Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Amplifier Discussion => Topic started by: FenderDeluxe112Plus on April 12, 2022, 04:39:01 PM

Title: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on April 12, 2022, 04:39:01 PM
Hello, this is my first time posting on the forum, I have been using related posts on here to help me so far.  Apologies if I have posted this in the wrong place.

I have a Fender 112 deluxe plus that was blowing the internal 2a fuse on start up. The amp was also making a very loud buzz with all volumes at 0.
I disconnected the speaker and checked the output transistors. I found a shorted pair of tip142/tip147 so I replaced all four transistors using new micas and heat compound.  I pulled and checked the heatsink diodes which all tested ok with multimeter. I also replaced the .47 emitter resistors for each output transistor.  I checked the diodes in the power supply and Qs 3-9.

The channel switching was also playing up - turning on the clean and overdrive channels at the same time. I replaced the u4 ic after finding it shorted with a new 4560D and powered the amp up with a light bulb limiter. This seemed to have solved that problem and the channel switching works again. The bulb was glowing very dimly with no sign of shorts  and fuse not blowing, so I powered off and reconnected the reverb pan - still all good. Then I powered off and reconnected the speaker - the amp is still making the same very loud buzz. I have gone through the entire board with a multimeter checking for continuity between each solder joint and all seems well. I reflowed the solder on a number of joints including the filter caps just to be sure. The 4 ohm speaker measures as 3.97ohms, so I assume ok. Could this be filter cap/power supply cap related or an issue with the transformer? Or am I missing something else e.g. switching issues in the jacks? I know these amps are not worth much but I would like to get it running again.
Any help would be greatly appreciated!

Thanks,

Andy


https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Fender-Deluxe-112-Plus-Schematic.pdf
Title: Re: Fender 112 Deluxe Plus
Post by: Enzo on April 12, 2022, 05:38:20 PM
SO the amp works with no speaker?  At least not blowing fuses?  Measure for DC voltage on the output.  Or look at the speaker cone when it makes the loud hum.  Does the cone move one direction and stay there?  That is DC on a speaker.  It will damage a speaker so no more than a couple seconds please.

DC on the output with no load will not draw large excess current.  Put a speaker or load on it and it will.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on April 12, 2022, 06:12:28 PM
Thanks Enzo,
Yes, the amp powered on ok without the speaker (using the lightbulb limiter) and did not blow the fuse.

When I connected the speaker the noise was apparent but did not blow the fuse before I turned it off.  The lightbulb remained at a dim glow throughout.

I will check for dc on the output and let you know.
Title: Re: Fender 112 Deluxe Plus
Post by: Enzo on April 12, 2022, 10:37:23 PM
OK, if there is no DC, then my next suspects are the main filter caps.  The two standing side by side near the heat sink.  I tend to think they are OK, but may have cracked their solder.

The main rails are 40v each,  are both up to that?  Or is one substantially lower, like 25 or something?  That would indicate large ripple and lack of filtration.  Or certainly you can meter or scope for ripple.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on April 13, 2022, 03:57:17 PM
Hi Enzo,
Here is the reading across the speaker leads.

Thanks,

Andy20220413_205152.jpg
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on April 13, 2022, 05:10:29 PM
Test point readings

Tp3    39.10vdc
Tp4   -39.10vdc

Tp5 16.35
Tp6 -16.42
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on April 13, 2022, 06:03:19 PM
Tp1 30.22vac
Tp2 30.25vac
Title: Re: Fender 112 Deluxe Plus
Post by: Enzo on April 13, 2022, 08:14:13 PM
If TP 3,4,5,6 are right - and they are - then TP1,2 pretty much had to be.

OK, so no DC on output and rails are right.   You might look for ripple on the supplies, but looks like that isn't the deal.

Plug the guitar into the POWER AMP IN jack and dial it to zero.  Hum still?
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on April 14, 2022, 03:04:53 PM
The buzz is still present with all volumes on 0 and guitar plugged into the power amp in.
Title: Re: Fender 112 Deluxe Plus
Post by: Enzo on April 14, 2022, 04:30:52 PM
I meant the volume on the guitar itself, we were bypassing all the preamp.  I will assume you did that.

That puts the hum in the power amp.  CAn you scope or otherwise measure the ripple on the 40v rails? 

Q7 is the mute JFET at the input of the power amp.  SHort across it, does that kill the hum?  Or remove Q7.  Same question.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on April 14, 2022, 04:42:16 PM
Hello, yes that was with all volumes at   zero - amp and guitar.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on April 14, 2022, 04:44:24 PM
Unfortunately I don't have a scope.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on April 14, 2022, 04:58:06 PM
Tp720220414_215543.jpg
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on April 14, 2022, 05:05:16 PM
Tp820220414_220201.jpg
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on April 14, 2022, 05:16:51 PM
When shorting q7, should I wire across joining the three legs of the fet?
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on April 14, 2022, 05:19:43 PM
The test points for 7 and 8 are a long way off - does this also suggest a preamp issue?
Title: Re: Fender 112 Deluxe Plus
Post by: phatt on April 14, 2022, 08:00:15 PM
@ FenderD112Plu, Regards Test point 7 & 8
Those AC test points need to be taken with a signal at input.
Read Note 5 bottom left corner for clues.
Phil.
Title: Re: Fender 112 Deluxe Plus
Post by: Enzo on April 14, 2022, 08:13:22 PM
2mv hum is not a lot

The JFET is across the signal path.  I just want to know what happens when you short from r59/C33 to ground.  NO need to short the gate.   While the JFET itself can be noisy, I was thinking your noise is either coming into the power amp through that path or it is after that point
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on April 15, 2022, 02:15:52 AM
Thank you both for clarifying those points for me.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on April 15, 2022, 06:00:14 AM
@ Enzo, buzz still happening with r59 shorted to ground.
Title: Re: Fender 112 Deluxe Plus
Post by: g1 on April 16, 2022, 07:07:36 PM
Check supplies for ripple.  You don't have a scope so you can measure AC volts at TP's 3,4,5,6
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on April 17, 2022, 03:00:09 PM
Thanks g1,

Ac voltages

Tp3 0.1174
Tp4 0.1421
Tp5 0.0025
Tp6 0.0023
Title: Re: Fender 112 Deluxe Plus
Post by: phatt on April 17, 2022, 06:52:24 PM
When you reinstalled the PCB you may have inadvertently disconnected the Chassis Com from circuit com.
If the amp is otherwise working I would be checking that you have Circuit common in contact with Chassis.
If that connection is open it will cause hum.
Phil.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on April 18, 2022, 05:44:22 AM
Hi Phil,
The chassis ground is attached and I have checked the continuity of the pots and they are grounded to the chassis.

R1 is not showing a continuity to the chassis - is that as expected?
Title: Re: Fender 112 Deluxe Plus
Post by: Enzo on April 18, 2022, 01:08:54 PM
The schematic does not show the PA jack tip connected to R1.   The tip is pin 2, which when plugged connects through auxiliary contacts 5 and 6 down to R99, which is the power amp input.   R1 does not connect to ground, it runs through the other auxiliary contacts and then back over to the input jacks.  It is part of the mute circuit.
Title: Re: Fender 112 Deluxe Plus
Post by: phatt on April 19, 2022, 07:06:11 AM
It's a long shot but,,, maybe check you have continuity from Mains plug earth pin back to chassis. For that matter try another wall outlet they can indeed fail on rare occasions? (if earth pin in an outlet fails equipment still works,, just no earth :o )
Phil.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on April 19, 2022, 07:35:04 AM
Thanks Phil,
Continuity between earth pin and chassis is good and I have tried plugging into different plug sockets with same result.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on April 19, 2022, 09:52:43 AM
Tp16 and Tp17 give the expected voltages of +25.7 vdc and -25.7 vdc.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on April 24, 2022, 10:30:39 AM
Hello, does anyone have ideas about how I should proceed next? Is getting hold of a scope the best option?
Title: Re: Fender 112 Deluxe Plus
Post by: phatt on April 24, 2022, 09:39:20 PM
A scope may not help much if it's ground hum.
I'm assuming the amp is working and passing signal except for the hum?

Well as there is no ripple on the supply and voltages are within spec then the most likely issue is one of 2 possibilities.
1/ open ground connection somewhere.
If the star ground node of main supply is not connected back to chassis correctly then it can cause hum issues. (Which is the node between C47 & C48 or Conn CP4)
You can test simply by shorting the main Common back to chassis with a wire. If the hum stops then you found the issue,, if hum increases then you just created a ground loop,, which brings us to,, 2/ Something is grounded that should Not be grounded, which would cause a ground loop hum.
Ground plane issues can be a nightmare to track down.
sometimes in design phase one has to use an alligator clip to find the best ground path before committing to a layout.

Often components like input jacks, FX loops and speaker outputs,  even the PCB need to be *Isolated from chassis*
This will depend on how the circuit common was designed which is hardly ever noted on schematics.

Tiss easy to loose these little isolation washers or forget to reinstall them.

If you acquired this amp with issues then someone may have already worked on it and if small parts have been lost then you have no idea why it hums.

*I would be researching ground path issues long before replacing parts in hope.*

I got caught once, I did not notice an isolation washer dropped off a PCB post and the hum drove me nuts for a few days.
finally found it hiding in the chassis corner. >:(
Phil.
Title: Re: Fender 112 Deluxe Plus
Post by: SemiConductive on April 26, 2022, 03:11:48 PM
FWIW, I think Fender isolates signal ground from the chassis in all their small SS amps.
Title: Re: Fender 112 Deluxe Plus
Post by: joecool85 on April 26, 2022, 05:23:09 PM
Quote from: phatt on April 24, 2022, 09:39:20 PMA scope may not help much if it's ground hum.
I'm assuming the amp is working and passing signal except for the hum?

Well as there is no ripple on the supply and voltages are within spec then the most likely issue is one of 2 possibilities.
1/ open ground connection somewhere.
If the star ground node of main supply is not connected back to chassis correctly then it can cause hum issues. (Which is the node between C47 & C48 or Conn CP4)
You can test simply by shorting the main Common back to chassis with a wire. If the hum stops then you found the issue,, if hum increases then you just created a ground loop,, which brings us to,, 2/ Something is grounded that should Not be grounded, which would cause a ground loop hum.
Ground plane issues can be a nightmare to track down.
sometimes in design phase one has to use an alligator clip to find the best ground path before committing to a layout.

Often components like input jacks, FX loops and speaker outputs,  even the PCB need to be *Isolated from chassis*
This will depend on how the circuit common was designed which is hardly ever noted on schematics.

Tiss easy to loose these little isolation washers or forget to reinstall them.

If you acquired this amp with issues then someone may have already worked on it and if small parts have been lost then you have no idea why it hums.

*I would be researching ground path issues long before replacing parts in hope.*

I got caught once, I did not notice an isolation washer dropped off a PCB post and the hum drove me nuts for a few days.
finally found it hiding in the chassis corner. >:(
Phil.

Been there.  I did this on my Dean Markley amp years ago.  Took forever to figure it out!
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on April 28, 2022, 11:45:12 AM
I am reading around 24ohms resistance from cp4 to chassis ground.
Title: Re: Fender 112 Deluxe Plus
Post by: joecool85 on April 28, 2022, 12:45:13 PM
Quote from: FenderDeluxe112Plus on April 28, 2022, 11:45:12 AMI am reading around 24ohms resistance from cp4 to chassis ground.

What about from CP2 to chassis ground?
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on April 28, 2022, 01:31:21 PM

Cp4 24.13ohms

Cp2 24.24ohms
Title: Re: Fender 112 Deluxe Plus
Post by: g1 on April 28, 2022, 02:36:13 PM
R4 and R5 run in parallel from circuit ground to chassis (pot brackets).
As they are each 47 ohms, 24 ohms seems the correct reading from CP4 to chassis.
It's possible there is another connection to chassis somewhere but I don't see anything on the schematic.
You could try shorting across R4 or R5 as a test and see if it affects the hum/buzz. 
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on April 28, 2022, 04:17:18 PM
Thanks g1,
Have just checked r4 and r5 with multimeter and the grounding seems good and as expected.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on April 28, 2022, 04:26:12 PM
Checking grounding of some of the electrolytics-I am getting 757 ohm resistance to chassis  on pos lead of c53.
Title: Re: Fender 112 Deluxe Plus
Post by: Enzo on April 28, 2022, 06:25:06 PM
And at the other end?
Title: Re: Fender 112 Deluxe Plus
Post by: phatt on April 29, 2022, 02:23:11 AM
Just turn the amp on and with a length of wire with alligator clips on the ends and just probe the Known ground points of the pcb and related ground points back to chassis.
**Just make darn sure you don't short power nodes back to ground.**
Use the schematic to verify test points and ground nodes.

If hum increases then you have created a ground loop.
If it reduces hum then you know there is a ground missing somewhere.

BTW the speaker Neg terminal is NOT Ground. This is a current feedback system and spk NEG is lifted from circuit common via that big 10 Watt resistor, R76.

You can visually work out the ground path but often not obvious.
Hence the wire probe can help define the problem.
Maybe post some circuit pictures and we might be able to help more?
Phil.
Title: Re: Fender 112 Deluxe Plus
Post by: joecool85 on April 29, 2022, 10:09:02 AM
Quote from: phatt on April 29, 2022, 02:23:11 AMJust turn the amp on and with a length of wire with alligator clips on the ends and just probe the Known ground points of the pcb and related ground points back to chassis.
**Just make darn sure you don't short power nodes back to ground.**
Use the schematic to verify test points and ground nodes.

If hum increases then you have created a ground loop.
If it reduces hum then you know there is a ground missing somewhere.

BTW the speaker Neg terminal is NOT Ground. This is a current feedback system and spk NEG is lifted from circuit common via that big 10 Watt resistor, R76.

You can visually work out the ground path but often not obvious.
Hence the wire probe can help define the problem.
Maybe post some circuit pictures and we might be able to help more?
Phil.

I'm with Phil on this one.  I actually have an old test lead from a dead multimeter than I connected an alligator clip to for this purpose - works great.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 02, 2022, 07:02:38 AM
Thanks for the help,
Jumping the node at cp4 to chassis ground produces same buzz.  With neg lead of c53 shorted to chassis getting same buzz. I double checked Enzo's suggestion of shorting q7 at r59 but still buzzing.
I just plugged guitar in to check signal is passing through amp to speaker. Input 1 produces very distorted sound on clean channel. Input 2 is less distorted.

Clip of the sound attached...

amp noise.mp3
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 02, 2022, 01:21:14 PM
Hello Phil,
Here are some pics of the wiring from the Transformer and power inlet area of the circuit.

The dodgy(!) looking joints on the board have since been reworked and checked with a mm.

20220502_180723.jpg20220502_180624.jpg20220223_183000.jpg20220223_182954.jpg   
Title: Re: Fender 112 Deluxe Plus
Post by: g1 on May 02, 2022, 02:51:58 PM
The pot brackets in these amps are used to carry ground connections.  I've circled one example in your photo below.  I would just resolder them all.
Suggest you deal with the obvious audio problems first (ch.1 distorted), maybe you will get lucky and fix the hum while you are at it.


pot brckt.jpg
Title: Re: Fender 112 Deluxe Plus
Post by: phatt on May 03, 2022, 03:10:35 AM
Oh dear, Reset,, that noise is not ground hum/buzz that sounds more like a blown poweramp. :o
I asked if the amp is working and passing signal except for some hum/buzz,, if so you may have a ground issue.

Obviously this is NOT the case.

In your fist post you said you replaced the power transistors but there might still be a problem in the power amp.
My best guess is the opamp at the input of power amp stage might be stuffed.
Check the DC test points on U1. (pin7&4)
Phil.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 03, 2022, 06:15:02 AM
Thanks,

Tp16 (U1 pin 7) and Tp17 (U1 pin 4) give the expected voltages of +25.7 vdc and -25.7 vdc.
 
Could the U1 still be causing problems?

Title: Re: Fender 112 Deluxe Plus
Post by: phatt on May 03, 2022, 07:57:15 AM

Yes it's possible when the power Tr's blew it also effected U1.

U1 is the power stage input, those 4 Pwr Tr's boost the current needed to drive the speaker so both U1 and Pwr tr's could have failed.
The other 2 tr's (Q8 & Q9 I think) are part of the over current protection. 

Now assuming none of the passives or Q8&Q9 are at fault then replacing U1 Should fix the problem.

Hopefully others more qualified will chime in,, if I've missed something.

As you have noted that plugging into the Pwr Amp input still sounds distorted then the Power stage is most likely at fault.

If you want to check if the preamp is working normally then take the Preamp output to another amp (if you have one) if that produces the same Buzz then the problem maybe further up stream in the signal path.
Phil.
Title: Re: Fender 112 Deluxe Plus
Post by: g1 on May 03, 2022, 02:58:21 PM
As real MC1436 are hard to find, and I think the circuit is picky about the exact part, post your DC voltages for U1, pins 2,3, and 6.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 03, 2022, 03:57:37 PM
Thanks, will do. I do have an Mc1436 in my parts box from a supplier that I have bought good components from in the past. Can I test it in any way to check if it would be suitable?
Title: Re: Fender 112 Deluxe Plus
Post by: joecool85 on May 03, 2022, 04:05:08 PM
Quote from: FenderDeluxe112Plus on May 03, 2022, 03:57:37 PMThanks, will do. I do have an Mc1436 in my parts box from a supplier that I have bought good components from in the past. Can I test it in any way to check if it would be suitable?

Throw in a socket on the board and then try it.  If it works, you can leave it in the socket, if not, take it out :-)
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 18, 2022, 04:16:13 PM
Hi all,
I have finally got the amp out to work on again.I have installed an ic socket as suggested by Joe. Unfortunately the solder pad for pin 8 has lifted. Am I right in thinking this is not a problem as it is not on diagram?

20220518_210113.jpgScreenshot_20220518-205602_Samsung Notes.jpg 
Title: Re: Fender 112 Deluxe Plus
Post by: willpirkle on May 18, 2022, 05:42:59 PM
For the MC1436, pin 8 is a no-connect (N.C.) so you can ignore a connection there.

However, be careful. Often, the supply pins are omitted from the main schematic page(s) because layout techs usually put all the op-amp power supply connections and all of their bypass caps in one area, usually separated from the other pages. This makes the schematic cleaner and easier to read, but can be easily mis-interpreted.

- Will
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 19, 2022, 03:20:52 PM
With the new op amp in U1  the amp seems to be drawing more current through the 100w bulb. Not a huge amount, but the bulb is glowing slightly more. This is with the speaker still disconnected. What is a sensible way to proceed next? Does this suggest power amp diodes or q7 and 8 breaking down under load?
Should I connect the speaker at this stage? I would prefer not to blow the entire power amp again :)20220519_201117.jpg
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 19, 2022, 03:26:35 PM
20220502_104745.jpg
This is the bulb previously.
I did check all of the new solder joints with a mm for continuity to other components.
Title: Re: Fender 112 Deluxe Plus
Post by: Loudthud on May 19, 2022, 04:29:23 PM
Check for DC on the speaker output and DC Voltages around the preamp. Something must have changed.
Title: Re: Fender 112 Deluxe Plus
Post by: phatt on May 20, 2022, 03:58:47 AM
As mentioned yes check for DC on output but tiss a fair bet that now that the circuit is correct it maybe well be just drawing more idle power.
I would suggest, try a speaker  (with bulb still connected)
If the bulb glows very bright at idle (no signal passing) then there is still an issue.
Phil.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 20, 2022, 06:25:50 AM
Thank you both, will check for dc later.
Was hoping it might be the case that amp is now running correctly and just drawing more idle current!
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 20, 2022, 12:26:30 PM
Some very slight fluctuations. And has increased since opamp change.20220520_171613.jpg

Out of interest what would be considered acceptable dc?
Title: Re: Fender 112 Deluxe Plus
Post by: g1 on May 20, 2022, 02:02:04 PM
Less than a hundred millivolts DC at output is fine.  You have about 50mV so no problem.
On the bulb limiter with power on, connect the speaker.
Some amps won't start up right with load connected when on the bulb, so that's why I said connect speaker while power on.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 20, 2022, 02:58:46 PM
Just fired the amp up- still making the same noise.   :(
Bulb did not brighten any further with speaker connected.
Title: Re: Fender 112 Deluxe Plus
Post by: phatt on May 20, 2022, 08:32:08 PM
Sorry to hear that. :(
Ok I'm gonna take a stab in the dark and asking better minds to look at this.

Look at J1,J2 & J5 as they are all interconnected to the low voltage rails via R1 & R49 as well as CR32, C54 & C55.

Now IF??? one of those switching circuits inside J1,J2 or J5 was faulty then it could cause a big DC offset. yes/no?

I've never liked those encapsulated plastic sockets as they are rather fragile inside.

My thoughts,,If the input side of C33 was lifted that would completely isolate the power amp input from any interference.

If that resolves the Buzz then I'd hunting down the offending socket,, or just replace all 3.
Phil.
Title: Re: Fender 112 Deluxe Plus
Post by: g1 on May 26, 2022, 02:21:53 PM
So you now have no significant DC on the output, and the bulb does not get any brighter with the load connected.  That is good so far.

Connect your guitar to the 'power amp in' jack.  Do you get the same distorted sound?
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 27, 2022, 10:19:36 AM
Can confirm same sound with guitar plugged into power amp in, but sound is slightly quieter.
Title: Re: Fender 112 Deluxe Plus
Post by: g1 on May 27, 2022, 01:51:20 PM
Confirm that CR14 cathode (or CR13 anode) is still 0VDC with no load connected.
If so, connect load and recheck.
Also check DC volts at pin6 of U1.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 27, 2022, 02:31:38 PM
-0.036vdc at cr14 cathode and cr13 anode without load.

00.22vdc on pin 6 of u1 without load
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 27, 2022, 02:58:21 PM
-0.042 cr14 cathode with load
-0.044 cr13 anode with load
0.016vdc u1 pin6 with load
Title: Re: Fender 112 Deluxe Plus
Post by: g1 on May 27, 2022, 03:40:20 PM
Good so far. 
How about DC voltages for E,B,&C of Q1,Q10,Q2,Q11
Also Q8 and Q9
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 27, 2022, 04:07:35 PM
Q1
B01.22
C39.22
E-00.02

Q10
B01.22
C39.20
E-00.02

Q11
B-01.19
C-39.47
E-00.05

Q2
B-01.19
C-39.44
E-00.05

All without speaker load
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 27, 2022, 04:23:09 PM

Q8
E-00.03
B-00.09
C 02.92

Q9
E-00.04
B 00.02
C-02.59
Title: Re: Fender 112 Deluxe Plus
Post by: g1 on May 27, 2022, 11:17:58 PM
Double check those collector voltages for Q8 and Q9.
Where could they be getting a voltage higher than the base voltages of the outputs?
(ex. Q1 base is 1.2V, Q8 collector gets voltage from that line)
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 28, 2022, 04:13:57 AM
Thanks g1. Have just double checked the dc voltages.

Q1 base 01.23 vdc
Q8 collector 03.29 vdc

Q2 base -01.20 vdc
Q9 collector -02.74 vdc


Title: Re: Fender 112 Deluxe Plus
Post by: g1 on May 28, 2022, 01:47:46 PM
According to the schematic, that is impossible.
So something funny, or we don't have a schematic match.

Check resistance from CR39 anode to Q1 base.  And resistance from CR39 cathode to Q8 collector.
Same for CR40 to Q2 and Q9.

q8q9.jpg

Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 28, 2022, 02:07:53 PM
Continuity between cr39 anode and Q1 base.
Continuity between Q8 collector and cr39 cathode.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 28, 2022, 02:13:49 PM
20220528_191147.jpg

Test point pairs marked in red and green.
Title: Re: Fender 112 Deluxe Plus
Post by: g1 on May 28, 2022, 03:00:55 PM
Quote from: FenderDeluxe112Plus on May 28, 2022, 02:07:53 PMContinuity between cr39 anode and Q1 base.
Continuity between Q8 collector and cr39 cathode.
Please specify reading in ohms.  Continuity is a somewhat vague term that changes depending on model of meter.  If relying on a meter beep for continuity, it can range anywhere from zero to 200 ohms.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 28, 2022, 03:12:45 PM
Apologies .0002 ohms for both readings.
Title: Re: Fender 112 Deluxe Plus
Post by: g1 on May 28, 2022, 03:26:02 PM
That's good.  Now measure DC voltage at CR39 anode and cathode.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 28, 2022, 03:36:46 PM
With com clipped to chassis.
C39 cathode20220528_203143.jpg

20220528_203058.jpg 
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 28, 2022, 03:41:23 PM
Cr39 anode20220528_203036.jpg
20220528_203042.jpg 
Title: Re: Fender 112 Deluxe Plus
Post by: g1 on May 28, 2022, 04:06:44 PM
That is really strange.  There is no where for that extra couple volts on Q8 collector to come from. 
If it were a solder bridge, it's unlikely the same exact thing would show up on the negative side (Q9).
So I would guess it's either an oscillation that the meter is interpreting as DC, or it's just a ghost reading and irrelevant.

Do Q8 and Q9 check out ok with diode check in circuit?
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 28, 2022, 04:44:36 PM
Diode check Q8

Pos to collector com to base 1
Com to collector pos to base .6489

Pos to base com to emitter .6536
Com to emitter pos to base 1.0033

Pos to collector com to emitter 1
Pos to emitter com to collector 1.0390
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 28, 2022, 04:53:24 PM
Q9
Pos to collector Com to base .6760
Com to collector poss to base 1

Com to emitter pos to base 1.0018
Pos to emitter com to base.6780

Pos to emitter com to collector 1
Pos to collector com to emitter .8232
Title: Re: Fender 112 Deluxe Plus
Post by: g1 on May 28, 2022, 08:01:26 PM
Quote from: FenderDeluxe112Plus on May 28, 2022, 04:53:24 PMQ9
Pos to collector com to emitter .8232
That seems a bit suspect.  Have Q8 and Q9 been replaced?
I would remove them and re-check them out of circuit.

Also, have CR5 thru CR12 and CR39 thru CR42 been checked in circuit?  Just looking for proper .4 to .7V readings forward biased, and no shorts.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 29, 2022, 03:10:03 AM
Thanks g1,

Cr9-12 and cr39-42 read between 0.6 and 0.7 with a reading of 1 with leads reversed.

Cr5 -8 are within the heatsink so I can't read them at the moment - I did pull them to test when I changed the output transistors and they were good.

Agree about q9. I have a few 2n4403s so I will replace it next time I pull the board.


Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 29, 2022, 03:18:32 AM
Cr13 and 14 have a forward voltage of 0.5 which seems ok.

Reversed they read 1.9020 and 1.8064
Does that check out ok?
Title: Re: Fender 112 Deluxe Plus
Post by: g1 on May 29, 2022, 02:32:00 PM
Quote from: FenderDeluxe112Plus on May 29, 2022, 03:18:32 AMCr13 and 14 have a forward voltage of 0.5 which seems ok.

Reversed they read 1.9020 and 1.8064
Does that check out ok?
Yes that seems right.
In post #41 sound file, is that the sound it makes with no signal, or is that with a tone at the input?
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 29, 2022, 02:40:15 PM
Yes, that is the sound at switch on with nothing plugged in.

Title: Re: Fender 112 Deluxe Plus
Post by: g1 on May 29, 2022, 04:44:51 PM
So that sounds like oscillation to me.
Try jumpering CR3 anode (or CR4 cathode) to ground and check if the oscillation sound is still there.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 30, 2022, 07:31:21 AM
With a jumper to ground at cr3 anode the buzz is there but quieter.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 30, 2022, 07:35:32 AM
Similar to plugging guitar into power amp in.
Title: Re: Fender 112 Deluxe Plus
Post by: g1 on May 30, 2022, 10:03:08 AM
Quote from: FenderDeluxe112Plus on May 30, 2022, 07:31:21 AMWith a jumper to ground at cr3 anode the buzz is there but quieter.
Better if we can put some numbers on it.  Measure AC volts across the speaker terminals with and without the jumper.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 30, 2022, 11:28:17 AM
1.48vac with jumper to ground.
3.90vac no jumper

These are the readings that it settled at. Both readings started higher then dropped.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 30, 2022, 11:31:01 AM
Initial ac voltages
1.72vac with jumper
4.5vac no jumper
Title: Re: Fender 112 Deluxe Plus
Post by: g1 on May 30, 2022, 02:20:39 PM
That is a substantial difference.
I searched for "100Hz squarewave" on youtube and it sounds pretty much the same as your soundfile in post #41.
Are you on 50Hz 240V power?

Can you run the 'preamp out' to another amp and see if it has the same sound?
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 30, 2022, 03:52:34 PM
Yes, am on 50hz 230v mains in the U.K.
I looked up 100hz tones on youtube and thought it sounded close.

I don't have another amp here to test it at the moment. I might be able to dig out a sampler to test it though.
Title: Re: Fender 112 Deluxe Plus
Post by: Enzo on May 30, 2022, 04:00:38 PM
If you have a stereo receiver, you could even run the preamp out of the amp to an AUX input on th receiver for a listen.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on May 31, 2022, 04:40:24 PM
Have got the 100hz tone coming from the preamp out.
Title: Re: Fender 112 Deluxe Plus
Post by: g1 on May 31, 2022, 06:56:14 PM
So there is excess 100Hz hum in both the preamp and the power amp.  That pretty much narrows it down to the power supply.
Recheck again the DC and AC volts on the power supply rails.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on June 01, 2022, 07:38:42 AM
Tp1 30.31 vac 
Tp2 30.31 vac

Tp3 30.12vdc    00.04vac
Tp4 - 30.20vdc  02.45 vac

Tp5 16.40 vdc    00.02vac
Tp6 -16.42 vdc  00.01vac

Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on June 01, 2022, 07:43:18 AM
Just double checked ac on collectors of both tip147s
Q11 and Q2
2.34vac 2.35vac respectively.
Title: Re: Fender 112 Deluxe Plus
Post by: g1 on June 01, 2022, 10:07:32 AM
So it seems the problem is on the -48V rail.  I'm not sure why it didn't show up when you measured the AC volts there before.

Check that the solder is good and there are no broken pads at C48.  Sometimes the pads break near the cap and you can't see it because the cap is glued down.  Measure resistance from the (-)lead of the cap to Q2 collector just to be sure.
If no solder or pad/trace issues, replace C48.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on June 01, 2022, 10:41:09 AM
20220601_153320.jpg

Thanks g1,

I guess either I misread or recorded the ac voltages incorrectly, or something changed after I replaced parts.

I reflowed the solder on the main filter caps and checked resistance to all connections when I originally pulled the board so will assume c48 is bad. Will order new filter caps!

Thanks again
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on July 20, 2022, 04:30:00 AM
Hi all,

I finally got hold of the new filter caps and had time to install them. I also replaced q9 while the board was out.

Long story short - I still have the same or similar buzz happening.

After cap replacement I checked for dc on speaker 0.023vdc

Tp1 30.05vac
Tp2 30.00vac
Tp3 39.02vdc
Tp4 -39.03vdc

Ac voltages

Tp3 0.066vac
Tp4 0.209vac


Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on July 20, 2022, 04:36:02 AM
I also checked for broken pads/cracked solder and all checked out ok.

Thanks,
Andy
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on July 20, 2022, 04:57:05 AM
Checked the ac on speaker 3.59vac
It drops to 1.32 wiith a jumper from cr3 anode to ground.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on July 20, 2022, 10:24:31 AM
Just ran the preamp out into another amp. The buzz is there but it is quieter now and does not sound like the previous 100hz hum.
With the main filter caps replaced, could the next suspects be diodes breaking down under load in the power supply?
Title: Re: Fender 112 Deluxe Plus
Post by: phatt on July 22, 2022, 07:37:31 AM
Yes, maybe check them.
Phil
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on July 23, 2022, 03:29:19 PM
I am reading 30.00vac at r82. Does this   suggest the rectifiers are faulty? 20220723_202805.jpg
Title: Re: Fender 112 Deluxe Plus
Post by: Enzo on July 23, 2022, 08:56:56 PM
No.

Look at your schematic.  R82 connects directly to CP5, which the schematic says has 31.9va on it.  Close enough.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on July 24, 2022, 02:57:03 AM
Thanks Enzo,

Where would be the next logical points to test?
Title: Re: Fender 112 Deluxe Plus
Post by: phatt on July 24, 2022, 07:59:04 AM
I get the impression you are not clear on diode testing, If so then
You check diodes with power OFF and lift one end of the diode.
Meter set to diode test.

If you get a reading both ways it's dud.
To save writing it all,, go here;

https://www.fluke.com/en-us/learn/blog/digital-multimeters/how-to-test-diodes
HTH,Phil.
Title: Re: Fender 112 Deluxe Plus
Post by: FenderDeluxe112Plus on July 24, 2022, 10:09:07 AM
Thanks Phil,

This is the method I have been using so far, I thought there might be other ways to test in circuit.