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Author Topic: Fender Frontman 212R hum and volume 80% down  (Read 776 times)

Jude

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Fender Frontman 212R hum and volume 80% down
« on: February 05, 2021, 05:26:36 PM »
Hi folks,

I am trying to repair a Fender Frontman 212R, that powers on ok but has hum and low output volume (around 20% of what it should be at full volume). I am new to electronics theory and amp repair but I can competently use a multimeter and soldering iron!

I followed the steps in an earlier  post (https://www.ssguitar.com/index.php?topic=3741.0) to try and isolate where the defective component is and measured the following voltages with the amp powered on (some values do fluctuate):

1. has 44 VDC at the main power caps at C71 and C72 ( I have linked the schematic below)
2. 16.5V VDC at C75 and C76
3. R87 and R88 44V, R89 and R90 -44 VDC, R102, R104 and R106 -0.5 to 1 VDC
4. PNP transistors at Q9, Q10 and Q11

Q9 E: 0.55 VDC, C: 43 VDC B: -0.45 VDC
Q10 E: 0.56 VDC, C: 43 VDC B: 0.05 VDC
Q11 E: 44 VDC, C: -26 VDC B: 43.7 VDC. from what I read in the earlier post Q11 should read 42V at C, I did repeat readings and measured either 41V or -26V at Q11 C.

So Q11 seems suspect and with the amp powered off I checked the readings at Q11 E-C, C-B and E-B. Everything read OK apart from C to E which read 1.6V and not OL suggesting a short.

After all this poking around the amp now has now sound at all!

Would an pro members like to give me some quick guidance on this? I was planing to order a replacement for Q11 unless my conclusion is wrong.

cheers from down under

Jude

Schematic:

https://elektrotanya.com/fender_frontman_212r_sch.pdf/download.html

g1

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Re: Fender Frontman 212R hum and volume 80% down
« Reply #1 on: February 05, 2021, 07:31:51 PM »
Q11 collector goes to a few diodes and then down from there is R89.
Is R89 ok?

Jude

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Re: Fender Frontman 212R hum and volume 80% down
« Reply #2 on: February 05, 2021, 07:53:29 PM »
R89 -44 VDC and R90 -44 VDC. thanks

phatt

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Re: Fender Frontman 212R hum and volume 80% down
« Reply #3 on: February 06, 2021, 03:25:19 AM »
If there is  HUM ,, first check the DC voltage at the speaker terminals,, it should be very close to Zero.
i.e maybe around 100mV DC is ok.
Phil.

Jude

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Re: Fender Frontman 212R hum and volume 80% down
« Reply #4 on: February 06, 2021, 06:40:14 AM »
Yes I checked that too following the archived post at the start of this thread. It was close to zero but I didn't record the voltage.

So since the collector of Q11 is not stable around 42 VDC to ground (also reads -26 VDC) I will assume it is shorting and desolder to check it out of circuit.  Also to reiterate after all the poking around measuring voltages the amp still powers on but now has no sound at all. The measurements of R89 -44 VDC and R90 -44 VDC were on one side only, the alternate sides of both those resistors gave very low VDC to ground from memory.

cheers

g1

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Re: Fender Frontman 212R hum and volume 80% down
« Reply #5 on: February 06, 2021, 03:53:54 PM »
R89 -44 VDC and R90 -44 VDC. thanks
I was asking for you to measure R89 resistance, not voltage.
If R89 goes open, you will get improper voltage at Q11 collector, which is what you have.

Jude

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Re: Fender Frontman 212R hum and volume 80% down
« Reply #6 on: February 06, 2021, 11:58:04 PM »
I could not get a resistance reading at R89, the meter reads 'OL', meaning its open? infinite resistance, also no continuity across R89, I see it is a fusible resistor Fu470 value.

I de-soldered Q11 and checked it out of circuit and it behaves normally, at least using multi-meter testing in diode mode.

So replace R89 instead?  thanks

g1

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Re: Fender Frontman 212R hum and volume 80% down
« Reply #7 on: February 07, 2021, 02:07:23 PM »
Yes, replace R89, it is open.

Jude

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Re: Fender Frontman 212R hum and volume 80% down
« Reply #8 on: February 22, 2021, 07:21:14 PM »
I have replaced R89 + R88 and R89 as those fusible resistors were open too. All the remaining fusible resistors check ok. The amp is now humming very loudly when powered on and plugged into a speaker and reads 35VDC across the speaker output wires.

Q11 now reads to ground with the amp powered on:

E: ~+40VDC C: -42VDC B: ~+40VDC

so based on an earlier post I would assume Q11 is the likely culprit? I think Q11 should be C: +42VDC

Please see:

https://www.ssguitar.com/index.php?topic=3741.15

thanks