I have a Fender Frontman 65r. When i turn it on it makes an explosive noise. after about 30 seconds it works fine. Any ideas?

 

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Describe the sound of "explosive noise". How long does the sound last?

A "thump" on power-on and power-off is common with solid-state amplifiers. This is the sound made when the output coupling capacitor to the speaker charges and discharges.

The Fender Frontman 65R is DC coupled to the loudspeaker. There is no coupling capacitor.
Hence the noise is originating from someplace else.

Typically, high power amplifiers have a speaker relay that is energized after a few seconds. This is to allow for a safety check and avoids the turn on "thump".

 

a pop and a buzz. last about 30 seconds. with or without input plugged in.

 

Service manual here....
https://elektrotanya.com/fender_fm_65r.pdf/download.html#dl

 

Sounds like a warm up issue in the main power supply.
The usual suspects are the power supply filter capacitors.
I will have a look at the circuit schematic later when I get a chance.

 

Here is a snapshot of the psu and mute circuit using J fet..

 

We've got some serious diagnostics ahead of us.

Here is what we know.
1) Pop and buzz, lasts for 30 seconds.
2) With or without input plugged in.

More tests
1) After the buzz goes away, what happens when you turn it off and on again within 2 seconds? Does the pop and buzz reappear?
If not, how long do you have to wait with the power off for the buzz to reappear?

2) Turn VOLUME and DRIVE to 0.
Set CH SELECT button OUT.
Is the buzz still there on power on?

3) Insert a 1/4" patch cable into the FOOT SWITCH.
Is the buzz still there?

 

We've got some serious diagnostics ahead of us.

Here is what we know.
1) Pop and buzz, lasts for 30 seconds.
2) With or without input plugged in.

More tests
1) After the buzz goes away, what happens when you turn it off and on again within 2 seconds? Does the pop and buzz reappear?
If not, how long do you have to wait with the power off for the buzz to reappear?

2) Turn VOLUME and DRIVE to 0.
Set CH SELECT button OUT.
Is the buzz still there on power on?

3) Insert a 1/4" patch cable into the FOOT SWITCH.
Is the buzz still there?

1 same buzz
2 yes clean channel and dirty
3 same with foot pedal or not

 

There is not much here to make a diagnostic.
Normally, one would examine the power rails with an oscilloscope during warm up .

The first suspects to replace are C69 and C70, 47μF electrolytics. Install 25V-63V types.
The next suspects would be C65 and C66, 2200μF electrolytics. Install 50V-63V types.

 

Thanks, I will try that. Could it be bad input power. The AC outlet I mean

 

Thanks, I will try that. Could it be bad input power. The AC outlet I mean

No. Very unlikely.

 

Do you know anyone who has an oscilloscope?

 

I can find one

 

I can find one

In that case, look at the +42V, -42V, +16V, -16V power rails just after power on.
You are looking for excessive ripple at 120Hz that goes away after 30 seconds.

 

will do. again, thanks