Fender Super 112 - Low Output

[earache]

Hello

I am looking at a Fender Super 112 tube amp that has very low output, and only on the "Gain" channel when it's turned all the way up.

Owner said that the amp needed new input jacks and that's why he thought the output was nil, so I replaced those. and also touched up the solder at all the other jacks and knobs for good measure. That didn't fix the problem...

I sprayed all of the jacks (Ext Speaker, Line Out, PreAmp Out and Power Amp In) with Deoxit but that so far hasn't helped.

I have swapped all tubes for known-good ones in turn - no change.

Finally broke down and disassembled the amp, which I was trying to avoid. It's probably the WORST amp ever for servicing - all components are under the board which is screwed down to standoffs. The reverb and speaker cables are captured in Heyco-type strain reliefs which have to be undone before you can get much apart. No identifying component artwork or legends appear on the top surface of the board, which would at least let you get some voltage readings without disassembly.

I noticed that the pcb exhibits some heat related browning at R168 (10W 20K) and at R155 (1/2W 470 ohm). R168 is stood off from the board with some asbestos looking tubing. Once off of the board I found that this resister still tested within 10% and that heat had loosened it from the board, so I re attached it more securely, still up off of the board for cooling. R155 I replaced with a 1W 470 Ohm metal film type.

Should I just replace all of the jacks? The Power Amp In works OK, but I cant get anything out of PreAmp Out.

super_112_schem.pdf

[gui_tarzan]

Do you get any signal at C117 or U101A pin 1? If it's not getting that far I'd start by checking for signal there going backwards.

[g1]

What he said above, if there IS good signal level there, then it must be the loop jacks or traces/connections to them.
If not good level, check you have +/-15V to the IC's, then start checking other supply and tube DC voltages.
Here is a better schematic, and on the last page is a component layout. Maybe it will help you do checks from the foil side. Yes those amps are a real pain to work on. For simple faults (ex. blown screen resistor, Fender suggested replacing on foil side, rather than removing the board. Just remember to shake the bad part out .

[earache]

gui_tarzan I don't have a way to test for signal at C117 or U101 pin 1, other than a "pop" test, like touching those device points with the lead of my meter and hearing a "pop".
I don't hear a pop at either of those points...I can get a pop at pin 5 of U102, but not at any other points on the 2 opamps.

g-one Thanks for the better schematic, the additional artwork helped me get a few voltage measurements which I will detail below. The schematic states that I should input a signal to test AC voltages, and I don't have a signal generator at the moment so I kept to just the DC voltages.

The schematic states that I should get -15V at P1, 2, 3, 5, 6, and 7 on both U101 and U102...and I do.

I got some strange readings around V102 and V103.

V102
Pin1 expects +210 but reads -71
Pin3 expects +1.43 but reads 0
Pin6 expects +43 but reads +27

V103
Pin1 expects +219 but reads +163
Pin2 expects +43 but reads +24
Pin7 expects +43 but reads +27

I looked at the circuit around where these questionable readings are. At V102, R112 I measured +420 at one side (acceptable) and at the other side of R112 I measure -.7 instead of +210

At V103 R140 I have +419 on one side and +1.5 on the other

At R145 I have +419 on one side, and +164 on the other where I should have +219

What would cause the low voltages? Are both of these resisters bad, or is there something common to them that is making the readings low? I have previously swapped in known-good tubes at these spots (V102, V103) with no improvement.

[Jazz P Bass]

[QUOTE=earache;365166]gui_tarzan I don't have a way to test for signal at C117 or U101 pin 1, other than a "pop" test, like touching those device points with the lead of my meter and hearing a "pop".QUOTE]

You have a volt meter, correct?

Set it to read Volts AC.

Send a signal through & you can measure the signal voltage at the ic output pin.

[52 Bill]

You have a volt meter, correct?

Set it to read resistance and test the values of R112 (100K edit:220K), R140 (82K) and R145 (100K).

R140 and R145 are the plate resistors for the driver tube. It is a very common problem for the 82K resistor to fail in more modern Fender amps. If it read as wide open, replace it and the power amp will come back to life.

R112 is the plate resistor for V102A if it reads wide open, replace it and the preamp should come back to life.

Because it is such a pain to remove the board in this amp, I'd meter test all of the plate resistors before pulling the board to see if there are any others out of spec and then replace them at the same time.

And one Enzo trick is to unsolder the resistor from the top of the board and let is fall under and out of the amp. Then solder the new one to the trace side of the board.

Edit: R112 is a 220K 1/2 watt

[earache]

52 Bill

Thanks for that info - I found that R140 and R112 both tested open and so replaced them with 1 watt metal film types. The amp works great now!

I also followed your hint about unsoldering the resistors from the top, shaking them loose and installing the replacements on the top side. Worked awesomely well.

The schematic I have says that R140 is supposed to be a 91K, but I used an 82K anyway...think that makes much difference?

Jazz P - I understand what you said about just checking for a signal by treating it as an AC signal and measuring it with the meter. It's a great idea and I will put that in my mental "files" to use later. Only question - how would I know what is sufficient signal?

[audiopete]

Only question - how would I know what is sufficient signal?

It says it right on the schematic at different test points (if you can read it). You need to inject a test signal in at schematic specified amplitude and then measure from there.

[earache]

Audiopete. Thanks for the reply. The question was initially posed because I did not have a means to inject a signal at the test points. so the answer then was to just put a signal into the input jack and monitor at the preamp out jack.

[52 Bill]

The schematic I have says that R140 is supposed to be a 91K, but I used an 82K anyway...think that makes much difference?

What was in there originally a 91K? An 82K is withing 10% of the 91K value, so I doubt that it will make much of a difference.

[pierrescotland]

Can I just say THANK YOU to this thread - exact same amp, exact same symptom.

In my case R112 (220Kohm) was blown.

[Juiced]

and another Thank you.

R145 DOA. My "I will never sell it" Super 112 is super again.

[Enzo]

And a strong lesson here: People love to call things "common issues", and in these cases, each amp had a different thing failing, yet the symptom seemed to be the same.

[Familyortiz]

earache,
Good job on checking resistance values. It's one of those super simple things to eliminate variables.