Roberts R606-MB - No Operation on VHF

[TheMightyMadman]

Hi all,

I'm trying to repair a Roberts R606-MB transistor radio for a friend of mine.

The unit works fine on AM (MW/LW), but doesn't seem to output any audio on VHF, with AFC enabled or disabled. I've been doing some basic investigation but I'm a bit stumped now and would appreciate some advice and assistance if anyone has any!

This radio has a separate FM tuning module attached behind the tuning dial (RR 1412/3), and there doesn't seem to be any signal coming out of it, the output is sat at 0V (pin 5 of the module versus ground at pin 4). Pin 6 of the module is sat at 0Vdc, and pin 8 is around 3mV (possibly floating).

There's 6.5Vdc on the power rail going into it (pin 7 of the module versus ground at pin 4). I've replaced a bad electrolytic capacitor on the power rail (C5, 150uF 16V) but it doesn't seem to have made a difference.

I don't think that the module is oscillating correctly. It has a BF195 and a BF200, both of which seem to test OK out-of-circuit. I've also removed the shield connection on the BF200 in case it's suffering from tin whiskers. The primary and secondary of the antenna transformer aren't open-circuit, and the IF transformers also seem to be OK. The module seems to have three silicon diodes, two test OK in-circuit and the other tests short but it is across an inductor so it's probably OK too.

The connections all seem to test OK in each position on the MW/LW/VHF switch blocks and the AFC switch block. The AFC switch block seems to be damaged and won't lock into place when it's engaged.

Please see service information attached.

Many thanks,

Adam

[Silicon]

I have not heard of the tin whisker problem in the TO72 package.

However 'Lockfit' packages like that on the BF195 have been known to be faulty.

[TheMightyMadman]

Thank you The BF195 seems to test OK out-of-circuit, would this package cause problems in-circuit if it’s just a 3-pin?

[paulsherwin]

The Lockfit series including the BF195 is a notorious source of trouble. They have a failure mode where they look fine on a basic hfe test but are noisy, intermittent or low gain in circuit. As you've already dismantled the tuner head, I would change it anyway even if it's not responsible for the fault. Any of the common VHF/UHF types should be OK, but watch out for the leadouts.

[SteveCG]

How is the tuning capacitor assembly connected to the tuner block? Any chance of a dry joint there?

[agardiner]

Originally Posted by paulsherwin

The Lockfit series including the BF195 is a notorious source of trouble. They have a failure mode where they look fine on a basic hfe test but are noisy, intermittent or low gain in circuit. As you've already dismantled the tuner head, I would change it anyway even if it's not responsible for the fault. Any of the common VHF/UHF types should be OK, but watch out for the leadouts.

Totally agree. There is a strong chance that the BF195 has gone low gain. Replacing it should be your next call before delving any further.

[Silicon]

When they say replace the BF195, I think they mean replace it with a different type of transistor which is not in a Lockfit package.

Would a 2N2222a be suitable?

[Niechcial,Steve]

I would be very surprised if it's not the BF 195. THose lcok-fits are used widely in tv signal stages of the 70s and are now dropping like flies. I have had the BF200 go o/c in the past but that left plenty of white noise still coming from the osc.

[paulsherwin]

A general purpose jellybean transistor like a 2N2222 or 2N3904 will probably work, and is worth trying if that's all you have to hand, but a proper VHF transistor is a safer bet, certainly if you have to buy one. I keep a stock of MPSH10s for these RF Lockfit replacements, but it's usually not very critical. The MPSH10 doesn't cost a lot if you search around or buy in quantity.

https://www.alldatasheet.com/datashe...MI/MPSH10.html

2SC2999 would also be a good choice and is very cheap from China.

As said, check the leadouts.

[TheMightyMadman]

Thank you all very much indeed, I appreciate it a lot I have some MPSH10s on order and will try replacing the BF195, and I'll be conscious of the orientation/pinouts.