Fidelity HF35T (Transistor version). - Page 8 - UK Vintage Radio Repair and Restoration Discussion Forum

julie_m · 17th Oct 2025, 2:00 pm

A ceramic capacitor will be fine in the C4 position. There are circuits where ceramic capacitors are preferrable to polyfilm, but this isn't one of them.

Now, if C3 (50µF; 47 will be fine as a replacement, anything between 10 and 22µF will do for testing, just take care with the polarity) were faulty, that would cause a loss of gain, while all DC voltages appeared beyond reproach.

Boulevardier · 17th Oct 2025, 2:17 pm

Quote:

Originally Posted by HRichard

Q4 is dead.

Could the OP tell us what is meant by "dead"? How did he measure C4, and what resistance readings (if any) did he get (see posts 132 and 134)?
Mike

HRichard · 17th Oct 2025, 2:27 pm

Dead as in open circuit on the meter Mike.
I've dropped in an electrolytic as that was all I had of that value.
I have sound through the speaker but it is still very low volume, but at least progress.

julie_m · 17th Oct 2025, 2:43 pm

A capacitor will read open-circuit on a resistance meter, unless it is faulty (and NB, not all capacitor faults will cause it to have a finite resistance, so open-circuit doesn't necessarily mean it's not faulty).

The C4 position should not need to be an electrolytic capacitor, as it will be a low value (100nF?). What value did you use?

HRichard · 17th Oct 2025, 3:19 pm

Quote:

Originally Posted by julie_m

The C4 position should not need to be an electrolytic capacitor, as it will be a low value (100nF?). What value did you use?

It was 0.1uf but all I had until tomorrow.
I will also look at C3 but changing C4 has definitely made a step forward.

julie_m · 17th Oct 2025, 3:59 pm

A 0.1µF capacitor is very unlikely to be electrolytic -- non-electrolytic capacitors are readily available up to at least 1µF. Can you post a picture of it, please?

HRichard · 17th Oct 2025, 4:14 pm

Quote:

Originally Posted by julie_m

A 0.1µF capacitor is very unlikely to be electrolytic -- non-electrolytic capacitors are readily available up to at least 1µF. Can you post a picture of it, please?

A little fuzzy but I think you can make it out.

julie_m · 17th Oct 2025, 4:58 pm

Well, it looks like 0.1µF -- and yet it looks like an electrolytic!

Très étrange, as they say in Paris! Every day's a school day .....

Boulevardier · 17th Oct 2025, 5:03 pm

Has it got any markings to distinguish positive from negative leads?

HRichard · 17th Oct 2025, 5:11 pm

Quote:

Originally Posted by Boulevardier

Has it got any markings to distinguish positive from negative leads?

Yes it does

music-centre · 17th Oct 2025, 6:21 pm

Yes, surprising they make them in that small value. I saw these on the CPC website Today when ordering some parts though.

https://cpc.farnell.com/multicomp-pro/mcmhr50v104m4x7/capacitor-0-1uf-50v-radial-105/dp/CA08233

HRichard · 17th Oct 2025, 7:08 pm

As a side question, it looks like someone in the past has soldered an extension to the headshell wiring. Is this acceptable?

Boulevardier · 17th Oct 2025, 7:28 pm

Quote:

Originally Posted by HRichard

As a side question, it looks like someone in the past has soldered an extension to the headshell wiring. Is this acceptable?

Depends what it is. Can you post a very clear photo of this?

HRichard · 17th Oct 2025, 7:31 pm

The joint is covered with heat shrink sleeve so wouldn't show anything.
I only know that wires have been joined because the wire colour used doesn't match.

Boulevardier · 17th Oct 2025, 7:34 pm

Quote:

Originally Posted by HRichard

The joint is covered with heat shrink sleeve so wouldn't show anything.
I only know that wires have been joined because the wire colour used doesn't match.

Could be crucial. You'll need to cut off the heatshrink by slicing it along length with a sharp blade and then peeling away.

HRichard · 17th Oct 2025, 7:50 pm

In what way crucial, Mike?

Sideband · 17th Oct 2025, 8:25 pm

What is the original value of C4? You say you've used a 0.1uF (which seems a bit low) which is all you had.

Too low a value will likely cause low sound.

HRichard · 17th Oct 2025, 8:49 pm

Quote:

Originally Posted by Sideband

What is the original value of C4? You say you've used a 0.1uF (which seems a bit low) which is all you had.

Too low a value will likely cause low sound.

C4 (Mylar I think) is marked 0.1k 100

Station X · 18th Oct 2025, 7:21 am

The service sheet shows C4 as being 0.1uF. I think we've been round this loop a few times now.

Quote:

Originally Posted by Station X

If you're using the diagram attached to post #22, C4 is the capacitor connected to the wiper of the volume control. The service sheet for the HF35, where it's designated C3, gives a value of 0.1uF.

Sideband · 18th Oct 2025, 11:56 am

OK, it will be an ordinary capacitor...I wouldn't use an electrolytic here even if you do have such a thing at 0.1uF. There is no real polarising voltage available so just an ordinary Mylar or polystyrene type will be fine.

17th Oct 2025, 2:00 pm	#141
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 8,006	Re: Fidelity HF35T (Transistor version). A ceramic capacitor will be fine in the C4 position. There are circuits where ceramic capacitors are preferrable to polyfilm, but this isn't one of them. Now, if C3 (50µF; 47 will be fine as a replacement, anything between 10 and 22µF will do for testing, just take care with the polarity) were faulty, that would cause a loss of gain, while all DC voltages appeared beyond reproach. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

17th Oct 2025, 2:27 pm	#143
HRichard Tetrode Join Date: Sep 2025 Location: Coventry, West Midlands, UK. Posts: 74	Re: Fidelity HF35T (Transistor version). Dead as in open circuit on the meter Mike. I've dropped in an electrolytic as that was all I had of that value. I have sound through the speaker but it is still very low volume, but at least progress.

17th Oct 2025, 2:43 pm	#144
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 8,006	Re: Fidelity HF35T (Transistor version). A capacitor will read open-circuit on a resistance meter, unless it is faulty (and NB, not all capacitor faults will cause it to have a finite resistance, so open-circuit doesn't necessarily mean it's not faulty). The C4 position should not need to be an electrolytic capacitor, as it will be a low value (100nF?). What value did you use? __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

17th Oct 2025, 3:59 pm	#146
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 8,006	Re: Fidelity HF35T (Transistor version). A 0.1µF capacitor is very unlikely to be electrolytic -- non-electrolytic capacitors are readily available up to at least 1µF. Can you post a picture of it, please? __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

17th Oct 2025, 4:58 pm	#148
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 8,006	Re: Fidelity HF35T (Transistor version). Well, it looks like 0.1µF -- and yet it looks like an electrolytic! Très étrange, as they say in Paris! Every day's a school day ..... __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

17th Oct 2025, 5:03 pm	#149
Boulevardier Nonode Join Date: Sep 2017 Location: Bristol, UK. Posts: 2,310	Re: Fidelity HF35T (Transistor version). Has it got any markings to distinguish positive from negative leads?

17th Oct 2025, 6:21 pm	#151
music-centre Heptode Join Date: Sep 2007 Location: Brentwood, Essex, UK. Posts: 812	Re: Fidelity HF35T (Transistor version). Yes, surprising they make them in that small value. I saw these on the CPC website Today when ordering some parts though. https://cpc.farnell.com/multicomp-pro/mcmhr50v104m4x7/capacitor-0-1uf-50v-radial-105/dp/CA08233

17th Oct 2025, 7:08 pm	#152
HRichard Tetrode Join Date: Sep 2025 Location: Coventry, West Midlands, UK. Posts: 74	Re: Fidelity HF35T (Transistor version). As a side question, it looks like someone in the past has soldered an extension to the headshell wiring. Is this acceptable?

17th Oct 2025, 7:31 pm	#154
HRichard Tetrode Join Date: Sep 2025 Location: Coventry, West Midlands, UK. Posts: 74	Re: Fidelity HF35T (Transistor version). The joint is covered with heat shrink sleeve so wouldn't show anything. I only know that wires have been joined because the wire colour used doesn't match.

17th Oct 2025, 7:50 pm	#156
HRichard Tetrode Join Date: Sep 2025 Location: Coventry, West Midlands, UK. Posts: 74	Re: Fidelity HF35T (Transistor version). In what way crucial, Mike?

17th Oct 2025, 8:25 pm	#157
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 8,354	Re: Fidelity HF35T (Transistor version). What is the original value of C4? You say you've used a 0.1uF (which seems a bit low) which is all you had. Too low a value will likely cause low sound. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

18th Oct 2025, 11:56 am	#160
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 8,354	Re: Fidelity HF35T (Transistor version). OK, it will be an ordinary capacitor...I wouldn't use an electrolytic here even if you do have such a thing at 0.1uF. There is no real polarising voltage available so just an ordinary Mylar or polystyrene type will be fine. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....