Where to start fixing massive DC fluctuations?

J

jbpollock

Well-Known Member
The is a follow up to my thread about the Technics SU-V8 that I purchased.

Here are the symptoms and what I have done so far:

Day 1: Clean all of the switches and pots, works fine for several hours after this. Checked DC offset and all was fine.

Day 2: Works for 30 minutes or so, sends a pop to the speakers and goes into protect mode. Check DC offset and it is wandering...and spikes up to 60V (not mV) Listen through headphones and it is obviously very noisy before protect circuit kicks in.

Yesterday: Downloaded service manual and set out to make adjustments. DC offset adjustment gets the DC offset reading to about 2-10 mV with careful adjustment. It will stay this way, then start wandering up to 60 and then up to 120 mV or so. Then ultimately it will jump all the way up to 60V and eventually go back down.

Reading through the DC offset sticky thread, I have read that some wander could be due to faulty transistors. Is this the place to start on this amp? Something else?

This is a "New Class A" amp by the way. Don't know if whatever kind of circuitry controls this could the the problem as well.

Thanks for any advice.
 
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I believe the "New Class A" amps all use output ICs. The entire output stage is on the IC. So if there is a problem with the circuit itself, and it's only on one channel, you will probably end up replacing the output IC.
If it's only one channel that has this problem, check the solder joints on the output IC and resolder any that are grainy or have a ring around the component lead. Check the solder joints in the area of the IC. If there is a coupling cap on the input of the IC, replace it. Likewise if there is an electrolytic cap to ground on any of the IC pins, replace that as well. If that doesn't fix it, the IC is probably bad.
If the 60V offset is on both channels, check the negative power supplies to the ICs, and tighten all the screws that provide ground to the main PC board, and the rear panel jacks.
 
I can add, I had similar problems on the Marantz PM-630. It turned out to be bad solder joints on the driver hybrid IC. This has ugly pins which solder not very good.
But another possibility could also be a small driver transistor in the amp section, had that 2 times on a old SX-737.
 
My thought's excactly as written above!!!

Not to difficult to trace.

Let us know how it went :)
 
Last edited by a moderator:
I will check all of the solder joints and ground connections tonight, provided I have time. Is there a way to test the transistors using a volt meter or some other meter?

Thanks for all of the advice so far. Keep it coming!
 
You can test transistors with the diode test funcion of your digial volt meter, but you cant find these break-trough effects that way. This needs higher voltage than the multimeter will supply. You need to trace the fault with a scope on the running amp and exchange the bad transistor.
 
dr*audio said:
I believe the "New Class A" amps all use output ICs.
Click to expand...

I am not a tech, nor have I ever attempted to repair anything beyond a few solders, replacement belts, and contact cleaner. In trying to make sure I understand the advice I am getting and don't mess everyting up, I have been researching everything on the web. In regards to output IC's...these would take the place of transistors? If so, I believe this uses transistors. There are a total of 8 transistors in the main amplifier section(4 on each side) that are labeled as "Transistor, Power Amplifier" on the schematic.

The IC's listed are Equalizer and tone amplifier, super bass buffer and filter, and Protection.

There is some kind of special "bias equalizer" or something ont he main amplifier board. Can't remember exactly how it was labeled.

Thanks again for the advice and help!
 
Output IC`s are the big Hybrid Sanyo STK-xxxx type things, the early type of an IC, incorporating several transisors on one substrate. In general ICs in audio gear are 16 to 28 pin devices used in the amp or tuner section and nearly everything else in modern amps.
 
In the early class A - I believe the SVI STK were only used upto 50 Watts (V4 or V5) and higher output is discrete amplifier.
 
Sounds to me like you have discrete transistors, then, rather than ICs. This means 3 things:
1. Better sound.
2. Parts will be pretty much always available for it.
3. It might be trickier to find the problem.
First thing I'd like you to do is to check if the 60V offset is on both channels or just one and report back here.
 
Well, I looked at it tonight and was not able to find any obviously bad solder joints. The power transistor solders do have some dried, rust colored material around their bases.

I recalibrated the DC offset and got both channels to below 10mV. Let it warm up a little more and they started wandering. Both channels seem to get erratic together (jumping around to random values mostly between 20 and 180 mv). Once the left channel did get up to 60 V before going back down. The right channel measured 4 V at this point, but it jumps around so fast that it may have gone back down in the left channel before I measured the right. But overall they seem to be steady, or erratic at the same time.

One additional observation. It seems that when the DC values start to jump around and get high that if I touch the board that the DC adjustment pots are on with my finger or with the screwdriver with my arm resting on the face of the receiver, that the voltage goes back down. The DC adjustment pots are not located on the power amplifier board, but on the board immediately proceeding that the schematic calls the pre-drive circuit. Perhaps this indicates a grounding issue that I can't find, or perhaps I am crazy and imagining this phenomenon :)

The only other thing I have noticed is that in the calibration instructions for the DC offset it says "cut the jumper wire if adjustment is not possible" Well the jumper wire on the left hand side has been cut by an unknown previous person and the right hand one is still intact.

I think thats all I know. Thanks again.
 
Another update: Went back in looking for loose ground connections and discovered after removing the aforementioned pre-drive circuit board that the connection clip to the little circuit board that holds the input selector knob was loose. It was attached but only about 1/4 of the way. Attached it securely and so far the amp is working. Will let you guys know if this solves the problem. I sure hope so, but I feel like this was way too easy.
 
jbpollock said:
Another update: Went back in looking for loose ground connections and discovered after removing the aforementioned pre-drive circuit board that the connection clip to the little circuit board that holds the input selector knob was loose. It was attached but only about 1/4 of the way. Attached it securely and so far the amp is working. Will let you guys know if this solves the problem. I sure hope so, but I feel like this was way too easy.
Click to expand...

Sometimes it is easy! If it doesn't solve the problem, or even if it does, check the tightness of the screws that hold the pc board to the chassis and tighten any that are obviously loose.
 
Well, it seems like it was that easy. I left it on for about 2 hours last night and had no major problems. Set the DC offset and got about 5 +/- 2 mV of drift in one channel and 3 +/- 2 in the other. Got up this morning and after being on for about 25 minutes it was reading 12 and 10 in each channel. I readjusted it back down and each channel is now at about 2 mV +/- 1.5.I probably need to stop obsessing about little fluctuation, put my meter away, and enjoy the music.

Thanks again for the help
 
One more question if anyone is still listening:

Well, all was good with the amp until I made the mistake of changing the "super bass" turnover from 150 Hz to 75 Hz and back. Then, you guessed it, 60V DC to the speakers (again). I determined this by toggleing between the 2 switches while measuring the output DC. These were the only 2 switches that I think I neglected to clean with Deoxit. Well, I cleaned them and tested again, and still got the 60V, but not as frequently.

Can these symptoms be caused just by dirty switches? i.e. do I need to clean them again(or let the deoxit dry longer)?

The circuit board with these 2 switches (it uses 2 button switches rather than one toggle switch) also has 5 capacitors and 4 resistors. This circuit board then feeds into the circuit board that contains all of the tone controls, including the super bass tone control and the 2 IC's that control the super bass amplification.

My intuition (although it's not so good) tells me that the problem is not likely to be in the IC's, but located on the turnover switch circuit board since all was fine before I switched the turnover. This includes turning the super bass up and down without switching the turnover.
 
One more question if anyone is still listening:
Click to expand...
I am listening as I have an Onkyo receiver with a very similar problem on one channel... I am hoping you have a breakthrough, as it'll stimulate me to re-investigate the Onkyo!

I can't help but suspect it's an active component (transistor or IC) shorting and letting the DC rail voltage get to the output. This IS the conceptual problem with direct-coupled amps, if you ask me. So... did your speakers survive the 60 VDC jolt?
 
The speakers (Advent/1) seem to have survived. Luckily the relay kiked in and it was only a short burst. Next time I think I have this amp all fixed up, it will have to play through junk speakers for a good long time before I trust it on good speakers again. The OEM IC's for the super bass control are still available, so I might just relpace them and see what happens. BTW: It does seem like this is primarily affecting the left channel. But further testing will be required to be sure of this.
 
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