Pioneer A-7 amplifier - right channel distortion

allanmatters

Active Member
Recently picked up a pair of Pioneer HPM-700 speakers which i hoped to use with my vintage Pioneer A-7 amplifier. Unfortunately on the day i bought the two babies home my amp was giving distortion on the left channel. After thorough checking by my dad said he found the transistor legs(one of them connected to the left heat sink) was loose from its body inside!! Every time we used to press the mother board it use to come clear(no dry or loose solder- checked with meter for reading everywhere). But I wanted to do a good job so i bought 4 pairs of Sanken transistors which where equivalents, before changing all 4 i tried just one channel with the new transistors . so it was right channel old transistors (Toshiba 12 amperes) & the left channel new transistors (15 amperes), this was wrong i guess cause when i started it . the protection circuit dint even kick in! It went straight to the main fuse and BOOM. then i changed all 4 transistors to the Sanken(15amperes) ones and the left side was perfect, no distortion like before. Right side was giving distortion now. the fault was the transistor on the pre amp board A985. It was short, got a new one and changed it but still no difference. the new transistors are good even checked them by taking them out. But still stuck with distortion.

here below is a pic of the old transistors which were bleeding. I couldent find them anywhere so i had to buy equilants​


the new Sanken transistors which did the job well , almost . If only i had changed all 4 of them i wouldent have blown anything else


my dad has studied the diagram from the book too. But hes so slow & this is taking ages like weeks . i was thinking of buying all the small mini transistors for the right side which is on the preamp mother board to fix the problem cause the main board where i fixed all 4 transistors are perfectly getting the desired voltage except 1 single transistor on the right channel. SO frustrating

Pioneer a-7


this amp is amazing it really matches the HPM-700 i bought with a rich bass i have missed hearing for a long time, the midrange is simply beautifull , the sound of the vocals is so calming on a low volume especially . Never heard such a sound, reminded me of a couple of Bose speakers which i used to listen too . But the a-7 dangerous if you dont know how to use it you could burn the two speakers.
my dad burnt his jamo proffessionals with this same amp. they were rated at 200 wpc and maximum 280 wpc. this pioneer a-7 is 70 wpc with 75 or 78 being max

he was telling me the jamo woofer coils were burnt inside

here is the two new babies i cant enjoy completely only because of the distortion​

 
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To save some time and effort, power up the amp with a signal into the defective channel and with a pair of high impedance headphones (or an oscilliscope, trace the audio signal until you get distortion and deal with the affected transistor- replace it's corresponding unit on the other channel to maintain the same audio qualities.

When dealing with finals- always replace all 4 at one time.
 
To save some time and effort, power up the amp with a signal into the defective channel and with a pair of high impedance headphones (or an oscilliscope, trace the audio signal until you get distortion and deal with the affected transistor- replace it's corresponding unit on the other channel to maintain the same audio qualities.

When dealing with finals- always replace all 4 at one time.
that's what I'm trying to do but the jumper wire from the mother board(heatsink) to the pre amp is limited I can't take out the pre amp board and test with the meter. Need to create some kind. Of extension, thanks for your opinion.
 
Are you able to get at the leads enough from above to use a test prod? I only mentioned from the bottom side because there are some transistors that have a nonconductive oxide coating- which can be scraped enough for making a contact, but tends to be a pain when the boards are populated real tight, but yours do not look too bad, and usually you will only be able to hear signal on 2 of 3 leads on a transistor. So if the boards need to stay put- if you can reach 2 leads of the transistors, that is something at any rate, but you should be able to narrow down to the transistor with even that limitation.

The heat sensors for now can be removed from heat sink for testing purposes. But the other leads can be an issue.
 
headphones? where should I connect them

Are you able to get at the leads enough from above to use a test prod? I only mentioned from the bottom side because there are some transistors that have a nonconductive oxide coating- which can be scraped enough for making a contact, but tends to be a pain when the boards are populated real tight, but yours do not look too bad, and usually you will only be able to hear signal on 2 of 3 leads on a transistor. So if the boards need to stay put- if you can reach 2 leads of the transistors, that is something at any rate, but you should be able to narrow down to the transistor with even that limitation.

The heat sensors for now can be removed from heat sink for testing purposes. But the other leads can be an issue.
In the first picture where I displayed the amplifier open from the top, you can see the board facing you. green in colour with the jumper wire on top, my dad took it out many times half the way with the jumper still on without the heat sink cap wires. Thing is he being a technician takes his own time, he keeps telling me if he could take out the whole board with the jumpers connected he could find the problem with the meter while the systems running, is the headphones thing you suggested make things easier? If so where do I connect them? I have no oscilloscope btw
 
Essentially one lead would attach to the chassis, the other end with the test prod would be used in the same manner you would use a scope probe.

The headphones need to be high impedance type similar to theold telegraphers headsets. If you have newer low impedance types, you can use an interstage transformer to step up the impedance.

In some ways, the headphones approach can work faster than a scope, you do not need to take your eyes off anything to look at a scope face, you just need to listen for sound quality. The old headphones may be low fidelity, but you will hear speech clearly through good tranistors, and those that are a problem will be distorted and garbled.
 
tried something different

Essentially one lead would attach to the chassis, the other end with the test prod would be used in the same manner you would use a scope probe.

The headphones need to be high impedance type similar to theold telegraphers headsets. If you have newer low impedance types, you can use an interstage transformer to step up the impedance.

In some ways, the headphones approach can work faster than a scope, you do not need to take your eyes off anything to look at a scope face, you just need to listen for sound quality. The old headphones may be low fidelity, but you will hear speech clearly through good tranistors, and those that are a problem will be distorted and garbled.

alright will try that out. mostly with the meter
 
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Well we figured out the problem regarding distortion form the right channel, 2 zenner diodes were shot dead followed by the ordinary diode 1s1555, I got the other zenner diodes but couldn't get the. ordinary , so I looked at its equivalent in the pionner a-7 manual. 1s2473. Couldn't find that to instead. Found on audio karma here itself a post saying that 1s2473 is also as good as 1n4148. so I put the 1n4148. wow!!!y my a-7 was completely working, thanks to this forum and all the great people and technicians here on audio karma, I'm testing it now past 6 hours. Beautiful simply!!
 
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pioneer a-7 amplifier fixed

Now with equilant transistors on the heat sink and new diodes. It's back to life!
 

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A "A-7" up and going again = nice.
Yup Hitachi 1S2473 replacement is a 1N4148
BTW there is a Pioneer section on this forum with loads of help
 
A "A-7" up and going again = nice.
Yup Hitachi 1S2473 replacement is a 1N4148
BTW there is a Pioneer section on this forum with loads of help

i shall search that pioneer section , thanks for the tip. Yea the A-7 FINALLY UP !! like after years maybe 5 years it ws just kept somewhere and ignored until i bought a set of HPM series i guess that motivated my dad even though he took more than a month

only thing my VU meter on the pioneer a-7 does not bounce all the way up inspite of the volume really high, its showing incorrect readings i guess because the new Sanken transistors are rated at 15 amperes while the old faulty Toshiba transisters were rated at 12 amperes , Just worried about not over driving the speakers, now i look at the woofer cone movement and hear the sound too untill i know the speaker is bouncing toomuch

Is there some way i can keep a level indicator on the speakers? like a light to know if i am over driving or not.. after all i can also remember the VU meters too , like till where they are bouncing up to the limit i am enjoying the loud classic music
 
i shall search that pioneer section , thanks for the tip. Yea the A-7 FINALLY UP !! like after years maybe 5 years it ws just kept somewhere and ignored until i bought a set of HPM series i guess that motivated my dad even though he took more than a month

only thing my VU meter on the pioneer a-7 does not bounce all the way up inspite of the volume really high, its showing incorrect readings i guess because the new Sanken transistors are rated at 15 amperes while the old faulty Toshiba transisters were rated at 12 amperes , Just worried about not over driving the speakers, now i look at the woofer cone movement and hear the sound too untill i know the speaker is bouncing toomuch

Is there some way i can keep a level indicator on the speakers? like a light to know if i am over driving or not.. after all i can also remember the VU meters too , like till where they are bouncing up to the limit i am enjoying the loud classic music

oh yea also want to do finishing touches on the transformer inside. its making a "timer running" type sound, like the timers u get in the old top loading washing machines only thing more louder.. My dad says it needs to be given for re-lamination . Any idea on this??
 
vr101 and vr102 adjust the level indicator
full instructions are in the service manual pdf page 27 ..fig 11.2
 

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transformer "timer running irritating sound" fixed

oh yea also want to do finishing touches on the transformer inside. its making a "timer running" type sound, like the timers u get in the old top loading washing machines only thing more louder.. My dad says it needs to be given for re-lamination . Any idea on this??
well i would like to share this with any one else who is facing the same problem as i was, If there is anybody with the same problem . As my previous post here says about the big transformer inside my Pioneer A-7 amplifier. Its making a noise. Usually there is always a "default" humming sound given by the transformer when its on and running. This sound is not necessarily heard. But if you go very close enough to the transformer or touch you ear to it you can here this default humming sound. My pioneer A-7 amp was quite old. Dad picked it up in 1981, Now the system was just fixed and the transformer was the last thing on my mind . It gives a high frequency irritating timer type of sound after 30 mins of use . this was irritating as i couldn't enjoy medium soft music as that transformer sound was disturbing. So before i could give it to the shop for them to do a re-lamination of fresh liquid, my dad did it himself

1st off we had too take out all the unwanted plastics around the laminated strips of the transformer. then also a thick copper strap and took off the side plastic brackets

then we took out the whole transformer and he used varnish liquid wood which was like lightish brown , like liquid sugar. He said thats the ultimate insulation as the ones inside the transformer are gone so old its all dried up and hence causing the sound

We made the transformer nice and hot almost as if you couldn't touch it for long , then he poured the varnish in the sides where the liquid would insulate deep in. I used a hairdryer to keep the heat hot as hell at the same time. then we flipped it on each side and did the same for 45 mins to 1 hour while the system was on so it would be very hot followed by the heat i put by the hairdryer. Then we waited for it to insulate nice inside.

after this we let it cool for half an hour, and then we started the Amplifier, Unbelievable the sound has been minimized completely. Only the humming default sound can be heard now , that too its not heard at all. Its become so silent.. have tested it today for past 5 hours with music playing loud too the transformer makes no sound at all.. this is useful in case you guys out there having similar problems cause these type of vintage sets is tough to get the same transformer, well at least in my area
Ive only uploaded pics of all the unwanted plastics my dad broke off the transformer and copper we kept away

 
Well my pioneer a - 7 dint work for long, it works when it wants eg after 2 days it worked , the problem is on the left chanel where I changed the diode using 1n4148 as the substitute of 1s2473 or 1s2555 , at times when it works when I raise the volume high to a 100 watts output the left chanel slowly decreases followed by distortion and cuts off. Then it happens like a cycle the next time I switch it on, and when it works the transitions on the left chanel don't get as hot as on the right chanel. Even then the same for the heat sink transistors. The 1n4148 substitute is causing this ? :
 
Is the diode part of the bias circuit? if so, usually the diode is thermally attached to the heatsinks near one of the transistors. You should check how the other channel diode is placed and do the same with this one. Do you have pics of each diode?

Can see there are other parts with 2 wires running to the heatsinks, those are probably the bias diodes or transistors. Check those for operation and the passive parts around the bias circuit.
 
My left channel is dead completely. Here is how it finally passed out , while I was listening to music , it started distorting and reducing . I turned the volume higher . It went lower in distortion and finally when dead . From that day it never randomly worked . Dad says it's got to be dry capacitors.. I've seen many forums solving similar dead channels by cleaning relays only thing what relays are they . My start up relays are working as u can hear the clicks and my protection circuit shows green light . My plan now is to take pre out put from pre-amp board to a power amplifier I have outside separately , as I don't wAnt to waste more money on transistors as they all got burnt slowly one by one

Any one on any tips ?? I successful fixed it once , I can do it again with a little help
 
Any body got any idea on what I gotta do ?

Yes, instead of just replacing transistors and diodes, you're going to have to check the passives (the resistors and caps) as well.
First off, to get some of these numbers straight, the 1S2555 you spoke of earlier is not a standard
diode. It's a 14 volt 1 watt Zener diode, and that number doesn't even show
up in the service manual, so which diode are you talking about? But if that number was one of the stock diodes,
you certainly cant replace it with a 1N4148. However the 4148 can be used in place of the 1s2473 just fine.
You're going to need to pull what you know is bad. Then test the others transistors for short or open. Make sure
to check all resistors, caps, ect. in the other surrounding areas as well. Remember, when transistors blow
they generally don't just fail without something else taking a hit. Its a good bet you will find collateral damage to
other parts as well. Miss just one and all of your work and new parts can end up in smoke.
 
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