp.

3. One output transformer with 4/8/16 Ω connections

4. One power transformer rated at approx. 80W.

I was wondering if these boxes are worth "cannibalizing" for parts. The RadioShack manual claims this is a 35W mono PA amp with 70V capability (not sure what that 70V means). The 4558 is of course a very cheap but certainly usable IC plus due to the age of these boxes it may worth a bit more (could it be an original 4558 ?). The D718 transistor, I was not able to find much on that, except the attached datasheet. It appears they are either used in power boosting applications or as output transistors in power amps (as in this case).

Can you please advise if there is some justification in salvaging some parts? IMO the boxes and the buttons/pots etc on them can be reused in other projects. Nothing serious, just amateur DIY stuff and mostly for fighting ...boredom while learning something new of course!

Thank you!

Amplifier Discussion / Re: Modifying a First Act MA104
« on: November 13, 2011, 04:19:36 PM »
OK, if you have not had enough of me today, just one last question, which I should have asked from day one:

What would be a FREE, easy to use, i.e. no steep learning curve, PC program to draw schematics with?

I mean, I've tried MS paint but -obviously- this program is not made for that.

Thank you!

Amplifier Discussion / Re: Modifying a First Act MA104
« on: November 13, 2011, 03:25:46 PM »
Hello all!

Has anybody been able to open the MA104Final.png attachement Koreth has posted in the very beginning of this topic?

I am trying to compare his schematic with the one I have traced from my MA1248 amp and I cannot open the file, every program I tried say things like "invalid format", "corrupt file", etc.

Can anyone repost this file please?

Also, Koreth, is this your final, I mean FINAL, version?

I've compared my MA1248 with the MA107 (schematic posted earlier by DartPlayer170) and even a novice like myself can see there are differences, e.g. some R,C values are different but over all they are very similar.
I also know for a fact that the MA104 and the MA1248 are identical, at least two units I compared recently side by side. Koreth has mentioned that his is using the TDA2003 while mine is using a TDA2030 (and yes, as stated before this is not a typo) plus from another schematic (posted here: it appears it's using two JRC4558 instead of one. So I am inclined to believe that the MA104 Koreth bought back in 2009 is slightly different than a MA104 currently sold by first act.

I reviewed both data sheets:



and other than 4Watt ouput power difference, both seem similar to me.


So what I am trying to do is compare side by side three schematics:

1) Koreth's (year 2009) MA104

2) DartPlayer170's MA107


3) Mine MA1248

Once I have the closest match I can produce a final version of the MA1248 schematic, which will help with all future discussion.


Anyway, is is possible -anyone- to repost the MA104Final.png schematic, please?   

Thank you all!   

Amplifier Discussion / Re: Effects Loop Questions
« on: November 10, 2011, 03:32:59 PM »
Hello -

I do not mean to distract you from this very interesting conversation, I just have a quick question for all but especially for Enzo and what he said here:

Most FX loops are simple breaks in the signal path through the jacks.  Some fancier ones have a little drive circuit for the send.

I have a very cheapo practice amp I am experimenting with and being a novice, destroying basically a toy is not the end of the world. ;)

It appears that this amp is build around a JRC4558 for the preamp and a TDA2030 for the power amp (please show mercy I use the terms loosely. :-[ . There is also a FET at the amp input acting -I think- as a buffer.

The toy-amp has one Volume, one Gain, and one Tone control. 

Assuming that I can identify the point where the signal is handed-off from the preamp to the power amp and given the cheap-o-ness (read: simplicity) of the design, is it safe to assume that simply by cutting into the pcb trace at that point[/u] and inserting two 1/4" audio jacks as SEND/RETURN will provide me with a crude yet functional FX Loop ?

From Enzo's post, I understand that some (more sophisticated) FX loops use a drive stage at the output (i.e. return) to compensate for any signal loss. In my case, this is not applicable, I can't build a drive stage plus I think I do not need one.


I plan to use an FX unit that has separate stereo in/out level controls, plus a mix pot DRY/WET and -if the unit documentation is not lying- a true bypass foot-switch.

So, what my diy FX loop will be lacking, the FX unit will provide thus compensating for the crude hack.

I tried to put the FX unit first, e.g. after the guitar and before the toy-amp but it sounds, well, just bad...

Again, sorry for the intrusion, aoresteen is doing exactly what I want to do (of course mine is extremely simplistic) so I was hoping to ask if this would work.

Thank you!         

Amplifier Discussion / Re: Speaker impedance matching
« on: November 08, 2011, 12:46:24 AM »
Very interesting thread, I wish more will contribute. Funny thing is I was recently looking at this --> website while researching on cabinet speakers.

I have a parallel question to what you're discussing here.

Say you have to chose between a single speaker or a combination of 2 (or more) speakers to achieve the desired Ω resistance for your amp. If -for example- a 2X8" combination has -almost- the same (cone) area as a 1x12", or to make the question more ..."provocative",  if a 4x4" has exactly the same area as a 1x8", why the 1x12" would be the choice of most people. Is it the better bass response of the larger 12" speaker or what?

I am not sure how I come up with these questions, probably because for the last few days I've been sick as a dog and have too much free time just lying in bed fighting this nasty cold. :'(


Amplifier Discussion / Re: Noisy Cricket problems!
« on: November 01, 2011, 02:04:30 PM »
ΟΚ, this is probably a waste of time for you all, it was a good exercise for me.

Here's what's going on:

There are two build versions posted by BeavisAudio. One is a PCB based one the other is the RadioShack (RS) version. BTW, I am building the RS version since at this point I do not have what it takes (knowledge and tools wise) so I cannot print and etch my own PCB.

Each version includes (at least) the following:

1. Parts List

2. Schematic

3. Parts Layout

Here's what the error is (regarding the V, G and T pots):

In the PCB version:

The "Parts List" agrees with the "Schematic" and both disagree with the "Parts Layout"

In the RS version:

The "Schematic" agrees with the "Parts Layout" and both disagree with the "Parts List".


Of course, all this is immediately recognizable by all you experienced people, unfortunately in my case it is a "show stopper" since I realize the discrepancy but I am unable to distinguish between RIGHT and WRONG.


However, by reading your posts and doing some research myself, here's what I will do:

VR1 = Volume = 10KΩ

VR2 = Gain = 1KΩ (*)

VR3 = Tone 100Ω


Νow, because, RS do not carry a 1KΩ pot, and since phatt(*) already suggested to change it with a 10KΩ,  is it OK to use -instead- a 5KΩ ( just happen to have one)  so I do not have to buy it? I do not want an explanation why (to save you time) just a YES or a NO.  ;)


Last question for today: This design calls for a 100nF "poly film" capacitor at C3. JMF suggested I change it to a 10μF (right?). I cannot find this value in film but I can in electrolytic. Can I substitute film with electro? Again, I am asking for a YES or NO answer, obviously I'm interested in the WHY, please answer only if you have a short and simple one, anything longer just post a link on the net, let me then do the reading.  :tu:


How am I ever going to pay you all back for your patience, kindness and your hard-acquired knowledge offered for free? My family owns a small "Bed & Breakfast" motel in the Greek island of Santorini, so if you ever visit my country, please drop me a line, I might (actually for sure) be able to work something out for you. But please hurry, given the current financial instability and judging by the (poor) quality of the Greek politicians, I am not sure if in the near future Greece will still belong to the Greek people or owned by some big, inconspicuous financial conglomerate.  :'(   

Amplifier Discussion / Re: Noisy Cricket problems!
« on: October 31, 2011, 07:10:06 PM »
Hey, how about this:

Looking in the NC schematic (RadioShack build or PCB build):

VR2 is the Gain pot, rated for 1KΩ

VR3 is the Tone pot, rated for 100KΩ


Looking at the parts list (in both builds) it appears that these values have been inadvertently reversed:

Gain is still VR2 but rated at 100KΩ


Tone is still VR3 but rated at 1KΩ.


So which is correct? The Schematic or the Parts List?

(Pls, see attached!)

Amplifier Discussion / Re: Noisy Cricket problems!
« on: October 30, 2011, 02:47:27 PM »
Hello all!

Ι was building the NC today and I think I got a little impatient (i.e. careless) towards the end, I believe I "fried" the MPF102 with too much soldering heat. I wish I'd bought a socket for it, just like I did for the LM386 IC. I mean there is a socket for this part, right?

I bought my parts from RS (radio shack) and that particular store did not have a socket for the transistor. I mean the attendant in the store I went (Rt. 83 in Des Plaines) probably knew less about electronics than -even- me... I don't know, from the first time I stepped into a RS, it did not feel right somehow.

So, anyone from the Chicagoland area? Do you know a decent and novice-friendly (yes, that's me!) retail electronic parts store?

Thank you!     

Preamps and Effects / Re: Mono guitar to stereo effects to 2 amps !?!
« on: October 29, 2011, 04:51:47 PM »
We are in DIY SSGuitar here, want to keep things within our average Member´s reach.



Funny, probably while you were typing your last message, I found this:  <-- (look at the last post of that thread)


I was about to ask you if that's what you meant BUT you beat me to it!

I learned a lot today, I am getting off of my computer NOW, or I 'll be abusing your kindness.

Muchas Gracias!

Amplifier Discussion / Re: Noisy Cricket problems!
« on: October 29, 2011, 04:38:10 PM »
C3=10μF and no pin7-5 circuit at all.

Shall we say this is FINAL?

(at least for today?) :D

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