Hacker RP38A transistors - UK Vintage Radio Repair and Restoration Discussion Forum

2nd Dec 2021, 4:19 pm

I know my answer, almost any modern silicon transistor will do, but what would be the very best replacement transistors for the Lockfits (BF194, '195b,c,d) on the IF board?

I ask because those Lockfits are known to fail (eventually) and I have the back off at the moment charging the batteries (a six monthly task). Next time I would like to change them.

All triggered by todays visit to the Maidenhead Heritage Centre for the new "Maidenheads Car Industry" exhibition. I found the Hacker display was twice as big including some Dynatron radios too. Odd to find ones day to day radio in a museum.

paulsherwin · 2nd Dec 2021, 4:48 pm

BF494/5 are probably the 'best' choice, but there isn't much point in trying to track them down (and paying lots of money too) when lots of other types will work equally well.

I tend to use MPSH10s or 2SC2999s which seem to be fine. I once did a Roberts R606MB with BC239s as an experiment, and it was also fine despite these being BC109 type AF transistors.

Radio Wrangler · 2nd Dec 2021, 5:59 pm

The prime selection factor is 'What have you got to hand?'

David

Graham G3ZVT · 2nd Dec 2021, 6:25 pm

Quote:

Originally Posted by Radio Wrangler

The prime selection factor is 'What have you got to hand?'

David

And what is the cheapest, most ubiquitous bulk buy from China, which is arguably the 2N3904 at the moment.
I have a bag of 100, plus one of the PNP compliment, the 2N3906.

Oh, and a short bandoleer each of the SMD versions.

paulsherwin · 2nd Dec 2021, 6:42 pm

Yes, it's always worth keeping a bag of cheap general purpose types to hand. I do normally replace Si VHF transistors with similar VHF types, but it's not really necessary in domestic AM/FM sets, and if I didn't have any to hand I'd happily use 2N3904s and not expect problems.

2nd Dec 2021, 6:43 pm

Quote:

The prime selection factor is 'What have you got to hand?'

Loads! Just wondering about the very best to use. More of a thought experiment than reality.

cathoderay57 · 2nd Dec 2021, 10:06 pm

Personally, I'd leave well alone if the set is working well at present. I have had no issues with the lockfits in my RP38A; no noise, no failures. If it ain't broke don't fix it ! Jerry

Hybrid tellies · 3rd Dec 2021, 10:44 am

Quote:

Originally Posted by cathoderay57

Personally, I'd leave well alone if the set is working well at present. I have had no issues with the lockfits in my RP38A; no noise, no failures. If it ain't broke don't fix it ! Jerry

I totally agree. If it's working leave well alone. These Lockfit Si transistors are known to cause problems due to moisture ingress but if they have not been exposed to high humidity levels and damp they will be ok.

My Hacker Hunter RP38A is working really well with its original lockfit transistors and I would not dream about replacing them whilst the radio is working so well.

paulsherwin · 3rd Dec 2021, 11:42 am

It's true that the BF194/5 transistors are less prone to failure than the AF Lockfits, but they do still fail, sometimes with mysterious symptoms. I wouldn't change Lockfits in a fully working radio, but tend to change them if the set is in pieces for any reason and it's easy to do. I view it as cheap and easy preventative maintenance. Obviously opinions will differ about this, as with wax cap replacement in valve sets.

3rd Dec 2021, 2:35 pm

Thanks all for the replies, I was more thinking "what would Hacker had used given todays components", probably one less gain stage!

2nd Dec 2021, 4:19 pm	#1
Guest Guest Posts: n/a	Hacker RP38A transistors I know my answer, almost any modern silicon transistor will do, but what would be the very best replacement transistors for the Lockfits (BF194, '195b,c,d) on the IF board? I ask because those Lockfits are known to fail (eventually) and I have the back off at the moment charging the batteries (a six monthly task). Next time I would like to change them. All triggered by todays visit to the Maidenhead Heritage Centre for the new "Maidenheads Car Industry" exhibition. I found the Hacker display was twice as big including some Dynatron radios too. Odd to find ones day to day radio in a museum.

2nd Dec 2021, 4:48 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,957	Re: Hacker RP38A transistors BF494/5 are probably the 'best' choice, but there isn't much point in trying to track them down (and paying lots of money too) when lots of other types will work equally well. I tend to use MPSH10s or 2SC2999s which seem to be fine. I once did a Roberts R606MB with BC239s as an experiment, and it was also fine despite these being BC109 type AF transistors.

2nd Dec 2021, 5:59 pm	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,896	Re: Hacker RP38A transistors The prime selection factor is 'What have you got to hand?' David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

2nd Dec 2021, 6:42 pm	#5
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,957	Re: Hacker RP38A transistors Yes, it's always worth keeping a bag of cheap general purpose types to hand. I do normally replace Si VHF transistors with similar VHF types, but it's not really necessary in domestic AM/FM sets, and if I didn't have any to hand I'd happily use 2N3904s and not expect problems.

2nd Dec 2021, 10:06 pm	#7
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: Hacker RP38A transistors Personally, I'd leave well alone if the set is working well at present. I have had no issues with the lockfits in my RP38A; no noise, no failures. If it ain't broke don't fix it ! Jerry

3rd Dec 2021, 11:42 am	#9
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,957	Re: Hacker RP38A transistors It's true that the BF194/5 transistors are less prone to failure than the AF Lockfits, but they do still fail, sometimes with mysterious symptoms. I wouldn't change Lockfits in a fully working radio, but tend to change them if the set is in pieces for any reason and it's easy to do. I view it as cheap and easy preventative maintenance. Obviously opinions will differ about this, as with wax cap replacement in valve sets.

3rd Dec 2021, 2:35 pm	#10
Guest Guest Posts: n/a	Re: Hacker RP38A transistors Thanks all for the replies, I was more thinking "what would Hacker had used given todays components", probably one less gain stage!