no clue here... how can i identify a suitable transformer for an old kay 704 amp?

[pinkjimiphoton]

hi everybody,
i've been working on an old kay 704, beautiful sounding all germanium guitar amp from 1963. when i first got it, i recapped it with new caps, and it sounded clear and nice, with a touch of grit and that beautiful germanium midrange warmth.

which of course means after months of using it, one day it just kinda crapped out. i opened it up and found a cap that had failed, and replaced it. fired it up, and it worked, but sounded weird.. at certain frequencies (kinda in between f# and g on the low e string of a guitar) it would break up and almost crap out, sounding kinda like a blown speaker or a rattly tube plate.
so i went thru and checked everything in the amp, ultimately replacing almost all of the passives in it. no dice. voltage readings on one of the transistors seemed weird, and something seemed shorted to ground. went thru the whole thing, and discovered it has what i assume is an output transformer. i disconnected it and checked it out, seemed to read ok... primary i forget the resistance, maybe 40r or so tho i'd have to check again cuz it's been a while. each of the dual secondarys that drive the output transistors read about 1.8r almost exactly. i checked for shorts between the primary and secondarys, and to ground from them. all appeared good.

i put it back in the amp and reconnected it, and got the same results. voltages were ok on one transistor, about 34v and 17v, but the other one i was reading maybe 600 to 700mv. i checked it with the meter, and that secondary was reading shorted to ground. so i'm guessing the transformer is toast.

i hate to give up too easy, so i'm hoping someone can lead my newb arse thru the darkness of how to find a replacement.

i looked at the hammond website, but couldn't find anything even remotely close there.

how do you find an odd duck like this? should i try and unwind it and find the short myself and repair it? have hammond or someone custom wind one?
i hate to put crazy money into an amp that only i seem to like.

any advice is appreciated!!
thanks
jimi

[Jazz P Bass]

It would help if you labeled the test points.
Base, Collector, Emitter.

Anyway, the transformer is an interstage transformer (not an output transformer).
Which is being used for phase inversion to the two power transistors.

You must study that schematic again.
The lower IT winding IS 4.7 ohms from ground.
And the Q13/Lower Emitter will be 1.5 ohms from ground.

It appears to me that the two Q13 Base readings will be 1/2 of the B+ Vdc.

[The Dude]

...... voltages were ok on one transistor, about 34v and 17v, but the other one i was reading maybe 600 to 700mv.......

Are you saying there's no collector voltage on the lower transistor? Also, those are PNP's with a negative supply. Voltages should be negative.

[pinkjimiphoton]

thanks Jazz P Bass, i will go downstairs shortly and check it out. thanks for the information. i will get ya some voltage readings from it.
from what i recall, B+ is about 34 volts, base is about 17 but i'll have to check to be sure, i had to take it off my bench to do a few gigs and have a memory
like a lobotomized flea

stay tuned, and thanks so much.

[Jazz P Bass]

What's up with that 'Skype' message.

It overflows into the text.

[pinkjimiphoton]

skype message? dunno what you're talking about, sorry!

[pinkjimiphoton]

What's up with that 'Skype' message.

It overflows into the text.

oh, duh! i see it now. no idea at all, i haven't used skype in years, and even then maybe only 3 times in total.

will go downstairs and fire it up and take some voltage readings... be back shortly, thanks guys

[pinkjimiphoton]

Are you saying there's no collector voltage on the lower transistor? Also, those are PNP's with a negative supply. Voltages should be negative.

i will check, this thing (for me, at least) is kinda hard to follow. but yes, the lower transistor is reading around 600 700mv. for some reason the minus/dash key doesn't work on this computer since one of the latest microsoft "critical" (f***)updates.

be back shortly, thanks dude

[The Dude]

On a side note, you should put a grounded AC cord on that thing and clip out the death cap if you haven't already.

[The Dude]

What's up with that 'Skype' message.

It overflows into the text.

I believe the forum lost its icon link for Skype or it's association with the icon. That is why it's displaying the text. It should normally be just a little graphic symbol something like this (only smaller):



I've taken the liberty of alerting tboy to the problem.

[pinkjimiphoton]

still trying to debug this thing, microsloth says its a software issue... arrrgh... yeah, a software issue caused by their damn update process.

haven't gotten to putting a grounded cord on it yet, first i need to get it working. death cap? there's no polarity switch on it, which one would the death cap be?

still trying to get downstairs. it's hard to post negative voltages without a minus key

[The Dude]

Death cap is C7 from the AC input to ground.

[Jazz P Bass]

I wouldn't sweat the polarity.

Simply post the voltages & indicate where you took the reading.

[pinkjimiphoton]

Death cap is C7 from the AC input to ground.

i will remove it, i just figured it was there to short rfi and crap to ground. thanks for the headsup!

EDIT: thanks dude, i removed it last nite

[pinkjimiphoton]

ok, i got some voltages. i also read the dc resistance of the transformer.
primary is 162.8r, secondary 1 (top output transistor) reads 18.1r. beeper shows no continuity to ground. secondary 2 reads 18.8r, but shows a short to ground that reads about 23r. to me, it seems that's gotta be the problem.

i didn't bother to take the voltages at the oscillator for the tremolo, i can if it's helpful.

ok, voltages, starting at
d1 anode -35v, cathode reading between 0.0 and 0.1v, but could be my shaky hands
tr1
c -.511
b -.140
e 0.0

tr2
c -.20.4
b -.511
e 0.0

transformer
top primary -20.4
bottom primary -24.1

top secondary (1-4 going down from top)
1 -34.8
2 -17.4
3 -17.3
4 -.10 - -.12mv

tr3 1 (top)

c -17.3
b -34.8
e -17.3

tr3 2 (bottom)

c -17.3
b -.10 - -.12mv
e 0.0

upper c8 cathode -17.3
node at c8 upper anode and lower cathode -17.3
lower anode 0.0

upper c9 cathode -35.2, anode 0.0
lower c9 cathode -35.6, anode 0.0

all voltages taken to chassis ground, volume tone full, speed and intensity off, speaker connected and output transistors in. if i take them out, the voltages go up substantially, but tr3 lower stays about the same.

thanks for the help, i hope the above gobbledegook makes some sense.

i DO have a pair of 2n554 output transistors if these ones end up fried. pretty sure i can sub pretty much any pnp ge into the other slots, as the nte replacement equivalent is the ubiquitous and crappy nte 102 or 103 (i forget, but boy they suck in a fuzz face )but i'm thinking that freekin transformer has gotta be the issue.
i've been at this thing on and off for way too long, so i REALLY appreciate the help, thank you so much