Hacker RP33 Autocrat

[Joe]

After getting a bit of confidence on the R200s I've had a quick look at the Hacker Autocrat I bought from Golborne in November.

There's obviously something amiss as it sounds like it's either not properly tuned in or that the speaker is shot. The audio board is fine (to put it mildly!) so that can be ruled out.

It can't be whiskers or it wouldn't work at all so I'm hoping it's just dodgy electrolytics.

Another possibility is the alignment, though there's no sign of previous meddling. I've looked into the drill for this and it's radically different to other procedures I've read about and performed.

   

It requires an audio millivoltmeter and "Frequency modulated sweep generator" neither of which I have, nor even know what the latter is.

Is this just a superior method of doing it and would blasting 470Kc/s in and twiddling for loudest output still work? Also would this way get better results out of other sets?

As for the millivoltmeter, I have a 100μA ammeter so could this me modified with a suitable resistor and diode?

Hopefully it won't come to it.

- Joe

[Mark R]

"Frequency modulated sweep generator"

sounds like a wobbulator to me.

cheers Mark

[tiv48]

it is a wobbulator ,i have alighned a Hacker Autocrat going for peak response at 470 k/cs and sounded ok to me.(no instability)
it will at least get it working. i would eliminate every thing else first if it does not look like any one has been there beforhand
rob t

[Skywave]

Hi Joe,
The procedure that is being referred to is called sweep alignment. I'll explain the general idea. It's a method of realignment that sweeps the test signal from the signal generator over a defined range of frequencies and the detected R.F. is displayed on a 'scope in synchronism with the sweep. So you get an instantaneous picture of the passband on the 'scope. Compared to the traditional 'single freq. input and tune for max. signal' method, this method literally gives you the 'full picture' of the I.F. amp. response curve and you can thus see the effects of any adjustment as you make them - in real time. A 'ganging oscillator', a 'sweep generator', a 'sweeping signal generator' and a 'wobbulator' are basically all the same animal.

Having said all that, that method of alignment is rarely necessary for domestic broadcast radios - except for 10.7 MHz I.F. amplifiers, where it is essential. The reason for this difference is the required bandwidth of the I.F. amplifier. For A.M. broadcasts, a bandwidth of about 6 kHz at - 3dB and 25 kHz at -20 dB is quite adequate; the bandwidth is relatively small. However with F.M. broadcasts, we require a much wider pass-band. It is the superior audio quality that is achieved with F.M. that requires this broader bandwidth.

With comms. receivers (more my line of interest) - especially those in the 'complicated / expensive class' - sweep alignment is usually de rigueur; the 1st. I.F. of a Racal RA-17 being a good case in point.

Some of what I have written above is a bit simplified in places (I expect the purists will pull me up on one or two points ), but the overall concepts are valid.

O.K., lecture over! So to your set: I would think it's highly unlikely that the I.F. cores have been tampered with. I would only attempt I.F. re-alignment (and that using the single freq. method too) as a last resort. If there is a fault in the I.F. strip, then defective decoupling capacitors and / or changed value resistors - along with the possibility of faulty AFxxx transistors - seems the most likely cause . . . if the fault is in the I.F. stages; you may have an A.G.C. fault. An A.G.C. fault on sets like this can play havoc with the I.F. stage - including causing dynamic mis-alignment (and that can wait for lecture no. 2) . . . which brings me back to where I came in.

Al.

[Mark Hennessy]

I don't have a wobulator, so I use a "Frequency modulated sweep generator"

Basically, a wobulator consists of an oscillator than can be swept, and another oscillator that does the sweeping. I have plenty of oscillators, so hook them all together. A wobulator would take much less space, but wouldn't look as cool...

Hacker advise against peak tuning, but it will get the job done.

It might be whiskers - unfortunately, when a whisker makes contact, it might cause a small amount of leakage rather that the dead short we've come to expect. It just depends. In that R707 I did recently, only the first transistor in the module had any obvious leakage (a few 10s of K-Ohm IIRC), but it was enough to upset it when used in AM mode, where said transistor is the oscillator. So, I'm in the habit of treating them first - you can't apply logic until you *know* that there is no funny business going on...

I've tried lining these up in the past, and have never succeeded in getting the RF alignment spot-on on both wavebands.

Oh, and suspect any ceramic disc capacitors that are square (with coloured lines to mark their value). These things pretend to be resistors - or worse, they sometimes pretend to be resistors! Only last night one nearly got me, but I'm wise to their evil ways now

[Joe]

Thanks, all - especially Al.

I certainly had no intention of jumping in feet first; my question was prompted more by curiosity about the method.

I was surprised to read Mark's warning about mischievous tropical fish as thought, along with mustards, that these were good. Another handy piece of knowledge.

I've now had a better look at the set.

There are only two electrolytics not on the audio board: C13, specified at 40mfd and C21 - 300mfd. Neither of these were healthy in terms of ESR and C13 also leaked like a tap. In practise C13 was 10mfd and C21 350md, the latter I replaced with a 470mfd.

These changes made no difference so I tried playing the set through my bench amp and it was fine. So, despite my earlier investigations the fault must be on the audio board. This certainly makes sense as there are far more electrolytic capacitors thereon and if he two I've tested are at the end of their days odds are at least some of these are too.

My main problem with this set is that it's far too good condition. There's all sorts of fun I could be having if it was a battered old wreck.

Thanks again for the info.

-Joe

[pwdrive]

Al has explained the basics of sweep alignment very well for those not familiar with that procedure, as for a full blown sweep generator you can get away with a relitively simple homebrew sweep gen if you have a 'scope with timebase out, you can use the sawtooth waveform from this to vary the voltage on a simple varicap/osc affair, somewhere recently I saw the article on this little box of tricks, I can not remember the center frequency ranges in the circuit but I'm guessing 455 khz would have been one of them. I will try and find the article in the mean time unless somebody beats me to it, I think it was a FET + NPN and that was about it. If you have never done sweep alignment Joe, then giving it a go will be quite rewarding both in terms of performance and further understanding of that side of things.

Lawrence

[pwdrive]

Here is the simple wobbulator link

http://www.youtube.com/watch?v=l4QZ5c8gqNE
(Edit) Hmm this link does not seem to work, anyway the the article was in radio bygones mag, I expect someone will come to the rescue?

[Mark Hennessy]

Oh, and suspect any ceramic disc capacitors that are square (with coloured lines to mark their value). These things pretend to be resistors - or worse, they sometimes pretend to be resistors! Only last night one nearly got me, but I'm wise to their evil ways now

I was surprised to read Mark's warning about mischievous tropical fish as thought, along with mustards, that these were good. Another handy piece of knowledge.

Just to clarify, I wasn't referring to "tropical fish" capacitors - these are indeed very reliable. The ones I mean are ceramic discs - but unusually for a ceramic disc, they are square, and have 3 coloured lines to indicate their value. These lines don't go around the whole of the capacitor (like they do on "tropical fish" caps) - rather they are short strips on one side.

I'll try to remember to take a picture later tonight...

[Mark Hennessy]

I've now had a better look at the set.

There are only two electrolytics not on the audio board: C13, specified at 40mfd and C21 - 300mfd. Neither of these were healthy in terms of ESR and C13 also leaked like a tap. In practise C13 was 10mfd and C21 350md, the latter I replaced with a 470mfd.

C21 failing O/C is a fairly common occurrence, and can cause the set to squeal and whistle like mad. C13 is the AGC capacitor, and this causes audio distortion when O/C... I'm sure that changing these will have helped slightly, even if you can't hear any change just yet - and if not, at least they're done now...

These changes made no difference so I tried playing the set through my bench amp and it was fine. So, despite my earlier investigations the fault must be on the audio board. This certainly makes sense as there are far more electrolytic capacitors thereon and if he two I've tested are at the end of their days odds are at least some of these are too.

On these boards, I've found the wire-wound pre-set to be 100% unreliable - although this same component is used on a number of other sets and I've not had them fail. But this normally causes bad crossover distortion, and is worth checking out. The normal failure mode is to fail O/C.

Depending on the vintage, electrolytic capacitors are definitely in the frame, although I don't recall having to change too many of these.

The germanium transistors can fail. One of the output pair going low-gain seems quite common, and also they can develop tin whiskers. For the purposes of testing, the types aren't all that critical...

When working well, I quite like these. But I'm probably repeating myself, so will shut up now