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Old 7th Jul 2019, 2:09 pm   #1
dekor8tor
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Default Hacker GP42 and AL42 restoration help

Good afternoon everyone. I have decided to restore the two amps in the 41 and 42 and I would like to ask for your help from all of the knowledgeable people on this forum
I have read lots of forum posts regarding these amplifiers and with that knowledge I set about taking some readings and i would like some help in interpreting them please.

Thank you.

All of the readings are voltage.

R15 52.3v
R10 6.2v
R14 4.95v
R13 7.4v
R8 1.3v
R16. 0v
R5. 0v
R4. 0.85v
R21. 10.7v
R18. 0.01v
R19. 0.01v

C3. 0.87v
C5. 1.3v
C9. 7.3v
C10. 7.5v
C8. 236v
C16. 253v
C7. 137v
C6. 136v
C14. 0v

I hope some one can help me as I don't want to go changing things that don't need changing these readings are from the AL41.

Many thanks.
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Old 7th Jul 2019, 2:20 pm   #2
ms660
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Default Re: Hacker GP42 and AL42 restoration help

You don't specify with respect to what or whether any of them are AC...

Valve electrode voltage measurements (voltages on the valve pins with respect to chassis) are the best ones to post up, keeping in mind that the valve heaters are AC fed.

Lawrence.
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Old 7th Jul 2019, 2:22 pm   #3
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Default Re: Hacker GP42 and AL42 restoration help

The readings you have are ambiguous, were they taken across the component, one side to chassis (which side) etc?

Best thing is to get the manual and use the voltages in that, also if there any Supamold capacitors you will probably find they are faulty, replace the ones connected to the control grids of the output pentodes.

Crossed with Lawrence.
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Old 7th Jul 2019, 2:35 pm   #4
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Default Re: Hacker GP42 and AL42 restoration help

In addition to the two previous posts I wonder if the two amps actually work? It would be useful to know that.
Cheers
John
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Old 7th Jul 2019, 4:02 pm   #5
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Default Re: Hacker GP42 and AL42 restoration help

It is also a good idea to state which service information you are using and hence what the component numbers refer to.

Mike
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Old 7th Jul 2019, 4:23 pm   #6
dekor8tor
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Default Re: Hacker GP42 and AL42 restoration help

Hi thanks for that i will post the voltages against the ones in the manual that i downloade from here all the voltages are dc
Many thanks
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Old 7th Jul 2019, 6:23 pm   #7
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Default Re: Hacker GP42 and AL42 restoration help

Please note what the first two posts asked, this is important to fully understand your voltage readings.
Cheers
John
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Old 7th Jul 2019, 11:03 pm   #8
dekor8tor
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Default Re: Hacker GP42 and AL42 restoration help

hi all i have measured dc voltage as mentioned in the service manual as follows

H.T.1 C12 280vdc
H.T.2 C11 264vdc
H.T.3 C8 237vdc
V1a pin 9 anode 150vdc
V2a pin 9 anode 167vdc
V1b pin 6 anode 258vdc
V2b pin 6 anode 260vdc
V1a pin 2 cathode 0.8vdc
V2a pin 2 cathode 1.3vdc
V1b pin cathode 7.3vdc
V2b pin 7 cathode 7.4vdc

R15 52.3v
R10 6.2v
R14 4.95v
R13 7.4v these are resistor voltages across the resistors dc
R8 1.3v
R16. 0v
R5. 0v
R4. 0.85v
R21. 10.7v
R18. 0.01v
R19. 0.01v

C3. 0.87v
C5. 1.3v
C9. 7.3v
C10. 7.5v
C8. 236v these are capacitor voltages across the capacitor dc
C16. 253v
C7. 137v
C6. 136v
C14. 0v


i hope these make sense to someone because if any components have failed i would like to change them before i use the equipment
many thanks for you help

https://www.vintage-radio.net/forum/...d.php?t=111739

Last edited by dekor8tor; 7th Jul 2019 at 11:09 pm.
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Old 7th Jul 2019, 11:08 pm   #9
dekor8tor
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Default Re: Hacker GP42 and AL42 restoration help

this was the thread i was following to get my voltage readings

https://www.vintage-radio.net/forum/...d.php?t=111739
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Old 8th Jul 2019, 1:03 am   #10
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Default Re: Hacker GP42 and AL42 restoration help

The voltages are not that far out, I have not taken note of the voltages across the resistors and capacitors only the ones from the manual.
Check C6 and C7, those being faulty can cause damage, the other slight differences in the voltages will be small changes in the value of some resistors and degraded emission in the valves but they are not that bad.

Has John asked do the amplifiers work and if so how well?

If C6 and C7 are good and the amplifier works well I would leave it as it is.

See what other opinions are.
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Old 8th Jul 2019, 6:18 am   #11
dekor8tor
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Default Re: Hacker GP42 and AL42 restoration help

Good morning Frank thanks for that yes both amplifiers work i was just making sure there were no problems as i do get a buzzing sound from both
Looking at the amps c6 and c7 both look like mustard mullards
How do i check the secondary voltage thanks and heater to V3 and V1 i dont know the pin layout on that valve it doesnt show that in the manual
Many thanks
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Old 8th Jul 2019, 8:03 am   #12
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Default Re: Hacker GP42 and AL42 restoration help

The three valves all have their heater connection on pins 4 and 5, the ECL86 have two valves in each envelope, triode and pentode, their heaters are connected to pins 4 and 5. Your meter on AC volts 10v or similar setting depending on what ranges your meter as and test across pins 4 and 5 on all three valves.
The rectifier V3 is the same but it has its own heater winding on the trannsforner.

The “secondaries “ this is the supply to the anodes of the rectifier, meter set to AC volts range 1000v. One meter lead to chassis the other taking separate readings on pins 1 and 7, both should be in the range of 250VAC.

I will be surprised if there is much wrong with them.

Need more information on the buzzing.
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Old 8th Jul 2019, 8:04 am   #13
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Default Re: Hacker GP42 and AL42 restoration help

The heater pins on all B9A base valves are pins 4 & 5. Viewed from underneath, count clockwise from the gap. Nominal heater voltage is 6.3VAC. measured across the pins i.e., between pin 4 & pin 5. The secondary voltages (also AC) are measured across the Transformer windings. In the case of the HT secondary, the voltage can be measured between earth and each end of the winding, although, as the HT DC voltages are reasonable, there is no real need to measure the HT secondary voltage at the transformer. Likewise, if the voltage at the heater pins of each valve is approx. 6.3VAC, there is no need to measure that at the transformer. (Edit; cross-posted with Frank (Nuvistor)
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Old 8th Jul 2019, 8:11 am   #14
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Default Re: Hacker GP42 and AL42 restoration help

I presume you mean a “buzzing sound” when you touch the input terminals, relatively silent otherwise?
Cheers
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Old 8th Jul 2019, 9:30 am   #15
Michael Maurice
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Default Re: Hacker GP42 and AL42 restoration help

Whenever I service these amplifiers, I replace the following on sight before powering up:

The two 220K anode load resistors on the top tag strip.

The two 820K and 1M resistor around the valve holders.

The two 2K2 and 220R resistors on the bottom tag strip.

The for 47uF capacitors on the bottom tag strip

The two mustard capacitors are almost always OK.
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Old 8th Jul 2019, 11:10 am   #16
dekor8tor
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Default Re: Hacker GP42 and AL42 restoration help

Good morning Michael thankyou for tha information it is greatly appreciated Can you point me in the right direction as to where to buy these components from please
Many thanks
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Old 8th Jul 2019, 11:47 am   #17
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Default Re: Hacker GP42 and AL42 restoration help

I find radio spares a good source for parts
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Old 8th Jul 2019, 12:00 pm   #18
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Default Re: Hacker GP42 and AL42 restoration help

That's RS Components nowadays- unless you're also a bit vintage

(Rebranded in 1971)
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Old 8th Jul 2019, 12:04 pm   #19
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Default Re: Hacker GP42 and AL42 restoration help

I use Vishay 2W Metal resistors available from RS. They are rated at 500V, modern carbon films are not rated for high voltage.

I also use Vishay axial capacitors.
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Old 8th Jul 2019, 12:21 pm   #20
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Default Re: Hacker GP42 and AL42 restoration help

Even after you've changed all the components mentioned above, you'll still have the 'buzzing', as none of the ones mentioned will be the cause of that!

It's absolutely no good whatsoever just jumping in and replacing components randomly without actually knowing why you're changing them and whether they're actually faulty in the first place, as it's been proven many times on here that it'll only lead to disaster!

Proper fault finding and an analytical and logical approach is what's needed. Firstly, the buzzing sound, is it a buzzing sound or is it more of a hum?

Is this 'buzz' or hum regardless of volume setting, ie, does it disappear when the volume control is turned right down to minimum?

It's possible you may have a bad signal ground problem at amplifier input stage, or if the 'hum' is there regardless of volume control then you could have a problem with one of the smoothing capacitors, but you MUST do proper fault finding!!
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