Pioneer SA-7800 replaced output transistors ....

70smidfiguy

70smidfiguy

Active Member
Hello,

I have a nice Pioneer SA-7800 that came to me with two different pairs of output transistors!
One pair were the 2SA1075/2SC2525 the other pair were Sanken 2SA1215/2SC2921.

The amp worked but with an intermittent left channel (the 1075/2525 pair) plus that channel was really heating up the amp.

I had a new pair of the Sanken so I replaced that channel to match the two pairs.

Prior to power up I insured that both VR5 and VR8 were turned fully counter clockwise.

After doing that, using a DBT, the light dims and the relay clicks in but both meters pin and then subside to like 25% and 10%.

Off the DBT, unit powers on, lamps are fine, no relay click.

Thoughts?

Thanks
 
Sounds like something maybe oscillating, the meters should be at zero with the volume set to zero.
 
You say you have meters showing you 10% and 25% load with no load? I would suspect that as well.
 
Keep in mind that the meters are skewed only when plugged in to the DBT.

If plugged in normally, meters are at zero as would normally be expected.

My concern is, when then amp is not on DBT, the relay does not engage, could that simply be that DC offset or BIAS are not correct.

The reason I ask is that most amps I have worked on, when those two settings are skewed, the protection circuit usually still clicks in.

I dont want to leave the amp on to set those settings, if it will cause damage?

Thoughts?
 
The DBT is to arrest catastrophic damage, and once the dragon's tail is tickled, is no longer needed. In fact it is deleterious at that point.

tickling the dragon's tail is a trial setting of the idle current while on the DBT. it is returned to minimum after the test as it will be TOO high without the DBT.
also only cone channel is done at a time - to avoid OTHER DBT related scares. and we are talking at least 100 if not 150 watts on the DBT.

one of the things we fear is an OLD idle current pot going open circuit (dirty) which turns on HARD both sides of a push pull amp at the same time.
blooie best describes the output transistors and emitter resistors in that event.
 
Does anybody have a transistor substitute for this amp, im having a very hard time finding REAL information.

The subs I have seen are all EOL on Mouser.

Example: q23 q24 2sc1913 to-220 150v 1.5a 15w 65-330hfe 120mhz
757-2SC5171(Q,M) npn to-220 bce 180v 2a 20w 200mhz 50-320hfe 200mhz nsa driver

or

q25 q26 2sa913 to-220 150v 1.5a 15w 65-330hfe 120mhz
757-2SA1930(Q,M) pnp to-220 bce 180v 2a 20w 200mhz 50-320hfe 200mhz nsa driver

Help!?
 
Progress, after replacing the 2 x 2SA913 with 2 x MJE15033 and 2 x 2SC1913 with KSC2073 the relay circuit engages both on and off DBT.

Off DBT, the issue now is that both meters are pinned, and im getting 9V on the left channel idle current measurement. This voltage sustains even with the amp turned off. Right channel is correct at about 60mv

Thoughts?
 
Broken :)
with power off, make sure the big ecaps are discharged ( use a ~>1000 ohm >= 2W R across them to discharge),
using your ohmmeter, (in diode mod to test semi's), compare each node to its counterpart node, on the good working right channel. test until every node measures equal within a ~5 % tolerance. do not power up until you get a 100% comparison.
When you describe what you have done, pls reference the comp #, so we know what you changed?
I am not sure why you are mixing up MJE15033 with ksc2073, these are not complements of each other and generally not a good idea to use together in push-pull class A-B type amps.
MJE15032 and MJE15033 should be used together as complementary drivers
KSC2073 and KSA940 go together, review these component datasheets for more info
 
Last edited:
Hi

So I have swapped the KSC2073 with the MJE15033.

As I was testing all components, I found one of the 2W .22 resistors was completely open on the problematic channel.

I had a 5W .22 in stock and after I replaced it, I now have a direct short again, as indicated by the DBT.

Strangely, If I take the 5W .22 resistor completely out of the circuit, leaving it open (uninstalled), the DBT dims again !

Im hoping this means that in having the dead resistor replaced, the circuit is shorting on another bad component I have not found yet.
 
I have now put the original 2SA1075 and 2SC2525 back in, same thing short on DBT.

If I lift the tabs on left channel output tr's, light dims and relay clicks.

I tested voltage at output tr's and they are consistent on DBT +10V -10V -10V +10V

Final test was that I replaced each tr on the left channel with appropriate replacements save the two 5 pin units.

Same result.

All resistors seem fine. Only thing left are diodes and ceramic caps and the above two 5 pin tr's

Thoughts?
 
you need to spend more time with the ohmmeter and test out everything, compare channels so they match comparing the same nodes, with the ohmmeter first before you apply any power.
70smidfiguy said:
The amp worked but with an intermittent left channel (the 1075/2525 pair) plus that channel was really heating up the amp.
Click to expand...
You are working on a difficult amp, with little or no experience, not a good recipe, above is either a bias issue or an oscillation issue.
 
Hello all and thanks for your help.

After removing the amplifier board (which I should have done in the first place) I was able to replace all of the transistors and test every component. Once that was completed, I also replaced the 4 emitter resistors with .27ohm.

Put the board back in, rewired, setup the DC offset and all was working and it sounds great!

If anyone has any questions about the Tr subs please feel free to reach out to me.

Cheers
 
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