Need help from someone at PG on an MQ430

[Eric D]

I know this may not be the place to post this, but I need help from someone at PG on this issue.

I just did some work on a Frank Amp 'N Stein for a member here. When finished I decided to check the MQ430 section to verify its output level.

I get just a tiny bit over 3V RMS from the MQ430 prior to clipping, nothing more.

The amp has 13.8V of input to the power supply. I am feeding a 5V RMS input signal to the RCA jacks, and I get about 4V at both outputs of OP-AMPS U7, and U10 (pins 1 and 7) when I detect clipping.

I have gone over this amp in great detail and nothing appears wrong with it anywhere. It sounds great, other than having such a low output. All 4 channels are behaving the same.

The amplifier measures 26V positive and negative for the main rails. This amplifier is REV-C.

Everything about this amp is telling me does not have any voltage gain as it should. What resistance values should I check to verify this?

I have another MQ430 board which is REV-C, and I painstakingly compared all the resistor values between the two, they look to be the same.

Any help would be greatly appreciated.

[Eric D]

Well, after spending a ton of effort this afternoon, I think I solved the problem.

I found two resistors per channel which directly affect the amplifier's voltage gain. I tried other values for these resistors and eventually managed to get 16V of clean signal out of the amplifier.

Considering I started with only 3V, going to 16V is a massive increase.

I have been all over this amp in great detail, and nothing looks to be replaced. All the values for all the items are stock, and match that of the other REV-C MQ430 board I have on hand.

I now need to change all 8 resistors, and then do some listening to the amp.

I now have a question for all you MQ430 owners about your amp. Do you notice you need to turn the volume on your stereo WAY up to start getting anything from your MQ430? If so, your amp has the same issue as this one. This also means your amp is likely clipping a ton at normal listening levels.

Maybe this issue is just related to REV-C versions of this amp. I know both of the amps I have here have this problem. I also know about a year ago I worked on another MQ430 and noticed it took nearly all my deck and line drivers output to get the amp to do anything, but I did not think anything of it and just forgot about it.

Unless I am making some horrible mistake (I have made mistakes this big before), the MQ430 is the most gutless amp PG ever produced until you mod it to do what it should have done in the first place.

[Eric D]

Been listening to the amp for over an hour now with the modifications in place. Totally different amp.

Before it was beyond weak. Now it sounds like a MS2125 or better (I am running it bridged into some 8 ohm home speakers).

[wooferdog]

so it's not gutless anymore what exactly did you have to replace?

[ttocs]

some resistors he said. Might have to do that on mine since it is already apart....

[wooferdog]

were they bad or the wrong value ? what value were the replacements and the location number on the board so we could check them.

[Eric D]

Listen to your amp, does it sound gutless? It was night and day difference for me with this amp. Two channels off a MQ430 should sound similar to a MS275. Prior to this mod, my M25 had more output than the MQ430 did when bridged.

I am still a bit confused by this, as the resistors I replaced were not bad. They were also not the wrong value. It is as if from the factory this amp just had no output.

I don't really want to start telling people to go and change something on their amp until more is known as to what is going on.

This amp is a REV-C board. It does not have two smaller 35V caps in it for the rails, as ttocs mentioned his amp did. I am wondering if the other style MQ430s do work correctly and this was just an issue localized to the REV-C models.

[wooferdog]

i'll let you know how my mq-430 sounds. i'll be trying it out in a couple of days. i also have one with the two extra caps on the rails.

[Eric D]

If you hook up the MQ430, and don't connect any speakers to it, you can then figure out if it is working well or not.

Play a 1kHz tone, and measure the output of one channel on the MQ430. See how much you can get. You should be able to get at least 16V RMS if it is clipping or not. I am guessing with the amp the way mine started, it would never come close to 16V RMS, even with max clipping (square waves).

[wooferdog]

i did the test with the 1k test tone with the gain all the way down(2 volt). my power supply was holding at 13.02 volts. the rails were 25.5 volts. my readings on the outputs were 13.88-14.01 per channel. this was checking 1 channel at a time so my lowest was 13.88 and highest was 14.01 steady output. i don't have a o-scope to check for clipping so i did not try moving the gain to see what it would do. rev b board. i ended up hooking up a speaker to 1 channel to listen for clipping and it gets up to 16.23 volts when testing on the output with no speaker. the gain is about half way when it starts clipping.that should work out to 65 watts at 4 ohms.

[Eric D]

i did the test with the 1k test tone with the gain all the way down(2 volt). my power supply was holding at 13.02 volts. the rails were 25.5 volts. my readings on the outputs were 13.88-14.01 per channel. this was checking 1 channel at a time so my lowest was 13.88 and highest was 14.01 steady output. i don't have a o-scope to check for clipping so i did not try moving the gain to see what it would do. rev b board. i ended up hooking up a speaker to 1 channel to listen for clipping and it gets up to 16.23 volts when testing on the output with no speaker. the gain is about half way when it starts clipping.that should work out to 65 watts at 4 ohms.

Sounds like this issue is only with the REV-C boards then.

[stipud]

Quite the mystery...

[Eric D]

I am quite confused overall.

When an amp fails, 99% of the time one channel fails. I cannot find anything wrong with this amp, and I cannot believe somehow this amp was damaged in such a way as to sound just fine, but have no physical evidence, and all 4 channels have only 3V of output prior to clipping.

That would be something like a 1 in a billion failure.

If anything could fail to reduce output, I would assume it would not be even across all channels. One channel might do 3V, another 2V, and the last two 4V. All 4 being the same is very odd.

Then what makes it even more confusing is I go in and replace 8 resistors (which were not damaged when removed), and the output jumps up to where it should be.

For those of you with this amp, what are the values of R112, R114, R212, R214, R312, R314, R412, and R414?

[wooferdog]

can those be measured properly still in the circuit ? if so i'll have some readings tomorrow when it warms up in the garage. it's already 32 out at 9 pm. i thought florida was the sunshine state. it's going to be warmer in indiana tonight than here

[Eric D]

You don't need to measure them, you just look at the color combination on them and that will tell you what their nominal value is.

[marko]

maybe there was a dry joint on one or more of the resistors altering the overall value?

[Eric D]

maybe there was a dry joint on one or more of the resistors altering the overall value?

It would have to be all 8, and by the exact same amount.

Besides, when I removed the resistors, I measured them out of the circuit and they all measured fine.

[wooferdog]

all 8 are the same at 510 ohms.

[Nils]

For those of you with this amp, what are the values of R112, R114, R212, R214, R312, R314, R412, and R414?

I'll check the two MQ's that i have later today. i think one of them is REV C

[ttocs]

mine is a rev - b

112-510
114-510
212 and 214 - 510

can't find the 412 and 414 yet...

[Eric D]

If they are 510 ohm, I am pretty sure the amp won't do what it should.

On the XX12 resistors, one end is connected to the last op-amp which feeds that channel. The op-amp can only swing 15V + or -, nothing more (as limited by the rails for the op-amps).

I found on my amp the signal at this point was plenty strong, but after leaving this point the gain of the actual power amplifier stage was not enough. The bad part is all this really requires an o-scope to verify.

[ttocs]

what size should be in there?

[Nils]

Now i have checked mine

REV B # 430846 all are 510ohm 5% tollerance (this one has two extra railcaps, small size)
REV C # 431934 all are 510ohm 5% tollerance (this one have only the big railcaps)

[Eric D]

Now i have checked mine

REV B # 430846 all are 510ohm 5% tollerance (this one has two extra railcaps, small size)
REV C # 431934 all are 510ohm 5% tollerance (this one have only the big railcaps)

Have you listened to both of these amps? Do they have the same output?

[Nils]

Now i have checked mine

REV B # 430846 all are 510ohm 5% tollerance (this one has two extra railcaps, small size)
REV C # 431934 all are 510ohm 5% tollerance (this one have only the big railcaps)

Have you listened to both of these amps? Do they have the same output?

Sorry, i haven't done that yet, but it seems that i have to do that prior to your mystery I'll try to do a A-B test during the weekend, but i won't promise anyting but i will try to manage it