M70 amp getting HOT - and distorting

Status
Not open for further replies.

wills15

Member (SA)
Jan 29, 2012
142
0
16
Australia
Hey Guys,

I have the world's worst M70 right here, and amongst other things it has an issue with the left channel amp.

Amp gets extremely hot, and sound from the left channel is not as good as the right; quieter, lacking in bass, distorted if you crank it.

I've swapped amp chips and recapped the left channel amp to no avail. I also tried adjusting the bias.

Nothing has made a difference.

Any ideas?
 

Superduper

Member (SA)
What do you mean you swapped amp chips? Left to Right, or did you replace it with a new one? Also, what do you mean you attempted to adjust the amp bias. Since the amp it a sealed module, and not constructed of discrete components, I'm not aware of an output amplifier bias adjustment. If you speak of bias adjustments in the tape section, that has nothing to do with the main amp. And you say you recapped the left channel..... which caps specifically? And why only one channel? In my experience, wholesale capacitor replacement alters the sound characteristics to some extent and it's best to recap symetrically so as to retain left to right balance.
 

Cpl-Chronic

Member (SA)
May 14, 2012
2,029
31
48
Windsor, Ontario, CANADA
It sounds like a partial short in 1 of your woofers or tweeters, from your limited description. That can happen if a voicecoil rubs & sometimes shorts when it moves the wrong way. If you physically swapped amp IC's & the caps for the amp circuit, which I have to assume is true then the only obvious common denominators in the equation is pre-amp, source or speaker. Look there for your answers & see if maybe a woofer or tweeter is partially blown. maybe a tweeter cap has an intermittent short.

Just an opinion... :hmmm:
 

wills15

Member (SA)
Jan 29, 2012
142
0
16
Australia
Left amp IC only is getting hot.

Swapped amp ICs left to right, but the left one still gets hot.

Recapped left power amplifier only, because the left was the one getting hot. Changed all the caps in this section.

There are a couple of bias adjustments in the amp section on the circuit board, so I assumed these were for the amp. Regardless, they had no affect.

No speaker coil is rubbing, and all speakers measure 8 ohms...

Thanks for the help, let me know if you guys have any other ideas.
 

Superduper

Member (SA)
Well, I don't know what you mean when you say all caps. If you want us to presume you mean all electrolytics and you did not miss any, then the only thing I can think of is that your amp is oscillating which will definitely cause the amps to run very hot. The oscillation can be caused by failing or failed caps in the nf or pf range, so not just electrolytics. Might also be caused by other things too.

If you mean you adjusted the bias and frequency prescribed in the service manual, that procedure applies only to the cassette amp, which does indeed use discrete transistors. It has nothing to do with the main output amplifiers, which use sealed modules and are internally biased. I know of no adjustments to bias the line or preamplifiers but if you suspect them, you can do a VR analysis of those circuits. The service manual schem shows the expected values at various test points. If those readings are off, it usually indicates a problem in that circuit.

Blown or shorted tweeter cap could affect HF sound or endanger the tweeter but will not cause, nor have any effect on the amp itself.

When you are replacing a single failed cap, that's one thing. When you are replacing them in groups, it's best to do both sides to retain symmetry in the audio. Like I said, every boombox I've recapped has resulted in audible sound differences. However the change, you would want it equal on both sides, is why I said do both. But I guess that's up to you.
 

wills15

Member (SA)
Jan 29, 2012
142
0
16
Australia
Sorry, I meant just electrolytics. Have not tried testing or replacing any of the non-electrolytic caps or resistors.

I will eventually replace electrolytics on both power amps, I just did the left one for the moment, because I thought they might be contributing to the amp getting hot.

I didn't adjust the bias as per the service manual, I just tried a few different positions on the pot that was within the amp section on the board and marked Bias (L).

Could problems in the preamp or power supply result in the power amp getting hot or oscillating like this?
 

Superduper

Member (SA)
If the amplifier is oscillating, it will definitely run very hot. The oscillation could be outside the audible range which the speakers would be unable to reproduce anyhow so you may not hear it. I'm talking could even be 50khz to 100khz, or anywhere for that matter. To measure/detect it, you would need an oscilloscope. No other meter will do the job here. If you are technically proficient, you can try google oscillation + hot + amplfier and see what you come up with. Be forewarned that much discussion will be for the nerdy tech geeks and while the discussions are interesting, might not be very helpful to someone that needs handholding or without test equipment. For what it's worth, I recently fixed an M70 that had hum, poor sound quality, and one amp chip running excessively hot. Replacing the the amps were the fix. Internal fault of the module.

Indiscriminately turning any pot on the boards are not recommended. Even if you return the settings to where it appears to be at the original approximate location, it could still affect the bias adjustments which are very picky and in the adjustments are usually in the mV range. If you've ever connected a meter while making such adjustments, you'll know what I'm talking about... these board mounted pots are not the accurate 20 turn bournes units.... they are crude 2/3 turn pots that could have different readings even at the approximate same spot. But you've already done that and it matters not to the issue at hand anyhow.

Yes, it is possible that oscillations can originate in the preamp.

No, it is doubtful that the power supply would cause the oscillation. If it did, it would be present in both amplifiers and not just the one.
 

wills15

Member (SA)
Jan 29, 2012
142
0
16
Australia
Superduper said:
For what it's worth, I recently fixed an M70 that had hum, poor sound quality, and one amp chip running excessively hot. Replacing the the amps were the fix. Internal fault of the module.
This accurately descibes the problem, except as I mentioned, I swapped the amps for left to right, but the left channel remains the bad channel.

I might try recapping the preamp.

I don't have an oscilloscope, but I've been thinking about getting one. I might have to if I can't solve this.

I probably won't be using the tape, so tape bias isn't an issue for me.

Thanks for the help.
 

Cpl-Chronic

Member (SA)
May 14, 2012
2,029
31
48
Windsor, Ontario, CANADA
:hmmm: I still think its a bad cap on the tweeter or a partial voicecoil short. Start by swapping woofers or tweeters & see if that's the fault. Way easier to start there before you recap the whole smack. It's worth a try.

Good luck & keep us posted as to your progress.
 

BoomboxLover48

Member (SA)
Dec 3, 2010
5,738
252
83
Chicago, IL
Cpl-Chronic said:
:hmmm: I still think its a bad cap on the tweeter or a partial voicecoil short. Start by swapping woofers or tweeters & see if that's the fault. Way easier to start there before you recap the whole smack. It's worth a try.

Good luck & keep us posted as to your progress.

Connect a headphone and see if the amp still runs hot.
 

347 Tony

Member (SA)
Feb 11, 2013
108
0
0
Moncton
If you put your meter on the woofer/speakers and they measure right at Idle, push on your speaker to duplicate It working motion, every time you see a zero, that a short, the more It move's(short's) the hotter your amp Is gonna get, I cooked lot's of woofer's doiing car audio, that's what I measured once I started having problem's with a set of driver's. 6 Orion 15's and 6000 watt's to run them In an 83 Mercury Cougar. 161.4 Db.
 

tshorba

Member (SA)
May 10, 2009
551
1
0
South West Victoria
347 Tony said:
If you put your meter on the woofer/speakers and they measure right at Idle, push on your speaker to duplicate It working motion, every time you see a zero, that a short, the more It move's(short's) the hotter your amp Is gonna get, I cooked lot's of woofer's doiing car audio, that's what I measured once I started having problem's with a set of driver's. 6 Orion 15's and 6000 watt's to run them In an 83 Mercury Cougar. 161.4 Db.
:huh:
The voice coils wire on these drivers in very thin gauge compared to a 15" Orion subwoofer, they will either be open circuit (and dead) or closed circuit (and working)
By testing with the headphones you are only putting a very very small load on the amp, connect some external speaker to the sp outlets or rig up a dummy load and see if the problem is still present. I think the issue is upstream and not the drivers.
 

347 Tony

Member (SA)
Feb 11, 2013
108
0
0
Moncton
Sorry I just thought a speaker was a speaker big or small, worked the same way on Mid range speaker's In the car audio stuff.. My bad. However I have a set of M70 speaker's coming and will certainly test them this way to see what I get.
 
Status
Not open for further replies.