Roberts RM40 - the dreaded Lockfits! - UK Vintage Radio Repair and Restoration Discussion Forum

G6Tanuki · 28th Dec 2023, 3:30 pm

My RM40 started making a strange low-frequency 'fizzling/burbling' noise through the speaker.

The burble's level didn't change with the volume setting - indeed it continued unabated even when the volume control was at zero so I knew it had to be in the audio output stage rather than in the preamp/tone-controls or the audio part of the AM/FM/AUX signal-source switching.

Looking at the circuit diagram revealed that the first audio amp was a BC149.

A-Ha! A Lockfit! So without further ado it was excised and replaced by a nice new-old-stock SGS branded BC109C.

Tweak the bias pots to get the voltages spot on, reassemble, and the burble has gone.

I love it when a fault is easy to fix.

paulsherwin · 28th Dec 2023, 4:04 pm

Yes, that's a classic bad Lockfit fault. As you say, fortunately it's an easy fix, especially if you've encountered it before.

G6Tanuki · 28th Dec 2023, 4:20 pm

I've come across it before; these days I treat Lockfits with the same level of visceral dislike as the AF11x type whisker-transistors and that other early-1970s horror the Texas Instruments SN7600x range of audio-output ICs which have a horrible tendency to self-destruct.

Maarten · 3rd Jan 2024, 11:54 pm

I'm wondering what the production code on the lockfit transistor is. Did you by any chance make a note of it?

G6Tanuki · 4th Jan 2024, 2:25 pm

Alas I didn't bother with recording any details of the failed Lockfit before I threw it in the bin.

28th Dec 2023, 3:30 pm	#1
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,410	Roberts RM40 - the dreaded Lockfits! My RM40 started making a strange low-frequency 'fizzling/burbling' noise through the speaker. The burble's level didn't change with the volume setting - indeed it continued unabated even when the volume control was at zero so I knew it had to be in the audio output stage rather than in the preamp/tone-controls or the audio part of the AM/FM/AUX signal-source switching. Looking at the circuit diagram revealed that the first audio amp was a BC149. A-Ha! A Lockfit! So without further ado it was excised and replaced by a nice new-old-stock SGS branded BC109C. Tweak the bias pots to get the voltages spot on, reassemble, and the burble has gone. I love it when a fault is easy to fix. Attached Thumbnails __________________ A proper amplifier dims the lights when you switch it on.

28th Dec 2023, 4:04 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 28,712	Re: Roberts RM40 - the dreaded Lockfits! Yes, that's a classic bad Lockfit fault. As you say, fortunately it's an easy fix, especially if you've encountered it before.

28th Dec 2023, 4:20 pm	#3
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,410	Re: Roberts RM40 - the dreaded Lockfits! I've come across it before; these days I treat Lockfits with the same level of visceral dislike as the AF11x type whisker-transistors and that other early-1970s horror the Texas Instruments SN7600x range of audio-output ICs which have a horrible tendency to self-destruct. __________________ A proper amplifier dims the lights when you switch it on.

3rd Jan 2024, 11:54 pm	#4
Maarten Dekatron Join Date: Dec 2007 Location: Haarlem, Netherlands Posts: 4,323	Re: Roberts RM40 - the dreaded Lockfits! I'm wondering what the production code on the lockfit transistor is. Did you by any chance make a note of it?

4th Jan 2024, 2:25 pm	#5
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,410	Re: Roberts RM40 - the dreaded Lockfits! Alas I didn't bother with recording any details of the failed Lockfit before I threw it in the bin. __________________ A proper amplifier dims the lights when you switch it on.