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Old 3rd Feb 2019, 3:18 pm   #61
ms660
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Default Re: Hacker Hunter FM volume R4

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Originally Posted by Findlay View Post
I can't work out from the circuit where the 4 connections are to the air capacitor!
Your green wire is connect to C8 and your substitute for C6, the mauve wire is connected to C23 and C22, the red and the black wires are connected to the tuning gangs rotor via the rotor springs and also to the ground line on the PCB, that's the thick horizontal line in the schematic.....So far as I can tell.

Lawrence.
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Old 3rd Feb 2019, 5:02 pm   #62
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Default Re: Hacker Hunter FM volume R4

Thanks Lawrence. C6 is 22p - I'm not sure of the value of my substitute as it isn't marked - is this value critical? Maybe I should order an exact replacement before going any further?
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Old 3rd Feb 2019, 5:06 pm   #63
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Default Re: Hacker Hunter FM volume R4

It’s part of a tuned circuit so is critical.
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Old 3rd Feb 2019, 5:25 pm   #64
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Default Re: Hacker Hunter FM volume R4

Thanks Frank. Will have to order the capacitor - hope you guys will be around in a few days when I get going again!

Cheers
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Old 4th Feb 2019, 3:28 pm   #65
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Default Re: Hacker Hunter FM volume R4

I found a 22pfd capacitor and soldered it into my "flying lead" arrangement with the tuner. Still no reception so I unsoldered all 4 leads and mounted the FM tuner back on top of the air capacitor. It is very tricky on this model aligning the posts on the capacitor with the slots/holes on the board - grateful for any tips!. Anyway, after soldering the 4 connections I can now get R2 at least, though feebly. I'm wondering if the board needs to be close to the tuning capacitor to tune up ok? I've ordered the transistor that was suggested earlier so will fit this when it arrives hopefully later in the week.
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Old 6th Feb 2019, 2:06 pm   #66
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Default Re: Hacker Hunter FM volume R4

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You will almost certainly have an hfe test function on your meter. That's all you need to confirm the pinout. You're just as likely to find an 'odd' leadout with a supposed 2N3904 as you are with anything else.

Adjust the replacement leadout to conform to the original while the original is still in place, and recheck the leadout with your meter to make sure you've got it right. A bit of PVC insulation stripped from some wire will help prevent short circuits where the leads cross, though this isn't strictly necessary if you take care.

I don't understand why you're so nervous about this. Nothing will go bang if you get the leadout wrong, it just won't work.

If you really don't want to try the BC549, an MPSH10 is a closer substitute electrically and has the same leadout as the original.

https://www.ebay.co.uk/itm/10x-MPSH1...9/140992887505
https://www.datasheets360.com/part/d...0907630689128/

The MPSH10 have just arrived in the post - bought 10 just in case! - but I'm now confused as the pin out seems reversed to the BF224. With the flat side facing me the MPSH10 seems to be BEC. I bought the tester recommended and this measures up as 160 on the hfe test. This is what the datasheet says. Looking at the BF224 data sheet this appears to be CEB with the flat side facing. Any help much appreciated!

Last edited by Findlay; 6th Feb 2019 at 2:16 pm.
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Old 6th Feb 2019, 2:30 pm   #67
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Default Re: Hacker Hunter FM volume R4

I don't understand the problem. Just fit the transistor appropriately.
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Old 6th Feb 2019, 2:59 pm   #68
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Default Re: Hacker Hunter FM volume R4

Thanks Paul - it is just that it is not clear looking at the board what the configuration is and I thought this would just be an easy substitution!
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Old 7th Feb 2019, 4:00 pm   #69
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Default Re: Hacker Hunter FM volume R4

Before I replace Tr1 I thought I'd better check - someone mentioned earlier about the voltage at terminal 1 being way too high. I just measured R2 which is marked s 56K but measured up in circuit as only 2.8K. On my good set I measure 12K. I can't for the life of me find R1 on the board as I wonder if this could be causing the high voltage to? Could this be reducing the supply voltage?
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Old 7th Feb 2019, 4:24 pm   #70
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Default Re: Hacker Hunter FM volume R4

Is the resistor colour coded 56k or are you referring to the circuit, there was a change, earlier sets used 15k R2 and 2.2k for R1, see circuit diagram for the tuner bottom left updates.
Check the colour code on the resistor.
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Old 7th Feb 2019, 4:28 pm   #71
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Default Re: Hacker Hunter FM volume R4

Measuring resistors still connected in circuit is bad practice unless they are DC isolated or you know what to expect in combination with any parallel resistances of a known value that might be connected across it.

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Old 7th Feb 2019, 4:38 pm   #72
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Default Re: Hacker Hunter FM volume R4

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Is the resistor colour coded 56k or are you referring to the circuit, there was a change, earlier sets used 15k R2 and 2.2k for R1, see circuit diagram for the tuner bottom left updates.
Check the colour code on the resistor.
Hi Frank. It's marked 56K - green blue orange gold - just as the one in the other set.
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Old 7th Feb 2019, 4:47 pm   #73
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Default Re: Hacker Hunter FM volume R4

Then has Lawrence states, the only way to be sure is to isolate it to test, it’s unusual for 56k resistor to reduce in resistance unless overloaded and I don’t think it has been in the tuner.
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Old 7th Feb 2019, 4:54 pm   #74
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Default Re: Hacker Hunter FM volume R4

OK I'll isolate it and test. I've got to wait a few days as I can't lean over the set at the moment to work on it due to a back problem! If the fault does lie with Tr1 could it be giving this low reading?
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Old 7th Feb 2019, 4:59 pm   #75
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Default Re: Hacker Hunter FM volume R4

Wouldn’t have thought so, Tr1 isn’t in circuit,has said earlier, there could be more than one fault.
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Old 8th Feb 2019, 1:31 pm   #76
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Default Re: Hacker Hunter FM volume R4

Success! I have just replaced the transistor and the set is working brilliantly on FM. The voltage at point 4 is 12.2V. Voltage at point 2 has gone down to 1,9V. I've yet to screw the tuner board down firmly - I'm not sure what to do about the copper sleeving that used to go around the transistor - I couldn't use it as the replacement has to be fitted the other way around. Have just pushed it to one side - is it important?

Thanks everyone for your kind help and for giving me the confidence to fix this
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Old 8th Feb 2019, 2:15 pm   #77
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Default Re: Hacker Hunter FM volume R4

Great news, well done, presume the 1.9v is on point 1 not 2.

I don’t know about the copper screen, not seen that tuner for many years.
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Old 8th Feb 2019, 2:19 pm   #78
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Default Re: Hacker Hunter FM volume R4

Thanks Frank. Yes, I meant point 1!
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Old 8th Feb 2019, 4:09 pm   #79
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Default Re: Hacker Hunter FM volume R4

Yes, good news. It actually only takes a few seconds to sub a transistor, and that's the most likely cause of poor operation and strange circuit voltages in a set of this age.

I wouldn't worry about the copper screening if the radio is working well.

This set uses BF195s in its IF stages, and they are known to be unreliable, so you might want to consider changing those while you're feeling confident. Do them one at a time. Your MPSH10s are good substitutes.
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Old 8th Feb 2019, 5:49 pm   #80
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Default Re: Hacker Hunter FM volume R4

Just coming back to this, I'll add my congratulations as well

Working on FM tuners is always difficult because of the physical way they are put together. Mounting the PCB directly on the tuning capacitor might seem like an odd choice, but up at those frequencies, the inductance (and stray capacitance) of even a relatively short length of wire can cause all sorts of issues. For what it's worth, Hacker are one of the better manufacturers in this regard - I've seen many that are much nastier to work on.

So well done for getting there! Working on the large IF PCB will be much easier by comparison!

Regarding that screen, off-hand, I think that particular tuner in the RP38 is the only place I've seen it. It certainly wasn't there in the A model, nor in the similar tuners used all the other sets that I can recall. Perhaps experience showed that it wasn't needed; perhaps they couldn't get one for the Lockfit cases that the other tuners used? As Paul says, if it's working, don't worry. Leaving it in place for a future restorer is a good plan.

All the best,

Mark
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