ALTEC LANSING 1715B MIXER AMP

[DynacoGreg]

Hi Im new here. been foolin with electronics since the 60s. I have a soft spot for audio equipt and collect 100s of pieces. ive fixed probably 200 amps in the past both tube and semicon and a retired semiconductor engineer.
recently been fooling with one of a pile of Altec PA units and one of them is giving me some low level noise. its an ALTEC LANSING 1715B MIXER AMP .
I cant find a schematic for the B unit, only rhe C unit and it looks different in circuit layout.
there is a distortion low level. it sounded fine when on voice PA but playing backround its distorted. i bypassed all pre stuff and looked at and set the bias and dc offset.
but theres a neg side distorted wave when puttin in a 1k sinewave... i will attach a scope pic if i can. when i sig trace it seems to disappear. something in the neg cyc is messin up. what you guys think
thanks Greg aka MR Crown, AKA mr Dynaco LOL

 

[Nigel Goodwin]

Well there's no size shown on the scope pictures, no mention of DC levels, and no mention of how much signal so it's difficult to say. However, as you already know, the negative half cycle is seriously missing - could be the output stages, could be anywhere in the DC chain.

For a start, check the waveforms on the bases of the output transistors.

 

[DynacoGreg]

thanks, ill relook and list the levels later today. there is a neg waveform its just slightly squeared off
the pic is center of pic is 0 volt level. I have parkins shake and now have alot of trouble probing and gettin in to what i used to all the time so nowadays more online play. also my typing gets real bad sometimes

 

[KeepItSimpleStupid]

Use th bi-sect technique when signal tracing. Look at possible coupling cap problems. Physically examine the amp. Watch the 1 uf and 10 uF electrolytics in the signal path.

 

[schmitt trigger]

The waveform looks like seriously incorrect bias.
To what KISS mentioned, if this is a stereo amp and one channel is functioning correctly, you have a standard to which compare good vs bad.

I would start by measuring DC bias, starting at the input and progressing towards the output. At some point you'll find a discrepancy.

 

[Nigel Goodwin]

The waveform looks like seriously incorrect bias.
To what KISS mentioned, if this is a stereo amp and one channel is functioning correctly, you have a standard to which compare good vs bad.

I would start by measuring DC bias, starting at the input and progressing towards the output. At some point you'll find a discrepancy.

It's a PA amp, so only mono - I would also suggest it looks nothing like bias problems, which would cause either cross-over distortion at low volume, or the amp to overheat and blow the output stage. A common technique when repairing amplifiers is to short the bias out (so as to reduce it to zero), to reduce the chance of thermal runaway.

As I said above, check the signals on the bases of the output transistors - it 'could' be an O/C bottom output transistor, or emitter resistor. As always, be careful, as it's a DC coupled power amp, and easily destroyed by accidentally shorting something out with a probe.

 

[schmitt trigger]

Ok, agree with you