Fender Frontman 65R

[Gingertube]

A friend dropped his Fender Frontman 65R to me to look at.
When it is powered on it hums/buzzes for about 2 to 3 seconds and then is OK.
This one of the cheap SS units.
Is that typical or is this a "fault".
I down loaded the manual which has a schematic but says "non-servicable" on the front page.
Should I just give it back and tell him to put up with it till it eventually quits entirely or is it worth some effort.
My "knee jerk" reaction is that this a slow rail start up thing, delaying the power amp DC feedback control.
Any guidance gratefully received.
Cheers,
Ian

[The Dude]

My bet is a filter cap starting to go. Is this on every power up? In other words, is it worse after sitting a while and not as noticeable if it's on for a bit- then powered down and back up? Or is it the same regardless of how long it's been running?

[Gingertube]

On every power up. Happens if I power down and then back up again in a few seconds.
Cheers,
Ian

[Enzo]

Non-serviceable means only that warranty claims will not be paid. The company prefers to replace the unit instead of repair. It has NOTHING to do with actual repairabilty.

Look at schematic, bottom center. the power up mute is immediately to the right of the transformer. Q21,22,8. I see two caps in that circuit. My first suspects are the caps. Perhaps the circuit wakes up slower than average.

Having said that, I rarely worry about a turn-on noise.

[Gingertube]

Gotcha,
Holds off the phase splitter tail current source during startup.
Will check around that circuit.
Cheers,
Ian

[DrGonz78]

I had a very similar issue with this exact same amp back in August.
http://music-electronics-forum.com/t42712/

You can read through and see what I parts I replaced as the amp had failed needing repair. Once repaired it would sizzle/hum for 2-3 seconds at turn on, which just drove me nuts. I agree with Enzo that it is probably not a big deal but OCD kicks in for me. Anyhow I measured about 750mv to 1 volt DC on the output in that 2-3 seconds while turning on the amp. I replaced some transistors that I suspected had been stressed by the amps failure. This worked in my case to fix the problem. On the first 2-3 seconds I would measure only 200mv or so at turn on after I fixed it. Not sure any of this will apply to your problem but the symptoms read very similar.

[Enzo]

Of course, many things could cause this. Just as my slow PU reset could be at fault, so could a slow transistor not turning on crisply.

[DrGonz78]

Yup, definitely one thing that always bugs me about symptoms like this one is that it is hard to track down. If this is the only Fender 65r that you have encountered then you initially think it could be just the way the thing works. Now that I fixed the one I worked on I know that it can be fixed. So we know it is a common "Fault" and not serious. In my case I was more curious about fixing the amp than concerned about the time it would cost me to try to fix it.

[J M Fahey]

Look at the speaker cone when you turn it on; if it moves forward or backward and stays there for 1 or 2 seconds, and at the same time it hums/buzzes, no big deal, it´s just that it takes that couple seconds to stabilize, during which it applies DC to the speaker.

Large PA type amps have a speaker output relay so this is hidden, many SS amps have a delayed turn on mute to do about the same in a cheaper way, what Enzo suggested is that it might be out of timing, but again, no big deal.

[Gingertube]

Hi Guys,
This Power Amp pushed one of my buttons.
I haven't physically attacked it yet but thought it would be a good idea to analyse the schematic so I knew what I was doing.
Immediate flags all over the place.
The speaker -ve goes to ground via 0.1 Ohm so there is current feedback, positive current feedback will reduce the output impedance and negative current feedback will increase he output impedance and emulate a valve out put stage , so which is it.?
By my analysis it is negative current feedback.
The output is a Complementary Emitter Follower Pair {CFP} consisting of (Q15,Q18 and Q17,Q19).
Flag #1
For a CFP the drive signals to the 2 sides MUST be in phase so why the hell do we have a phase spitter (Q6,Q7)?
The drive to the top of the push pull output is from Q6 collector, the drive to the bottom of the push pull is from Q9 collector BUT Q9 is a simple inverter off the opposite side of the phase inverter (from Q7 collector).
So drive to the output complementary pair is in phase at it should be.
This is where I may go wrong with confidence.
There are actually 2 amplifiers here.
The second one has output transistors Q10 and Q11. It NEEDS the phase inverter. If you check the phases of the drives (I found) then you find that drive via R84 is an amplified copy of the input signal - same phase.
It feeds Q12 and Q13 which are output transistor drive clamps via voltage limiting zener + diode thus limiting drive voltage). This clamp is also triggered if the output transistor current exceeds about 3 Amps (R103 and R105 =.22 Ohms.)
Notice That the signal voltage clamp is triggered by comparing the amplified input voltage (at R84) with the voltage at the speaker +ve.
Q14 and Q16 are also output transistor drive clamps. R89 and R92 are bootstrapped via C53 and C54 and so their effective impdedance is way more than their 1k5 value. That means that R95, R97 are pretty much a true divide by 2.
So we have 3 effective output drive clamps, Output transistor overcurrent, and overvoltage clamp triggered by difference between speaker drive voltage and a copy of the input signal, and a 3rd (Q14,Q16) one which seems to be triggered when the output tranny drive voltage is more than 2X the expected speaker output voltage (open speaker protection?).
We are now back to that phase splitter.
This is where they have been real clever or perhaps just too clever for me.
Positive (left side of the diff amp / phase splitter is driven by U7-A.
The Negative (right side us driven by what?
Well U7-0B is doing its bit to provide a say 10 Ohms output impedance with its AC phase the same as the +ve diff amp input. It is going to force the Q7 base to act like a summing junction).
So what are we summing in there?
1) The input signal which exceeds +/- 0.65V *D15,D16)
2) DC and AC feedback (R70)
3) DC feedback (to keep the speaker +ve at 0V average , via R71,R72, filtered by C48,C49 Fh = 1,8Hz) - So when I get physically to the amp, given to fault description, I will check C48 and C49 first).

CAVEAT: That U7-B OpAmp has a gain of (1 + Rf/Ri) = 4 up to a Fh of 159 Hz and a gain of 2 above that frequency. So there is an additional mess with the effectiveness of the summing junction with frequency.

Then there are 2 additional protection circuits.
Q5 mutes the power amp drive when no guitar is plugged in.
Q11 etc. I'm fairly sure is simply an over temperature switch but every analysis I did suggests that over temperature results in turning UP the diff amp current ???

Now this was typed up after the 2nd or perhaps the 3rd(whose counting) Rye Whisky which increase the chances that I missed something critical.

Cheers,
Ian

Cheers Ian

[J M Fahey]

Hi Guys,
This Power Amp pushed one of my buttons.
I haven't physically attacked it yet but thought it would be a good idea to analyse the schematic so I knew what I was doing.
Immediate flags all over the place.
The speaker -ve goes to ground via 0.1 Ohm so there is current feedback, positive current feedback will reduce the output impedance and negative current feedback will increase he output impedance and emulate a valve out put stage , so which is it.?
By my analysis it is negative current feedback.
The output is a Complementary Emitter Follower Pair {CFP} consisting of (Q15,Q18 and Q17,Q19).
Flag #1
For a CFP the drive signals to the 2 sides MUST be in phase so why the hell do we have a phase spitter (Q6,Q7)?
The drive to the top of the push pull output is from Q6 collector, the drive to the bottom of the push pull is from Q9 collector BUT Q9 is a simple inverter off the opposite side of the phase inverter (from Q7 collector).
So drive to the output complementary pair is in phase at it should be.
This is where I may go wrong with confidence.
There are actually 2 amplifiers here.
The second one has output transistors Q10 and Q11. It NEEDS the phase inverter. If you check the phases of the drives (I found) then you find that drive via R84 is an amplified copy of the input signal - same phase.
It feeds Q12 and Q13 which are output transistor drive clamps via voltage limiting zener + diode thus limiting drive voltage). This clamp is also triggered if the output transistor current exceeds about 3 Amps (R103 and R105 =.22 Ohms.)
Notice That the signal voltage clamp is triggered by comparing the amplified input voltage (at R84) with the voltage at the speaker +ve.
Q14 and Q16 are also output transistor drive clamps. R89 and R92 are bootstrapped via C53 and C54 and so their effective impdedance is way more than their 1k5 value. That means that R95, R97 are pretty much a true divide by 2.
So we have 3 effective output drive clamps, Output transistor overcurrent, and overvoltage clamp triggered by difference between speaker drive voltage and a copy of the input signal, and a 3rd (Q14,Q16) one which seems to be triggered when the output tranny drive voltage is more than 2X the expected speaker output voltage (open speaker protection?).
We are now back to that phase splitter.
This is where they have been real clever or perhaps just too clever for me.
Positive (left side of the diff amp / phase splitter is driven by U7-A.
The Negative (right side us driven by what?
Well U7-0B is doing its bit to provide a say 10 Ohms output impedance with its AC phase the same as the +ve diff amp input. It is going to force the Q7 base to act like a summing junction).
So what are we summing in there?
1) The input signal which exceeds +/- 0.65V *D15,D16)
2) DC and AC feedback (R70)
3) DC feedback (to keep the speaker +ve at 0V average , via R71,R72, filtered by C48,C49 Fh = 1,8Hz) - So when I get physically to the amp, given to fault description, I will check C48 and C49 first).

CAVEAT: That U7-B OpAmp has a gain of (1 + Rf/Ri) = 4 up to a Fh of 159 Hz and a gain of 2 above that frequency. So there is an additional mess with the effectiveness of the summing junction with frequency.

Then there are 2 additional protection circuits.
Q5 mutes the power amp drive when no guitar is plugged in.
Q11 etc. I'm fairly sure is simply an over temperature switch but every analysis I did suggests that over temperature results in turning UP the diff amp current ???

Now this was typed up after the 2nd or perhaps the 3rd(whose counting) Rye Whisky which increase the chances that I missed something critical.

Cheers,
Ian

Cheers Ian

I down loaded the manual which has a schematic

OK, so WHERE is the schematic?
I don´t see it anywhere and it´s needed to follow your long list of complaints.
Please post it here so anybody can actually answer

[Enzo]

http://bmamps.com/Schematics/fender/FM65R_Schematic.pdf

[nickb]

Hi Guys,
This Power Amp pushed one of my buttons.
I haven't physically attacked it yet but thought it would be a good idea to analyse the schematic so I knew what I was doing.
Immediate flags all over the place.
The speaker -ve goes to ground via 0.1 Ohm so there is current feedback, positive current feedback will reduce the output impedance and negative current feedback will increase he output impedance and emulate a valve out put stage , so which is it.?
By my analysis it is negative current feedback.
The output is a Complementary Emitter Follower Pair {CFP} consisting of (Q15,Q18 and Q17,Q19).
Flag #1
For a CFP the drive signals to the 2 sides MUST be in phase so why the hell do we have a phase spitter (Q6,Q7)?
The drive to the top of the push pull output is from Q6 collector, the drive to the bottom of the push pull is from Q9 collector BUT Q9 is a simple inverter off the opposite side of the phase inverter (from Q7 collector).
So drive to the output complementary pair is in phase at it should be.
This is where I may go wrong with confidence.
There are actually 2 amplifiers here.
The second one has output transistors Q10 and Q11. It NEEDS the phase inverter. If you check the phases of the drives (I found) then you find that drive via R84 is an amplified copy of the input signal - same phase.
It feeds Q12 and Q13 which are output transistor drive clamps via voltage limiting zener + diode thus limiting drive voltage). This clamp is also triggered if the output transistor current exceeds about 3 Amps (R103 and R105 =.22 Ohms.)
Notice That the signal voltage clamp is triggered by comparing the amplified input voltage (at R84) with the voltage at the speaker +ve.
Q14 and Q16 are also output transistor drive clamps. R89 and R92 are bootstrapped via C53 and C54 and so their effective impdedance is way more than their 1k5 value. That means that R95, R97 are pretty much a true divide by 2.
So we have 3 effective output drive clamps, Output transistor overcurrent, and overvoltage clamp triggered by difference between speaker drive voltage and a copy of the input signal, and a 3rd (Q14,Q16) one which seems to be triggered when the output tranny drive voltage is more than 2X the expected speaker output voltage (open speaker protection?).
We are now back to that phase splitter.
This is where they have been real clever or perhaps just too clever for me.
Positive (left side of the diff amp / phase splitter is driven by U7-A.
The Negative (right side us driven by what?
Well U7-0B is doing its bit to provide a say 10 Ohms output impedance with its AC phase the same as the +ve diff amp input. It is going to force the Q7 base to act like a summing junction).
So what are we summing in there?
1) The input signal which exceeds +/- 0.65V *D15,D16)
2) DC and AC feedback (R70)
3) DC feedback (to keep the speaker +ve at 0V average , via R71,R72, filtered by C48,C49 Fh = 1,8Hz) - So when I get physically to the amp, given to fault description, I will check C48 and C49 first).

CAVEAT: That U7-B OpAmp has a gain of (1 + Rf/Ri) = 4 up to a Fh of 159 Hz and a gain of 2 above that frequency. So there is an additional mess with the effectiveness of the summing junction with frequency.

Then there are 2 additional protection circuits.
Q5 mutes the power amp drive when no guitar is plugged in.
Q11 etc. I'm fairly sure is simply an over temperature switch but every analysis I did suggests that over temperature results in turning UP the diff amp current ???

Now this was typed up after the 2nd or perhaps the 3rd(whose counting) Rye Whisky which increase the chances that I missed something critical.

Cheers,
Ian

Cheers Ian

Oh, some of that is just bizarre. The output of U07A is going to be badly distorted. The temperature compensation doesn't do anything other than of a transient nature as there no DC connection between Q22 and Q21. One would (usually) expect it to mute the power amp by turning off the current source in the tail of the diff amp.

I therefore posit
(a) the schematic contains more than one drawing error
and / or
(b) I need some of that rye...

[DrGonz78]

The schematic has at least one type of error... Anywhere that is lists 2N4001 or 2N4003 it really is 2N4401 and 2N4403.

[J M Fahey]

I felt the same in the beginning, for a long time I didn´t understand it , even helped Lowell and a couple more repair theirs without fully understanding it and IF I had received one to repair with a short time to deliver a working unit (typical: "it´s my only amp and I need it by Friday 6 PM at most, today being Thursday") AND wouldn´t have found the problem by Friday noon, I would have pulled all the nonsensical Mods and turned it into a plain vanilla amplifier ... which to boot would have had the exact same sound.

Yes, there is a very conventional amplifier in there, clumsily covered in unneeded extra clothing and makeup.

The very conventional and fully adequate amp would be:
* Q6/Q7 differential input pair; audio input should go straight to Q6 base, without the U7 nonsense; NFB should go to Q7 base, period.

* Q10 is the voltage gain stage (Vas) , Q11 is its active load.
A normal amp would add a biasing transistor or a few diodes there, top collectr would drive the top driver transistor, bottom collector would do the opposite, but no, they had to do it "a different way" so they do not put the bias there.
OF COURSE it´s needed, so they have to add it later, one biasing transistor for the top half : Q14 , and its partner for the lower half: Q16.

At the beginning I believed that diodes D33 and D34 contributed to biasing, but not, they are inside a local NFB loop (R95/97 and R98/100) so total biasing is still "4 diodes" , like all Fender amps in the last 15/20 years.

* then we have very conventional discrete Darlingtons Q15/18 and Q17/19

* Q12/13 are short protection transistors, fed from a complex diodes and resistors net which tries to make them tightly follow power transistor second breakdown curve. Not bad but must be adjusted to the actual speakers they will use.

* NFB is practically constant current drive, and using the Randall trick of splitting the NFB resistor and grounding the centerpoint , practically hidden because of all the added stuff, such as the un necessary input buffer U7a and the only part I do not understand: U7b but which I suspect is a DC servo (U7b is an integrator so it does not pass Audio signals) , maybe needed because output DC would drift too much because of all the add-ons.
Or maybe they can not live with 10/20mV offset at the speaker out and they want to reduce it to less than 1mV , don´t know.

* throw in a dozen diodes to separate the add ons to interact too much or maybe to correvt some tiny tiny piece of a waveform, dunno.

Of course it works, there is a real amplifier inside after all

At the beginning I thought the add ons and mods were added to make it clip in a rounded way, like Tube amps are *supposed" to do, but I discarded that idea.

In any case, adding non linear elements such as diodes inside the amp is self defeating because NFB treats any difference as an error and corrects it.

* the circuit around Q21 and Q22 is just a thermal overload protection, I hope PTC RT1 is in direct contact with the heat sink.

Edit:
* D15/16 won´t make U7a distort because they are neither in U7a feedback loop (should go from output to -in) or, say, from output to ground.
I also thought so at the beginning but now I see those are just the differential pair input to input protection .
Why on an input which will never ever see the outside World?
Who knows, this design looks like a basic sensible one, with extras copypasted from various sources.

We have a popular saying for such add-ons: "más inútil que cenicero de moto" , meaning: "more useless than an ashtray in a motorcycle"