good call -

replaced drivers and pre-drivers and we are all good!

thanks, Enzo!

scratching my head a bit with this one -

got it in the shop with a transistor missing from the back (TO-3 sized hole on the chassis...) -

replaced that and replaced one shorted power transistor in the power amp -

with no load plugged in i get a good looking waveform on the output but once i plug in a load not so good - flattened out and clearly there's a problem -

some voltages are not quite as they should be - none of the transistors (power or driver or otherwise) in the power amp seem to be forward biased - even though they are all measuring fine diode-test wise - in and out of circuit -

the +/- rails are good, but the bases and emitters are all stuck at 0V or thereabouts - the symmetry of this amp is confusing me so i am having a hard time trying to isolate the problem without just shotgunning the whole thing -

any suggestions?

the amp I have does not jive with any of the schematics ive found online.

(http://el34world.com/charts/Schematics/files/polytone/Polytone_Schematics.htm)

someone has definitely dug around in here before. first warning sign was that both output transistors had been replaced with 2n3055. I know occasionally youll run across a power amp that uses two NPNs but this amp has +30 and -30 rails and none of the polytone designs I've seen resemble a dual NPN thing. there was also a shorted driver. but before I dig too deep id love to see a schematic...

this amp has the proper To-3 heatsinks for the power amp. it says Polytone 102 70W on the back. its a 1x12 combo. the power amp pcb has a pair of NPN/PNP drivers for each output. currently they are 2n3053 and 2n4037s. other differences from existing schematics:uurather than being .15 ohms, the resistors on the output are 15 ohms. and rather than being 100 ohms,uthe resistors on the bases of the output transistors are 2.2K

oh yeah and important detail: there is an 8 pin TO-5 old-style IC at the input of the power amp. unfortunately all the text has rubbed off of it....

I can post a pic soon - any thoughts in the meantimeu

hey!

any thoughts for a suitable replacement heat sink for an ampeg ss140c?

got a salvage amp here and the heat sink is just *gone!*

someone must have removed it used it for.....something???

like the one in this pic: https://gbmedia.azureedge.net/usercontent/gear/3421921/p2_uhxjnxoft_ss.jpg?maxwidth=500

i doubt ill find that part - so im thinking maybe some of you guys have thoughts on what a good material would be to use/where to get something easy to fasten on there??

yeah - called em. no dice. amp is still under warranty.

he can send it back but i think he's a little hesitant to do so seeing as he has already - doesn't feel like going through the headache again and would rather have an amp that doesn't need to be mailed back to fender every few months!

i wonder: with the variable voltage thing going on in here - would it be possible to bypass the fancy  fet-controlled variable voltage stuff and just rewire this amp like a good old-fashioned normal tube amp? i think the variable voltage is just controlling the screen voltage and the bias (bias tracking or whatever its called) - anyone have any experience doing something like that?

its gone back for a warranty repair already -

the owner wants to make sure whatever happens doesn't keep happening!

fender is probably gonna give me the 'you're not an authorized service center' spiel and the cold shoulder

wondering if anyone has any documentation/schematics on this amp?

everything works and ive got a solid swing coming from the PI to the power tube grids -

but the output section is not happening - im getting normal plate voltage but ZERO screen voltage and -3VDC or so grid voltage - problem is this amp has a pretty complex thing going on with the variable output and mosfet control B+ (or whatever it is!) -

i know schematics are hard to find for these - anyone have thoughts on a similar circuit i could use for reference?

so this thing was working fine and then blew again WTF...

doing some research I came across this:

"The one limiting factor is the design of the amplifier output. If it is a single-ended design, meaning that the negative side of both the left and right output channels have a common reference (ground) and as a result the positive side of each channel is the only audio energy source, all you have to do is connect the sleeve to the common reference, the ring to the right channel positive speaker output, and the tip to the left channel positive speaker output. But   if the amp is a dual ended design and both the positive and negative speaker outputs are presenting audio energy, you cannot sum the negative outputs from each channel (which is what happens when you connect to the sleeve of the headphones) or you will fry the amp."

(found on this thread: https://www.gearslutz.com/board/geekslutz-forum/54486-powering-headphones-power-amps-how.html)

now this Yamaha is a dual-ended design and it was being used in the studio for the stereo headphone mix - meaning that the sleeve of the headphone jack on the output was indeed connecting both negative poles of the Yamaha's output. is this why the amp is blowing up???

and that rig will be sufficient even for a much higher power output amp like this Yamaha?

J M,

a 220 or 470 in series with the + lead of each earphone jack or with both the + and - lead of each jack?

what wattage would you recommend for those 220s or 470s? 10? 20? and are you saying instead of the 4 ohm or in addition to?

I think 1-3 phones -

but the impression I am getting is I should talk him out of using the Yamaha as a headphone amp?

I am not totally sure why he is hell bent on that, but seems to me like not a great idea, if not a terrible one.

we did it!

alright rebuilt most of this damn thing -

the heatsink may have been a big issue - i noticed some spots where it had either melted or blown up a little bit and i think this may have been causing some arcing/shorting!

but ran both channels at 75 watts for quite a while and we're all good.

NOW for the final question:

i was fixing this up for a studio in town. they are using it as a - and i'm not kidding - headphone amp. im wondering, should i talk them out of this usage altogether? or is there a way to make it work? they had their tech put a 4 ohm 25W resistor in series with the + speaker lead and otherwise just hook it directly into headphones. the headphones they are using are 38 ohms that can handle 1.5W. now if they run it at low wattage it should be okay, BUT

1) the VUs on the front are for watts/8ohms - with this setupt the amp is seeing 42 ohms  - is there a way to calculate the adjusted wattage
2) would it make more sense to also wire a high wattage 8 ohm resistor across the output jacks so that the impedance matches more closely?
3) is this a silly path to take? i think the engineer just really likes this amp but im not sure i see the point of using a high wattage power amp on headphones....

k think I'm gonna go with the insulators -

man this amp fix is never ending!

no one have any thoughts on where to get green heatsink tape?

all the stuff I'm finding is double-sided and made for ICs - comes in squares

the tape on the Yamaha output transistor heatsink is greenish and one-sided and clearly comes in a roll -

alright guys we are almost there!

re-transistored the whole amp (the bad channel, that is) and we seem to be powering up and no-load idling just fine at the same idle draw as the other channel.

only thing is, before i send anything through it, i'd like to replace the green thermal tape behind the output transistors. its gone through some trials and tribulations and there was at least one bare spot that shorting B+ to B- through the case of one of the output transistors. i dont have any of this stuff around - RIP radioshack and the electronics store in town doesn't carry it -

any suggestions where to look for it (non-internet) or what kind of tape works for this purpose?