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Old 13th Feb 2018, 10:52 am   #1
Keith
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Default Hacker RP25 - service sheet voltages?

Currently trying to sort the above. Audio is OK at low level but oscillates when the bass is turned up. Quiescent current will only come down to about 6mA, drifting up slightly, using RV4. Mid point voltage at about 6.7V. I'm confused by the voltage tables in both manufacturer's and R&TVS sheets. They seem incorrect for an 18V supply wrt chassis. Am I doing something wrong?
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Old 13th Feb 2018, 12:02 pm   #2
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Default Re: Hacker RP25 - service sheet voltages?

The oscillation is most likely caused by the 5-pin plug/socket...

The numbers for each transistors are: Vce, Vcb, Vbe and Ic.

Assuming the resistors are close enough in value and no caps are leaking, then leaking transistors is the most likely explanation. T4 gives the most trouble in this regard, but anything is possible...
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Old 13th Feb 2018, 12:13 pm   #3
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Default Re: Hacker RP25 - service sheet voltages?

Thanks for the reply - I've been reading your excellent articles on Lockfits, etc. I'll try some Deoxit on the socket, although it's been in and out a few times now. My confusion with the voltage tables is that both T5 and T6 collectors are shown as approx. 9V. Am I wrong in assuming the chassis is at 0V (i.e. lower battery negative) as shown?
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Old 13th Feb 2018, 12:50 pm   #4
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Default Re: Hacker RP25 - service sheet voltages?

Yes, it's a negative earth chassis.

Those numbers in the table aren't voltages with respect to chassis - they are voltages across legs of the transistors. So it's right that there is ~9V across the collector and emitter of T5 and T6 - together, they add up to 18V.

As well as the 5-pin connector, check all earth paths back at the chassis. Also check the power switch measures 0 ohms when closed. And that the batteries (or PSU) are good.
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Old 13th Feb 2018, 2:31 pm   #5
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Default Re: Hacker RP25 - service sheet voltages?

Yes, I'd guessed that was what they meant. It was the "with respect to chassis" that fooled me!
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Old 13th Feb 2018, 2:38 pm   #6
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Default Re: Hacker RP25 - service sheet voltages?

Yes, that's a mistake in the Radio and TV Servicing book. Easily done, I suppose...
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Old 13th Feb 2018, 8:09 pm   #7
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Default Re: Hacker RP25 - service sheet voltages?

Thanks for the advice, Mark,
I cleaned the 5 pin connector, remade some of the dry-looking joints and gave the board a good scrubbing with IPA to get rid of the flux and everything is now working correctly. There was was a certain amount of noise with no signal so I replaced T1 with a BC109 (couldn't find a BC108). A definite improvement. Can't get the inter-station muting to work at present but I guess this could be a switch contact problem. There's a bit of a crackle on the bass control - could this have been caused by DC via C5? I shall try Deoxit first.
The main cosmetic problem is the lack of covering on the handle. Presumably this was the same blue rexine that covers the case.
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Old 13th Feb 2018, 8:26 pm   #8
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Default Re: Hacker RP25 - service sheet voltages?

That's good news

No problem using a BC109 instead of a BC108. Did that solve the problem with the quiescent current and mid-point voltage?

Check C5 is 6u4 rather than 40uF. If you change it, 4u7 or 10u will be fine. A little bit of crackle from the bass pot in the first few seconds of use is normal, as that cap charges.

The inter-station mute is normally reliable. I'm assuming you've tried twiddling the user-control inside? IIRC, the only fault I've had with that was caused by a diode inside the can for L4.

Yes, the same rexine as the case. There's a dilemma here: given that it's easy to clean up the old handles (if the old tape is still stuck on, leave it in a bowl of water overnight), does a bare handle look worse than a handle with something on it which doesn't match the case? Personally, I lean towards the former...
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