DBT Troubleshoting Results SX-6

pcxmbfj

Active Member
I continue to try to grasp the concepts of repairing audio equipment beyond the mechanical on some throwaway pieces like this Pioneer SX-6 receiver.

Had the unit playing with air and CRC cleaning but the desire to do more resulted in a dead unit.

So I took it off the shelf and put it on the DBT to see what I could learn.

I learned that I don't know the best method for the DBT or what it means.

Started with a 150w bulb because I thought that was the most protection, bulb doesn't seem to light.

Put in a 40w bulb and bulb lights, doesn't dim.

What is this showing?
 
The 150 should have a very, very dim glow to it. Might be hard to see.
 
Thanks, I believe that may be the case.
Do I now use a 100w bulb or should I know enough from the condition of the two bulbs to get out the DMM?
 
I don't mind messing with the broken units that have no real value. I have learned the skills needed to tackle the projects more worthy of the time.
 
When I got the SX6 it was dirty and only had tuner on the right channel. I had the unit playing on both channels after a couple of CRC baths and working the balance and volume POTS so I put the case back on and used it for a few days.
Open it a couple of weeks later and did some more CRC bath and discovered the two AA batteries. Replaced them and left overnight.
When I powered the next morning no sound. Figured the batteries shorted something because the unit wasn't completely dry. The batteries I replace had been so old they were dead and didn't do damage the first cleans.

So now just trying to learn what happened and maybe recover sound.
 
normally dbt glows brightly briefly than dims ti very low glow when sized correctly.

bright glow is big caps charging. once they are charged, we NEED the glow to be very dim.

The ONLY thing a DBT does is save your output transistors if the amplifier tries to make them do something very damaging. Other fuses will still blow.
 
answer: you have to specify a model of amplifier or receiver.

a dim glow is FINE and EXPECTED, tickle the dragon's tail successfully and the DBT can be dispensed with until the innards are messed with/worked on again.

The BAD thing is if it starts to glow brightly AGAIN or NEVER dims. THAT's when there's problems.
 
No lights on SX-6 with either bulb.
Doesn't dim on 40w so off power now.
Looking at the two fuses not obviously burnt.
Will put meter on them to be sure.

Gonna go with a) as you say the 150w is more conductive.
 
please play this game in your own thread and not confuse this thread - it's hard enough with everybody piling on, without two separate conversations in the same thread.

It's been too damn long since I got any bench time in.
 
Pcxmbfj, pay close attention to the advice given by Markthefixer. He knows what he is talking about and will supply you with good information.

"Started with a 150w bulb because I thought that was the most protection."

Not exactly.

Pcxmbfj, in simple terms, the wattage rating of the bulb used in the DBT is the maximum power that can be supplied to the unit being tested.

A 40 watt bulb will only allow 40 watts of power and a 100 watt bulb will allow 100 watts to be supplied to the piece of equipment being tested.

In general, starting will the smaller wattage rated lamp provides the most protection as it limits the maximum power to the unit being tested to 40 watts. But, depending on the piece of equipment being tested, 40 watts may not be enough to enable correct operation of the device and the lamp may stay lit more than just dimly and or the piece of equipment may not operate correctly. This is why a range of sizes of bulbs may be needed.

I left out the discussion of current, voltage and changes in the resistance of the bulb with the brightness of the bulb to keep this simple. The purpose of of the DBT is to limit the maximum power supplied to the unit being tested to help prevent additional damage related to defective internal parts.

As markthefixer correctly pointed out, the actual wattage rating for the lamp chosen is dependent on the equipment being tested. And he correctly described the operation of the DBT.

For those interested there are many threads on AK and the internet on the technical ins and outs of the operation of a DBT.
 
Tested two glass fuses on DMM, seem good.

Power on without DBT, hear delayed click (protection circuit?), no smoke, no lights.
 
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IIRC, and it has been awhile since I have been inside one of these, there is a 3 terminal 5 volt regulator that supplies power to most is not all of the LEDs and the BCD decoder integrated circuit for the station indicator LED'S and possibly other things.

This same 5 volt regulator also supplies, via a separate diode isolated line, voltage to the same place as the backup batteries.

There is another three terminal voltage regulator, 12 volts.

Both of these regulators are supplied by their own fuse.

Simple question, but I have to ask, because not everyone does this, but did you remove the fuses when you checked them with your DMM?

If the fuse for the three terminal voltage regulators is bad, no LEDs. If it is good then on to other things.



Again, IIRC this unit has a number of power supply voltages.

The protection relay, if that is what you are hearing click, is driven from the positive 35 volt power supply rail and will turn on even if other voltages are missing, because it is biased on from the same 35 volt line that drives the relay. It is turned off if there is DC voltage on the speaker output or if there is excessive voltage drop on an output transistor emitter resistor (excessive power output).

It is not a bad thing to start with measuring all of the power supply voltages.



BTW, the backup battery supply is diode isolated from the 5 volt regulator and diode polarity protected for the battery back up voltage. Unless the protection diodes are bad or leaky, batteries, no batteries, weak batteries or batteries installed incorrectly should not cause a problem.

If all power supply voltages are present and correct, then it is on to other things.

An addition, again from memory, but if you select tape one and the 5 volt regulator is doing is job, the tape LED should light. IIRC this is just a connection to ground on one of the terminals on the tape selection switch.

Power up the unit and push the tape 1 button and see if the tape monitor LED comes on. If it does, then at least one of the 5 volt lines from this regulator is working.

If loopstick can post the complete service manual with voltages and parts values that will help.
 
@JBL
Much appreciate your input.
Yes, I took the glass fuses out to check continuity.
My DMM has Ohms but no speaker so I'm planning to get one with continuity testing.
My meter skills are minimal but will try to do the measurements you so graciously suggested.
 
Give the tape button test a try.

If you are using the basic test probes that come with a DMM, tape all of the conductive end of the probe but the very end to help prevent accidental shorts in case a probe slips.

If you have the small insulated gripper type test leads, they are safer to use.

If you have to find a problem, hopefully it will be a power supply problem or a bad connection.

The control integrated circuits may be difficult to find as replacements. although others here may have more information about the availability of replacement parts.

Good luck with your testing and let us know what you find out.
 
Did the tape button test, no lights.
Took the front panel off and redid 1. thought I might see the led better and 2. because I've never had it off.
No light but many insects.
I'm using a SX-5 manual because can't find an SX-6.
Have several schematics but no SX-6.
Have clips and hook probes so will use them cautiously when I get oriented to the test points you address.
Thanks again!
 
Stereomanuals.com has the service manual, as a reproduction. Rick's products are excellent, and if you are serious about getting this working, it is essential.
looks like $18 + 3
 
"Did the tape button test, no lights."

That indicates at least one missing 5 volt power supply line.

If that is all that it is that would be good news.

This voltage regulator and the 12 volt voltage regulator have their own separate power transformer secondary winding and fuse.

If you can find that section of the power supply, check for AC voltage on this secondary winding.


You can follow the circuit from one end of the fuse that is not connected to the main power switch directly to the input of the three terminal +5 volt DC regulator. There should be about + 5 volts DC on one of the other pins of this regulator. It is either not there or not making it to the rest of the circuit.

These voltage regulators have short circuit shutdown, and depending on the value of the fuse, could have a short to ground and not blow the fuse.

Again most of this is from memory.

Bottom line, see if you can find the +5 DC volt three terminal regulator by following the line from the fuse that is not connected to the main power switch and find a voltage greater than 5 volts DC by a few volts and a voltage close to +5 volts DC.
 
I would say yes, but this is from memory. Neither end of this fuse will be connected to the main AC power switch.

That fact that the one that your are referring to is a smaller fuse makes sense.

Follow the leads from the other fuse and they probably go to the main power switch and it will be a larger fuse.
 
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