Old Pioneer Amp - Waveform explanation

[Nevillet]

Hi All,

I have been cleaning up a Pioneer A5 amplifier (C.1981) by cleaning connections, pots and switches. ESR on the electrolytics was all fine. I adjusted the bias as per the service manual to 0.31V.

I put a 8 ohm, 100W resistive load and and compared the channels under noticed a kind of distortion on one channel, not clipping as such, but distortion of the sine wave.

Can anyone please explain why one channel would show this kind of distortion and where I should look for a fault.

The screen grab shows L&R channels displaying a 1K sine wave.

Many thanks.

[petehall347]

Re: Old Pioneer Amp - Waveform explanation

hope you didnt really mean 310mv ?

[petehall347]

Re: Old Pioneer Amp - Waveform explanation

check further back in the circuit to see where its coming from .
also try without a speaker load .
and try without an input

[redwire]

Re: Old Pioneer Amp - Waveform explanation

There's three different Pioneer A-5 service manuals:
HE, HB - Europe, UK models ART-618 but I found ART-621.
YP, S, KC - Australia, General Export, Canada models ART-619
KU - USA ART-617 (April 1981), ART-620.
Circuit Description- use A-6 service manual ART-672.

The schematics have big circuit differences. Older models used JFET Cascode with DC-coupling and servo, then changed to BJT and AC-coupling later models to lower cost. Which model do you have?

I would say the amplifier has a problem - not a bias or offset issue. To narrow it down, I would very carefully take DC voltage measurements at the transistors and compare with the service manual's values or the other channel that works.
For signal tracing, best with the scope set to DC coupling, trigger off the signal generator. It's difficult to troubleshoot because these are negative-feedback amplifiers which are always trying to correct for the internal problem and the scope won't easily reveal where the problem is. You could have bad capacitors because ESR is not the whole picture and I see a phase-shift (delay) between the two channels, it could be the tone controls though.

Setting the power amplifier's bias is done with no load, good warmup and after DC offset has been checked. Across the dual emitter resistor 0.22Rx2 should have (between TP1, TP2) 31mV +/-13mV. 31mV is 70.5mA
If you had trouble dialing that in, or it's quite unstable then problems with the bias diode. Japanese "varistor diodes" are notorious for failing in old gear. D409, D410 is STV2H, if it goes high Vf or open, bias current will go off the charts and damage the amp.

[Nevillet]

Re: Old Pioneer Amp - Waveform explanation

Hi,

Sorry I meant 31mA. I'm trying to work out the version of A5 amp it is.

On the solder side of the PCB there is the marking "ANV-200-0" on the component side there are no identifying marks. The serial number is "BI 9502292 S"

The main output transistors per channel are C2579 NPN (Sanken) & A1104 PNP (Sanken)

Also the only adjustment pots on the board are the bias controls

[redwire]

Re: Old Pioneer Amp - Waveform explanation

Maybe post a pic of the pc board, the newer version A-5 seems to have parts numbered transistors Q400-up, caps C400-up and the old JFET version was like C16, C17 with a few extra op-amps for servos.

[petehall347]

Re: Old Pioneer Amp - Waveform explanation

looking like its he hb amp . its NSA . i know this design is very picky about components being changed .

i would try and rule out the pre amp by checking wave-forms at the vol pot .

[Nevillet]

Re: Old Pioneer Amp - Waveform explanation

Oh cr*p, I have a more immediate problem. It's started blowing the 1.25A fuse. When I power it through a variac and a series 120W lamp it lights brightly and draws 1.8A.
The relay does not energise.
I'll get back to it tomorrow.

[petehall347]

Re: Old Pioneer Amp - Waveform explanation

check if any outputs shorted . if so its maybe the bias trimmer went open circuit after moving it .

[Nevillet]

Re: Old Pioneer Amp - Waveform explanation

Hi,
the outputs don't appear to be shorted.
Measuring the current at the internal fuse it draws 480mA at 230V
The relay does not energise

[petehall347]

Re: Old Pioneer Amp - Waveform explanation

check the bias and dc offsets

[Nevillet]

Re: Old Pioneer Amp - Waveform explanation

Hi Pete,

I've reset the bias to 30 & 30 mV and the current has dropped to 100mA.

The only two adjustment pots on the PCB are the bias ones.

The DC offset on each channel is 1mV or less.

I am noticing that the bias voltage is wandering all over the place. If I return to it after a while it's increased significantly.

The relay remains un-energised.

[Nevillet]

Re: Old Pioneer Amp - Waveform explanation

When I momentarily turn the amp off>on the bias jumps up to around 500mV and the quiescent current to 500mA

[petehall347]

Re: Old Pioneer Amp - Waveform explanation

where are you measuring dc offsets ?
set bias after warm up .

[Nevillet]

Re: Old Pioneer Amp - Waveform explanation

DC offset measured at speaker outputs.

When the amp is warmed up and I set the bias, why would it them be so radially different following a quick on-off-on?

[petehall347]

Re: Old Pioneer Amp - Waveform explanation

you wont see any offsets with the relay being off ..you need to measure before the relay .i mostly look for the coils near the output transistors .

[Nevillet]

Re: Old Pioneer Amp - Waveform explanation

We seem to back in business.
The amp is working as normal.
The relay engages.

With the speakers disconnected and measured at the outputs, the DC offset measure 146 & 150mV.

Quiescent current 150mA 230V

[petehall347]

Re: Old Pioneer Amp - Waveform explanation

offsets are a bit high . you could sub in a pair of transistors for each diff pair .

[Nevillet]

Re: Old Pioneer Amp - Waveform explanation

Are you referring to the large power transistors (2SA1104 x2 & 2SC2579 x2)

[petehall347]

Re: Old Pioneer Amp - Waveform explanation

no the differential pairs .. i think these are 5 legged ones .
you can use closely matched pairs that are correct spec to do the job .