DC OFFSET question

[calman46]

The actual body of the TIP 36c is melted from an iron. I had the pinned VU meter problem before I changed the 2N4410's but it went away so I thought it was something just might be shorting with me moving the boards around. Board 2 is a small board that is wired to the output board but not the rectifier board.The +63vdc/ -63vdc are here . I'll post some pictues .

 

[K7sparky]

The VU intermittent is the type of problem caused by the rivets loosing connection to the PCB. Might not be that at all, just something that has bitten me repeatedly over the years.

Look closely for the solder being a dull gray as that is one of the indications. If you do radios, it happens to PIN diodes also if bias is high and leads too short.

 

[calman46]

Here is shot of board 2 from the unit I'm not working on .The red wire is +63vdc and the green is -63vdc. The orange wires are from the TIP36C emitters. I put Ac meter on the speakers terminals and is high AC on the channel in question .

 

[K7sparky]

With no input and AC on the output over a few MV AC that your meter is picking up from the AC lines running around your house that channel is oscillating.

I would expect no change, but short the input and see if the AC on the output changes or goes away. I doubt it will make any difference.

Curiosity only, do you have any to measure the frequency of the AC on the output?

Where in the circuit is the resistor that appears to be broken off the upper left of the board appeared to go to the 10 ohm and is above the cap above the 22K?

 

[calman46]

All the resistor's are in place not sure what picture you're looking at. I have a frequency counter. I'm concerned about leaving the meter pinned for to long ,should I disconnect it ? I shorted the input and no change. What do think is causing the oscillating ?

 

[K7sparky]

I need a picture of the 40494 to see if it is the case The only spec sheet I have found calls out.

I am guessing that the 40494 and 595 collectors are tied and are the drivers of the TIPs. Tie the input of the TIP to ground with a capacitor. .01 to .1 or anything around that you. That should kill the OSC. Try you counter and find the frequency. not sure that will help or not.

I thought the pinned VU was an intermittent. Not so?

Your post #6 (11-17) has a picture and that is the one I am referring to. The resistor is tipped so the color bands can't be read and looks broken off the board. The board has a copper colored dot that looks like the resistor broke off there.

I would replace the TIP with the case burn same as you plan just to make me feel better. It the one there works, then it is probably an exercise in futility. But that never stopped me from doing something that makes me feel better.

Trouble shooting remotely is a challenge. Pop the meter or add a resistor in series with it if you have a bunch of parts. You can read AC on the output to see when you have an an oscillation. No idea of how big a junk box you have to grab parts from.

 

[calman46]

The 40594/40595 are TO5 case . The C of 40494 is connected to B of 40595 .The C of that 40594 goes to the B of the TIP36C. I don't know if that any help I realize its tough working this way. Those resistors in post #6 I lifted to check I have either replaced or re-soldered them. I have parts kind of a lot ,flea-market freak you know .
Yes the meter stays pinned .It was doing it before but it had stopped so I thought it something was shorting it by me moving the board around . I think I need to remove the boards to get a good schematic drawn.

 

[rockur]

The HfE on 2n4410 are 73, 75 , 290. What about the wattage on the resistors in the picture ? There are 3 sizes I'm guessing 2w , 1w, and 0.5w ? I had 48Vdc on the new filter caps.

Are all your parts on the board? on left middle area, there seems to be space for a missing possible 47ohms 2W like the other side.

 

[Jon_Logan]

Is the hum a new problem? Sorry, I haven't looked carefully at the whole thread. Is anything getting warm? That would indicate that the transformers are clacking due to high currents coursing through them. If no extra heat and no other signs of high current consumption, then the issue could be that the transformer's laminations are just 'clacking' because they are loose. ....sometimes exacerbated by 'dirty' line voltage. Harmless, but annoying.

It "sounds" like the amplifier's topology did use a differential stage, judging from the before and after results with transistor replacements and because there was no offset pot in the circuit.

Since you've been able to solder in the replacements, look for the two transistors (that you've replaced) having the Emitters tied together. That will be your diff pair.

By you saying that the polarity of the offset flipped, tells me that it is (still) a diff pair issue and that you need better matching and/or more gain from the transistors. Look for a ZETEX sub for the Motorola (On-Semi) MPSL01. By 20, or so and select a pair that has the largest and closest matching hFe.

If the DC offset had stayed the same polarity, no matter which pair of differential transistors you replaced, then you might take a look at the high value shunt capacitor at the inverting input. This amp does incorporate negative feedback, yes? If the feedback shunt cap gets leaky, it will rob some of the DC correction signal, giving the differential pair incorrect information about the DC conditions. A properly working differential amp will try to "correct" for this false imbalance, creating a DC offset.

 

[calman46]

This side is now oscillating ,I'm getting high AC on the output terminals and the VU meter pins.

 

[calman46]

I think the TIC126 rectifier maybe bad . Its 220 case K A G leads

 

[Jon_Logan]

That sounds like an SCR (Thyristor) that might be used as a crowbar protection device.

IMO, that's probably not the cause of an oscillation.

I'd go back and re-install your original transistors and see if the oscillation stops.. If not, we can start looking for an unintended short or open, possibly as a consequence of working on the set.

 

[calman46]

I wasn't getting the same voltage readings as the other side's TIP126c This is dual mono design. I did notice if I pull the +63vdc(right off the+ terminal of the filter cap) fuse the VU pins

 

[Jon_Logan]

They are two different animals. A transistor is a more linear device that amplifies base-emitter current into a potentially larger collector-emitter current.
An SCR behaves more like a latching switch. When the Gate-k(Cathode) *I think* is biased on, the Silicon Controlled Rectifier conducts A(Anode) to K(Cathode).

Anyhow, these SCR's are used as "Crowbars" that, when biased on, will provide a short across the power supply.......... .....causing the fuse to blow, but protecting the circuitry.

So, no, they won't measure the same, or be in the same locations in the circuit.

 

[calman46]

Its two separate circuits. Left channel, right channel

 

[K7sparky]

I'm interested to know the approximate frequency. Careful of the input of your counter. I expect the oscillation is rail to rail so about 126V P-P. If you are not sure of the counter max input and feel OK building a voltage divider give it a try. My curiosity at this point is not worth taking a chance on frying your counter input.

On your drawing base of Q2 one resistor is 2.2K what are the other 2 and the 2 caps. At this point I am looking at that area the cause of oscillation. There is a + end of the cap to ground so I am guessing it is the little electrolytic and maybe leaky / open / failing to low MF. I also guess the cap between the 2 resistors is a disc and they tend to not change or degrade.

I need to try to redraw and add the finals when I get a bit more time.

 

[K7sparky]

I agree with Jon. The SCR is a crowbar. Either OVP (over Voltage protection) or to blow the opposite supply fuse if one side fails. Either way protecting other circuitry. With a DC source, once a SCR triggers it will stay on until it blows or the current through it goes to 0 (OK under latching current).
That is my only guess lacking a print of your circuit for it's function in a stereo. I would doubt it is used on the input of the power transformer as a voltage regulator. Only one would only cut one side of the sin wave - bad news for the transformer.

Probably off topic a bit, but was what happened in an unbalanced SCR application.
In the way back machine they used 1/2 diodes and 1/2 SCR (called a modified bridge) in DC drives if my memory serves me up to 50 or 75HP. SCR were MUCH more expensive than diodes back then. It caused a LOT of input transformer heating. I think they had to double the KVA rating to keep them from frying. The DC was also ragged and the motors also had to be at least 1 frame size larger to not smoke. That was a short lived design concept.

 

[calman46]

I have this rough drawing of the 2 positive outputs

 

[K7sparky]

I couldn't guess enough to get a workable print. Power amp sections are mostly the same with different trimmings and protection schemes. Making a print from the PCBs is a trial for your patience and a great mental exercise.

Since there are 4 emitter resistors on your board picture I suspect there are 2 TIP35C with collectors tied to the -67 rail and 2 TIP36C with collectors tied to the +67 rail. The TIP come with and without isolated tabs. If your originals have isolated tabs then you need the same or add isolation to your replacements. Remember the mounting screw torque is probably around 6"# (snug not tight) and thermal grease needs to be vary lightly applied.

The .1MF from the output through a 10 ohm to ground seems a bit high but not out of reason. It is a high pass to bleed high frequency from the output. If the frequency of oscillation is as I suspect in the 100K up range then either (or both) the .1mf or 10 ohm is blown. If the oscillation is caused by something other than this filter being blown, it will smoke again.

I suspect the inductor in series with the output is shunted by something in the order of 10 Ohms. The inductor is a choke to prevent frequencies over audio range from going out to the speakers.

Here is a rather plain vanilla amp module.

Your amp should be a variation on this form. The print is a jumping off point and maybe will help with the flow. You can also grab a few manuals off the AK Data Base to get more variations on the PA theme. Don't loose track of the forest looking at the trees. Point 6 is +18V. Point 10 gets tied to point 9 when the protection relay picks up and is the feedback for DC offset balance. Things on example to ignore: R25 to R28 are part of the protection circuit and the wires from them go to offset detection. D2 and RV1 are the bias adjust, but you will have a variation on this maybe 2 ea 1N3754 diodes you mention on the heat sinks and a resistor. Points 13 & 14 are headphone outs

I agree with you that board 2 is most likely your protection circuit. Is that where the SCR is located? Any chance it also has the VU meter driver of voltage divider?

When I am trying to solve oscillations I often use lab supplies for the rails with current limit at 100 mils or so. Since you have a complete amp for the other channel you could pop the +/- 67 from the oscillating channel and use a variac on the opposite channel for variable voltage and fuse the leads around 1A. I usually add temp fuses to the rails on the amp under test for a period until I am satisfied I have cured the oscillations. At the minimum use a dim bulb as a CYA.

 

[calman46]

I found two shorted outputs transistors 1 TIP36C and 1 TIP35C ,I think that might have been also causing the excessive hum.I have new ones to put in . Is there a socket for the TO-218 case? The original TIP's were soldered in the rivets .Do I have any other options for mounting the outputs ? picture1
Thanks for schematic this will help a lot. There are no relays in the amp just fuses and some of them are original MTH-4 .The SCR is mounted on the rectifier board which sits on top of the filter caps picture2