Welcome to Solid State Guitar Amp Forum | DIY Guitar Amplifiers. Please login or sign up.

March 28, 2024, 06:10:32 AM

Login with username, password and session length

Recent Posts

 

Altec 770

Started by gbono, February 04, 2016, 05:46:00 PM

Previous topic - Next topic

gbono

Question about grounding on this "electronic" crossover amplifier from Altec Lansing. See page 1/8 - where does this amp design return/use chassis ground. I have several of these amps and I notice on 1/3 units that if a measure the +52 supply referencing chassis ground I get get strange reading on a DMM - like I'm reading AC? The transformer, rectifier didoes and C13 test good. Same thing happens at C12. When I check chassis ground continuity (screw on transformer) to a point like negative terminal of C12/13 there isn't any. The other two working amps have continuity between the above points and the supply voltages measure fine when using chassis ground as reference.

Enzo

The first question, though, is does this amp do anything wrong? 

If there is no continuity between circuit common and chassis, then taking voltage readings from chassis to any place in the circuit will be meaningless.

What readings do you get when you measure directly across those two caps?

As to why two amps are different, take any readings with NOTHING connected to the amp other than mains power.  I see terminal strips for input power and output.  On the input side, beyond the two mains screws, there is the center tap for the power supply.  It also is connected to pin 9 of the power control board.  presumably that board, well... controls power.

On the output side are two terminals marked common, which are the circuit common.  I might guess that unless the commons and the center tap are not connected together, the supply doesn't provide any current to the system.  Note that pin 10 of the power control board is connected to common.

I'd see if on the two "good" units, if pins 9 and 10 of the control board are connected together.  Now pull the control board and see if those two pins are still connected together.  In other words is the connection between the two on that board instead of out in the chassis.  And in fact, when power is turned off, on the good units, does that chassis to common continuity still exist?

The control board seems to have an SCR between 9 and 10.


gbono

Sorry for the delay but things pile up around here fast   ;)

I stopped working on the amp with the chassis ground issue since I could measure correct DC voltages across the filter caps but the chassis is isolated from circuit ground on this particular amp.

I have 6 of these amps now. Using a triage strategy to repair the most functioning chassis first.

So the most promising candidate has the following problem: when the amp is run for 15-20 min with program the LF amp output will start to distort. The issue can be traced back to the output of Q3 on the power control board. When the amp starts to distort I change to a sine input and the negative cycle of the sine wave is heavily clipped. I can let the amp "cool" off and use a heat gun to reproduce the issue. The signal at Q1 and Q2 are fine just the output of Q3 is distorted.

I measure 22.5V at the collector of Q3 under all conditions but VB starts at 1.2V and drops to .99V when "hot" and VE goes from .6V to .27V also when hot. I replaced Q3 with no improvement. I check the resistors and capacitors around Q3 but I don't find an issue. If I use another control board from a different amp I get a good stable output even at temperature. Stumped.


Enzo

Use the common terminals at the output as your ground for test equipment.

Is it loaded?  Take the load off, does this till happen?  Is there any difference between loaded and unloaded at the amp output?  Have you subbed the large speaker cap?


Heat guns and freeze spray can be very selective.  Have you tried freeze spray?  If heating or cooling an area changes the symptom, then narrow it down.  If I am spraying chiller, I am chinning an area.  SO I can use a piece of card stock or a 3x5 card to make a shield.  I can curl it around as little as one transistor.  Spray at that isolated part and it cools only that part.  That can help isolate the thermally sensitive part.  I can do the same with heat, but I have to be careful not to light the paper on fire.

gbono

The amp plays fine until it warms up for 15-20 mins then the LF channels distorts. I tracked down the issue to Q3 on the power control board (this is where the input signal comes into the amp and EQ is applied before routing to the LF and HF amps). If I use a heat gun on the amp when it's cold I can get the output of Q3 to distort (negative part of sine wave is clipped) - Q3 drives the signal into the LF driver board - if I use freeze spray on Q3 the problem will go away.

The above is done without a load attached though I've found the problem way up stram of the driver and output stages. Q3 has been replaced and the issue is still there. Still stumped

Enzo

Then look one stage back from Q3.  Also look at every resistor asociated with Q3, and the connections between them.  How about the outpout transistor Q3 is trying to drive?  It might be fighting and Q3 overheats trying to do its job.

gbono

#6
The signal flow to Q3 is rock solid. I scoped the input signal through to the base of Q3 and their is no distortion present when the output of Q3 is clipped. I replaced the capacitor coupling Q3 to the slider with no change. I will swap out the LF driver board and see what happens.

edit - substituted another power control board and there are no issues with heat soaking the board. So what ever is "upstream" doesn't appear to be an issue since the sub board works without issue.

stumped ???

Enzo

I don't recall,, did we check bias?  Did we check mains draw for excess?

And did we check the resistors around that circuit?  If you have good boards, take voltage readings at each end of each resistor, then compare to the bad circuit once it acts up.

gbono

I measure 22.5V at the collector of Q3 under all conditions but VB starts at 1.2V and drops to .99V when "hot" and VE goes from .6V to .27V also when hot. I replaced Q3 with no improvement. I check the resistors and capacitors around Q3 but I don't find an issue. If I use another control board from a different amp I get a good stable output even at temperature.

I originally thought the "B" voltage Vcc for Q3 was drifting down but it remains steady at 22.5V. Oddly enough the "bad" waveform that Q3 shows, after heat soak, looks like the output at Q3 when bringing up the mains voltage with VARIAC and you are still below 60VAC.

gbono

So I looked at the voltages around Q3 on a good board and I get Ve=Vb-.6V as expected but Ve is around 8V. So the bad board has much lower voltages at Q3. All the resistors around Q3 are identical for both boards (within 10%). When the bad board reached 75F the output clips since Vb will go below 1V. I've looked at the signal at the base of Q3 when distortion occurs on the bad board and it is a exact replica of the input signal. Replace Q3 on the bad board with two other transistors with no improvement. Nothing is "loading down" the output since the good board functions in the same chassis as the bad board. It's a puzzle.