no sound from Fender Princeton 650

[smackoj]

Hi amigos; I have a question re: Fender Princeton 650 ss amp which come as a combo with one Celestion 12" spkr.  I bought it as a 'parts' amp to restore and keep or sell? It had one burned spot on the board where a 5 watt wire wound R got too hot. I replaced that with a better quality ceramic cylinder type which is well above the pcb now and should keep it cool enough. I am still not getting any sound to the speaker. This amp has quite a lot of dig FX which makes the pcb chock full of sister circuits. Running it on a current limiter lamp it looks to be drawing the right amount of current (low glow thru the 100 watt bulb). I tried using a listening amp probe and I found signal at several of the op amps. I tried touching the probe on the power transistor legs and  got a horrible buzz so I stopped the probing. I hooked up my multi meter to the speaker leads and  get close to 15 vac on the spkr leads but no sound. Would that suggest that the output Ts are working but they are not getting the guitar signal to them? Thanks amigos   

[PRR]

> 15 vac on the spkr leads but no sound.

15V AC over a wide range of frequencies is "a sound".

Is the speaker good? Jacks? Internal jumpers?

[smackoj]

hi PRR; I have tried 3 different speakers. It doesn't use a jack for the speaker leads, it's just wired straight from the pcb. And I'm not exactly sure what you mean by internal jumpers? Do you mean like the grey multi wire connectors they use to connect different pcbs?

[shooter]

tried 3 different speakers

when you found 15vac, was that measuring speaker + to -, or each to ground?  15vac on a speaker should be making the cone, buz, move, something.

[shooter]

I can't blow up the schematic to read it but it looks like you have a pre out and PA in
have you used them to 1/2 split the amp?

[PRR]

> I can't blow up the schematic to read it

[smackoj]

Hi Shooter; I measured the + to the - spkr lead. Should I be testing each lead to ground? I was intending to try the Pre Out jack but haven't had time today. I will give it a try and report back. thanks

[st]

Hard to read the schematic... but a problem I once had on a Princeton 65 (similar design without the dsp fx) was that the channel switch button and footswitch jack had bad connections to the board with no sound as result. I remember from googling around that this was common problem (as were the burnt power resistors). Check connections in those areas and/or try plugging in footswitch to diagnose. No idea how this rhymes with your speaker out voltage readings though.

[smackoj]

Thanks ST that's good information. I did test the signal thru the "Pre Amp Out" jack and it's Fully Functional. I will do so snooping around the channel foot switch. That has potential as a problem area.

thx amigos

[shooter]

had bad connections to the board

I believe that's what PRR was referring to in post #1
basically anything that can, or should move, switches, jumpers, connectors, etal.
+/- is what I wanted measured, just verifying

[smackoj]

I am not sure where to go from here. The PreAmp Out has everything working. But something is keeping the signal from getting thru power amp to the speaker? Anyone have any more suggestions?     Thanks

[PRR]

> I am not sure where to go from here.

You really are not giving us much to work with. Did you DO checks on all internal connectors? Peer, poke, wiggle? Did you work the PA In jack to clean its switch?

1)

I can't blow up the schematic to read it but it looks like you have a pre out and PA in have you used them to 1/2 split the amp?

2) Go back to speaker wire measurement. You say you have AC. You should hear something. Are you sure it is not DC? Read both.

[smackoj]

Hallelujah! It's working! PRR; I am sorry for my lack of good info to work with. 

The speaker wire had a break inside the plastic pincher (spkr wires soldered right to the pcb and then routed thru the aluminum chassis) but it was not obvious because the wire insulation did not show a break. Man, that was a tricky one to find. I suppose that Fender saves cost to build by not using a 1/4" jack for the spkr out?

Thank you for all your help amigos.  Smacko Jack