Hacker Hunter RP38A

[ronbryan]

Anybody come across this problem with the RP38A discriminator stabilising capacitor? I am aware of two examples so far, both on pcbs marked A/504/9.

Ron

[Mike Phelan]

Is that with a station tuned in, Ron, and are the two diodes OK?

[ronbryan]

Hi Mike

It's tuned in to BBC R4, which is reasonably strong around here.

I've checked both diodes and they are ok. I've also traced out the pcb tracks and confirmed that the diodes and associated components are wired as drawn on the Hacker circuit diagram CD/38/55 - e.g. D2 cathode feeds R25 (1k), then on to C27 ; D1 anode goes to the AM balance pot, then on to C27.

The problem seems to be that the pcb silk screen incorrectly defines which way round C27 is fitted, and the Hacker assembly workers have followed the silk screen markings, causing C27 to be reverse biased.

I wonder if this mistake was ever detected. It doesn't seem to make much difference, even though the (electrolytic) capacitor should be leaking to some extent when reverse biased and presumably have no capacitance to speak of. (Is this right?)

Perhaps forum members could check their RP38As to see if the silk screen was changed, or a mod was done to reverse the capacitor C27, compared to my photos.

Ron

[Darren-UK]

I've just checked mine, also marked A/504/9, and it's the same as in your picture. I might, sometime today, reverse the polarity and see what happens out of curiosity.

[ronbryan]

Hi Darren

Thanks for looking at your set. Did you get a chance to confirm that C27 was in fact reverse biased?

Ron

[Darren-UK]

Yes, it is.

[ronbryan]

I removed C27, 6.8uF 40V and tested it. Reverse biased it read about 50k on the AVO ohms x 100 range (15V) and normally biased it read >20M after a few minutes. The Peak ESR tester showed an ESR of 0.79 ohms and a capacitance of 6.7uf, so it was apparently undamaged by being reversed.

I refitted the capacitor in the set with it correctly biased (i.e. reversed according to the C27 silk screen polarity markings). The DC voltage across the capacitor when tuned to BBC R4 increased to 4V from the previous reading of 3.5V. I'm not sure whether the lower reading was due to leakage or lack of capacitance in the reverse biased mode, but it unsurprisingly indicates that a small improvement can be achieved by fitting C27 the right way round!

Ron

[newlite4]

Yep, you're right Ron! Just checked mine (board no: 504B) exactly the same positioning. Reads 0v on MW and -3.15V on FM R4. Set still sounds OK, any ex Hacker blokes around? Howard, show us yours.
Neil

[mhennessy]

Hi Ron,

My one is the same (serial number 51555)... I know that a little reverse-bias is occasionally OK, but 3 volts permanently can't be good. Interesting that it reads OK on the Peak, so I guess it's a non-issue, even after all these years, but well done for spotting it! Out of interest, what caused you to be investigating that part of the radio - has it a fault?

Mark

[ronbryan]

Hi Neil and Mark

Thanks for checking your sets. It does seem that the 'problem' remained undetected for some time. The serial number of the set here is 56674.

Out of interest, what caused you to be investigating that part of the radio - has it a fault?

After first fixing a faulty BF195 3rd IF transistor and changing a faulty MW trimmer capacitor, I noticed the FM RF needed adjustment to get the dial calibration right. I was using the voltage across C27 as an output level indication when adjusting the FM tuner trimmers. I saw that the AVO indication went backwards when the leads were apparently connected correctly. If I had used a DVM I expect I would have overlooked it!

Ron

[Darren-UK]

Very strange, with the capacitor mounted as per Hacker I was only getting a reading of 1.9V but with the cap reversed I got spot-on 4V

[ronbryan]

Darren

Does the AFC work any differently with the greater output from the discriminator. I thought my set had a greater capture range, but it may be imagination.

Ron

[mhennessy]

Originally Posted by ronbryan

After first fixing a faulty BF195 3rd IF transistor and changing a faulty MW trimmer capacitor, I noticed the FM RF needed adjustment to get the dial calibration right.

FM alignment. Only one thing to say: yes, don't they all

I must admit that I hang a 'scope across the volume control and look for the strongest, best shaped sine wave.

And I've had to change a fair few "Lockfit" transistors - they either go noisy (e.g. TR1 in the audio amps) or low gain.

I'm planning to dig out my restored Hunter to give it some exercise, so will make a point of reversing that capacitor and checking the AFC action. It's probably worth cross-posting this to the Hacker Radio Yahoo group, as I'm sure it's worth making this known to the people across there

Thanks again for mentioning it!

Mark

[Darren-UK]

Re #12,

The best way I can word it is to say that the AFC now appears to be much 'keener'; as soon as the radio finds a station it grabs it loud and clear.

I wouldn't say the capture range is greater though. It's currently downstairs with Manx Radio, MW, babbling away but I'll have another play with FM in view of what you've said.

[howard]

Originally Posted by newlite4

......, any ex Hacker blokes around? Howard, show us yours. Neil

The above set is mine Although Ron is keeping it once restored.

Two others here are wrong as well.

Howard

[ronbryan]

Thinking about the AFC action, I quickly measured the voltage across the 2.5uF AFC decoupler C21. On the BBC R4 it varied from between +0.9V to -0.75V as the tuning control was varied to the extremes, just before the AFC lost 'capture' on that station. In a perfect world, it looks as if that cap should really have been a non-polar type, but ...

Ron

[Darren-UK]

There's a picture of an RP38A RF board halfway down this page. It appears to be a modern radial job which the author fitted, but it's unclear what the polarity is.

[ronbryan]

I think the Mullard C27 has been replaced with a modern axial cap - it's the one next to and in line with the skeleton pot. I believe it has been replaced following the polarity on the silk screen, as the crimped bit of the can retaining the rubber seal is near the +ve label on the pcb.

Ron

[Crawfish]

Originally Posted by Darren-UK

There's a picture of an RP38A RF board halfway down this page. It appears to be a modern radial job which the author fitted, but it's unclear what the polarity is.

That's my set! And my blog too!

Yes, when I recapped it I used mainly radial caps as axials are more difficult to find these days. I am positive (ha ha) I installed the new cap following the marking on the board.

To be honest, I hated to touch it at all, but it had developed a terrible 120Hz hum. I did keep all the original capacitors though instead of just pitching them.

I love my RP-38A. Wish I could get Radio 3 on it , but I use it often for an excellent classical station nearby. It sounds wonderful. Definitely the best-sounding transistor set I own (and I own more that I will admit to).

I have 5 or 6 other Hackers in my long restoration queue, as well as a Roberts set. They're quite rare in the US and I treasure all of mine.

-Kevin

[SteveCG]

Crawfish,

Welcome to the Forum.

The USA uses 75uSec emphasis on FM, but the UK (home of Hacker and vintage-era Roberts) uses 50uSec. I wonder what your Radios were set-up for?