Hacker Helmsman

[RobinBirch]

Hi All,
I'm just starting on a Hacker Helmsman. Physically good, slight ware to the printed strips on the loudspeaker cover and no sign of being mucked about .

I've got it to power up on all bands but on LW and MW the audio quality is very poor and when tuning through stations on long wave it behaves as though there's something oscillating that shouldn't be - a heterodyne note either side of the stations - ideas please?

Robin

[mhennessy]

C21 on the IF board is the usual suspect...

[RobinBirch]

Thank you, I shall look

[RobinBirch]

Hi,
I've fixed the strange oscillation problem, yes it was C21 but a number of others were changed as well. Realised that this had been like this for a while as many of the electrolytics had been changed. However, despite this success I still have crap audio. As the volume is increased the sound becomes lumpy and then breaks up entirely.

I tried doing a static set up and whilst I could get the mid point of the resistors set for voltage (RV1) the static current is nearly zero and cannot be adjusted (RV2). As a result I think that the complementary pair (AC128, AC176) are shot. Any other views anyone ?

Kind regards

Robin

[mhennessy]

One or both might well be below par, but I don't think we can reach that conclusion based on what you've reported. More data needed

Is the preset (RV2) OK, is T2 OK, are the resistors in that area reasonably within spec (say, within 20%)?

The cheap Chinese component testers seem to do a reasonable job with germanium transistors - worth picking one up if you don't already have one...

[paulsherwin]

There is an emerging problem with tin whiskers in Mullard transistors like AC128s. Search the forum for lots of threads about this issue, which was previously only thought to affect AF11x RF transistors.

Mark, have you tried a BC327/337 conversion in these sets?

[Radiocruncher]

I recently had a very similar problem. I could set the mid point voltage but could only get the quiescent current to just below 2ma. I also had noise at minimum volume. It turned out to be resistors well out of tolerance. I also changed the capacitors which improved the audio considerably.

Good luck

Graham

[RobinBirch]

Thanks guys, I shall look further. One curiosity is R11 and R12. The circuit says 2.2 ohm. On inspection they appear to be bent pieces of plated metal that read as dead shorts. Replacing them with 2.2 ohm makes no difference btw.

[mhennessy]

Some of these sets have the dreaded change-on-sight Daly capacitors, which can definitely upset the DC conditions. Later ones use the generally reliable "Philips blue", which to me are innocent until proven guilty.

I can't immediately think of a tin whisker fault that would give the reported symptoms, but anything is possible, and it's easy to check by removing the heat sink clip to remove the ground connection to the cases. It's also worth checking T3. And if the bias transistor is a later "T2" rather than the OC71 then that might have tin whiskers shorting emitter and collector, or collector and base.

Noise can be caused by T1, which is usually a Lockfit BC148 rather than the BC108 shown in the diagram. Of course, carbon-comp resistors go noisy for fun

I've never done it, but a silicon change ought to be pretty straightforward here. The LS is 15 ohms, so the 800mA BC327/337 pair will be fine. Naturally, T3 needs to change as well. The hardest thing will be mounting them neatly on the heat sink.

Of course, bad audio can be caused by the loudspeaker, or even the headphone socket. And what about the volume pot? Always eliminate the easy stuff first

[Herald1360]

I've used BD139/BD140 for silicon output and bias control. OTT but easy to mount on heatsink, though legs need "crossing" (use wire extensions) to fit PCB. Biassing needs to be played with.

See:

https://www.vintage-radio.net/forum/...r+Herald+RP37A

[RobinBirch]

Guys, thank you, it's fixed. Some drifted resistors and one of the output transistors was blown (AC176)

Cheers

Robin

[mhennessy]

Good news! So, in what way was the transistor "blown", and what led you there?

If it was short-circuit from collector to emitter (a very common failure mode), then the quiescent current would have been sky-high, and there wouldn't have been any output. If it was open-circuit in the collector circuit, that could explain the symptoms, especially as the preceding AC128 would have been able to supply limited positive output current via the B-E junction. The B-E junction could have failed short. Another failure mode - which I've seen a few times - is simply very low Hfe, which gives similar symptoms.

[RobinBirch]

It was open circuit in the collector. I checked the various bias points around the OC71 which looked ok and varied how I would expect as RV2 was tweaked. It then had to be either the o/p transistors or the AC128 prior to them and the sound symptoms suggested one side of the output chain.

Robin