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Old 19th Nov 2023, 11:25 pm   #1
TheMightyMadman
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Default Roberts R606-MB - No Operation on VHF

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

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Old 20th Nov 2023, 10:01 am   #2
Silicon
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Default Re: Roberts R606-MB - No Operation on VHF

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.
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Old 20th Nov 2023, 11:47 am   #3
TheMightyMadman
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Default Re: Roberts R606-MB - No Operation on VHF

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?
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Old 20th Nov 2023, 11:55 am   #4
paulsherwin
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Default Re: Roberts R606-MB - No Operation on VHF

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.
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Old 20th Nov 2023, 11:59 am   #5
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Default Re: Roberts R606-MB - No Operation on VHF

How is the tuning capacitor assembly connected to the tuner block? Any chance of a dry joint there?
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Old 20th Nov 2023, 12:10 pm   #6
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Default Re: Roberts R606-MB - No Operation on VHF

Quote:
Originally Posted by paulsherwin View Post
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.
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Old 21st Nov 2023, 2:07 am   #7
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Default Re: Roberts R606-MB - No Operation on VHF

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?
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Old 21st Nov 2023, 7:20 am   #8
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Default Re: Roberts R606-MB - No Operation on VHF

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.
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Old 21st Nov 2023, 11:08 am   #9
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Default Re: Roberts R606-MB - No Operation on VHF

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.
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Old 21st Nov 2023, 11:45 pm   #10
TheMightyMadman
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Default Re: Roberts R606-MB - No Operation on VHF

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.
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Old 30th Nov 2023, 9:29 pm   #11
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Default Re: Roberts R606-MB - No Operation on VHF

Hi all,

I've replaced the BF195 with an MPSH10 (taking care to match up the collector, base, and emitter), and unfortunately VHF is still dead. I've even replaced the BF200 with a new-old-stock part, again with no change in symptoms.

Can someone please confirm that the new part is installed correctly for me, please? Does anyone have any other suggestions before I call this one?

Many thanks,

Adam

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Old 30th Nov 2023, 9:55 pm   #12
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Default Re: Roberts R606-MB - No Operation on VHF

Do you have a signal generator? It would be good to establish that it is a front end issue by injecting a 10.7MHz IF signal into the rest of the set.
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Old 30th Nov 2023, 10:06 pm   #13
TheMightyMadman
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Default Re: Roberts R606-MB - No Operation on VHF

Quote:
Originally Posted by agardiner View Post
Do you have a signal generator? It would be good to establish that it is a front end issue by injecting a 10.7MHz IF signal into the rest of the set.
Thanks Adrian I do have a signal generator, yeah - should I disconnect the output of the FM module and put a 10.7MHz sine wave into the input to the radio?
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Old 1st Dec 2023, 1:37 am   #14
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Default Re: Roberts R606-MB - No Operation on VHF

Have you tried cleaning the wave change switches ? Roberts are also notorious for poor soldering on the switches, the R707 often suffers from poor joints on the switches and intermittent operation, a little switch cleaner and working the switches often cures it. I have had a couple where the solder just wouldn't take well to the switch pins, I had to remove them and clean the pins to get them to take.
Maybe the R606 suffers the same problems?
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Old 1st Dec 2023, 1:24 pm   #15
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Default Re: Roberts R606-MB - No Operation on VHF

Quote:
Originally Posted by TheMightyMadman View Post
Quote:
Originally Posted by agardiner View Post
Do you have a signal generator? It would be good to establish that it is a front end issue by injecting a 10.7MHz IF signal into the rest of the set.
Thanks Adrian I do have a signal generator, yeah - should I disconnect the output of the FM module and put a 10.7MHz sine wave into the input to the radio?
Yes indeed. Probably don't need to disconnect the module though, just inject the IF signal and see if it is making it through.
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Old 5th Dec 2023, 11:20 pm   #16
TheMightyMadman
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Default Re: Roberts R606-MB - No Operation on VHF

Quote:
Originally Posted by agardiner View Post
Quote:
Originally Posted by TheMightyMadman View Post
Quote:
Originally Posted by agardiner View Post
Do you have a signal generator? It would be good to establish that it is a front end issue by injecting a 10.7MHz IF signal into the rest of the set.
Thanks Adrian I do have a signal generator, yeah - should I disconnect the output of the FM module and put a 10.7MHz sine wave into the input to the radio?
Yes indeed. Probably don't need to disconnect the module though, just inject the IF signal and see if it is making it through.
Thank you, I'll give that a go!
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Old 5th Dec 2023, 11:21 pm   #17
TheMightyMadman
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Default Re: Roberts R606-MB - No Operation on VHF

Quote:
Originally Posted by slidertogrid View Post
Have you tried cleaning the wave change switches ? Roberts are also notorious for poor soldering on the switches, the R707 often suffers from poor joints on the switches and intermittent operation, a little switch cleaner and working the switches often cures it. I have had a couple where the solder just wouldn't take well to the switch pins, I had to remove them and clean the pins to get them to take.
Maybe the R606 suffers the same problems?
Thank you I have cleaned the band select switches, and they all seem to have the correct continuity in both positions, unfortunately. All of the solder joints on them look OK to me, but I can try reflowing them.
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