Hacker RP33 Autocrat

[AlanBeckett]

I've never played with a wobulator, but do they offer the option to stop the sweep at any frequency so you can check calibration?

Professional ones usually have a 'Manual' option so that you can vary the frequency slowly, or not at all, and observe the results.
Alan

[Joe]

Well, I think I've sorted it. One cap had an ESR of 7Ω an another couple measured around the 3Ω mark so I changed those.

If anything it works a bit too well as I suffer from "Next-door's SMPS Syndrome." In an attempt to get away from this I put two PP3s in it and it then seemed generally better than on my power supply. This is the first time that that has been found wanting.

   

I tried playing 6 Music through it and it sounded fantastic, it was difficult to believe it was only in mono.

I've now got to lock my drill up before I succumb to the temptation to start modifying it.

- Joe

[pwdrive]

Having done this, it's really quite simple...

Picking up the signal post-detector seems to be pretty standard practice in these sorts of sets, and works well. I've got a diode probe that does the same job, though I can't remember the last time I used it...

I mentioned earlier that I don't use a wobulator; instead I use two oscillators. One is a Marconi TF2008, which generates the 470kHz. This is swept by a Wavetek 162, which is a lovely old function generator. This also drives the X input of the 'scope, of course.

The Wavetek can be stopped at any point in the cycle, and that's really convenient. It means that I can set the output to 0V, then centre the spot on the centre of the screen using the X-shift, then adjust the Marconi to give exactly 470kHz. Then, I can move the Wavetek output to the peak of its output, then adjust the amplitude so that the Marconi is giving a sensible frequency - say 460kHz. Then I adjust the X-gain on the 'scope, so that the spot has moved 4 divisions left. That means 1 horizontal division equals 2.5kHz. Once all that is done, the Wavetek is set to oscillate...

Estimating the -3dB point is quite easy, as you can see the peak at the centre, and the baseline. If the difference between the peak and the baseline is 8 divisions, then the -3dB is 5.6 divisions above the baseline.

It's harder to explain than to actually do

I've never played with a wobulator, but do they offer the option to stop the sweep at any frequency so you can check calibration?

That sounds a good set up Mark.

Lawrence.

[Mark Hennessy]

Well done

Knew you'd be impressed when you got it going!

I'm not immediately seeing how the ESR would cause the exact symptoms - were either of them leaking? Those Philips capacitors are generally pretty reliable in these sets, so I'm always happy to see them.

And yes, the earlier Hacker sets are quite fussy about their power supplies...

So, the RP18 next???

Cheers,

Mark

[Joe]

The RP18 was posted at dinner-time so will hopefully be here by the w/end.

I don't pretend to know why the ESR would matter but just took it as in indication that all was not well within. The two Daly ones leak 100mA at 12V, at which they are rated, and one gets hot to boot. The Philips ones may be OK and I could have been lead astray a bit by the interference and PSU issues. That said, it's all practise.

As to being impressed I'm not so sure as, at least at present, I prefer the Roberts sound for MW radio, though would freely accept that the Hacker is 'Better;' the amplifier would have been designed to get the best out of wider band-width signals and it's great on digital. Shows what a subjective thing audio is. I'll certainly be interested to hear the Sovereign on VHF (I nearly added "Subject to the ECC85 playing ball"!) and see what it's like with the full tone controls.

I'm certainly happy with it even if it's ends up being something to trade in towards another one.

- Joe

[Mark Hennessy]

Ah well - if the bad ones were Daly, then that's the answer. I change these on sight now. As I say, the Philips are good, but the older red Electromolds are a mixed bag - sometime OK, mostly not, but I take each of those on their merits...

Depending on where they are, a bit of leakage can be bad - typically upsetting the DC conditions throughout the amplifier. If they are just supply decoupling, they might just run the battery down a bit faster, or drag down a rail if there is a resistor between them and the battery - the answer is "it depends". But they need to be changed either way.

As for ESR, I've had leaky capacitors that have read really well on an ESR meter. Including some that were totally short! With capacitors, you have to measure all 3 (value, leakage - at the working voltage - and ESR). Arguably, ESR is the least useful in a radio, though as you know, it's public enemy number 1 in a typical power supply

When the RP18 arrives, you'll probably find that the amplifier is full of those Daly capacitors, and will probably not work as a result. I'd recommend a blanket swap, but if you decide to apply logic (why not - it's a good way to learn), you ought to change C7 and C10 - even if the set appears to work. Otherwise, they'll put DC onto the bass control, causing crackles that might eventually damage the track.

How are you getting 6Music into it - straight in at the volume control?

Cheers,

Mark

[Mark Hennessy]

That sounds a good set up Mark.

Lawrence.

Thanks - I've been using it this evening, and it works well. It would be nice to make it more compact and streamlined, because it does take a little while to set up, and I'm feeling inspired by the articles and projects that Al and David posted. One day...

Cheers,

Mark

[Joe]

How are you getting 6Music into it - straight in at the volume control?

Yes, that's how it sounds really good. It's also better through my pantry Tx than Absolute is off the air which is what made me comment on the lower band-width on AM these days.

I too have had dead short caps give very healthy ESR readings - well they would wouldn't they? - and o/c ones pass a leakage test with flying colours.

- Joe

[Joe]

Well I seem to have finished this off well and truly.

I thought I'd got it fixed until I tried to actually listen to it. I don't have anything to judge it against but thought there was no way it should sound like it did - like it's either not properly tuned in or that the speaker is physically damaged.

I checked all the AF117s for whiskers and they were all fine and also the square disc capacitors with strips on one side for leakage or being o/c. Again no problems.

I was thoroughly confused as my MP3 player or DAB radio played through it via the volume control sounded fantastic and the Autocrat when played through my bench amp seemed fine. Yesterday I also tried it through the Sovereign and it played OK through that, albeit quietly.

As the distortion seemed like that caused by overloading I had a go at the trimmers on the audio board. I'd not bothered before as the original anti-tamper paint was un-disturbed. I also don't have an output meter but as setting the basic current and voltages made no reference to readings thereon I thought I could manage without.

I managed to make an utter mess of this job and have blown the output transistors (T3, T4 and T5). I changed T3 and T4 for BC337-25s, T5 for a BC327-25.
   
It works undistorted like that but is drawing 60mA through the test link and the BC337s are getting hot. Adjusting RV2 can get the current down to 20mA but it doesn't work at that setting - it oscillates and takes Radio Two off the air.

I'll see if I can get hold of some BD131s and 2s cheap but it looks like I'll be knocking something up with an IC and putting it on e-Bay.

I should never have started to mess with Dark Forces which I don't understand. That said, long-standing members will recall that this is just how I was with valve sets a year ago.

- Joe

[Tedzed1]

That's transistors Joe, the work of the DEVIL.