Another Pionner sx 1250 repair

[Joe Stem]

Hey I'm Joe, I live in Sw Florida and I've noticed from looking around and searching the forum that it does not appear that there are any shops to work on these Vintage receivers. This week my dad gave me his 1976 Pioneer SX1250... It has not been used in at least 20 years... This amp is the reason i love music, my dad used to play Dire Straits "Brothers in Arms" cd for me when i was little and "Money for Nothing" is the 1st song I ever remember hearing and it came out of this AMP, so it is very special to me. I got it hooked up at my house and It was brilliant for about 30 mins. I had the music on pause and heard a small "POP" about as loud as a muffled cap pistol. Now i have no audio output ANYWHERE, not A,B,C or head phones... I have found a shop in Naples that may be able to help me (Probably VERY Expensive because Naples is full of rich people) but I would also like to know where to start my own diagnosis, as I am reasonably competent with a meter and have the tools required. I build custom computers for a living and my dad worked for IBM for 40 years and has a degree in electronics so between us we have the skills i think to do it our selves. I don't want to pay an unknown (to me) shop if it is something simple... If it is major work best left to a PRO please say so as well...

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• Joe Stem
• Mar 9, 2017

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• Joe Stem
• Mar 9, 2017

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• Joe Stem
• Mar 9, 2017

 

[markthefixer]

Welcome to AK, and the Pioneer Audio Forum.
The font and motherlode of Silver Faced Pioneer DIY.

You have a leg up on a lot of DIY'ers - what model is that Tektronix scope, 485?
You provide the hands and eyes, we'll supply the guidance.
Many have done full restorations by following directions (and not jumping ahead blindly - THAT'S the sin) and our (MY) parts list(S).
SEE my 20,000 posts - that's a decade of repair and restoration done by non-experts.

First get the service manual - (free downloads or get our GOLD standard - a reproduction paper manual from Rick at stereomanuals.com - it's WORTH it - NO affiliation just a multiple satisfied customer)
READ it carefully, learn how to identify the boards (zzz-999) and then the components (q123, R34, C45 etc)
Then find and pull the power supply board and visually inspect it for the source of the pop. Get used to looking at it.
Working on the SX-1250 with it on it's side standing up on a lazy susan seems to be most efficient, and I get a long slice of plywood that I screw onto the side crosswise as a foot using the case screw and case screw hole to keep it from ever tipping over.

BE VERY CAREFUL WHEN POKING AROUND, THE AMPLIFIER DRIVER CARDS HAVE A COMPONENT UP TOP WITH WHITE WIRE COVERINGS THAT IS CONNECTED TO IT, YET MOUNTED ON THE HEAT SINK NEXT TO IT, AND THOSE WIRES L O V E TO BREAK OFF AT THE BODY OF THE (STV-3H) DIODE.
So, loosen the screw holding it (The stv-3h thermal bias diode) on, and carefully move it to safety over the amp card, taking extreme care to NOT BEND the last 1/2" of wire going into the diode. Grasp it 1/2 inch down the wire and make SURE all bending happens on the other side of your grasp.

Look around inside for the source of the pop. After we find it, then we shall evaluate how to proceed.

Generally we rebuild the four cards first for a solid foundation: power supply, protection and amplifier (driver) cards.
Then we go after the front panel boards that carry the sound.

You WILL be doing a LOT of reading, of 1250 repair and rebuild threads, you will encounter our parts lists.
If you can drive a DMM (and scope) and have (or develop) good desoldering and soldering skills, you are golden.

WATCH OUT - DIY REPAIRS ARE ADDICTIVE...

 

[zebulon1]

One of the four boards in the back is where the pop probably emanated from. The two inboard ones can be removed and inspected by unplugging the Molex connections at the bottom of the boards and unscrewing the mounts. The inner right board is the stabilizer and is the power source for most of the receiver. The two outer boards are the amps and are not as easy to access but also can make pop noises. You may have to loosen the heatsinks from the chassis to access what you need to inspect.

Welcome to to the right place!

 

[Joe Stem]

thanks for the replies guys.... forgot to mention the 1st thing i did was acquire the full service manual.... should have probably stated that... the scope is an old IBM branded scope Ill go to the shop in a bit and get more info on it... It is my dads but he left it for me to use... I'm hoping someone can help me figure that thing out as well... its older than I am lol so is the 1250... I'm going to read and re read Mark's reply a couple of times before proceeding... FYI the 1250 makes one of the most amazing guitar amps i've heard.... blows my Marshall stack out of the water.... we shall start on repairs tomorrow...

 

[Joe Stem]

i can solder, haven't done much de-soldering, and I have never ran the scope... But with plenty of guidance I can manage... And I already stumbled across the parts list...

 

[Joe Stem]

@markthefixer the scope is a 453... Ive finally got my bench ready and got my tools together and got her opened up... lets start from the begining... Step 1 lol... remove inner cards and loosen heatsink to get at outer cards, yes?.. then Step 2 carefully remove Stv-3h while not breaking the lead going into it... remove molex connectors and the two screws.... am i correct with all of this?

 

[Joe Stem]

removed ALL 4 boards and documented Step by step and how many screws and where they came from. Put screws in corresponding labeled and numbered bags matching the parts they came from. I documented the direction and order of all Molex plugs and did not break the leads going to the heat sink Documented board numbers...... Everything looks squeaky clean apart from a bit of dust.. all fuses intact on 107 board on bottom of unit... what do i test 1st and what equipment do i need to use for said test and on what settings? All Molex pins are squeaky clean as well EXCEPT those connecting transistors in the heat sink, those look like they got pretty hot. everything else looks like the day it walked off the pioneer assembly line.. is it possible i just popped an output transistor or 2... to be honest there isn't even that much dust considering the unit has been stored for 20+ years... although i do live in a fairly humid area in SW Florida... the only other think that looks a bit odd is looking down on unit from top the left most board cover is discolored compared to the other two... I believe that is the tuner board correct?

 

[markthefixer]

the plastic wheel sticks out of it? no heat in that one, it's just the luck of the draw as regards to finish and aging.

popped parts are subtle, usually there are no smoking craters. discoloration and carbon.

don't be fooled by assembly glue at the base of the e-caps on the boards.

re: pins - look up my eraser trick. shine em up.

testing is all boards installed, then DC voltages on (connector or wire wrap) pins.
Chassis ground for the black dmm lead, red dmm lead gets all but the tiniest portion of the tip insulated.
if meter is autoranging you are golden, otherwise start high (200 v) and shift down until several significant (non-zero) digits are read.

look up awr-107 in search, then awr-106 stabilizer, will find lists of pins and expected voltages and a plethora of troubleshooting threads.
also pay attention to the reporting formats, to take it easy on the help. mental whiplash isn't fun.
Include expected and measured voltages with board and pin number.
0v is not acceptable. 0.0v and 0.000v is. sometimes 0.1v is important - and to borrow a phrase, "huge".

once we know (or made) the awr-106 board is ok, then we will move on to the protect board awm-091 which will also tell us a lot.

 

[Joe Stem]

i tested the 22000uf capacitors and one of them (The one back right if looking down on the unit) gave me a short... all others gave me a number which proceed to increase as expected...

 

[Joe Stem]

i tested the 22000uf capacitors and one of them (The one back right if looking down on the unit) gave me a short... all others gave me a number which proceed to increase as expected...

sorry i dont have the Part numbers in front of me my notes are in the shop

 

[zebulon1]

The big caps can go bad. They have a cork at the bottom between the terminals. The vented mess will be seen on the inside of the bottom cover.
If the shorted filter cap is the case a blown fuse might be found as well?
I have extra 1250 caps from restored sets if you need one to continue troubleshooting.

As Mark stated a check of the voltages maybe next.
No talk of a DBT? (Dim Bulb Tester)
Got one?
If not, you will need to build one before continuing the troubleshooting.

Wondering:
Does the set light up, dial lights working?
Do the function indicator lights illuminate? AM, FM, Phono, Aux?
When selected to FM, can you tune and see the signal meter's working as if you could tune in a station?

 

[Joe Stem]

@zebulon1 I do not have a DBT i will have to build one... ALL other aspects functions as they should... EVEN the switches and pots are clean no static or noise coming from them... before "the pop" the unit sounded Amazing... I would be willing to purchase a filter cap from you for a decent price if it is known to function properly... is it possible for the cap to let go without spilling electrolyte? tuner picked up stations in resonable quality even without FM antenna... all lights illuminate and the meters function as well...

 

[zebulon1]

I'll PM you about the cap. Postage is all I need for compensation. This site is about helping each other.

You'll have to horse the set back together so you can check voltages on the stabilizer board and protection board.
Readings will be needed from some test points on the boards and from the backsides of the Molex plugs. All Molex plugs should be reconnected. A pointy DMM probe is helpful. You will probe the connectors/sockets that are crimped to the wires in the Molex plugs.

As Mark said once again: The set should be placed on its side, so you can access both top and bottom of the boards.

It's not that I don't trust your filter cap discovery but it is a long shot that its that bad. None the less, I will send you an original cap to replace it and we can continue the repair.

When looking at the scat, the big caps are part of the unregulated power supply circuit. They provide the DC voltage for the big outputs mounted on the heatsinks and a few other circuits in the amplifiers. If bad there would be some real issues.
The set is turning on but not blowing fuses. (You did check all of them?)
This is a good sign the repair could be minor but this receiver can fight back. Cross your fingers and don't proceed if your unsure.

I would get the set running again (first) but you can take the opportunity to do a service on the set. Like lube the controls and switches, repaste the mounted transistors.

Once the DBT is built the troubleshooting can continue.

 

[Joe Stem]

Results when Testing Filter Cap Results when testing the other cap on right side Double Checking results from Test 1 on Bk Right Cap Testing the meter Cap Removed from circuit prior to testing Board ID for Problem 2 "-" end of C13 on board 106A Dim Bulb Tester

 

[Joe Stem]

@zebulon1 so after fiddling around with this thing for the last day or so, Ive discovered some interesting observations..
1. 2 of the four filter caps will give me a reading in ohms... 2 of them will only do it sometimes and usually with the cap removed from the system
2. i reflowed the solder on C13 on the stabilizer board and that seemed to help but i still think it needs replaced as it is the only component so far that shows any visual signs of stress
3. i re installed Filter caps and the two power boards, plugged the unit into DBT, threw the switch and bulb light bright and dimmed instantly, then 4 seconds later "CLICK" just like i want.... HOWEVER, i turned unit off and let it sit for 10 sec., and tried again, just a glowing bulb this time no click.
4. let unit sit unplugged for couple hours and tried again (NO CHANGES WHAT SO EVER) Bright light instant dimming, "CLICK" with a constant dim glow...
5. Try again and bright light no dimming no click... it will go on in this cycle...
6. While testing various things I discovered that if I manually slowly discharge the main filter caps to 0.0V i can sometimes flip the switch and the unit will turn on and click like it is supposed to... other times it won't
7. Also with unit on DBT i can read ~36Vdc on the filter caps...
8. when i check immediately after power down it reads the same and caps discharge as expected

Hope i havent jumped around to much but I didn't touch anything that I wasn't 100% sure what it was or what it's function was....
I await further instructions please...

Unit is currently sitting on bench with both power boards installed but no amp boards, please let me know in detail what exactly to test next...
thanks for ya'lls continuing guidance and patience...

 

[zebulon1]

Let's check the voltages on the stabilizer board.
Like I mentioned earlier about probing the Molex plugs.
With the set on the DBT ckeck the voltages at those Molex connectors/plugs.
There's going to be AC and DC voltages.
Pins 1 thru 5 are AC.
Pins 6 thru 15 will be in DC.
Probe the back or the connector. Insert the probe into the little cavity where the wire and crimp connector reside and get a reading. Black probe to chassis ground.

The next check is the protection board. That is the other left inboard board. The one with the relay on the top side of the set.

 

[Joe Stem]

will do and ill record the readings and report back soon

 

[Joe Stem]

@zebulon1 Test results for Inner Right board
Pin Voltage Color
1 -32.9 white
2 32.0 Brown
3 30.9 Black
4 0.5 Red
5 0.5 Red

6 90.1 Pink
7 23.4 White
8 22.1 Blue
9 20.9 Green
10 45.0 Brown
11 87.3 Red
12 0.0 Black
13 0.0 Orange
14 0.0 White
15 0.0 Blue

Which pins are Ac and DC on left board... are they the same as right board i.e 1-5 VDC 6-15 VAC

 

[zebulon1]

I steered you wrong on the AC voltages but you gave enough readings to see a few things.
If you are not getting a good reading you can switch the meter over to AC and check the for voltage.
Looking at the scat you will see the ground connections and know there won't or shouldn't be any voltage.

Check all the fuses by removing them and ohm them out, making sure that none are open on board AWR-107.
The negative regulated voltage is missing from your readings. Board AWR-107 supplies them.
Fuse F2 is OK? (1A)
All the big diodes on the board should be checked. Their kinda hidden between the big ceramic caps.
The picture of the axial cap you imaged is part of that circuit.
A good AWR-107 board will supply +85 volts DC from pin 9 and -85 volts DC from pin 10.
12 volts AC from pin 12 (Lighting).
+23 DC volts from pin 8.
Lets check board 107 for voltages.

 

[Joe Stem]

@zebulon1 pulled Fuses and they all check out....

Pin1 0.1
Pin2 0.1
Pin3 36.8
Pin4 37.6
Pin5 10.2
Pin6 10.2
Pin7 Grnd
Pin8 22
Pin9 85
Pin10 0.0
Pin11 4
Pin12 3.8
Pin13 46.8

Going back out to do diode check in a min