Frontman 25R Channel Switching/Footswitch Problem.

[dmbspook]

I was given this amp not too long ago. It seems to work except that it is stuck on the Drive channel. I do not have a footswitch for it and thought that it could possibly be the cause. I took it to my local guitar shop and they plugged in a peavey switch (my fender dealer is an hour away) and all that it did was turn on and off the channel select light, but had no effect to the channel... neither does the channel select button on the unit itself. I am not super familiar with electronics repair, but given the amp was free, i have time, and im not stupid, I would rather fix it myself than spend $80 on it. I got the schematic, and though not sure what I am looking at will be staring at that until I get some advice, so please help.

Thanks in advance,
Phill

[52 Bill]

...all that it did was turn on and off the channel select light, but had no effect to the channel... neither does the channel select button on the unit itself.

Do you mean that the led goes on and off but the signal doesn't change channels?

Go to the Fender site and download a schematic.

There is one half of an 4560 op amp that controls the switching, and a couple of fet's that turn on and off the channels. If the led goes on and off, then I would guess that the op amp is ok, so I'd check the fet's and the circuit wiring to them.

[dmbspook]

Do you mean that the led goes on and off but the signal doesn't change channels?

Thats correct. So i suppose I have to check the "fets" and the circuit wiring to them. I looked up fets and assume you are talking about PhotoFETS (newbie here remember ) However I dont know how to check them, what they should look like. I am assuming I will be using a multimeter before and after the fets on the circuit wiring to check for changes in voltage when using the switch to determine where the signal is being lost. But some more information on what fets are, (especially how they are usually are denoted on schematics) would be incredibly helpful (or it may not, thats to your discretion) But really, and more direction you can give me would be fine. Ive already got the board out of the amp so I can look at it and compare to the schematic, but any advice from you pros would be great

[Enzo]

He refers to JFETs. The channel switching signal goes to the gate of several JFETs. ALl you need do is meter each gate while flipping the panel button. The voltage toggles off and on there or it does not.

It seems stuck in Drive. Turn the Drive Volume to zero and turn up the clean volume. Anything come out? The way this amp works is Q2 is a series element, so it turns OPN to pass the drive signal. On the clean channel Q1 is a shunt element, so it turns ON to short the signal to ground. In other words the drive shannel is let to pass to turn it on or off, while the clean channel gets its output shorted to turn off. SO if Q2 is shorted, you couldn;'t turn of the drive channel, but you could stull turn its volume to zero. The clean channel might stil work, but you'd never hear it under the drive channel. By zeroing the drive channel we can hear if the clean is stil switching on and off.

OR...

1 meg R54 might be open leaving the control cignal disconnected from the switching JFETs. And that would leave you wondering.... R54 where are you?

(You have to be a certain age to get that)

Little C32 could be shorted, thus grounding off the control signals, and that would leave you stuck in drive.

ANd finally D5,D6 route the control signal to each gate. One of them could be open or shorted.

[dmbspook]

Alright, thanks. It would appear i have quite a bit of testing to do... as I assumed I would. I should have time to mess with this next week so Ill let yall know what I find out. THanks again.

[Enzo]

Look up the schematic, the circuit is a lot simpler to look at than describe.

[dmbspook]

as it turns out i dont know how to use a multi-meter... i cant get any relevant readings... i have a gb instruments GDT-11 using the red pin in the 10A DC as opposed to the V"omega"mA and the black pin in the COM slot (im fairly certain thats correct. I have the dial set to measure voltage and the V has a - with three dots under it as opposed to the tildae. and its set to 200m. I have tried them all ... nothing but zeroes and random ones...
... any advice would be great.

Anyway when I touch the pins to both sides of C31 and R50 the drive light lights up. also if i touch with just one pin both U5-7 and U5-8 it lights up as well. aside from that is just small details but im not sure what im doing with the multi-meter and why its no getting any readings so i can find out anything specific.

[Enzo]

Omega is the symbol for ohms. The hole marked volts/ohms/milliamps is the one to use for everything except high current readings. The 10A DC hole is for one thing only - reading high current. That means 10 amp DC - nothing else. So your readings are not working because yopu have the red probe in the wrong hole.

[dmbspook]

I feel like I should have been able to figure that one out on my own, but no. Thanks Enzo

[52 Bill]

Did you check as Enzo suggested to see if both channels were in fact working at the same time?

To check the switching circuit:
Set your meter to read DC voltages. The maximum voltage that you'll be reading is around 30vdc, so set the range accordingly.

Hook the black lead of your meter to the metal chassis, carefully turn on the power and read the voltage at either end of R54. Now change the channel switch and read the voltages again. In clean mode there should be a positive voltage of around 13-14v. In distortion mode there should be a negative voltage of around 13-14v.

Let us know what your findings are.

[dmbspook]

Hook the black lead of your meter to the metal chassis, carefully turn on the power and read the voltage at either end of R54. Now change the channel switch and read the voltages again. In clean mode there should be a positive voltage of around 13-14v. In distortion mode there should be a negative voltage of around 13-14v.

Let us know what your findings are.

Upon testing R54 (with results this time thanks to you both), I have a postive voltage on either side of 13-14v in DISTORTION mode, in clean I am getting negative 13-14v to the right side (from the top of the board, controls on the left) of R54 and negative .4-.5 on the left side. Not sure what this means, but something is definitely screwy. I am getting ready to retest everything i tested yesterday, although, I wont know what im seeing.. thanks

[dmbspook]

Althought neither of you mentioned It I checked TP-18 and TP-19 to ensure the voltages were correct as listed in the schematics and they are. If that helps at all.

EDIT: Here is the updated info. All dials set to schematics specs.

    Clean                                       Drive
         L          R                               L          R           (refers to side of each "circuit" [Q2 is top and bottom in that order])
c32     0        -.47                                0      +13.2
D5     -.37      -.47                              +.04     +13.2
D6     -.06      -.47                                0       +.03
Q2       0       -.4                                 0      +13.2
R54    -.47      -14                                +13     +14

[52 Bill]

Althought neither of you mentioned It I checked TP-18 and TP-19 to ensure the voltages were correct as listed in the schematics and they are. If that helps at all.

Yes that is good, but that's why I asked for the readings off of R54. Same information, the problem lies in Q1 or Q2 or D5 or D6. My guess is Q1.

Unplug the amp, and set your meter to read either low resistance or to the diode test function. Attach one lead to the chassis and read the resistance from the unbanded end of D6 to ground. You should get either a high or low reading, but not zero ohms. Reverse the red and black meter leads and you should get the opposite reading from the first one. If the first reading was high, the reading should now be low.

If my guess is right Q1 is shorted, and you'll get the same low or zero reading either way.

[dmbspook]

i see the black bands the whole way around d6.. they are thicker to the right and thinner to the left .. so I am assuming the left side of D6 is unbanded. the readings i get there are (ohms set to 20k) 5, and reversed 5 as well. the opposite side, (incase im wrong) are an error and reversed 13.

[52 Bill]

i see the black bands the whole way around d6.. they are thicker to the right and thinner to the left .. so I am assuming the left side of D6 is unbanded. the readings i get there are (ohms set to 20k) 5, and reversed 5 as well. the opposite side, (incase im wrong) are an error and reversed 13.

Does the "5" mean 5 ohms or 5K ohms?

My guess is still with Q1 being shorted. How are your soldering skills?

If you remove Q1 from the circuit, then the amp should have both channels on or just clean channel on. It should also bring back the full -14 volts when in clean mode.

If you have a pair of side cutters that will cut close to the board you could just clip out Q1, leaving enough lead so that it can be resoldered back in later if it tests ok.