Resistor Question. Hacker Talisman GP46.

[paul1962]

Hello Everyone,
I have a question that some of you knowledgeable people may be able to answer ?
I have in the main amplifier circuit, an 820 ohm ceramic resistor and are trying to work out the wattage and a suitable replacement (Carbon, Wirewound, etc) It is taking a supply of 30v DC.

The dimensions of the original is 10mm L X 4mm W

I attach a pic of the original type in the hopes it may help in some way, as it appears that they are hard to come by these days and I need to source a suitable alternative if necessary.

Could anyone please advise ?

Kind Regards

[Station X]

What model Hacker is it?

30 VDC across an 820R resistor implies a power of 30V X 30V/820R or around 1.1 Watts.

[peter_sol]

That is a carbon half watt resistor.
what is wrong with the original?

[mhennessy]

Those green 5% types are carbon film. Not sure what their rating is, but if pushed, would guess at 1/4 watt.

I've had a quick scan of my service manuals, looking for a model with two 150k resistors in close proximity to a 22k, but no joy.

[Nickthedentist]

Presumably a Talisman or Grenadier? Unless it's one of the stereos?

[mhennessy]

Looking some more, could it be a GP46 Talisman?

If so, the 820R could be R17. This is one of the two resistors providing a load for the VAS (inside the IC). The junction is connected to the usual bootstrap capacitor C15. In which case, a 1/4 watt resistor is fine here...

Edit: partial schematic attached

[Nickthedentist]

Yes, previous posts from Paul say he's the proud owner of a Talisman

[mhennessy]

Posts crossing like arrows

Thanks - I think that confirms it, and tells Paul all he needs to know

[Station X]

That fits in with the 30V supply rail although the voltage across the resistor will be lower than that.

I'll update the thread title.

[Radio Wrangler]

That resistor has to carry the quiescent current of the voltage amplifier stage, PLUS it has the full output AC voltage of the amplifier applied to it via C15, so it has a moderate amount of power dissipated in it.

Those resistors with the pushed on end caps are not the best. the green 5-percenters are better than the carbon composition types in terms of noise, but the push-on connection lugs and the film construction make them none too reliable. I'd suggest a half-watt metal film or metal oxide axial lead type as a reliable alternative. It doesn't run with 30v across it, otherwise that 3.9k in series with it would have about 120v on it.

THe two resistors will have about 15volts across the pair, DC. So 15/(820+3900) = 3.1mA

so I^2 * R = 8mW

Say 5W output into an 15 ohm speaker (wet finger guess) and 820 Ohms will take a third of a watt from the output. So a half watt resistor would be OK.

David

[mhennessy]

The official output power is 9 watts into 8 ohms, which is a voltage swing of 8.5V RMS - entirely plausible given the regulated 30V supply and output stage topology. Into 820R, that's 88mW unless I've dropped a digit somewhere

What I think of as a 1/4 watt resistor - the sort that measures about 7mm long and fit nicely in holes spaced at 10mm - is actually 0.6W according to many. I'm sure one of those would be generous here.

It would be interesting to learn if the original failed because of the metal end caps failing (quite common on these) or the film itself failing (also common, but in my experience, more so with higher values that see higher voltages).

[paul1962]

Originally Posted by mhennessy

Looking some more, could it be a GP46 Talisman?

If so, the 820R could be R17. This is one of the two resistors providing a load for the VAS (inside the IC). The junction is connected to the usual bootstrap capacitor C15. In which case, a 1/4 watt resistor is fine here...

Edit: partial schematic attached

You are absolutely right mhennesy .. it is indeed a GP46 talisman

[paul1962]

Originally Posted by peter_sol

That is a carbon half watt resistor.
what is wrong with the original?

Hi Peter_sol .. the issue is it is cracked. Voltage one end, nothing the other and amplifier dead of voltage supply after the resistor, with ohms tester reading O/C

[paul1962]

Many thanks to all of you knowledgeable people. Once again, you have been absolute stars !!

I feel that a 1/2w as suggested would be the best solution as I feel it's always best to not get too close to maxing the current. I will fit that and let you know how it goes

[mhennessy]

It's absolutely fine to use a resistor with a higher power rating than is necessary. Sometimes, that choice might be driven by appearance, as modern components end up looking a bit "lost" among the original ones.

However, in this case, we've established that the power dissipation varies between 0.01W and 0.1W (rounding for simplicity).

So you're nowhere near the rating of a 1/4W resistor. Especially as you'd only ever get close to the upper end of that range if you played a sine wave at full volume (which you simply never would - it would be dangerous for both the speaker and your hearing). Real music has a much lower average power compared to the peak value - anywhere between 10 and 20dB, which is a factor of 10 or 100 respectively. Meaning that if the peak power is 9 watts, the average power - which is the power than causes heating in things like resistors and voice coils - will be anywhere between 0.09 and 0.9 watts, depending on how compressed the recording is.

As I say, use whatever looks right - no harm in over-specifying here - but please don't assume that you're getting close to overloading even a small 1/4W resistor. Always best to know what you're doing

[paul1962]

Well everyone .. the saga continues I guess

Obtained and fitted a 1/2w resistor and the player is now operational (after a fashion) The only resistor I could obtain had a 1% tolerence, whether this is the issue, I'm not sure.

It seems that for the first 5 seconds or so when powering up, there is a nasty crackling through the speaker that also comes and goes at random during power up, almost like a dry solder joint, but isn't.
I have replaced the smoothing cap on the main amp board and this is becoming so annoying that I am considering a complete Electrolytic cap change across every board.

On powerup . guaranteed to happen. I'm thinking that it's possibly when a cap is charging from power on. The sound is almost like rubbing your finger across the speaker cone, mixed with arcing.

[paul1962]

After much searching, I seem to have found the issue. It appears that it was the "classic issue" of these type of resistors. R10 (22k) had the end cap loose.

Replaced with a 1/2 W Metal Film type and also replaced R9 next to it with the same. The horrible noise has gone, but now it has resulted in only half volume from the player.

How replacing a 22k resistor with a 22k resistor can result in this issue is doing my head in

Any ideas on this all you knowledgable people ?

[mhennessy]

Well, you haven't replaced a 22k resistor with a 22k resistor!

The original resistor was clearly not 22k. Maybe it was some of the time, but who knows?

R9 and R10 are responsible for setting the DC conditions of the amplifier. The DC conditions must be correct before you think about applying the AC signal. When the DC conditions are off, anything could happen, depending on the exact design - which, of course, is largely hidden from us inside the IC.

By contrast, should R16 go high, the AC gain of the amplifier will be correspondingly high. It works in conjunction with R11 to establish the AC gain. But I realise you've not touched either of these - just by way of explanation.

It is possible that a high R9 and/or R10 did change the gain because of the loading effect on the previous stage, but it's hard to quantify that for sure.

What you need to do is establish if the gain is correct. Was it too high before, and is it now correct? Or, is it too low now?

To answer that, you need to take some measurements. See page 3 of the service manual.

If you are unable to make those measurements, you're simply guessing. And so are we. Do you have the test gear necessary (signal generator and AC voltmeter, or a 'scope)?

Such gear often comes up for bargain prices in the "for sale" section of the forum. It's about time I did a bit of thinning down of my test gear collection, so I might be able to help in the future...

[paul1962]

Originally Posted by mhennessy

Well, you haven't replaced a 22k resistor with a 22k resistor!

The original resistor was clearly not 22k. Maybe it was some of the time, but who knows?

R9 and R10 are responsible for setting the DC conditions of the amplifier. The DC conditions must be correct before you think about applying the AC signal. When the DC conditions are off, anything could happen, depending on the exact design - which, of course, is largely hidden from us inside the IC.

By contrast, should R16 go high, the AC gain of the amplifier will be correspondingly high. It works in conjunction with R11 to establish the AC gain. But I realise you've not touched either of these - just by way of explanation.

It is possible that a high R9 and/or R10 did change the gain because of the loading effect on the previous stage, but it's hard to quantify that for sure.

What you need to do is establish if the gain is correct. Was it too high before, and is it now correct? Or, is it too low now?

To answer that, you need to take some measurements. See page 3 of the service manual.

If you are unable to make those measurements, you're simply guessing. And so are we. Do you have the test gear necessary (signal generator and AC voltmeter, or a 'scope)?

Such gear often comes up for bargain prices in the "for sale" section of the forum. It's about time I did a bit of thinning down of my test gear collection, so I might be able to help in the future...

Many thanks for the input mhennessy

According to the schematic I have and also the original 4 band resistor (red, red, orange and brown tolerance = 22k 5%) and it's replacement 5 band resistor ( Red Red Black Brown and Brown tolerance = 22k 1%), it should be correct apart from 1% tolerance, which should make it a correct fitment.

It's current output is definately weak, as on full tilt, if you close the door, it is barely audable. Certainly not the rated 7W output, whereas before the resistor change, it would come close to making your ears bleed. The fault was almost like scratching your finger nail over the speaker cone, caused by the arcing on R10 where the metal end cap was loose, but apart from this, it had full output and was working correctly.

I do have the necessary test equipment (Signal Generator, AC voltmeter and also Hameg HM 205-2 scope) to hand, but the service manual I don't have. I have only the schematics of the circuits to reference.

[electrogram]

Replacement resistor red red black brown = 2200 ohms ?