Hacker Hunter FM volume R4

[Findlay]

Thanks Lee,

I found a resistor which seems to be dropping the 18V supply to the tuner after the FM switch - R6 on the schematic. It is supposed to be 1.5K and measures up as 1.35K. There is 12.11V across it when switched to FM, which maybe explains the 6.69V at the tuner? On the good set it measures 1.48K. Not easy to see the decoupling caps in the FM tuner - how do I check them?

[Nuvistor]

C28 and C45 are not in the tuner, a leak in either could cause that voltage drop.

[Findlay]

Thanks Frank. I'm not sure how to check capacitors. I've measured the resistance across C28 and C45 with the DVM and it comes up as around 2.5k for both. On the good set I measure about 3.3K.

[Nuvistor]

Probably need to disconnect one end the capacitor to test, see if the voltage returns to a more normal level.
One voltage that is a worry is pin 1 being very high on the faulty radio which could be a collector base leak on the RF amp transistor but test the easy items first.

See what Mark as to say, he as repaired many of these.

[mhennessy]

I've dug out my notes. Please note that most of my repairs have been on the A model, which has a slightly different front end...

On an RP38A, the IF HT, as measured across C20, should be about 13V on FM. The supply to the front end should be around 8.3V on an RP38A.

However, on an RP38, the front end supply should be more like 12.3V. I don't have any numbers on the IF schematic, suggesting that I've never had to chase a low-gain fault on one of those yet. The voltage readings on the front end tell me that the only low-gain faults I've found on a non-A model were restricted to that. For info, here's what I have written down:

TR1:
E: 0.9V
B: 1.63V
C: 12V

TR2:
E: 1.22V
B: 1.79V
C: 11.95V

I appreciate that some of those numbers are different to what's in the manual - they are what I've measured over the years

Checking the DC conditions is always the first step of serious diagnosis. Transistors need the right operating currents and voltages before they'll consider thinking about amplifying signals.

Returning to the IF board, R6, R7 and R8 are the first ones to check. I've also had R14 go high. And lots more besides all that, but it's late

[Findlay]

Thanks for this Mark.

I've disconnected C28 and C45 and there is no effect on the supply voltage at the tuner.

R6 = 1.43K, R7 = 1.12K and R8 = 0.95K
manual states 1.5K for R6, 4.7K for R7 and 1.2K for R8 - so R7 is low but I measure the same on my good set.

I can't see how to measure the Tr2 voltages without taking the board out - do you just undo the one screw?

Tr1 measuring left to right looking down on the board comes in at 5.52V. 0V and 4.24V so I think this is

E = 4.24V
B= 0V (can't seem to get any reading and signal fades when I try)
C = 5.52V

On the good set I get:

E = 1.56V
B = 0.88V
C = 11.93V

Cheers!

[paulsherwin]

I think TR1 is bad. I don't know how easy it is to access, but you can replace it with a general purpose silicon type like a 2N3904, at least temporarily. If this appears to clear the fault then you can consider a permanent replacement with a proper VHF transistor, though the 2N3904/whatever may be fine.

Be careful to check leadout differences and fit appropriately.

[Nuvistor]

Data for a BF224 transistor.
https://alltransistors.com/pdfdatash...a/bf224rev.pdf

The centre lead out according to above is the emmiter not the base, with 4v on the base and none on the emmiter would indicate an O/C base emmiter junction.

You need to check what each lead on the transistor is connected to and confirm which is CBE.

Crossed with a Paul.

[paulsherwin]

Do these sets use a BF224? I thought they used a BF194 in that position. They are also CEB though, as are quite a few VHF types.

[Nuvistor]

My Hacker RP38 Hunter service manual states BF224, although Hacker could have used other types in production.

[paulsherwin]

They seem to be the same transistor in a different encapsulation.

[Findlay]

Thanks guys. Looking at the data sheet for the BF224 and that central emitter it seems I messed up my readings. I think they should now be:

C = 5.52V
E = 0V
B= 4.24V

I guess it is easy enough to lift the circuit board out to fit the 2N3904 when I get hold of one?

[Nuvistor]

What I don’t understand is why the collector voltage is so low, with no conduction it usually goes high unless it also is leaking to the base as well as the emmiter being O/C.

[paulsherwin]

I only mentioned a 2N3904 as an example of a cheap general purpose small signal transistor. Anything similar you have to hand could be used.

[Nuvistor]

It’s also possible that there is more than one fault.

[Findlay]

Originally Posted by paulsherwin

I only mentioned a 2N3904 as an example of a cheap general purpose small signal transistor. Anything similar you have to hand could be used.

Would a BC549 be any good?

[julie_m]

Well, it's NPN and Silicon, which is a good start. The "C" in the number indicates an audio-frequency device as opposed to an "F" for an RF device, so it might have too low a turnover frequency. But it might be worth a try anyway, if you can replace it without risking damage -- bearing in mind that you might have to replace the BC549 again, if it does not make an improvement.

[Findlay]

Thanks Julie. I need to summon up some courage to do this - I'm not that good at replacing transistors.... I don't want to wreck the set completely. At the moment it works ok with really fresh batteries so so I suppose I could just learn to live with it until it starts to fade after about 10 hours or so of use and run off the rest of the batteries in the other set. A bit of a nuisance though and I guess it might deteriorate further. Because I'm so ham-fisted with transistors I'd rather put the best replacement possible in!

[paulsherwin]

Originally Posted by julie_m

Well, it's NPN and Silicon, which is a good start. The "C" in the number indicates an audio-frequency device as opposed to an "F" for an RF device, so it might have too low a turnover frequency. But it might be worth a try anyway, if you can replace it without risking damage -- bearing in mind that you might have to replace the BC549 again, if it does not make an improvement.

General purpose transistors with BC prefixes have hfe performance easily good enough for use at band II VHF. I normally use HF transistors like MPSH10s or 2SC2999s, but that's because I have them to hand and some of the other parameters (like capacitance) are a better fit. A BC549 will be fine here though, certainly as a test.

Silicon transistors are pretty tough and you're unlikely to damage them during replacement. It's not like the old days with germanium types. The likeliest problem is you'll fit the new transistor incorrectly despite checking the leadout (done that) or you'll overheat the copper tracks causing them to lift.

[Findlay]

Thanks - I'll have a go over the weekend when I'm feeling calm and relaxed, hopefully....