polygon 102 schematic

[ilyaa]

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

(Polytone Schematics - Tube amp Schematics)

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: rather than being .15 ohms, the resistors on the output are 15 ohms. and rather than being 100 ohms, the 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 meantime?

[vintagekiki]

Polytone Schematics

To start looking here

http://music-electronics-forum.com/t34687/

Emitter resistors R20 / 21 (0.15 / 5W) must be the values indicated on the schematics polytone 378 pwr amp because they define a bias for the limiter

The output transistors 2N5880/ 5882 can be freely replaced with the MJ15003/ 15004, of course with the previous predefined bias.

[J M Fahey]

Well, I´m quite certain that the amplifier fits one of the known schematics, but poorly replaced parts do not.
Just match the PCB, no matter what´s fit on it, with the known schematics.

To begin with, Mr NO CLUE has been there.

1) he probably thinks "hey, TO3 power transistors? ... THE workhorse is 2N3055 so let´s fit a pair there"

You compare output stage with both known compound transistor styles, they are NOT the same:


On a real complementary circuit, both NPN and PNP are Darlingtons,
* top one has an NPN driver emitter feeding NPN power one base. (true Darlington)
* bottom has a PNP driver emitter feeding PNP power one base. (true Darlington)

On a Quasi complementary circuit, NOT both NPN and PNP are Darlingtons,
* top one has an NPN driver emitter feeding NPN power one base. (true Darlington)
* bottom has a PNP driver collector feeding NPN power one base. (Sziklai pair or quasi complementary)

so download your transistor datasheets, confirm pinout and triple check whether both powrr transistor base are fed by driver emitters or the bottom one is fed by its driver collector.
That´s an universal truth and needs no Factory schematic to be confirmed one way or the other.

As a side note: I have on my bench one of my old (as in 19 y.o.) 300W Bass amplifiers, sent by a no clue Tech who gave up trying to repair it, fed up with blowing transistors.
I must believe him, it´s a +/-65V rails amplifier, powered by industrial workhorses 2N3773 , Hometaxial rough but robust as h*ll, designed to drive solenoids and electric motors, Audio performance was secondary, and Mr no-Tech replaced all with linear but weak ST Epitaxial 2N3055, absolutely unsuited to the job.

No wonder they exploded on turn on.

Only good thing he did was always use the proper mains fuse; otherwise the amp woud have caught fire.

2) 2N3053 and 2N4037 look correct , problem is whether they are still alive, check them for shorts.
If needed but unobtanium (or horribly expensive), they certainly can be replaced by modern available TO126 plastic versions or equivalents.

Again: compare PCB layout to known schematics, PCB tracks can not be altered by anybody, while actual parts fit there can be anything.

3)

other differences from existing schematics: rather than being .15 ohms, the resistors on the output are 15 ohms.

so think which one is correct:
a) "this is a mystery Factory variation, I must hunt this hard to find schematic or else"
b) "Mr No Clue can not read a Schematic" ... he ignored the dot before the 15 in the schematic so instead of .15 he fit 15 ohm.
In fact it looks more and more like a Musician, not a Tech of any kind, just got a soldering iron and tried to save a few bucks by repairing it himself.

4)

rather than being 100 ohms, the resistors on the bases of the output transistors are 2.2K

it´s very common that when output transistors blow (generally short), emitter resistors blow open and full supply voltage drops across 100 ohm base resistors, making them catch fire ; I guess No Clue started rising value until he found 2k2 stands 30V across them ... while actually NOT repairing the amp; as Enzo says "if a resistor burns, something else caused that"

5) Just checked the EL34 schematic site:
* there is a "55W amplifier" , fitted to "102 model" but it is all discrete, no ICs whatsoever.
* more important, NO Darlington outputs but BOTH (top and bottom ones) are quasi complementary .... a weird choice but if that´s what you have, so be it.
For now check whether power transistor bases are fed from driver emitters (which would make them true Darlingtons) or driver collectors (which would make both quasi complementary as in this schematic):


Count how many transistors you have on PCB and heatsinks, the schematic above shows 7 in total ... how many do you have?
Also post a couple sharp well illuminated PCB pictures.

How many legs does the mystery round IC have?

Is its footprint round or rectangular ? (as in modern DIP packages).

[J M Fahey]

Checking other EL34world schematics, found this one, which *might* hold the key:



notice input transistor pair is shown as a dual package, not 2 separate transistors, yet they claim to use 2 separate (but somehow joined) regular type 2N5210 ones.

But there is an asterisk besides them, which leads to Notes at the bottom left of the schematic.
As Enzo says: "always read the notes, they are there for a purpose!!!!!!"

The asterisk one reads: "Q1/Q1a represents Q1 replacing PT1583"
and what is mystery unavailable even way back then PT1583?
I found more modern Japanese version 2SC1583, a dual transistor indeed.



this one has a typical Japanese plastic package (notice: 5 legs) but I can easily think that the original Polytone version used an early American dual transistor, housed in a round TO5 metallic case ... which you think is an IC.
It might have 5 legs ... or 8 but actually use only 5 .

IF so, you may have a "Polytone 102" ... model numbers are kept for Marketing reasons even if guts are modified, (just witness Peavey Bandit, made for over 35 years and of which some 15 different versions exist), but such an early version that it used metallic TO5 dual transistor input, the rest probably matching the schematic I posted above.

Compare the PCB parts placement with what you have, and post a picture showing yours, we might have a hit.

In general, manufacturers do not fit *anything* at random inside their amplifiers, but have a couple pet ideas or preferences and use them all the time ... your amp must match or be very close to some of the others.

EDIT: Notice both schematics I posted are basically the same, almost can be built on the same PCB, just the bottom one has beefy MJ802/4502 transistors to be able to produce 110/120W into 2 ohms, while the top one has smaller ones, good for 55 or 70W into 4 ohms.

Proof of what I said about recycling ideas over and over.
Fine with me, it simplifies production, parts procurement and servicing.

In any case, I suspect the input transistor pair is fine, it works under very low stress; damaged ones are power ones, often their drivers, and sometimes the Class A predriver .

[ilyaa]

damn! thanks J M -

will look into this asap and see what i uncover. i suspected this may have been a partial 'repair' but there were so many parts that seemed off i thought no way would someone have mucked things up THIS bad -

but i guess anything is possible!! thanks so much for your detailed and thoughtful response -

[ilyaa]

getting back into this thing -

here are some pics of the power amp PCB

[ilyaa]

and here are two more

[ilyaa]

as you can see, the transistors on the left have their legs bent, which makes identifying what the intended PCB traces from there actual ones a bit difficult. the other two I removed but they were oriented the same way as the ones on the left -

the top and bottom traces go off-board, as you can see, some to V+/V- and some to output transistors. what the hell is going on with that purple capacitor near the RCA jack (which is the preamp -> power amp input node) I have no idea.

any thoughts on this monstrosity?

[ilyaa]

J M - any ideas?