Marshall MG 100HDFX-Clean switching?

[retrorod]

No worries! I appreciate ya' getting me this far without the amp in front of you. I have more of a grip on what I,m shooting for thanks to your help. I have ordered the DG212CJ and I will do some further testing. Man, I long to work on a '65 Super Reverb....! Guess I better get over it! This is what its come to....

[retrorod]

Marshall MG100

Hey again. I thought I posted a 'no worries' comment after your last one. I must not have sent?!
Update** The zener diodes test good. I managed to unsolder and remove IC2 and rechecked the voltages. Still the same as before.
I have ordered another DG212CJ. When removing the IC2(What a pain), I noticed that it is marked DG212BDJ. Not sure if that is significant!!
At any rate, I know it is difficult to diagnose something that you don,t have in front of you, and I am pleased to have got this far. I now have a better understanding of the function of IC,s and the way they are notated on the schematics!
Thanks and I will carry on and let you know of the outcome. Meanwhile, I take comfort in knowing that there is an old VibroChamp coming in next....something I can better wrap my brain around--Ha!

[Jazz P Bass]

Marshall Channel Switching

If after removing the IC you are still getting no change in Sw B input, there is something wrong.
On page 2 of the schematic there is the switching circuit.
It has 5 Vdc on all the SW A B & C.
TR1 2N2700 is a MosFet n channel switching transistor. Marked D (drain) S (source) & G (gate)
S is at ground potential.
D is at +5Vdc.
When the gate (not marked G) goes high, SW B goes high, D now goes low (to ground) which should pull SW A low through the 100K resistor.(R34).
When the gate goes low, Sw B will be low & SW A will go high.
Number 1: is the gate going high & low when the channel selector is activated.
If it is, and SW A & SW B are still staying high, then the TR1 is at fault.

[retrorod]

Thanks for the last info, I believe your on to something now. I am checking this out and have parts on the way. Will let you know as I progress, Rod

[retrorod]

Hey, Thanks Coach, I'm happy to report that TR1 was indeed at fault! I am now getting channel switching and panel light goes on/off as its supposed to. I haven't messed with the amp much, but have not really dialed in 'nice tones' yet. Perhaps, its 'operator error' or other issues with this amp.
Should I assume that the audio amp TDA7293V is a 'functioning or non-functioning' device? Or are there 'degrees' of 'compromised function'? It come up as an issue in other posts.
Thanks again, Rod

[Amp Kat]

I would change the footswitch jack and the problem may go away.

[Prairie Dawg]

That unfortunately is normal from what I've experienced
[ATTACH]7704[/ATTACH]
Here's the scat. Hope that helps.

Remove the fan. Peel the paper label off of it and you will see the bushing that the shaft rides in. Place a single drop of sewing machine oil or 3 in 1 on the end of the bushing and twirl the fan with a little jet of air from a can of dust off-not much. Then replace the paper label and reinstall. Your fan will be nice and quiet.

[captainlee]

I was thinking of adding a additional larger fan????

Since the unit needs cooling I am thinking of adding a fan to blow air inside of the metal box under the circuit board this should cool the amp even more.
also thinking of putting a heat sensor with a readout on the front panel to keep an eye on it with a led that comes on when temps start getting higher.????
any other suggestions from marshall MG 100hdfx owners would be appreciated. Cap

[Prairie Dawg]

Since the unit needs cooling I am thinking of adding a fan to blow air inside of the metal box under the circuit board this should cool the amp even more.
also thinking of putting a heat sensor with a readout on the front panel to keep an eye on it with a led that comes on when temps start getting higher.????
any other suggestions from marshall MG 100hdfx owners would be appreciated. Cap

I don't own one but I worked on one for a guy and I had it here a few weeks. The only place that a fan is needed is right where it is, because the power amplifier chip generates a lot of heat and if not cooled will destroy itself in short order. That fan is dedicated to this one task. You might put a temp probe on that heat sink and see how hot it's actually getting and see how close that is to the rated maximum, which is something I did not do. If I owned one I would probably have a spare fan handy for emergencies-it's easy enough to change.

[retrorod]

Yes P Dawg, You hit the nail on the head. Spare fan and spare power amp. while ordering parts, I ordered a spare power amp. After reading all the posts about this amp, that seems to be a frequent problem. Oiling the fan shaft was I good idea also...to quiet and run efficiently.
My problems with the MG100 seem to be over for now. Hope these threads will help others as they have me....Rod

[captainlee]

How to baby your MG100HDFX

Retrorod where did you order a spare power amp and how much was it??


I think it does little good to have a back up fan if the fan takes a dump it will fry the chip so better to have a fan built in that will come on when it senses a high heat rise and should have a led on front to alert the user that the secondary fan has come on. how hard would it be to fashion such a device, and how much would it cost if we bought the components from a supply house?? maby a temp sensor for the heat sink with a read out on the front would be nice after all this is a computer like component.cap.

those are my thoughts what yours???

[retrorod]

My thoughts on adding a second fan? Not necessary, and too much trouble! A 'backup replacement' fan would be good too. I suppose if the IC amp burns it would be because of a non-functioning or 'dust clogged' fan. I just ordered a back-up power amp IC. I do some amp repairs. Since that IC seems to be the 'achilles heel' for these amps, I suspect that one day I will have another to repair! I ordered it from Mouser Electronics. It was $4 but they really jacked me for shipping. I ordered 3 other light parts. Total was $11 something....shipping $12 something....OUCH! Regards, Rod

[Prairie Dawg]

My thoughts on adding a second fan? Not necessary, and too much trouble! A 'backup replacement' fan would be good too. I suppose if the IC amp burns it would be because of a non-functioning or 'dust clogged' fan. I just ordered a back-up power amp IC. I do some amp repairs. Since that IC seems to be the 'achilles heel' for these amps, I suspect that one day I will have another to repair! I ordered it from Mouser Electronics. It was $4 but they really jacked me for shipping. I ordered 3 other light parts. Total was $11 something....shipping $12 something....OUCH! Regards, Rod

The funny thing is how much power comes out of there, too. The one I worked on was functional but had a few things wrong with it, and I traded the work for an odd bag of tubes the owner had from a long gone Boogie. I steeled my nerve, opened up, and there was....nothing much.

After a while I figured out how to talk to it and I was stunned at the volume it produced. I'm like "Where's the iron?"

I think that overheating of the IC amp is the achilles heel, if you will, and that makes fan health a sine qua non.

Now....how many times have you heard Latin on an amp chatboard?

[captainlee]

How to baby your MG100HDFX

Hi Prairie Dawg,


Yes it has some serious volume , I only use the clean channel but when I turn on the crunch it gets real loud I know something is wrong there i wished i knew so i could fix it as i did the peavey xr 500s i have. My gess is its one of those little ic chips is boosting the volume on the crunch circuit??

I really like the effects and reverb they are awsome I have a 5 effects pedals but don't need them now.

So now you know what my prob is what can i do to fix it and what other tweaks can I do to help it out.?? Thanks in advance Cap.

[retrorod]

I think that overheating of the IC amp is the achilles heel, if you will, and mathat kes fan health a sine qua non.

Now....how many times have you heard Latin on an amp chatboard?

Not very many....Et tu Brute'.....Donimus Gofriskum....vini.vidi,vici....and hopefully less in the future... ;-)