Fidelity HF35T (Transistor version). - Page 5 - UK Vintage Radio Repair and Restoration Discussion Forum

Sideband · 6th Oct 2025, 12:55 pm

Quote:

Originally Posted by HRichard

Outputs - one is marked AC128 but the other has no markings so assuming AC126?

No AC126 is PNP and lower power. More likely AC127 which is NPN complement to AC128. I'm not at home at the moment so can't check the circuit. It could also be AC176

Sideband · 6th Oct 2025, 6:37 pm

If you need a couple of transistors, as I said in post #74, I can let you have a couple for cost of postage. Actually the AC176 and AC128 is a very common pairing and the AC176 has a higher collector current rating than the AC127 so it's a better choice. You would be better changing the pair since the AC128 may have been stressed as well. Also check the two 2.2 ohm emitter resistors. When the output stage is working correctly, you should be able to measure approximately half the supply rail at the junction of the two resistors.

I thought I had a circuit for the HF35 but its the HF25.....

HRichard · 6th Oct 2025, 6:42 pm

Many thanks sideband.
I will message you now.

HRichard · 13th Oct 2025, 12:17 pm

The kindly donated transistors have made no difference at all.
What next

Sideband · 13th Oct 2025, 12:46 pm

Take some voltages around the output transistors. What is the mid-point voltage?

HRichard · 13th Oct 2025, 1:46 pm

Apologies again for my lack of knowledge, but how do I measure midpoint voltage exactly?
(I did use the search function but couldn't find a how to explanation).

Boulevardier · 13th Oct 2025, 3:48 pm

Quote:

Originally Posted by HRichard

Apologies again for my lack of knowledge, but how do I measure midpoint voltage exactly?
(I did use the search function but couldn't find a how to explanation).

Meter set to DC Volts; black probe on negative lead of C7 (500 microF) and red probe on junction of R9 and R10 (both 2.2 ohms).

Another point, there are 3 electrolytic capacitors in this circuit, and unless they've already been replaced they are ca. 50 years old - even if they haven't failed yet, they probably will eventually. Sensible to just replace them as a precaution (those meters [post#72) can give strange results with electrolytics, and you can destroy the meter if they aren't thoroughly discharged, so safer and more thorough just to replace).

Mike

HRichard · 13th Oct 2025, 5:25 pm

Hi Mike,
can I ask to which circuit are you referring please?

Boulevardier · 13th Oct 2025, 5:45 pm

I was looking at this schematic. Have I got the right one?

Doc6.docx

Keith956 · 13th Oct 2025, 6:26 pm

Quote:

Originally Posted by Boulevardier

I was looking at this schematic. Have I got the right one?

That schematic has an error in it - the junction of R2 and R3 should be connected to the base of TR1, else it will be floating!

Boulevardier · 13th Oct 2025, 6:29 pm

Quote:

Originally Posted by Keith956

Quote:

Originally Posted by Boulevardier

I was looking at this schematic. Have I got the right one?

That schematic has an error in it - the junction of R2 and R3 should be connected to the base of TR1, else it will be floating!

Yes agreed - if it wasn't connected to there there'd be no point in having two separate resistors!

Station X · 13th Oct 2025, 6:30 pm

The circuit was reverse engineered back in post #22.

SPICE details in post #78.

HRichard · 13th Oct 2025, 6:56 pm

Unfortunately I don't understand or have any knowledge of SPICE and for just a simple amplifier circuit where everything appears to test OK is just frustrating me immensely. I'm starting to think the fault is somewhere before the amp....

Edward Huggins · 13th Oct 2025, 7:18 pm

The only thing before the amp is the cartridge and we've been there already. After 93 Posts on one of the most simple amplifiers in the World, maybe it's time to get a second opinion, hands-on, from a local Member near to you?

Boulevardier · 13th Oct 2025, 7:22 pm

Quote:

Originally Posted by Edward Huggins

The only thing before the amp is the cartridge and we've been there already.

I can't remember whether this has been mentioned earlier in the thread, but did the OP take a soldering iron to the connector pins on the new cartridge, or solder the leads to the slide-connectors while they were still on the cartridge pins (which makes it easier to fix them while soldering to them)?

Just a thought...

Mike

Sideband · 13th Oct 2025, 7:27 pm

Irresepective of which circuit, please measure the voltage at the junction of the two 2.2 ohm resistors. These should be easily identifiable.

HRichard · 13th Oct 2025, 8:02 pm

Quote:

Originally Posted by Boulevardier

Quote:

Originally Posted by Edward Huggins

The only thing before the amp is the cartridge and we've been there already.

I can't remember whether this has been mentioned earlier in the thread, but did the OP take a soldering iron to the connector pins on the new cartridge, or solder the leads to the slide-connectors while they were still on the cartridge pins (which makes it easier to fix them while soldering to them)?

Just a thought...

Mike

Nothing soldered on or near the cartridge.

HRichard · 13th Oct 2025, 8:03 pm

Quote:

Originally Posted by Edward Huggins

The only thing before the amp is the cartridge and we've been there already. After 93 Posts on one of the most simple amplifiers in the World, maybe it's time to get a second opinion, hands-on, from a local Member near to you?

Happy to do that if anyone is around the Coventry / Leicester area?

HRichard · 13th Oct 2025, 8:04 pm

Quote:

Originally Posted by Sideband

Irresepective of which circuit, please measure the voltage at the junction of the two 2.2 ohm resistors. These should be easily identifiable.

Will do tomorrow.

HRichard · 14th Oct 2025, 1:25 pm

Quote:

Originally Posted by HRichard

Quote:

Originally Posted by Sideband

Irresepective of which circuit, please measure the voltage at the junction of the two 2.2 ohm resistors. These should be easily identifiable.

Will do tomorrow.

I have 27v

6th Oct 2025, 6:37 pm	#82
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 8,354	Re: Fidelity HF35 If you need a couple of transistors, as I said in post #74, I can let you have a couple for cost of postage. Actually the AC176 and AC128 is a very common pairing and the AC176 has a higher collector current rating than the AC127 so it's a better choice. You would be better changing the pair since the AC128 may have been stressed as well. Also check the two 2.2 ohm emitter resistors. When the output stage is working correctly, you should be able to measure approximately half the supply rail at the junction of the two resistors. I thought I had a circuit for the HF35 but its the HF25..... __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman..... Last edited by Sideband; 6th Oct 2025 at 6:47 pm.

6th Oct 2025, 6:42 pm	#83
HRichard Tetrode Join Date: Sep 2025 Location: Coventry, West Midlands, UK. Posts: 74	Re: Fidelity HF35 Many thanks sideband. I will message you now.

13th Oct 2025, 12:17 pm	#84
HRichard Tetrode Join Date: Sep 2025 Location: Coventry, West Midlands, UK. Posts: 74	Re: Fidelity HF35 The kindly donated transistors have made no difference at all. What next

13th Oct 2025, 12:46 pm	#85
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 8,354	Re: Fidelity HF35 Take some voltages around the output transistors. What is the mid-point voltage? __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

13th Oct 2025, 1:46 pm	#86
HRichard Tetrode Join Date: Sep 2025 Location: Coventry, West Midlands, UK. Posts: 74	Re: Fidelity HF35 Apologies again for my lack of knowledge, but how do I measure midpoint voltage exactly? (I did use the search function but couldn't find a how to explanation).

13th Oct 2025, 5:25 pm	#88
HRichard Tetrode Join Date: Sep 2025 Location: Coventry, West Midlands, UK. Posts: 74	Re: Fidelity HF35 Hi Mike, can I ask to which circuit are you referring please?

13th Oct 2025, 5:45 pm	#89
Boulevardier Nonode Join Date: Sep 2017 Location: Bristol, UK. Posts: 2,310	Re: Fidelity HF35 I was looking at this schematic. Have I got the right one? Doc6.docx

13th Oct 2025, 6:30 pm	#92
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 22,853	Re: Fidelity HF35 The circuit was reverse engineered back in post #22. SPICE details in post #78. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

13th Oct 2025, 6:56 pm	#93
HRichard Tetrode Join Date: Sep 2025 Location: Coventry, West Midlands, UK. Posts: 74	Re: Fidelity HF35 Unfortunately I don't understand or have any knowledge of SPICE and for just a simple amplifier circuit where everything appears to test OK is just frustrating me immensely. I'm starting to think the fault is somewhere before the amp....

13th Oct 2025, 7:18 pm	#94
Edward Huggins Dekatron Join Date: Sep 2015 Location: Southwold, Suffolk, UK. Posts: 9,180	Re: Fidelity HF35 The only thing before the amp is the cartridge and we've been there already. After 93 Posts on one of the most simple amplifiers in the World, maybe it's time to get a second opinion, hands-on, from a local Member near to you? __________________ Edward.

13th Oct 2025, 7:27 pm	#96
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 8,354	Re: Fidelity HF35 Irresepective of which circuit, please measure the voltage at the junction of the two 2.2 ohm resistors. These should be easily identifiable. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....