Fender 212r repair: distorted sound output and bump on power on

[vincenzodesanctis]

When I power on the amp there is a bump. And when I play guitar the sound is reduced and distorted, like a transistor fuzz.

I tested the power rails:

TP1 is 32VAC
TP2 is 32VAC
TP3 is 41VDC but there is also 93VAC!!
TP4 is -41VDC but no VAC
TP5 is 16VDC and there is 36VAC!!
TP6 is -16VDC but no VAC

I tried to change the electrolitic caps C71-C72 (4700) but same problem. I tested the diodes value and I think they are good (D53-D54-D55-D56-D57-D58). Even C75 and C76 are good.
I don't know what is the problem and what kind of test I can do.

[Dave H]

The power rails look OK. Make the AC voltage readings again with the meter probes reversed. If you want to go with this new thread delete the other post(s).

[vincenzodesanctis]

Thank you Dave for your reply. I tested with the meter probes reversed on TP3 and TP5 and now there is no VAC.

[vincenzodesanctis]

Do I have to procede to test TP7-TP8-TP9...and so on?

[Dave H]

...I tested with the meter probes reversed on TP3 and TP5 and now there is no VAC.

I thought so. It's your meter. I have one that reads the same on the AC ranges. It's half wave rectified. On AC it reads 2.22 x the DC voltage with the probes one way and zero with the probes reversed. It's only any good for measuring true AC (no DC component) so you can't use it for measuring power supply ripple.

[vincenzodesanctis]

And if I will use a good one? I mean for example fluke!!?

[vincenzodesanctis]

and how to measure power supply ripple?

[J M Fahey]

I'm not aware of Fluke making poor quality meters, so any of them should do, but in general it's not a "brand" problem but a manufacturing savings one.

Cheapest meters typically have only 2 AC scales, say 200VAC and 500 or 750VAC, not *real* AC meters but simply a diode in series with the existing DC meter, a 15 cents solution , and which causes scaling problems if you use it to measure DC (what you did).

Good meters have a real dedicated AC measuring circuit inside, which does *not* read DC (good) and so can read ripple.
They typically have a full AC scale, such as 200mV - 2V - 20V - 200V - 500 or 750V .

And for safety you first set them to 200VAC and touch the rails, then if necessary increase sensitivity step by step until you get a good reading, say try 20VAC>2VAC>200mVAC (if needed) .

I think that unless one main filter got unsoldered (happens sometimes because of vibration and they are heavy) you should not have a ripple problem ... but check of course.

[Mick Bailey]

Make sure the solder joints on R109 and R112 (0R22 ohm) are good, and the resistors themselves measure OK. A bad joint can cause the symptoms you describe.

[vincenzodesanctis]

Thanks Mick Bailey, I tested R109 and R112 and one of them is broken (the internal connection is open).
There is one thing that sound me strange, the others 5W 0R22 5W resistors measure are from 2.2ohm to 1.8ohm , it's strange because I think the correct values should be 0.22ohm. Isn't it? Is my multimeter not good? The multimiter scale I used is the lowest 200ohm.The resistors I have tested are R109, R112, R114 and R115.

However ever if I don't have at the moment another 0R22 5W I repaced R109 and R112 with 2 x 1,8ohm 10% 5W colber. I know that the values are different but just to understand if the amp behavor is the same.....and magically now I don't hear the classic bump when the amp is on, but when I play guitar the sound is distorted, but it's better than previously.

Now I'm thinking to do another test: to invert R109 and R112 with R114 and R115.

My cheap multimiter (20€) are able to measures correctly resistors but I suppose it's not good for the 5W resistors. For example, even measuring the 1.8ohm 10% 5W colber the value is 36ohm and not 1.8....strange.

Can you confirm the value of 5WR22J to 0.22ohm? waht does should be correct? 2.2ohm or 0.22ohm?

[vincenzodesanctis]

Reading the scheme, R114 is in parallel with R115. So I think that I can remove R115 and leave only R114. And replace R112 and R109 with the correct values 5WR22J

[Enzo]

It is hard to measure 2 tenths of an ohm with a meter. Touch your probes together and measure that. That is the resistance of your probe wires, now subtract that number from the reading of the resistors. Those are wire wound resistors, and for the most part they are either OK or open. If they measure something real low, they are likely OK. if they measure high resistance, they are open.

5w r22 means 5 watt 0.22 ohm.

[g1]

5WR22J is .22 ohm, not 2.2 ohm.
For testing, you can remove R114 or R115, but do not run at loud volume.

[vincenzodesanctis]

I removed R114 and leave R115. And I replaced R112 and R109 with the correct values 5WR22J.
This is the situation:

Nu bump when I power on (very good).
The level of the guitar when I play is little bit more comparing before, but it's distorted like a soft fuzz.

I connected a cable to the pre out outlet and connected it to the new other amp input and in this way the sound is good. I think it means the pre stage is good.
Do you have any suggestion how do we go about this?

[Mick Bailey]

Can you measure the voltage across R109 and R112 with no signal going through the amp?