President HR2510 distorted RX audio

[Alexis Mercado]

Hi. I have a President HR2510 with a 5 pin microphone. RX audio is ok with volume at 9:00 am position. When I turn volume past 9:00, audio begin to get distorted and garbled. The more you turn the volume up, the more distorted it gets.

What can be the problem ?

 

[Handy Andy]

Locate C95 - and Pin 1 of your TDA1905 chip -

From Pin 1 of TDA 1905 to ground - reads 220 ohms? Then Chip ok, else you're going to have to dig out that schematic and fix some blown parts.

C95 is the output cap - so Locate Pin 1 of TDA1905 (again) and Ω measure back to the Speaker - one side of speaker should test nearly equal the other.

Now check speaker terminals to ground - one should be the ohms stated on the back of the speaker, while the other would be zero. (Board ground)

Now remove unsolder the speaker wires and check speaker terminals at speaker to ground make sure they are open.

Then measure Ohmic - from Speaker wires to board ground - again one should be a dead short - but the other should show OPEN - if not, replace C95.

Then check that jumper pin header for speaker Pins 7 and 1 are jumpered for internal speaker.

 

[Alexis Mercado]

Thanks Andy. I’ll verify that.

 

[Handy Andy]

In my experiences, the speaker gets old, the ohmic seems to stay, but the cone wears out. Used in damp Field Day conditions and other conditions as such can ruin the speaker.

So, if it's original - that may be a signal that it is on the way out - just plan on replacing it.

 

[999]

In my experiences, the speaker gets old, the ohmic seems to stay, but the cone wears out. Used in damp Field Day conditions and other conditions as such can ruin the speaker.

So, if it's original - that may be a signal that it is on the way out - just plan on replacing it.

I would think there is also the possibility that the speaker is just blown. These radios are getting on 30 years old and used paper cone speakers.

 

[Alexis Mercado]

I used another speaker and sounds the same

 

[SuperLid]

Hi. I have a President HR2510 with a 5 pin microphone. RX audio is ok with volume at 9:00 am position. When I turn volume past 9:00, audio begin to get distorted and garbled. The more you turn the volume up, the more distorted it gets.

What can be the problem ?

Bad speaker? Have you tried an external speaker yet? Also, look for metal filings that are stuck to the cone around the areas close to the magnet. Usually around the dust cap and where the cone meets the spider.

 

[sp5it]

Put sine wave on mic socket, than observe on the scope how it looks on input and output of audio chip.

 

[SuperLid]

Put sine wave on mic socket, than observe on the scope how it looks on input and output of audio chip.

I would start at the simple obvious, like the speaker.

 

[sp5it]

I would start at the simple obvious, like the speaker.

I used another speaker and sounds the same

 

[Alexis Mercado]

It has a bad audio chip.

 

[Alexis Mercado]

By the way, Is this a first generation HR2510 ?
A modification was made by the previous owner. Do you have any idea of the
modification ?

See pictures.

 

[Handy Andy]

Good call on the bad audio chip...

Saw this - this is interesting - this is a Mute "mod" Unden installed for better control of the TDA1905 - I have one too.


The main PCB board - to me - has several variants - this is one of the LATER ones. the circuit shown on that riser card is this on the schematic.



They took it (the circuit) out for some reason - the board still functions without it - but the riser card only serves to replace those parts missing from the subsequent board.

The earlier ones the Pin 4 works like a simple mute but the cap resistor combo helps with the opening up of the amp to operate - for most of the time if the noise threshold didn't hold it up - the TDA's big amp used for the audio - remained off or at quiescent current to keep the radio cool inside - else the audio chip ran hot from audio drive all the time - the Pin 4 works with Pin 5.

They never imagined the caps used like this could dry out and cause even more issues.

 

[Alexis Mercado]

Handy Andy, Which one is the TDA1905 chip ?

 

[SuperLid]

Handy Andy, Which one is the TDA1905 chip ?

The AF amp IC is not visible in the photo. It is a PDIP packge TDA1905, obsolete now, but you might find them around. Worst case, NTE7000 is supposed to be a sub.