Quote from: nosaj on October 22, 2016, 01:42:50 PM
Tell me how it's a calculated guess when your troubleshooting an amp based on other threads.   You for get some of the people here have been in the business for a while and a cost benefit thing to them is to replace only the bad part.  If the test are not reliable may it is the person performing the tests.  These guys will not steer you wrong when giving you a procedure.

If you want the best chance at success with the least effort maybe you should take the amp to a qualified tech rather than bucking good advice.

my 2cents and then some

nosaj

I don't understand your first sentence, are you suggesting that I follow advice on this thread but discount advice regarding similar issues with the same amp on other threads?

I know how to troubleshoot the amp. That's not what I want to do with this amp. Time is a cost, troubleshooting is time consuming, it will probably cost a lot more in time to chase down an intermittent issue than the cost and time to replace two caps and a transformer.

A qualified tech would tell me unless it's quick and easy, it's probably going to cost more than the amp is worth to find and fix the problem -- which is what I'm trying in vain to explain on this thread.

I appreciate the expertise on these forums, on this particular occasion I was asking for some advice about a transformer upgrade.

Quote from: Enzo on October 22, 2016, 12:45:49 PM
Fair enough, but the simple tests I suggested take very little time and might tell you something.  Why replace a $30 transformer when it needs a $2 jack?

It's a cost/benefit call, from what I've read upgrading the power supply fixes this amp for a number of people. There's a great many other things it could be, but that takes time to test, and some of those tests are not reliable, meaning I may end up replacing other parts which test intermittent and not fix the problem and start sinking time into it. Upgrading the power supply components will, without doubt, make this amp more reliable. If it turns out not to solve the issue I'm experiencing then the risk I took will have bitten me.

Most testing and diagnosis result in a calculated guess as to the cause of the problem, that's all I'm doing. For an amp that routinely goes for $50, I'm hesitant to pull anything off the board unless it has a fair chance of fixing the issue -- which without much testing, upgrading the power supply would seem to be the best chance at success with the least effort.

Sure, cleaning some jacks is certainly in order, but from what I've experienced with the amp I don't believe it's them.

The amp is not pricey, and not that important too me, numerous folks have reported upgrading the caps and transformer solves the type of issues I'm experiencing with it, ie, sporadic volume drops and cut outs.

I cleaned the jacks, visually checked components, and the amp works, just here and there cuts out, volume drops, which seems like it could easily be the amp stressing the power circuit and IC's taking a powder because they're under powered. If upgrading these components doesn't fix the issue I may not bother with more because extensive troubleshooting is probably not worth it for me, for this amp.

I'm reading 22.3 VAC across the output of the transformer, so I guess it's a 110VAC to 24VAC, not sure what amp rating I should be looking for.

I picked up an MG30DFX, for free actually, because it had intermittent issues for the previous owner and they got fed up. I've read it's most likely an weak power supply.

This thread folks recommend replacing the power caps and the transformer.

http://www.marshallforum.com/index.php?threads/marshall-mg30dfx-problem-volume-fade-instability.36882/#post-1346279

The caps are no problem, but I can't find any specs on the transformer. I don't have much experience with transformers specifically, the markings are sparse and I can't find a part number. Obviously it's not the best amp, but would be nice to have to take places where folks might abuse it. Any idea what would be a good upgrade for the transformer in this amp?

http://www.amparchives.com/Amp%20Archives/Marshall/Schematics%20&%20Layouts/MG%20Schematics/Marshall%20MG30DFX.pdf

Replaced Q13 with the KSC3503, had to put it sideways due to the different pinout -- and CH1 came back to life! So I guess Q13 was bad after all, even though it tested alright. Thanks for all the help everybody, especially phatt!

https://youtu.be/vbaU8HK2h1I

It's got some noise oscillating, but I'll sort that later -- it's got chorus!

For now I've ordered some KSC3503 DSTUFS-ND to replace Q13's 2SC1628Y. Different pinout but should be able to make it fit.

I did test D1 out of the circuit and it's fine. Just in the circuit it's readings differ from the good channel's D1.

As for Q13, the diagram calls for 2SC1628 with EBC pinout, I had asked earlier in the thread (not noticing the pinout) if I could use a 2SC1624, which is BCE pinout. I had hoped a couple days back that this was my issue, however after testing the original and swapping back to it the problem persists. Its orientation matches that of the good channel Q13. Attached in an image of the good channel, CH2.

One of the things I've noticed is that when I test D1 on the bad CH1 board I get 1.8 M voltage drop the wrong way when using the diode test, the correct way is .56, off the board I get OL for the wrong way. On the good CH2 board I get OL the wrong way and .56 the right way. I've substituted a 1N4148 for the IS2473 on the bad board. I've checked the traces around D1 and they all seem fine. Getting a reverse drop on a good diode mean anything to anyone? Apologies if I'm using the wrong wording for the situation.

Quote from: phatt on September 25, 2016, 08:37:27 AM
Ouch,, If all actives have been replaced then a failed Feedback resistor R66 (47k) can cause that problem. If it reads ok  then check for broken tracks which connect to the nodes either side of R66.
Phil.

R66 tests fine and its connections to adjacent components are good. Attaching a photo of the board as it stands in case there's something obvious that I'm missing.

Here are the voltages in the area, the bad channel in red, the good channel in green.

Update: Added more measurements.

Quote from: phatt on September 24, 2016, 07:12:15 AM
Then print out that power amp schematic you posted and write down all the voltages from the working circuit and then cross reference those voltages with the faulty circuit. Somewhere in that first section you have a failed part or broken track or even a cold solder joint.
Hairline cracks in the copper tracks can be right next to the solder and are hard to see. Bright sunlight and a magnifying glass will help.

Could even be a tiny fray of wire bridging a track or solder point which can't be seen by eye, I've had that happen before.
A good gentle scrub of the track side with a tooth brush can dislodge those offending little critters. :tu:
Phil.

I've done the toothbrush thing, but yeah there's been a lot of debris with all the de-soldering. I'll try more voltage comparisons as you suggest.

Quote from: phatt on September 23, 2016, 08:59:49 AM
Then Q11 & Q12 are likely the problem.  Check the pinout, the replacements might be different to the originals.
Phil.

Unfortunately the pinout is the same and the transistors are all good (old ones and new ones). Haven't found any additional failed components on that board.

Quote from: phatt on September 23, 2016, 08:33:32 AM
Yeah,,, BUT How do those readings compare with the working amp voltages?

I assume you mean that the 6 Volt side of R63 is the Emitters of Q11 & Q12?
6VDC on the emitters seems high,, I'm guessing it should be less that 1Volt.
If Q11&12 are working you should have about ~100mV on the bases of those two transistors.
Phil.

CH1 (bad)
R65 30 VDC
C32 0 / 30 VDC
R63 6 / 30 VDC

CH2 (good)
R65 31 VDC
C32 .001 / 32 VDC
R63 .8 / 30 VDC

R65 reads 30 VDC, C32 reads 0 VDC on the negative side and 30 VDC on the positive, R63 reads 6 VDC on one side and 30 VDC on the other.

I've also verified that the pinouts are the same Q11 and Q12, (replaced 2SA970GR with 2SA970GR) they're in the correct orientation and their connections to the next components in the circuit are good. Could it be that Q11 went bad as soon as I replaced it due to the bad cap on C32?

I get 0.6 VDC coming out of R1, 16 VDC out of Q1 and 25 VDC coming out of Q2, not the output Q1/Q2, the 2SK117's near the inputs, that's very early in the signal path no?

Replaced all the electrolytic caps on that board, C32 was definitely burnt, but replacing it (and the others) hasn't changed the behavior.

R61 and R68 are connected and have 0.5 ohms to ground, same on the good channel.