How do I test a transformer voltage

[CHRIS-F]

Hi, can anyone tell me how to correctly test a transformer output for correct voltage using a digital multi meter?

I know this is the wrong section to ask about record player transformers, but the help I have had in here has been excellent so far :-) The transformer is from a vintage record player I'm trying to fix for my dad. The problem is there is no sound, I am thinking the problem may be one or both transistors, but I started at the beginning and wanted to test the voltage getting to the pcb first, I have attached a picture below of the circuit diagram for the transformer, I followed the black wire which goes to chassis ground and there are 4 other wires, two are for the 6v lamp but there are two others going to two diodes then to one single pink wire and onto the pcb. I put my multi meter on 750v ac and attached my black probe to chassis gnd and the red probe to the pink wire at the pcb, but I am getting 58.9v see picture below, the problem is there is a smoothing capacitor next to the transformer rated at 30v so I would assume that the output from the transformer should be a tad under 30v why am I getting double the voltage is it because there is 2 outputs on the transformer? am I measuring it wrong? is this a half wave rectifier? I don't want to move onto testing other stuff till I know the pcb voltage is correct first.

Thanks in advance,
Chris.

[ed12]

ok to start with it is a/c volt range  u want to be on NOT dc
as follows

the mains side should show u 225vac >volts ac
there is 2 winding on the secondary side
1 set is 6.3 vav for the poilt light's
the other set will be before the 2 diodes will give u the full ac value of the transformer
for the second set of secndary side windings
from what i see it,1 of the windings is center taped >high voltage secondary side<
the other 2 wires are for light's 6.3vac,as clearly listed in your print

so math here comes to play
my total secondary  winding before the center tap >the 1 i am interested in is 36vac<
with the tap we divide by 2 = 18vac x 2 after the 1/2 wave bridge would give u about 25-vdc at the filter well 20-25 >load not taken in<
but these are your rough's

this is where u concern yourself first
then move on

ed

[CHRIS-F]

Thanks for a quick reply Ed much appreciated.

What you said makes some sense but I'm still a little confused, it's the centre tap bit I don't understand, have I measured the voltage correctly at 58.9v does that sound about right for a transistor record player chassis? the schematic has some test voltage points but it doesn't have a voltage for that test point at the pcb I just though 58.9v was double when the smoothing cap was only rated at 30v

Also is it normal for transformers to get hot? the reason I ask is that there was a waxy substance on the bottom of the transformer, I assume it was put there just to stop any wires shorting out etc, but some of it appears to be melted off onto the wooden base of the record player.

Thanks again,
Chris.

[ed12]

if the filter cap is 30volts dc
then i can ifer that it is a 12volt center tapped transformer
as i have shown u
2x12=24 or just about the were as of
well with in 30vdc range of the main filter

your picture i cannot get anymore ?
but i was going to list the leads for u..
it getting "hot" tell's me u are loading it
first desolder the 2 diodes
and then plug it back in and c if it heat's up
if it dose ? then un-hook the 6.vac light's line
and retry
this will narrow down your search
there is no way on the secondary side u will c 58 volt's
or that 30 volt cap would have blown up on u by now

ed

[CHRIS-F]

Ah! The transformer isn't getting hot now I felt it I think it must have gotten hot somewhere in the past as the wax is melted of it but it's not hot now, I checked the schematic again and did a bit of reading about center taps, there are two different schematics mk1 and mk2 one appears to have a negative center tap the other positive, one is fused and one isn't. I'm assume mine is a mk2 as there doesn't appear to be a fuse, on the mk2 schematic it says 27.5v and 26v on the mk1 between point 11 and 12 why would I be getting 58.9ac Am I measuring it wrong? after the transformer and filter cap/diodes should I be measuring in ac or dc, I've been trying to measure it in ac but one of those stupid moments just occurred to me if there are diodes to rectify and a smoothing cap should I now be measuring dc??

I left it at my dads but I will pick it up later and have a good look over it at the weekend, how would I check the voltage at the cap? is it safe to put the meter between the 2 legs of the cap or can that short something out?

Thanks ed,

I learned some electronics from books, but learned a lot more from you and Shawn on the forum in such a short time. Thanks! Thanks! Thanks! again :-)

[ed12]

at the transformer is ac
at the filter cap is dc
the 58 vac has me really confused
unless u are reading 1 side of your input ac
and 1 side of your secdonary side ?

your center tap acording to this schematic u posted is refed. to ground
so the filter cap negative side to either leg of the secdonary side  BEFORE the diode's
should give u the ac voltage,after the diodes u switch to dc volts

ed

[CHRIS-F]

Thanks Ed,

It was me being an Idiot, I was trying to measure AC at the test point I forgot about the voltage being rectified, I checked the voltage at the test point this time using DC I'm getting exactly 27.5v between point 11/12 it matches the schematic exactly :-)

Now that I know the psu is correct, how do I go about checking the transistors? I saw a pair of AD161/AD162 transistor for sale but would it be possible that something else has gone taking the transistor out, or is it normal for transistors to just die?

Cheers,
Chris.

[ed12]

if they are removable ? best way
go to either diode chk or low ohms
and measure across emiter to collector >normaly they would read open<
then u repeat the process between base and emitor and base and collector
at that point 1 way b-e/b-c u should get a reading,not a dead short but on a digital meter about .5,on a low ohms about 15 ohm's is a good call
then reverse the black and red probe and redo b-c/b-e they should read open
if they chk out then the outputs are in usually fine shape

ed

[RoyalScam]

Just a few quick ideas, on how I'd shoot this.  You know you have voltage from the power supply, good.  According to the schematic, you could touch a probe ( we're talking screwdriver tip, anything, not grounded) to the center tab ( wiper ) of the tone control pot.  If you hear any output, hum, thump, anything from the speaker, I'd suspect the amplifier was working.  If not, disconnect the speaker, apply voltage to it from a 1.5 volt battery and listen to see if the speaker "thumps".  Record players are more mechanical than electronic.  I'd suspect a bad connection from the stylus through the tonearm to the preamp.  BTW ground in your case is positive, red lead on your meter.   See the PNP output transisitors and note the polarity of the diodes in the power supply.

Good Luck,
Scam

[CHRIS-F]

Thanks scam,

I am about to take the transistors out to check them as Ed suggested, but before I do I just wondered if anyone could spot any faults from my schematic voltages, which could be causing the problem, I took a few readings and compared them to the schematic test points see pic below. Also are you sure the gnd is positive, as I posted two schematics, I was a little confused at first, but I'm sure this is negative ground (mk2 model) again see pic below, am I right in thinking the two diodes are only allowing negative side of the wave through to gnd and the positive is the center tap, I am still trying to understand which way round anode and cathode is Lol.

My smoothing capacitor has a stable 27.1v across it so that looks to be ok, anyone have any ideas about the low voltages or is it time to pull the transistor?

I checked the speaker which is 15 Ohm and got a reading of 14.3 Ohms and also heard static through it quickly applying a 3v coin cell across it, I also know the tone arm is in good working order as I hooked it up to my Technics amp and the sound played fine, so Stylus/Cart and wiring is fine.

Thanks again guys :-)
Chris

[ed12]

chk tr3/4/5
your feed voltage is spot on
27.vdc
but where u are getting such a low voltage on tr3/to the base of tr5 is of some conern
that is a common push/pull output stage npn/pnp
but that voltage on tr3'd collector is of concern

ed

[CHRIS-F]

Hi Ed,

I noticed that the output pair tr4/tr5 were attached by wires to the mainboard so I unsoldered them and hooked them up to a transistor checker see below, are those readings adequate or do I require further testing? I've never used a transistor checker before so not sure what to look for, I also had to de solder c11 as it was in my way, I checked that while it was off, the schematic says c11 should be 400uf it reads 472.9 is +18% ok for old caps like that?

TEST1. PNP  configuration 1(b) 2(e) 3(c)
B=99 UF=88mv

TEST2. NPN configuration 1(b) 2(c) 3(e)
B=121 UF=89mv

What is B? and UF?

Regards,
Chris.

[CHRIS-F]

The Image wouldn't upload before there was an error, here it is.

Chris.

[ed12]

ya the cap is getting stressed
there cheap just replace it

b=beta tech term of the gain
uf is the upper freq range

if they did not show a short,which i do not see in the screen shot
then back up 1 transistor to q3
there is something pulling 13.5vdc down to the low u posted.

again please list the to output transistor number's for me so i can do a fast chk for u

ed

[ed12]

ok i c better now

>    PNP germanium alloy transistor intended as power amplifier in AF applications. <

i had a nagging question in my mind why they were setup as collector followers
instead of emiter followers as u would normaly see in a push pull amp
hence i backed up to where the #'s were listed and ran them
>http://www.radiomuseum.org/tubes/tube_ad162.html<
lord and behold that is dang near your amp to a t

just watch germanium is about all i can say
they are not like normal silcon based 1's .5 foward and open reverse

ed

[RoyalScam]

Yeah, sorry tired eyes.  I'm not used to seeing a full wave center tap rectifier drawn this way.  I've always seen it with the center tap of the transformer at gnd and the diodes biased in the opposite direction.  In both schematics you'll have a negative gnd, my mistake. My next guess, TR3. Remove it from the circuit and check it for a shorted condition.

Scam

[CHRIS-F]

Guys you were both spot on about TR3, It's alive, just put a short clip on youtube.

 https://www.youtube.com/watch?v=PfyJjeHpX7k&feature=youtu.be

I want to thank you both, I checked all resistors, although some were a bit out of tolerance r10 was way out. as luck would have it I had a spare 470 Ohm in my box replaced that and I thought I would check TR3 whilst I had the board off. the leg on the collector of TR3 was snapped inside the coloured sheet, I used a small blue jumper wire and some heat shrink tubing, put it back together and to my amazement it sprang into life. Your help has really helped me out much appreciated :-)

now I can get back to sorting my arcade machine psu out Lol.

[ed12]

good work 

ed

[CHRIS-F]

Hi Ed,

Just a quick question, my dad was made up that I've fixed his record player, but he is a bit concerned, he said when he has played about 20 records the metal plate where the tone controls are gets quite hot, I noticed myself before I gave it back to him it was warm but I only played one record to test it and it was only on a short time. would it be normal for something of this age to get hot when in use, the AD161 and AD162 output pair of transistors do seem to be connected to the chassis as a heatsync and the tone controls are connected to the same chassis, but how hot should they get? should I be concerned?

Regards,
Chris.

[ed12]

i would asume 1 of the output's are leaky
what i would do for good measure is just replace them both
well u are there provide alot of heat-sink-compound,it is a grease u can buy in 1-oz tube's

tone arm's getting hot to the touch mean's something is over heating

ed

[CHRIS-F]

Ah!

Sorry Ed, It's not the tone arm that is getting hot it's the metal plate by the tone controls, bass/treble/volume buttons, see pic below, I would imagine, it may get warm as the transistors are using the chassis as a heatsync, but my dad said it was quite hot after playing for a while, and there is some plywood separating the chassis and the plate so I don't think it should get hot.

Can I test the transistors for leaking?? I think I might get two and replace them as you say with new thermal paste just to make sure, but I like to learn as I go so would like to know if they are bad.

Regards,
Chris.

[ed12]

as i explained before
gernamion trans are a pain in the butt to test
the golden rule here apply's,if in doubt replace..
no shame in it.
after 30 year's u would :stress-out-to:

ed

[CHRIS-F]

Thanks Ed, will replace them soon, think I got some thermal paste left over from my cpu or xbox gpu's somewhere, just wondered though where would I source new Ad161/2's? I've seen New Old stock still boxed advertised but will they be any good, do they still make them or is NOS the only way to get them?

Thanks again,
Chris.

[jennifer]

    Omg, that plate looks like its got heat damage, Jenn has never seen anything quite like that.

[ed12]

high
gernamion's went the way of the dodo bird about 20years ago
so if u can find good nos ?
that is your best bet

ed

[CHRIS-F]

Thanks again Ed,

P.S. Jennifer, the plate isn't heat damage, that's my dad's handy work trying to take it apart before we bought the service manual, it's just an aluminium plate glued onto plywood, but the glue isn't so sticky anymore. and now the plate isn't so straight Lol.