SS Amp Extreme Noise Channel 2 Only

[ThomasE]

Working on a Randall RG300. Channel 1 works fine. Channel 2 has an extreme hiss/hum. This one just about has me stumped, any assistance would be appreciated. Listed below are the symptoms and what I've already tried.

Symptoms - Amplifier has extreme hiss/hum only on channel 2. Noise does change when adjusting master volume, volume, treble, reverb. Noise present with guitar plugged in or no input. Also, reverb noise is present when turned above 4.

Tests performed

1. Swapped both treble and volume pots with channel 1 pots. No Change.
2. Replaced every electrolytic capacitor in amp. (several were bad) (filter caps too) No Change.
3. Swapped Q16 & Q17 with Q1 & Q2. No Change.
4. Replaced rectifier. No Change.
5. Replaced C23 & C28. No Change.
6. Lifted C40 just to take reverb out of the equation. Obviously no reverb noise but noise still present.

What am I missing?

I can attach additional schematics if needed.

Thanks

[shooter]

More schematic might be nice, can't tell if both channels become common after the tone stack. 
only thing I see is try and  lift one side of D1/D2 .  Maybe try un-soldering the mid pot from ground, (float the TS) to see maybe you have DC leaking around in there.
Can you *hear* guitar over the hiss?

[PRR]

> can't tell if both channels become common after the tone stack.

Mixed at R82 R47. Q3 is common to both.

How is the gain for guitar (ignoring crap)? Same, or different Ch 1 versus Ch 2?

Since they share a power node, hum suggests bad-from-the-start (unlikely) or a ground has come loose in the Ch 2 area. Input jack, R41, VOL wiper, and Mid would be the critical grounds.

Next bad guess is: disconnect the trem LDR. I can't imagine how that could go-bad in a way to hiss, much less hum (unless it has the bad ground), but easier snipped than thought-about.

In all this, I'd put a 1/4" metal stick in the Reverb Foot Switch jack (or jumper it on the back) to keep reverb out of the picture for now.

[shooter]

Q3 is common to both.

Thanks PRR, what I thought, I wasn't sure if there was *more* below the trem/verb section.

[ThomasE]

More schematic might be nice, can't tell if both channels become common after the tone stack. 
only thing I see is try and  lift one side of D1/D2 .  Maybe try un-soldering the mid pot from ground, (float the TS) to see maybe you have DC leaking around in there.
Can you *hear* guitar over the hiss?

Tried all those. Yes, you can hear the guitar fine. Just a lot of noise.

[ThomasE]

> can't tell if both channels become common after the tone stack.

Mixed at R82 R47. Q3 is common to both.

How is the gain for guitar (ignoring crap)? Same, or different Ch 1 versus Ch 2?

Since they share a power node, hum suggests bad-from-the-start (unlikely) or a ground has come loose in the Ch 2 area. Input jack, R41, VOL wiper, and Mid would be the critical grounds.

Next bad guess is: disconnect the trem LDR. I can't imagine how that could go-bad in a way to hiss, much less hum (unless it has the bad ground), but easier snipped than thought-about.

In all this, I'd put a 1/4" metal stick in the Reverb Foot Switch jack (or jumper it on the back) to keep reverb out of the picture for now.

Gain for the guitar is ok on both channels. It almost has a pink noise sound. Checked everything you recommended. Even pulled LDR completely out. Another goofy thing, when checking Q16, as soon as I put my meter on SOURCE I can hear a radio station. Any other suggestions?

[ThomasE]

Here are a few more things I did this morning. I decided to totally get trem and rev out of the circuit.

1. Pulled R59, R64 & R66
2. Pulled LDR
3. Pulled R77, R70 & Q23
4. Disconnected Rev Pot
5. Pulled C31
6. Pulled C24
7. Pulled C26
8. Replaced R44 (4.7K) with same value as R7 (2.2K)

Still has  A lot more noise than channel 1. I'm beginning to wonder if this was just noisy design from the factory. Also, channel 2 happens to be closest to the xformer.

[shooter]

It almost has a pink noise sound

sounds like you've about beat it to death, can't help thinking there is a ground issue or bad coupling cap.  The parts count looks small enough that the shot-gun, start all over price would be less than your time, if it's something you wanna keep.

[ThomasE]

It almost has a pink noise sound

sounds like you've about beat it to death, can't help thinking there is a ground issue or bad coupling cap.  The parts count looks small enough that the shot-gun, start all over price would be less than your time, if it's something you wanna keep.

Funny you say that. I was just thinking the same thing. The time I've spent on this amp has gotten ridiculous. But.......... it's gotten personal now. I'm going to figure out what's going on with this thing.

[shooter]

I'm going to figure out what's going on

You'll get it, but take your time, you don't want to *create* issues as you resolve issues.  know anyone with a scope?  This sounds like one of those times.

good luck

dave

[ThomasE]

I'm going to figure out what's going on

You'll get it, but take your time, you don't want to *create* issues as you resolve issues.  know anyone with a scope?  This sounds like one of those times.

good luck

dave

Yes, I own several scopes, a huntron too. Everything I've tested so far checks out good. That's what's so frustrating.

[ThomasE]

The plot seems to thicken......

I'm not so sure the issue is at the front end anymore. Injected 100hz into the amp, started at the output and worked my way back. Both channels seem to be effected (based on scope) channel 2 just a lot louder hum/hiss. Attached images show distorted sine-wave (when treble is turned all the way down it's a perfect sine wave, bass & mid look fine at any level) the pink highlighted areas show where the distortion is residing. What are you guys thoughts?

[ThomasE]

Just replaced Q7, Q8 & 9. Still noisy as hell. 

[PRR]

Have you measured any of the DC voltage check-points?

[ThomasE]

Have you measured any of the DC voltage check-points?

Thanks PRR for checking in. I have checked the points I thought were relevant. But not all. I will check all points tomorrow and post the results.

[shooter]

when treble is turned all the way down it's a perfect sine wave

Is that both treble pots or just ch2?
Does the distortion look the same with say 1khz vs the 100hz?

If your tone gen does square waves, run it once that way, I'm thinking you have a HF cap or blocking diode that is flaking out.  If you have "caned air" used for keyboards etc, shake it up, turn the can upside down and just focus the spray to a single component.  When the cold hits the *bad part* it'll get either worse or clear it up.

[ThomasE]

Have you measured any of the DC voltage check-points?

This is what I have so far -

1. xformer primary - 117vac
2. xformer secondary - 62vac
A. 21.7v (spec'd @ 25v)
B. 25.4v (spec'd @ 28v)
C. 42.0v (spec'd @ 43.5v)
D. -42.0v (spec'd @ 43.5v)

Q1 Points
D - 16.2VDC (spec'd @ 15VDC)
S - 1.1VDC (spec'd @ 2.5VDC)

Q2 Points
C - 21.2VDC (spec'd @ 25VDC)
E - 15.1VDC (spec'd @ 10.8VDC)

Q16 Points
D - 15.5VDC (spec'd @ 15VDC)
S - 1.16VDC (spec'd @ 2.5VDC)

Q17 Points
C - 21.0 VDC (spec'd @ 25v)
E - 14.5VDC (spec'd @ 10.8VDC)

[ThomasE]

when treble is turned all the way down it's a perfect sine wave

Is that both treble pots or just ch2?
Does the distortion look the same with say 1khz vs the 100hz?

If your tone gen does square waves, run it once that way, I'm thinking you have a HF cap or blocking diode that is flaking out.  If you have "caned air" used for keyboards etc, shake it up, turn the can upside down and just focus the spray to a single component.  When the cold hits the *bad part* it'll get either worse or clear it up.

Just ch2. Here are some screen shots injecting a square wave @ 1khz. All tone controls turned to zero. Master vol at 10, vol at 2 on both channels. BIG difference between 1&2.

[shooter]

That's some serious overshoot!

I decided to totally get trem and rev out of the circuit.

Did you do that at R46 and the reverb pot, or by pulling all the parts you listed?

I'd be about ready to use that amp as target practice
BUT, if you're still fighting it and wouldn't mind, could you repeat the same measurement at the wiper of both treble pots?  1khz square wave.

[ThomasE]

That's some serious overshoot!

I decided to totally get trem and rev out of the circuit.

Did you do that at R46 and the reverb pot, or by pulling all the parts you listed?

I'd be about ready to use that amp as target practice
BUT, if you're still fighting it and wouldn't mind, could you repeat the same measurement at the wiper of both treble pots?  1khz square wave.

Yea, it's being a PITA. I've got way too much time invested in it to give up now though. I'll take those measurements in the morning and post results. Just for the heck of it, I took some thermal images of the circuit earlier just to see if anything showed up. See pics.

TH1 -
 
SP1 - R26 - 140.8F
SP2 - R27 - 143.3F
SP3 - R6 - 136.7F
SP4 - Q9 - 115.6F
SP5 - Q7 - 120.0F
SP6 - R81 - 128.0F
SP7 - R20 - 122.4F
SP8 - Q2 - 103.4F
SP10 - Q8 - 105.7F

TH2 -

SP1 - R37 - 147.6F
SP2 - R43 - 126.7F
SP3 - R61 - 125.0F
SP4 - R80 - 111.9F
SP5 - Q17 - 106.7F

[ThomasE]

That's some serious overshoot!

I decided to totally get trem and rev out of the circuit.

Did you do that at R46 and the reverb pot, or by pulling all the parts you listed?

I removed LDR, removed C40, disconnected 10K reverb pot and grounded C11.

[ThomasE]

That's some serious overshoot!


BUT, if you're still fighting it and wouldn't mind, could you repeat the same measurement at the wiper of both treble pots?  1khz square wave.

Here is a shot of ch1 & ch2. 1khz square wave @ treble wipers. Top trace is #1.

[ThomasE]

Injecting the same 1k square wave. This is what I'm seeing on each side of C28.

[shooter]

Is your scope probe calibrated?  most scopes have a cal point, usually a 1khz 5v.  If yours has that, touch your probe tip to it and see if you have the overshoot, If so, hopefully your probe has an adjustment to make it look closer to a square wave.  I'm not sure if it's a probe or your amp.

The shots from c28, guessing left side of cap = top trace?

At this point, if it was me, I would disconnect the wire/s on both treble pot wipers(center pin), re-measure at the pot and look for any differences.  If they both look the same, I would take a shielded wire, center to treble wiper, shield to ground and hook up a small bench speaker or listening type amp and listen with both a 1 kHz and music.  that should half spit the amp, cuz I'm starting to get lost in the weeds.

[PRR]

> Is your scope probe calibrated?

One of those traces is quite decent (for a square wave through a guitar amp). I'd assume his probe is close enough for rock-n-roll.

C28 is the *treble* pot feed. It *should* just highs, no mids or lows, on the right side. And that is what the square-wave shows. That is working as expected. Find another red-herring.

Do *any* of those DC voltages look odd? (I stacked a ton of block today and am in no mood for number-lists.)

[ThomasE]

Is your scope probe calibrated?  most scopes have a cal point, usually a 1khz 5v.  If yours has that, touch your probe tip to it and see if you have the overshoot, If so, hopefully your probe has an adjustment to make it look closer to a square wave.  I'm not sure if it's a probe or your amp.

The shots from c28, guessing left side of cap = top trace?

At this point, if it was me, I would disconnect the wire/s on both treble pot wipers(center pin), re-measure at the pot and look for any differences.  If they both look the same, I would take a shielded wire, center to treble wiper, shield to ground and hook up a small bench speaker or listening type amp and listen with both a 1 kHz and music.  that should half spit the amp, cuz I'm starting to get lost in the weeds.

Scope is calibrated, I calibrate before doing any measurements. I'll check those pots tomorrow.

[ThomasE]

> Is your scope probe calibrated?

One of those traces is quite decent (for a square wave through a guitar amp). I'd assume his probe is close enough for rock-n-roll.

C28 is the *treble* pot feed. It *should* just highs, no mids or lows, on the right side. And that is what the square-wave shows. That is working as expected. Find another red-herring.

Do *any* of those DC voltages look odd? (I stacked a ton of block today and am in no mood for number-lists.)

The only voltages that look "odd" are almost identical on channel 1 also.  Since ch1 is fine I'll assume those values are "close enough".

[ThomasE]

Installed a .1k cap in parallel with the treble 50k pot. (not the wiper, the 50k resistor). No noise, silence at any volume. Treble works, mid and bass works. Guitar sounds great. Does anyone see an issue with this? It sounds better than ch1 now.

[shooter]

Find another red-herring

That's how I got in the weeds/ probe-chk.  the input to c28(top) is the "integrated?" waveform, before the TS, after, the right side c28(bottom), it's good square

If it works with a .1, don't fix it   Guessing you might be shunting a gremlin? to ground?
 

[ThomasE]

I hope this battle is almost over. I relocated the .1k cap from across the 50k resistor pot to the wiper and C29 (little better tone). Put reverb and trem back into the circuit. Powered up and it sounded like someone was pissing in the reverb tank. Horrible noise. To make a long story short Q18 fet  had crapped out. Replaced that and all is good. No noise, everything functions as it should. The only thing I've noticed is that even at idle Q10 & Q11 get significantly hot. Thermal images show 12,13,14,15 around 93F. Q10 & Q11 are around 140F. Does that seem normal? Voltages are within probably 3% of specs.

[shooter]

12,13,14,15 around 93F. Q10 & Q11 are around 140F

glad it's up, I'm guessing 12-15 are ||'d so they will probably run a little cooler, 10/11 do have 7W R's so that says to me it's gonna get warm.  Play it til it dies, throw it way then

[ThomasE]

Amp is back together and sounds fine. Been a hell of a fight with this thing. The fact that it had been "worked on" through the years didn't help matters. I found several components not correct based on schematics. Anyway, all is good! Thanks Shooter! Thanks PRR! I appreciate all the suggestions and following this post throughout the battle.

[PRR]

> Does that seem normal?

No. Or at least worth checking.

Note, at R31, 6mV across 0.6 Ohms. 10 mA. Assuming 40V supply rails, 0.4 Watts in each transistor.

If the 31C has no heatsink, 140F *may* be correct.

But it is likely to rise with output power, and with temperature. Such designs can go into thermal run-away.

There are several possibilities.

First check the bias. *IS* there 6mV (3mV-8mV) across R31? AND R32 (I suspect the "6 Ohms" is a typo for 0.6).

If so, play it, beat on it. Monitor the large heatsink temperature. It can get hot, too hot to hold, but not too hot to touch.

If you don't have the 6mV, *gently* trim the 250r trimmer so you get 3mV-8mV on R31. If it won't trim, something else is wrong.

I would also use a floating volt-meter (battery power DMM) across each of R33 R35 R34 R36 while strumming big chords. The voltage should be zero when silent but spike-up toward 0.5V near clipping. If all four do this, good. If any of them do not, the transistor above it is not pulling its share of the work. Perhaps dead, perhaps bad solder. A 4-device output stage may "work" on 3 devices, usually the one carrying double-work soon fails. That can put pure DC on the speaker. If the speaker survives, the main rectifier may not. There is also (again) no protection system, so as the outputs fail little Q8 Q9 try to pull the load, and die.