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Spec 4 with Problems

miklemke said:
Thanks for the substitute list.

The drift has gotten worse, it will go up to 90mv. I'm suprised protection hasn't kicked in.

I'm going to replace Q2 thru Q5 as well as Q10 & Q11. I'll see if that helps to settle things down.
Click to expand...

Check your meter and battery - see how stable the meter is while reading a battery. Get a new battery for the meter.]

WHAT is the make and model of the meter?
 
My meter is a Fluke 177. That's the one I primarily use.

I also have a GB Instruments GDT 294A that has a transistor checker that I use for checking transistors. It reads the HFE. Also used this meter to double check the Fluke readings and got similar results.

I haven't changed the batteries in either of the meters for a few years. I'll put in fresh batteries and see if the readings change.

Thanks for pointing that out.

As mentioned earlier this unit was worked on in the past. There are two added diodes on the back side of the PCB. One connects the two base pins of Q1 and I dont recall the other, but I intend to remove them to see if there is any improvement. From the way they were done, I doubt this was a factory modification.

When I tear it apart again, I'll note where the second one is connected. I did see another SPEC 4 that did not have these diodes.
 
The extra diodes on the back of the PCB are connected as follows on Q1 the 2SA798:

Diode 1 ==> Base 1 of Q1 to common emitter of Q1
Diode 2 ==> Base 2 of Q1 to common emitter of Q1

Can these be removed?

What purpose would they serve?

Could this be what's causing the drift in the DC balance setting?

Thanks
 
miklemke said:
The extra diodes on the back of the PCB are connected as follows on Q1 the 2SA798:

Diode 1 ==> Base 1 of Q1 to common emitter of Q1
Diode 2 ==> Base 2 of Q1 to common emitter of Q1

Can these be removed?

What purpose would they serve?

Could this be what's causing the drift in the DC balance setting?

Thanks
Click to expand...

Cathodes to the common emitter? They did something like that on the sx-3900, but they used 6.1 volt zeners. The easiest way to answer the question is since they were ADDED is to pull one or both channels worth, and see if that helps correct the condition.

It looks like a way to limit the excursions of the inputs to the bases of the differential, but with the impedance of the inputs versus the leakage of the diodes, who knows for sure - and diodes are not perfect.
 
The diodes they used were 1S2471's.

I will give it a try with one channel and see what impact it has.
 
Removed the extra diodes on Q1 on one channel. Had no effect. Still have the drifting of the DC balance. Do have new batteries in both DVM's

I'll replace Q2 Q3 Q4 & Q5 on one channel to see if that solves the drifting. I finally received the replacements. Must have gotten hung up in the holiday shipping.

Seems odd that both channels would have the same problem. I am taking my readings off of the speaker output terminals with nothing connected to the amplifier inputs and the attenuators set at infinity. Is that the correct procedure for a SPEC 4?

The issue with the meters was solved by recalibrating the right channel meter.

Getting there one problem at a time.
 
I did replace Q2, Q3, Q4, and Q5 on one channel. This had no effect on the drifting of the DC balance. Could the diodes be causing the problem?
 
As a reality check, I used the same DMM I've been using on the SPEC 4 on my SX1250. The readings for the SX1250 on both channels were between 0 and 1mv. On the SPEC 4 they run between +66 and -29mv, roughly the same on both channels.

So far on one channel, I've replaced Q1, Q2, Q3, Q4, Q5, Q10, Q11. I've removed the post factory diodes, replaced the trim pots, checked all the resistors, replaced all the electrolytic caps, and cleaned all the connectors. All with no effect on the drifting issue.

It seems all that's left are the diodes. What are the recommended replacements for:

D2 & D3 Varistor SV-04
D4 & D5 Varistor STV3H-G?

Thanks,
 
Spray those diodes with a very small amount of freeze spray, or just touch them with a finger, to create a slight temperature change. If the offset changes a lot when you do that, replace the diodes. You can buy the originals at partstore.com. I'll check the location in the circuit and see if you can use 1N4148 diodes to replace them.
 
I don't think that D3 and D4 are the likely culprits here. If they were, your bias would be drifting around. D2 sets the regulated voltage that is used to adjust the offset. Because this circuit has gain at DC, the voltage on D2 doesn't have to change very much to cause an offset. Try spraying D2 with a small amount of freeze spray. Also, did you try replacing the offset adjustment pots?
Actually, now that I think about it, a change in voltage at D2 should not cause an offset, because both collectors should change the same amount, and it would get nulled out. See if your bias is drifting around. If so, then try a bit of freeze on D3 and D4. You will have to use original parts to replace them, I think, if they are attached to a heatsink it would be hard to do that with 1N4148s.
 
I did replace the trimpots.

D2 and D3 are very sensitive to temperature, these are the SV-04's. Very quick and noticeable change in the DC balance measurement.

D4 was also sensitive, but not to the same extent. D5 didn't seem to be affected.
 
Well, I realized I made a mistake in my last 2 posts. I grabbed the wrong schematic. I had a Kenwood 600 schematic instead of a Pioneer Spec 4. Duh. So if anyone notices some stuff I said didn't make sense, that's why. Before you order new diodes, check the regulated supplies at pins 2 and 6 of the amp boards. See if they wander around. The DC offset adjustment comes off of these supplies and if they vary at all it will get amplified. This is where I was looking before, but I had the wrong schematic.
The regulated supply for this is located on the meter amp board. If you do see one of the supplies wandering around, check the solder joints on the transistors on the regulated supply, especially Q1 and Q2.
 
>D2 & D3 Varistor SV-04
D4 & D5 Varistor STV3H-G

I have not looked at the schematic. If these are diodes mounted on the heat sink then they will change the volt across the device with temperature change. Spraying them with freeze mist will make a quick change in amplifier bias. You can take them out and put them in an oven to plot their terminal volt vs temperature and compare that to spec. Keep the good side separate from the bad side and compare terminal volt. If they are the same through several temp cycles, problem is somewhere else.
 
He said the offset is fluctuating on both channels, so it is likely the regulated power supply, since it is common to both channels.
 
The voltage to pin 2 on the amp boards is a constant 66.7v and the voltage to pin 8 on the amp boards is a constant -68.1v.

The solder joints look OK, but I will reflow them.

Thanks
 
I resoldered Q1 & Q2 on the meter amp board. Still have the drifting of the DC Balance on both channels.

The power seems stable, but my meter reads in tenths of a volt on the scale I'm using. It does vary by 0.1. Could this be causing the drifting? If so, what can I do to correct it?

Thanks
 
Because the amp has gain at DC, it is possible that .1V difference on the regulated supply could cause a shift in the DC offset. The problem is either going to be caused by the diodes or the power supply. The next step is to see why the power supply is varying. Monitor the voltage on the base of Q1 and Q26, Q2 and Q27, Q3 and Q4, and D14 and D15 in the meter amp. If the voltage on D14 and D15 does not vary, then one of the transistors is probably flakey.
 
I took the voltage readings on the meter amp board as indicated. The readings are in order and were taken at start-up, 10min, 30min, and 60min.

Q1 = 66.9 = 67.7 = 67.8 = 67.8
Q26 = 67.6 = 68.3 = 68.4 = 68.4
Q2 = -69.2 = -69.5 = -69.5 = -69.5
Q27 = -69.9 = -70.1 = -70.1 = -70.1
Q3 = 24.8 = 25.08 = 25.12 = 25.13
Q4 = -24.27 = -24.36 = -24.37 = -24.37
D14 Cathode =24.2 = 24.48 = 24.52 = 24.53
D14 anode = .001 at all times
D15 Cathode = -23.57 = -23.73 = -23.72 = -23.74
D15 Anode = .001 = .000 = .000 = .000
 
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