On Friday, August 30, 2002 at 1:22:07 PM UTC+2, Shaun Davy wrote:
Am doing repairs on a Pioneer Stereo XR-P740M with amp problems. Its that
one with the 2 halves mounted around the fan!!Problem started when there was
trouble with the tape deck (owner induced) 2 buttons were pressed at once.
after that a tape belt started slipping when unit initialises or other deck
is used. in investigation of that (by owner) it started blowing fuses. Now
the output transistors are blown. Tape deck has now been unplugged from main
pcb. Replaced transistors and again blown.

Have looked at Amp section and looked for info on this amp and found that a
bad 220 ohm resistor can cause this fault, checked board and one of the 2
power transistors (2SA1264) has a bad resistor that looks blown and can't
read print on it (R7547) it is across B-E junction like the other power
transistor.

My plan of attack is to replace transistors 2SA1264 & 2SC3181, the smd
resistor and put in a 1A fuse to start with to make sure im not going to
blow transistors. Also seen info where a bad tape motor can cause these
transistors to blow. Can i try the above with the tape deck still unplugged?
and if so can i plug the tape deck back in and not be worried about blowing
op transistors again.
I know this amp is a beast but would really like to get it running again.

Any suggestions appreciated

Shaun

Hi Shaun

Did you manage to get the fault on the power amp?
I two have an xr-p740m that keeps blowing fuses.
If I switch the unit on without the power amp connected it doesn't blow the fuse , but as the power amp module also include the voltage regulators it might be something further down the power rail.
Perhaps a bad cassette motors like Lindsey and/or Aaron mentioned.
I have desoldered all the trough hole transistors and did not find any short on any off them.
My R7547 test 220ohm without me desoldering it.

This looks like a really awesome amp and would love to get it running again.

If you have any info to spare on the problem or unit I would really appreciate it.

Do you know if this unit would function normally if the tape and CD units are disconnected?

Sorry for my bad English :-(


Regards

Bennie van Eeden

On 11/01/2016 4:17 AM, wrote:
On Friday, August 30, 2002 at 1:22:07 PM UTC+2, Shaun Davy wrote:
Am doing repairs on a Pioneer Stereo XR-P740M with amp problems. Its that
one with the 2 halves mounted around the fan!!Problem started when there was
trouble with the tape deck (owner induced) 2 buttons were pressed at once.
after that a tape belt started slipping when unit initialises or other deck
is used. in investigation of that (by owner) it started blowing fuses. Now
the output transistors are blown. Tape deck has now been unplugged from main
pcb. Replaced transistors and again blown.

Have looked at Amp section and looked for info on this amp and found that a
bad 220 ohm resistor can cause this fault, checked board and one of the 2
power transistors (2SA1264) has a bad resistor that looks blown and can't
read print on it (R7547) it is across B-E junction like the other power
transistor.

My plan of attack is to replace transistors 2SA1264 & 2SC3181, the smd
resistor and put in a 1A fuse to start with to make sure im not going to
blow transistors. Also seen info where a bad tape motor can cause these
transistors to blow. Can i try the above with the tape deck still unplugged?
and if so can i plug the tape deck back in and not be worried about blowing
op transistors again.
I know this amp is a beast but would really like to get it running again.

Any suggestions appreciated

Shaun

Hi Shaun

Did you manage to get the fault on the power amp?
I two have an xr-p740m that keeps blowing fuses.
If I switch the unit on without the power amp connected it doesn't blow the fuse , but as the power amp module also include the voltage regulators it might be something further down the power rail.
Perhaps a bad cassette motors like Lindsey and/or Aaron mentioned.
I have desoldered all the trough hole transistors and did not find any short on any off them.
My R7547 test 220ohm without me desoldering it.

This looks like a really awesome amp and would love to get it running again.

If you have any info to spare on the problem or unit I would really appreciate it.

Do you know if this unit would function normally if the tape and CD units are disconnected?

Sorry for my bad English :-(


Regards

Bennie van Eeden


Hey Ben, did you check the date on the posting ??

On Sunday, January 10, 2016 at 3:17:51 PM UTC-5, wrote:

Hi Shaun

Did you manage to get the fault on the power amp?

If he did, that was 13 years ago.

As far as this amp goes; it's a toilet. Instead of using speaker protection output relays (which are at several cents U.S. each) they use a "blow line" to intentionally destroy the outputs which will open the fuse and save the speakers (maybe, depending on how many fuses were tried and of what current rating).

This is the famous "smoke chimney" that uses the fan to exhaust the smoke out the back of the amp. I still can't believe Pioneer built this bomb.

But anyway, usually changing the outputs *and* the smd emitter resistors will keep it from destroying itself long enough for the check to clear. I did close to a hundred of these years ago without buying the two board halves from Pioneer.

Hello guys i have read alot about this pioneer amp probs and seems all about same issue which is a bad design and mounting the amp inside is very weak depending on back plastic plate which make it easy to break and the whole heavy amp fall inside and causes a mess of short circuits at any unexpected vibration which happens to mine and i tried to fix it up by changing transistors many times and goes blown time after time despite the time waste by ordering those damn transistors and this stereo has a memorable time for me and my family i kept searching for a solution to bring back yo life i find this easy and very cheap instead of keep changing parts by doing this
1- disconnect the tape board ( black ribbon cable comming from main board)
2-buy 120wx2 power amp board(TPA3116D2)powered by 12v from the cable disconnected from tape
3-disconnect the grey ribbon cable from the old amp and split cables marked as RIN-FG-LIN and connect to new amp input then connect R OUT -L OUT to new amp +L +R only no negative need .
4- connect back to the old amp the cables of REF G and FAN and rest of cables keep disconnected.
Now you can bring life back to your old pioneer beast. ..
ENJOY YOUR FAVORITE OLD MUSIC

VERY IMPORTANT NOTE
1-if the amp is falling inside and the plastic plate is broken edges where screws goes in from the back side try to put it back in place and drilling new holes for the screws to hold back in its place and BE SURE THAT THE 2 HEAT SINKS of the transistors upper part is not touching the lower part to prevent short circuits between transistors (cable tie or isolated copper cables by tying them separately to the back plastic plate through vent holes)..
2-support the amp by using metal strip fixed internaly above the amp on the back of the case to the vent fan (there is an plastic arm on top of the fan where you can fix the other end of the metal strip) I use a slot metal cover from old pc and works well for the size and length this way will support the weight of the amp and prevent it ftom falling back inside .
3-MOST IMPORTANT ISSUE is to disable the high voltage from the main power transformer output that generate hi voltage to the amp (no more need for this hi voltage) by desoldering the 3 pins marked as MAIN on the output board of the main power transformer and be sure to discharge the hi voltage that connected to the old amp (grey ribbon cable) marked as -B +B can be discharged by connecting those cables to a 70 watt lamp CARFULLY THEY CONSIST HIGH POWER FROM THE 2 GIANT CAPS and then isolate those cables and leave them disconnected from the old amp.
This procedure must be taken before installing the above mentioned process
Good luck

On Saturday, August 10, 2019 at 1:37:06 PM UTC-4, wrote:
Hello guys i have read alot about this pioneer amp probs and seems all about same issue which is a bad design and mounting the amp inside is very weak depending on back plastic plate which make it easy to break and the whole heavy amp fall inside and causes a mess of short circuits at any unexpected vibration which happens to mine and i tried to fix it up by changing transistors many times and goes blown time after time despite the time waste by ordering those damn transistors and this stereo has a memorable time for me and my family i kept searching for a solution to bring back yo life i find this easy and very cheap instead of keep changing parts by doing this
1- disconnect the tape board ( black ribbon cable comming from main board)
2-buy 120wx2 power amp board(TPA3116D2)powered by 12v from the cable disconnected from tape
3-disconnect the grey ribbon cable from the old amp and split cables marked as RIN-FG-LIN and connect to new amp input then connect R OUT -L OUT to new amp +L +R only no negative need .
4- connect back to the old amp the cables of REF G and FAN and rest of cables keep disconnected.
Now you can bring life back to your old pioneer beast. ..
ENJOY YOUR FAVORITE OLD MUSIC


Why on God's green earth would anyone modify that hunk of garbage just to keep using it. I would be embarrassed for anyone to even see me throwing it out. Besides, I don't see how the 12V tape deck source will provide enough current to drive an output board, at least not above background level.

It may be a stupid design, but those Pioneer amps aren't hard to fix.

As i said earlier its a vintage deck from the days when my kids were young and used to sing karaoke on it now they are all above 20 y old and they are happy to see it working again and as for stupid design it is stupid because they didnt consider enough strength for mounting inside and there is no spare parts neither service manuals from the companey it self (i find one posted in a forum very thanks to that persone )without his manual i wouldnt be able to reach to this result .
Anyhow its just for those who like to keep on with thier memories

On Saturday, August 10, 2019 at 9:49:53 PM UTC-4, wrote:
As i said earlier its a vintage deck from the days when my kids were young and used to sing karaoke on it now they are all above 20 y old and they are happy to see it working again and as for stupid design it is stupid because they didnt consider enough strength for mounting inside and there is no spare parts neither service manuals from the companey it self (i find one posted in a forum very thanks to that persone )without his manual i wouldnt be able to reach to this result .
Anyhow its just for those who like to keep on with thier memories


I get it, but whereas that Pioneer was probably and honest 50WPC, you would be lucky to get a couple of watts per channel before the low current 12V tape deck source sags out or even fails completely. You should add a 12V transformer or use a buck converter and grab the now unused B+ source to supply it.

You still won't get the same power output as it had but you can probably get up to half of what it had before it failed.

Dear john
Regarding the 12v dc that i used is comming from the main huge power supply and its current output is more than what the new amp board need and i did try it for long time on the original 110w×2 8ohm speakers with no over heat on the new board same aswell with old amp althought the new board is very compact and not power monger it can run on 12v- 24v in general as iam not an electronic engineer just a hobbies it was a challenge for me to bring life again to this vintage machine .
Actualy this machine is made from 2 sets one for the amp and casset and radio and another set for the 50+1 cd player conneted to the main set by a ribbone cable and power cord and back again to the bad design both sets has a voltage selector 110-240v last time ayears ago the kids try to use it they change the voltage selector to 240v on main set and didnt notice that the cd set need to be changed at same the result was the cd power suply blown because there is no fuse there it was another challange to be fixed coase there is no info about that output voltage and there is no schematic diagrams for that set to know the output voltage of that power supply after hard time of searching for a replacement i couldn't find any then i decided to desolder the transformer from the board to try winding it i find there at the input section unusable pin not connected to any traces on the board i check out if its connected to other pins and there was there i felt tgere is a hope i connected it serial with 70watt 110v lamp to avoid further damage and the result is the deck works again on 110v then i disconnect the volt selector leaving it direct and i add a small stepdown volt transformer from 240to 110v (i used small 30watt traveller converter which fit inside the deck housing ) it was a real challange but thanx GOD i did it and i liked to share this experience with other people who face this prob with PIONEER products and they like to keep them functioning after many years.