Truvox TSA 100 Amp and OC81Z

[Impecunious]

One channel has lower gain than the other (about half peak to peak voltage at output). I have taken out the OC81Z (VT7 on diagram) and its Hfe is 10 ( measured on MK328) which to me seems miserly. In circuit, the voltages on its leads are fine, with base being about 200mV different from emmiter. The voltages around the circuit are pretty much as in the diagram.

The AF118 does get moderately warm, something I would not expect. The AF118 in the stage working properly is cold.

A few questions;
1) would the low Hfe of the OC81Z account for the output stage's overall low gain?
2) I appreciate that the OC81Z will have at least 25V across it before there is any signal, and if I were to replace it, how critical is it that the Vce of the replacement is 40V, like the OC81Z? It has few equivalents, only ASY48.
3) Could I put in an AC128 or an OC81 (Vce 20V) and hope that it doesn't notice that its getting a few more volts than its manufacturer intended.

Thanks David

[G6Tanuki]

From memory the Z-rated transistors were selected to handle a higher emitter-collector voltage than the generic parts.

This would be supported by the use of an AF118 - which I would more associate with first-generation transistorised TV deflection stages.

[Jez1234]

Assuming you've checked out the other stages and the fault is definitely in the power amp then I'd check the 50uF emitter decoupling cap on the AF118 and the 12.5uF bootstrap cap.

[SteveCG]

As G6Tanuki says the Z almost certainly means specially selected for higher Emitter to Collector voltage working. The book on Leak amplifiers comments the same about some of the devices used by Leak in their early transistor amplifiers.

[Impecunious]

Thanks for suggestions, an update.
1) OC81Z, Hfe 10 (VT7) on diagram was replaced with OC84D with Hfe 65. This made no difference, there is still a discrepancy between the gain of the left and right power stages.
I've injected identical signal to base of VT5 VT15 and the difference is still apparent at the output, so it cannot be attributable to the previous stages.

2) Electrolytics C20, C21, C23, C25 all removed, tested and all found to be good, but having removed them, I replaced with new ones anyway. Still no difference to gain of the left and right power stages.

3 Tested VT6, VT7, VT8, VT9 VT10 and all good. Mid rail voltage is 25V as it should be, so confirms that output pair are OK.

4) I have put gain difference to one side and have tackled noise. Both stages extremely noisy and have easily cured this by changing base bias resistors, R31, R32 on VT5 and the collector and emmiter load resistors, and all their equivalents on the other channel. From being dreadfully noisy, the left and right power stages are now virtually noise free. The originals were carbon composition.

5) Tone control stage very noisy and will also have its carbon compositions replaced. As well as bias resistors, I'll also replace collector and emmiter resistors.

This must be a later amp as VT1,2,3,4 and 5 are not OC germanium types, but 2N4058 Si.

I sometimes wonder about the wisdom of tackling amps with Ge and carbon comp resistors, but I will probably pursue. Thanks for suggetstions so far.
David

[DMcMahon]

You are doing well David, good logical fault finding/testing.

I guess this is a standalone amp not part of a tape machine so really should be moved by the moderators to the Vintage Audio section.

David

[Impecunious]

Ah yes, I now notice that I'm in the wrong place, and not for the first time in my life. The TSA is a 10W per channel standalone amp. Mods, please move me.
Ta D

[Station X]

Thread moved.

[dave cox]

Does VR52 adjust the gain ?

dc

[DMcMahon]

Do the transistor voltages on the output stage measure similar between left and right channels and are the voltages similar to the values shown on the schematic ?

David

[Impecunious]

Do the transistor voltages on the output stage measure similar between left and right channels and are the voltages similar to the values shown on the schematic ?

Yes

Voltages in the L and R Output stages are pretty similar and are as the diagram. I'm assuming that mid-rail voltage should be half supply, which it is.
This mid rail is altered by VR42, which also changes the quiescent current. Voltages on the output AD149s are all good, with the bases being about 150mV above the emitter. The same can be said of the OC81 and AC 127 drivers. The AF118 also has its base about 0.2V above that of its emitter. This suggests to me that the semiconductors are all good. The 2N4058, which is VT5 has its base 0.6V above emitter, which would make sense as it’s Si.

One thing I have noticed is that the quiescent current ( calculated by measuring the voltage across the 2.2 Ohm emitter resistors) for the Right channel, ( the one with lower gain ) is 6mA, whilst the left channel quiescent current is 3.9mA. Those are with mid rail set at half supply. I can of course change quiescent current, but that changes the mid rail voltage. Those current are accurate as I replaced all the emitter resistors with new ones and even checked that they were 2.2 Ohm before they went in. (Old ones I took out were, 2.7, 3.8, 7.3 and 13.0 !)

The circuit has 0.03 V across emitter resistors R49 and R50, and this gives current of 15mA. (Diagram has them as 2.0 Ohm, but in reality they were 2.2)

Should I try setting VR42 to the spec of 15mA current and just forget about mid rail voltage being half supply.

VR52 adjusts gain only minimally, not enough to correct for discrepancy.
I’m still stumped.
Anyway I replaced all the resistors in the tone control stage and what once was a roaring sea of noise on both channels is now just a whisper. So success there.
Again, thanks for help so far.

[frsimen]

With VR52 not making much difference to the gain, check that R38 is in good health. If it has gone high in value, the gain will be reduced and the range of adjustment from VR52 will be reduced.

C50, the bootstrap capacitor, will also have a big influence on the gain. You haven't said whether you have checked that but it was mentioned by Jez1234 earlier in the thread.

The voltage gain from the base of VT15 to the output should adjust from about 22 to 122, if all is working well.

Check also R104 on the other channel. If that has gone high, that channel's gain will be higher than it should without adjustment of VR105.

Paula

[Impecunious]

First of all, many thanks with your persistance with this... almost as great as mine !

With VR52 not making much difference to the gain, check that R38 is in good health. If it has gone high in value, the gain will be reduced and the range of adjustment from VR52 will be reduced.

Have done so, its around the 22 it should be. tested in circuit, both ways around so that no stray volts could upset meter.


C50, the bootstrap capacitor, will also have a big influence on the gain. You haven't said whether you have checked that but it was mentioned by Jez1234 earlier in the thread.

All electrolytics (C20, C50, C22, C23, C25, ) replaced. C26, 1000uF tested to be spot on.

The voltage gain from the base of VT15 to the output should adjust from about 22 to 122, if all is working well.

I'll go and take some measurements of voltage gain for each channel. You say it should be adjustable.... via VR52 ?

Check also R104 on the other channel. If that has gone high, that channel's gain will be higher than it should without adjustment of VR105.

I was wondering if I were chasing the wrong channel and that the fault was too much gain in one stage rather than too little in the other. I'm sure I've checked all the values of all the components except the 600pf caps, C24, C27, C51, C54

One last thing, I've been trying at all frequencies and the difference in gain starts to become less noticable above about 12KHz. By 30k ( yes I know the amp is only designed to 20k) the gains of L and R are both low, but the same. Makes me think capacitative...... somewhere.

Thanks again for help and hints.


Paula

[frsimen]

Yes, the gain should adjust with VR52 on the "bad" channel or VR105 on the other channel.

Paula

[Impecunious]

HOO ****** Ray
Sorted, at last and thanks for all your help.
The "bad" channel (with low gain) was indeed good and the baddun was overamplifying due to the pot VR52 having an openish track of 32k rather than the 2k it should have had. Anyway, found a preset to put in and both channels track beautifully with identical gain at all frequencies. So that poor channel that had been subject to nearly every component being lifted and tested and in many cases replaced, was in fact never the guilty party. Oh how wronged it must feel.

After I realised that most resistors were noisy I replaced nearly all and the attached pic gives an idea of what's come out of the earlier stages as well. The emmitter resistors aren't there as I'd tossed them early, but they came out too. After taking the pic I realised that I'd placed some of the wrong values down. They are all in the right position, but just not necessarily of the right value... Mr. Preview.
Anyway, there's still the phono amp input stage which is yet to have its resistors changed as it's noiser than it should be.
Oh and many thanks to the previous poster whose hand-drawn circuit I used.

Gain for the output stage turned out to be a bit under 100, for a 20mV RMS input, I got a shade under 2V RMS out..... at all frequencies. So thanks for help.
David

[Herald1360]

Originally Posted by Impecunious

HOO ****** Ray
Sorted, at last and thanks for all your help.

.....


Oh and many thanks to the previous poster whose hand-drawn circuit I used.


.....


David

That's a blast from the past

https://www.vintage-radio.net/forum/...29&postcount=6

A pink pocketfile folder comes to mind; where it is now though?