Ibanez TSA5-S heater supply issues.

[walkman]

Hi all,

These amplifiers, sometimes exhibit a problem, where after playing for sometime the volume fades. It can also be that there is no sound at all, then after some minutes of turning on and off it may jump to life.

The fault appears to be the heater supply.

With no tubes, measure the voltage to be 10.1v to the 6v6 heater. With a 6v6 in place the voltage drops to .5v

With only a 12ax7 present the voltage to pin 9 of the heaters is 1.4v

The high voltage supply is ok and with no tubes is 360+ volts.


I have found the schematic, and where i was expecting a voltage regulator correct.

TSA5_Schematic.pdf

There is a link to a document about TIP31a here https://www.egr.msu.edu/classes/ece4...edirects=0&d=1

How is this working to regulate the voltage?

The schematic shows 5v to the tubes heaters.

How low can you run the heater voltage?

I expect that this reduces the output from the tubes and had previously wondered why the amp was very clean on the tube section.

This is the original version , not the later one with reverb and more of a TV front styling.

I should be able to find a replacement TIP31a at an electronics store, and hope to have a look at it next weekend.

Any advice or discussion is welcome.

[Enzo]

It is just a pass transistor in a common circuit. Does the part test bad?

[walkman]

It is just a pass transistor in a common circuit. Does the part test bad?

I don’t know its in the circuit .. I expect it is the bad part. I found cracks in the traces on a couple of the legs and repaired that and tested.

[Enzo]

You can still test parts in circuit. Yes, sometimes the circuit clouds readings, but if it checks shorted, that is enough reason to pull it, and if it tsts open, same deal. Just basic test using the diode function of your meter.

[walkman]

Ok, will pull the board out it and do the tests.

[John G]

Hi, I have had several of these amps for repair over the years.
I usually find low heater voltages, so check that D15-D18 are RL207 diodes (early models were fitted with 1N4007 diodes). They must be of a two amp rating raised up off the board.
Also, check the series pass device along with the divider string R39 and R47.
John

[walkman]

Hi, I have had several of these amps for repair over the years.
I usually find low heater voltages, so check that D15-D18 are RL207 diodes (early models were fitted with 1N4007 diodes). They must be of a two amp rating raised up off the board.
Also, check the series pass device along with the divider string R39 and R47.
John

Thanks for that extra information.

Is pretty tight in the chassis and i want to pull it out as least times as possible!

[Greg Robinson]

It's not a regulator, it's a capacitance multiplier. The ratio of R39 and R47 - the Vbe of the TIP31 set the output voltage relative to the supply, and C34 provides additional smoothing. Googling capacitance multiplier should bring you explanations of the circuit.

[walkman]

Eventually got back to this amplifier and have now replaced the heater rectifier diodes D15 - D18 with RL207, replaced the R39 with a 1 watt 56r and C33 with 2200uf 16v.
Heater voltages measured with both tubes in are 5.59 volts across pin 4 and pin 9 of the V1 12ax7. The amplifier now works and has been tested for about 30 minutes at reasonable volume with out issues.

Will the lower 5.59 volts reduce the overall output volume by much? Is there much point in adjusting the R39 R47 ratio to get 6.3v?

[g1]

No point in raising it, 5.6V still leaves some room before sound will drop out. Lower heater voltage may actually be better for the tube life and for noise.

[walkman]

Thanks G1, will leave the heater voltage as it is now


BTW This was the same fault as in the link below
(schematic on page 2 )