is amplifier going bad?

[victor]

I have a 70's model stereo receiver that is still working as good today as it did when first purchased. That is except for the fact that it seems the left channel plays louder than the right. Is this a sign of the amplifier starting to die out? Could it be that it needs a good cleaning or adjustment? I can check to be sure that I have all the cables and speakers connected properly and in phase, but I'm almost sure thats not the problem.

[paul_n_oneal]

Is it with all sources or maybe just the tuner?

[davidyohn]

> I have a 70's model stereo receiver that is
> still working as good today as it did when
> first purchased. That is except for the fact
> that it seems the left channel plays louder
> than the right. Is this a sign of the
> amplifier starting to die out? Could it be
> that it needs a good cleaning or adjustment?
> I can check to be sure that I have all the
> cables and speakers connected properly and
> in phase, but I'm almost sure thats not the
> problem.

If this is a new symptom and nothing has changed in your system, then yes I'll guess you have a channel on the verge of failure. The louder one, probably.

[victor]

> Is it with all sources or maybe just the
> tuner?

Yes it is with all sources, not just the tuner.

[victor]

> If this is a new symptom and nothing has
> changed in your system, then yes I'll guess
> you have a channel on the verge of failure.
> The louder one, probably.

Oh, then maybe its time I start looking into a replacement. A friend of mine told me that its because I have 6 ohm speakers on "A" and a 4 ohm sub on "B". I didn't think that would affect it since I don't play it very loud, but the receiver does specify to use 8 ohm or higher impedance speakers.

[envisionelec]

> I have a 70's model stereo receiver that is
> still working as good today as it did when
> first purchased. That is except for the fact
> that it seems the left channel plays louder
> than the right. Is this a sign of the
> amplifier starting to die out? Could it be
> that it needs a good cleaning or adjustment?
> I can check to be sure that I have all the
> cables and speakers connected properly and
> in phase, but I'm almost sure thats not the
> problem.

No. There is a speaker relay that will need to be replaced or cleaned. This is a super common problem with Sansui and Pioneer receivers, but others experience it as well. You may also have dirty switches, like the Stereo/Mono switch or Loudness button or Tone Defeat. Any of these will cause your symptom.

Amplifier channels do not get quieter when they "go out" - they get heavily distorted.

[Paul O]

The most likely culprit is dirty switches or rotary controls. If cleaning does not fix it they may need to be replaced. Second most likely cause is cold solder joints. These usually occur at the connectors. As Aaron stated, amplifiers work as new until one day when the magic smoke gets out, then they sound real bad or make no sound at all.

[unknownuser]

> I have a 70's model stereo receiver that is
> still working as good today as it did when
> first purchased. That is except for the fact
> that it seems the left channel plays louder
> than the right. Is this a sign of the
> amplifier starting to die out? Could it be
> that it needs a good cleaning or adjustment?
> I can check to be sure that I have all the
> cables and speakers connected properly and
> in phase, but I'm almost sure thats not the
> problem.
When the caps dry out in later 70s amplifiers, the gain starts going down, usually in one channel at a time. In early 70s amplifiers the gain can start to go up in the channel that is going bad.

In either case it is usually a cap drying up.

Relays get distorted or crackle before expiring, sometimes switches do too(try cleaning).

Pots in tone circuits (and the turn-over point switches for same) can do what seems like volume changes. They usually crackle when moved(try cleaning).

If the unit takes a longer and longer time to turn on it is usually a cap in the relay circuit drying up.


(Originally posted by: djk)

[envisionelec]

> When the caps dry out in later 70s
> amplifiers, the gain starts going down,
> usually in one channel at a time. In early
> 70s amplifiers the gain can start to go up
> in the channel that is going bad.

> In either case it is usually a cap drying
> up.

> Relays get distorted or crackle before
> expiring, sometimes switches do too(try
> cleaning).

> Pots in tone circuits (and the turn-over
> point switches for same) can do what seems
> like volume changes. They usually crackle
> when moved(try cleaning).

> If the unit takes a longer and longer time
> to turn on it is usually a cap in the relay
> circuit drying up.

They'd have to be pretty dry. I have found that to be the case once in probably 150 units I have worked on. The relay delays increasing are more often caused by excessive DC offset at startup - caused by leaky capacitors, in my experience. The times I have suspected capacitors to be at fault, I've measured their value to be not only within spec, but their ESR to be a reasonably low, too. I think modern (< 30years) capacitor dry-out is overrated.

[unknownuser]

> They'd have to be pretty dry. I have found
> that to be the case once in probably 150
> units I have worked on. The relay delays
> increasing are more often caused by
> excessive DC offset at startup - caused by
> leaky capacitors, in my experience. The
> times I have suspected capacitors to be at
> fault, I've measured their value to be not
> only within spec, but their ESR to be a
> reasonably low, too. I think modern ( 30years) capacitor dry-out is overrated.

"once in probably 150 units I have worked on."

You've worked on that many units people complained about low gain on?

I doubt it.

My experience differs with yours. I ran shops for 30 years with several techs under me and didn't see that many. I find the most common item missed by techs in LOW GAIN situations is the cap in the feedback loop is bad.


(Originally posted by: djk)

[envisionelec]

> "once in probably 150 units I have
> worked on."

> You've worked on that many units people
> complained about low gain on?

> I doubt it.

> My experience differs with yours. I ran
> shops for 30 years with several techs under
> me and didn't see that many. I find the most
> common item missed by techs in LOW GAIN
> situations is the cap in the feedback loop
> is bad.

Ah, well - that was exactly the problem in the unit with the faulty cap. In the feedback loop. I think it was a Marantz.

Ok, I should have clarified. I meant that out of 150 units total (meaning 1970's era units) I have seen just one (OK, maybe two) with low gain attributed to a faulty capacitor. Many of those - I'd say about 50 had complaints of "left channel seems to fade in and out".
It turns out that some of these people only listen at background levels and oxidized relay contacts were to blame. Of course, this is technically NOT low gain - and I would certainly not exaggerate any claims to that effect.

Yep, this is just lil old me in my basement over the years. I was pretty hot on vintage receivers, doing about one to two a week a few years ago.

Thanks for your input. Seriously - I enjoy reading your posts.

[davidyohn]

> Oh, then maybe its time I start looking into
> a replacement. A friend of mine told me that
> its because I have 6 ohm speakers on
> "A" and a 4 ohm sub on
> "B". I didn't think that would
> affect it since I don't play it very loud,
> but the receiver does specify to use 8 ohm
> or higher impedance speakers.

Well that kind of misuse can certainly lead to early failure.

[victor]

I just checked out all connections today, tested caps and looked for any problems parts inside. So far the only problem I found was with the bass knob. It does need a good cleaning. But all fuses, relays, caps, resistors etc. are in very good condition. I do have the speakers all in correct phase too. It looks like what I did was used the wrong wire to connect the speakers with. Both speakers on "A" are connected with 18 gauge (sub on "B" with 16 gauge), but the length is where I goofed... the left is on about 1.5 foot of wire and the right is on about 10 feet of wire. I guess thats what the problem is and where I messed up?

Oh well, best way to learn I guess.

[davidyohn]

> I just checked out all connections today,
> tested caps and looked for any problems
> parts inside. So far the only problem I
> found was with the bass knob. It does need a
> good cleaning. But all fuses, relays, caps,
> resistors etc. are in very good condition. I
> do have the speakers all in correct phase
> too. It looks like what I did was used the
> wrong wire to connect the speakers with.
> Both speakers on "A" are connected
> with 18 gauge (sub on "B" with 16
> gauge), but the length is where I goofed...
> the left is on about 1.5 foot of wire and
> the right is on about 10 feet of wire. I
> guess thats what the problem is and where I
> messed up?

> Oh well, best way to learn I guess.

No, a wire length difference like that won't cause one channel to lose output. From your initial description, I still think you have a bad cap or diode in a feedback circuit, a bad cap in the power supply, or a bad MOSFET (or whatever is driving your output stage.)

[victor]

> No, a wire length difference like that won't
> cause one channel to lose output. From your
> initial description, I still think you have
> a bad cap or diode in a feedback circuit, a
> bad cap in the power supply, or a bad MOSFET
> (or whatever is driving your output stage.)

Oh, so then I should test the output transistors? I'm not sure MOSFET were used in 70's models, but the ones I have are the round ones with 2 solder lugs and the third (ground?) is screwed to the heat sink. I can post a pic if need be. How would I go about testing these parts though? I have both a digital VMM and an analog volt/ohm meter.

[envisionelec]

> No, a wire length difference like that won't
> cause one channel to lose output. From your
> initial description, I still think you have
> a bad cap or diode in a feedback circuit, a
> bad cap in the power supply, or a bad MOSFET
> (or whatever is driving your output stage.)

Diodes in the feedback circuit? Bad MosFets? Bad Caps in the power supply?

Where do you get this information? Nothing could be further from the truth.

[davidyohn]

> Diodes in the feedback circuit? Bad MosFets?
> Bad Caps in the power supply?

> Where do you get this information? Nothing
> could be further from the truth.

Hey back off, I thought he was describing one channel going into thermal overload and was simply giving him places to look. I guess you have a far better ability than I to diagnose a problem using text on the web. Me, I prefer to have a piece of gear on my bench. :shrug:

[envisionelec]

> Hey back off, I thought he was describing
> one channel going into thermal overload and
> was simply giving him places to look. I
> guess you have a far better ability than I
> to diagnose a problem using text on the web.
> Me, I prefer to have a piece of gear on my
> bench. :shrug:

Wow. You're incredible.

Show me an audio amp with diodes in the feedback circuit. Show me a home audio amp with MosFets in the output circuit. Show me a power supply with a bad capacitor that causes a weak channel.

Please. From your vast bench experience.

The truth is, I have a decent ability to repair by proxy - that is, to help repair or diagnose items without ever seeing them. Most people are happy with that ability, but it seems you're just jealous.

[davidyohn]

> but
> it seems you're just jealous.

Yep, that's it.